Ok to make direct connection between vehicle and leisure battery in a scrape? - Page 1
Forum section: electronics / beginners

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: May 13, 2025, 05:48:11 pm
content: Not meant as a usual practice but if the vehicle battery dies while on the road yet there is a huge surplus of solar power feeding the leisure battery.In this case can a direct connection be made between the two, even though different types, sealed/lead I believe for vehicle and lifepo4 for the leisure. 72ah and 105ah respectively.I was told on an off-grid forum this would be fine and if the vehicle battery is at 0% or too low to start then it both will equalize to 50/50. Is that right?I first thought about this idea when I got to thinking about using things like jump start packs or even standalone chargers, which could be charged from leisure or vehicle battery with the relevant connectors. However, of these solutions I came to realize that if not using mains supply to charge them, which I would not as the idea is to keep self-sufficient and off-grid, then the 3rd device is moot is it not? as you are just moving power from one or other of the batteries to another device to charge whichever one is low.As such I thought might as well cut out the middle man and just make sure either one has enough charge at any time to provide to the other if the occasion arises.I forgot about the idea for a few months as I didn't see any pressing incidence to use it. The other day though while driving my battery light was coming on during a drive indicating an alternator problem it seems. The alternator is still working when I have tested with voltmeter on terminals and the battery light does go off but the issue seems to be coming and going. No drastic drops in voltage yet on the battery and it is still indicating full charge when I finished the drive in question but just planning it trouble comes.So I would like to know in case this happened and the battery drained while driving. As the dc-dc charger I have is not bi-directional the solar power is not able to be utilized unless bypassing it. Since the wires already run between the two with that I was advised by the aforementioned folks to just bypass the connection with the positive of the vehicle battery and leisure battery. A switch was suggested for this but I don't want anything as permanent as that as I don't intend to have to use it except in exceptional circumstances if it was called for. Or would that be a more advised way to approach it?If so what is the proper name for such a switch as I am only familiar with on/off type switches whereas this will be to switch from one circuit to another so what is the technical name for that? Hmm just looking generally at switches for such amps they are expensive anyway so seems the clips are a much cheaper solution for something that would only be used extremely rarely if at all.Would it just be a case of clipping an alligator wire to an exposed post for each? There is already a switch to break the dc charger connection when I don't want it in use, like now when there is loads of solar, so I am thinking I could just flip the cap off that and connect to that and then to the positive busbar for the leisure battery.Is it right that if there was an issue with the alternator then the battery would not be able to run the vehicle for long, even if it started? So I might have to do this trick every few miles until I get to a place to better deal with the issue? Still better than being stranded and having to call to be towed away.If the theory is sound is it just a case of getting alligator wire which will take a continuous current over above 200a is that right or if not what is the right minimum amp rating to use?Below is an image I made a bit ago of the wiring of the system. Not the most clear but no way I am making another one as that took me ages and is accurate, if convoluted.

author: Gyro
author_rank: Super Contributor
timestamp: May 13, 2025, 06:29:14 pm
content: The secret here is limiting the current, this is where cheap Aluminium jump leads are a positive advantage. A leisure battery isn't designed to provide the high current required by the starter... and the jumper wire certainly won't be! What you need to do is put just enough capacity into the car battery that its low internal and circuit resistance is just sufficient to start the engine. You don't need to keep the link wire on until the batteries have leveled 50/50, just enough to allow the engine to start and let the alternator take over the task. This is the way the little Lithium starter packs work, there's no way they could ever provide enough current to run the starter, they just dump charge into the car battery and let it's own low internal resistance provide enough short term current to operate the starter.So yes, as long as the batteries share a common negative, a (well insulated) clip lead between their positive posts will put charge into the vehicle battery. You want to make the lead long to provide sufficient series resistance - remember the negative connection between the batteries through normal wiring looms won't be rated for starter current either!.Determine the length of jumper wire necessary to provide sufficient series resistance (from wiring tables). Use wire rated for around 20A and determine the resistance to provide, say, 10-15A. The vehicle battery should jump up to around 11V more or less immediately so you can use that as a basis for your resistance calculation. Clip to wire on for a couple of minutes, monitoring the wire temperature closely, then disconnect before starting (otherwise itwillsmoke!).You have the problem that the lead has a battery at each end, so you need to be very careful about accidental contact with exposed metalwork at either end when clipping and removing. Place it away from the vehicle on the ground until the other end is removed. As extra safety, you could add an in-line fuse-holder in the middle of the lead. The fuse rating needs to be just high enough that it doesn't blow on instantaneous connection current, maybe 30A, and keep spares.Finally, get to the bottom of the charging fault. Total discharge of a vehicle battery won't do it any god at all.EDIT: I just noticed the DC-DC charger in your diagram. If there is anything other than a direct Negative connection between the batteries, it could complicate thingsa lot. In this case (or if in doubt), you would be better to completely disconnect both terminals of the leisure battery and use both a positive and a negative jump lead.

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: May 13, 2025, 06:54:59 pm
content: Thanks for the comments.Quote from: Gyro on May 13, 2025, 06:29:14 pmWell you miss the main point. I am saying if worse came to worst and the alternator was not working at all! It might be intermittent but plan for the worst, would it work to just repeating the process over and over to 'bump' along a few miles at a time topping it up as much as I could from the leisure til they equalize and then driving a bit and repeat?QuoteIndeed that is why I thought it was just moving energy from one place to another. I am aware it would not require going to 50% in normal conditions but my question is about alternator total failure.QuoteHmm this series rating stuff is new to me. Won't the existing wires be fine which are used for the dc-dc charger already? They are rated at 110a I believe. I have both positive and negative running the full length. In other words not using chasis as common ground as it was strongly advised against by the dc charger manufacturer.QuoteWhy so low, I guess this is only for you answering to make mimumum charge but I am asking for equalization charge ratings. So, as above, will the dc charger wire not suffice? I read up in the meantime and it seems that the relay I would want is single pole double throw right? Or even simpler just another on/off switch coming out before the on/off switch to the dc-dc charger and after the positive out exit of the charger right? Then if it is switched it would be bypassing and if off it would just be inactive and the normal dc route would be active. What if both were accidentally on though? Bad things? Maybe better for a two inputs single output switch instead to avoid that?QuoteOf course. This is not a standard practice thing, as noted; it is an emergency use only scenario. The priority is always to solve issues but if otherwise stranded this is the 'if all else fails' scenario.See the image below.I am just reading on the subject and it seems this is actually a 'thing' already to link the batteries together 'in case of emergency' as the image indicates. I noticed that the switches I have already been using for on/off of batteries are also used for this. I now see why they are called battery isolator switches.QuoteWhat do you mean by anything between them? It is just like in the diagram. Not sure what is meant by the question. They are linked together via the negative busbar, as is everything else on the leisure side, which in turn are all linked to 'ground' point of metal on the chasis.

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: May 13, 2025, 07:04:46 pm
content: Ah just remembering the rules on parallel linking the existing wires would not be enough would they as parallel v stays the same but a increase so would have to have wire 177a + 20% right?The dc charger being rated at 60a I guess prevents this from going too high so if this middleman was removed it would be no good would it and for those rated wires? I have no intention of rewiring for this so maybe I could add some simple device to keep the amps low enough that the 110a rated wire would tolerate?What device in between would work for that?I have used step downs for voltage for tiny circuits, would something like that but for amps be the case here? Just want very simple, no special monitoring or whatever just a blunting of the amps passing through the circuit to within range of the wire, so what would be called for there? Might be a rudimentary answer but I am not versed on the nuts and bolts of the components of circuits.

author: jpanhalt
author_rank: Super Contributor
timestamp: May 13, 2025, 08:05:43 pm
content: QuoteThere are 2 points to be made: 1) Getting the car restarted; and 2) Keep it running. Unfortunately, very few cars have magneto ignitions.I had a 2000 Honda Accord and in about 2002-2003 the alternator went bad. There were no indications, until I was on an expressway and everything died. I was able to coast to the edge. Called a cab,* got a round trip home, and returned with a 12V marine battery + jumper leads. I was able to get it home, but was surprised how much current it took.How much power can your leisure battery provide for how long?*Finding a telephone was the hardest part. I didn't have a cell phone, but promptly got one just for use in such emergencies.

author: SteveThackery
author_rank: Super Contributor
timestamp: May 13, 2025, 11:53:35 pm
content: Where does the alternator connect in your diagram?

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: May 14, 2025, 04:54:04 am
content: Quote from: jpanhalt on May 13, 2025, 08:05:43 pmHow do you mean? It is 105ah as mentioned, or you mean the continuous discharge or 'cold cranking' rate? I don't understand that latter term but remember it was relevant for when I was briefly investigating using it to power a winch which I quickly abandoned when I found it totally unsuitable for the task due to the massive peak amps required in short bursts, which it was not designed for. Not sure what the relevant piece of information is buthereis the technical data sheet if you could find it.

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: May 14, 2025, 04:56:41 am
content: Quote from: SteveThackery on May 13, 2025, 11:53:35 pmWell that wasn't put in at the time of making it as I was only doing the leisure system electronics, with the exception of the link to the vehicle battery, and anyone I would show it would assume it was in the default place.It is just the normal vehicle alternator setup so would linked to the vehicle battery.

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: May 14, 2025, 05:08:47 am
content: Quote from: Gyro on May 13, 2025, 06:29:14 pmOh yea I was thinking about this more overnight. Any accidental contact with bare metal, which could easily happen if not being extremely careful means short circuiting doesn't it! Does not sound fun! Building a switched circuit seems much safer in light of this, yes?Another thought I had is although my dc-dc charger is not bi-directional, for charging the vehicle battery from the panels, I do remember seeing standalone products which do this and from what I remember they were not that expensive, so maybe an alternative option.EDIT: Ok just reading more and see there are products called 'battery balancers' which seem to do what I am wanting however looking at the output of the couple I have seen so far they only deliver 1a current so that would not be much help for getting you very far very fast would it!? You would have to wait like 48 hours for it to give enough charge to give about 1/2 charge to the starter right? Would take a long time to get very far like that unless I am missing something.

author: jpanhalt
author_rank: Super Contributor
timestamp: May 14, 2025, 08:11:39 am
content: Sorry, I missed the 100 Ah part. That should get you home. There's another thread about jumpers. I pointed out there that one must be sure they are long enough. Mine are quite long. They will reach the full length of a car. I ran them from the battery behind the front seat to under the front of the hood and then to the battery. That way, I could get the hood sort of latched for the trip home.

author: SteveThackery
author_rank: Super Contributor
timestamp: May 14, 2025, 09:04:52 am
content: If anyone is struggling with the OP's diagram, I've drawn it out properly (based on the original diagram).@electroniclearner820327: does this look correct?I don't know anything about how caravans/motorhomes charge their batteries, but I'm puzzled to see the leisure battery straight across the busbars and the starter battery connected to the charge controller. Isn't it usually the other way round?Am I right to assume that the busbars are connected to the alternator?

author: Gyro
author_rank: Super Contributor
timestamp: May 14, 2025, 09:37:05 am
content: The redrawn diagram is indeed helpful.The worry I have, as in the edit to my first reply is the behavior of that DC-DC charger. I don't know how that is triggered into charging mode - ignition turned on or rising vehicle battery voltage from alternator charging? Either way, the jumper wire is going to be shorting its output to its input, what happens in that case is unknown - especially if the alternator is intermittently cutting in and out.My suggestion of disconnecting both leisure battery terminals and using both positive and negative jump leads is equally applicable to 'jump' starting and emergency running in this case and avoids any potential harm to the charger. This is effectively what jpanhalt used to get him home with the battery on the rear seat.As for the image of the switch panel in reply #2, I'm afraid it leaves me rather confused about what it means by "Bank" in the context of the three switches. Can the vehicle battery be considered a 'bank'? Unless there is a decent manual describing the use of this panel, I don't think it's possible to comment one way or another about using the 'Emergency Battery Bank Interconnect switch'.Personally I would go for the total leisure battery disconnect and two jumper lead approach as it's least likely to break something.P.S. @SteveThackery, I think the busbars are purely internal to the caravan with leisure battery and all internal loads switched across it. In this context, the starter battery is just an input to this 'wiring system' via the DC-DC charger, which won't won't be operating when parked anyway. The Alternator ground is probably connected to the caravan -ve busbar via the chassis/ tow hitch and umbilical cable (if connected).

author: SteveThackery
author_rank: Super Contributor
timestamp: May 14, 2025, 11:09:11 am
content: Quote from: Gyro on May 14, 2025, 09:37:05 amYou make a good point. Later on today I will redraw it with the starter battery extracted and shown as an input. In truth we don't have enough information. The diagram must include the switches and confirmation of where the alternator connects before we can properly understand what is going on and give reliable advice.I hope the OP will find the extra details for us.

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: May 14, 2025, 11:41:21 am
content: Quote from: jpanhalt on May 14, 2025, 08:11:39 amWow so you were actually driving with them connected!?That goes one step beyond what I had imagined! I was just thinking of connecting and charging til equalized then disconnecting every few miles as it discharged then recharging again. With your improvement you could just drive without interruption until reaching the destination right?I am veering towards the jumper idea now as it seems like the most direct and effective way but how to minimize the risk of short circuiting to the chassis via accidental touching of metal which I can envision could happen extremely easily.Would an inline fuse, as suggested earlier protect everything, both equipment and self, from harm, or are there other things that could be added to make it even safer either by safety practices or somehow stopping errant alligator clips from straying? Knowing how those wires behave I just picture how easily they could flop away and touch metal.

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: May 14, 2025, 11:47:09 am
content: Quote from: SteveThackery on May 14, 2025, 09:04:52 amThanks for the diagram modification however I am unable to say if it is correct or not as I do not know what all the conventions are for such diagrams, except what I managed to cobble together with my one! I thought I was going from what few other images I had seen. I made no attempt to do a circular flow as you have but I was told by the dc-dc charger manufacturer who I sent it to, who is an electrical wiz in campervan stuff, that, while it took him some time to understand what was going on, it was technically sound.I am not sure what you mean by straight across. As mentioned I made no attempt at a circular flow and just drew things as I remembered them that is why they are seemingly in random places. I just made the jump sign, which I looked up online, to show they aren't connected when lines cross.The busbars have nothing to do with the engine battery, which is totally isolated from the rest of the diagram, except the link into the dc charger.As above the alternator has nothing to do with that system except being connected to the vehicle battery which then delivers power through the dc charger circuit.

author: SteveThackery
author_rank: Super Contributor
timestamp: May 14, 2025, 11:53:59 am
content: This is a second attempt at the diagram, in accordance with @Gyro's suggestion (I believe}.

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: May 14, 2025, 12:16:32 pm
content: Quote from: SteveThackery on May 14, 2025, 11:53:59 amThanks for the update. I can say again I do not understand if it is right or not based on my imperfect knowledge of such diagrams but if you are following @gyro's suggestions, after his explanation, they seemed correct to me.Starter battery, as he said, is just an input via the dc-dc charger and otherwise isolated from the whole system.You mentioned you did not know how campervans charge their systems. Many use mains hookup as well, but most have some sort of solar panels as the main charging source and dc-dc chargers are the most popular backup. As I am focusing on off-grid I use solar mainly, no mains, and the dc-dc charger is the backup for the deepest winter months when solar is not able to cover my daily usage. Only on a dc system personally.As it is only about 2 months of the year that solar is out of action there will nearly always be enough solar to shunt to the starter should the situation arise. I have been having the opposite problem of more solar than I know what to do with even since perhaps late january, after having got a third 100w panel, totaling 300w. Ironically I got this with the idea it would take me through winter but it turned out to be redundant as the low apogee (I think that is the right term?) of the sun meant even 33% more power made no meaningful difference. Solar was as good as 0w per day mostly. Not just the low apogee but also long runs of mist and otherwise thick cloud cover. Lots of storms back to back this last winter in the UK.I was told after I had already installed it I would have been better angling the panels to face 45 degrees during this period but they are all siliconed on now so not going to attempt adjusting them now. The dc-dc charger worked very well for this short period to keep me in electric via the alternator.

author: Gyro
author_rank: Super Contributor
timestamp: May 14, 2025, 12:35:36 pm
content: Quote from: electroniclearner820327 on May 14, 2025, 11:41:21 amThe cardinal rule with jump leads (assuming you go for two) is +ve connection made first and removed last, -ve (chassis) connection made second and removed first. That's about as safe as you can practically get. An in-line fuse in both leads might be overkill but it ought to give reasonable protection against either lead becoming unclipped at either end. [Edit: You just need to determine the minimum practical fuse value that doesn't blow when using the jumpersbutand so provides maximum protection - the batteries didn't ought to have a problem blowing a 100A or more fuse, what you definitely want is a value that pops before the leads smoke!]

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: May 14, 2025, 12:50:28 pm
content: Quote from: Gyro on May 14, 2025, 09:37:05 amWell I think I mentioned earlier, that I already have a disconnect for that circuit between the vehicle battery and the dc-dc charger so it is not active at all with that off. Well it is still getting power from the leisure battery but stays in standby mode. It can be activated by either of the ways you mention but I have it on voltage only mode which it comes on at the engine voltage raising above 13v or thereabouts. I have had it disconnected for a few months now since I am using solar power exclusively again. It was only needed for about 2 months of the shortest days when I was getting next to 0 solar power and the solar suddenly shot into action again, as I note in the previous post.So there probably won't be a case where it would be active while wanting to do this because the dc-dc charger requires alternator power anyway doesn't it so I wouldn't be using it if the alternator was not working! Likewise I would not be using solar if the charger was needed. Might be a very thin week or two either side of the heart of winter where both might be in use at the same time though.QuoteIs it an issue though, given the above, that the dc-dc charger's circuit will be switched off, or would the jumper cables make the circuit again? They shouldn't should they? If I connect them to the + and - busbars of the leisure battery? I would prefer not to have to mess with the actual battery posts as I made them to not be removed, that is why I got the busbars, so modification would be easier on them. The - on the leisure has a plastic cap which is taped down to avoid an exposed post, and the + already has a rubber cap which came with the mrbf 150a (I know this is above than is necessary or ideal for 100a battery but got it before I knew better and it is supposed to still protect against arcing and other similar cases) master battery fuse.I mean it is not super problematic to remove them both for this rare case but if it is a moot point given the dc-dc charger being disconnected anyway I would not want to.Oh I do have a master isolator switch as well between the leisure battery + and + busbar which switches off power to the whole leisure system. So I could just turn that to off right to be sure as an easier way than removing wires from terminal posts? Then connect the clip direct to the post on + and to the - on the busbar?QuoteAh I see that the campervan lingo is not universal to general electronics lingo then. Was not aware. They use bank to mean any array of batteries in parallel I think. Not paid it much notice myself to say 100% as I only have the 1 vehicle battery and 1 leisure battery, but I do understand it to mean whatever batteries you have in parallel, usually referring to leisure as there would be little cause I guess for a starter battery bank! You can ignore the bank reference really I think and just think of 1 vehicle battery and 1 leisure battery and that idolator switch to link the positives.That was merely shown as an example of van builders doing something similar to what I am proposing.QuoteThis looks right.Ye all the negatives go to the negative busbar, which is then linked to the chasis as a ground, which was advised as a best practice in the van building community.

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: May 14, 2025, 12:57:40 pm
content: Quote from: Gyro on May 14, 2025, 12:35:36 pmOk so nothing crazy should happen like electrocution or fires or explosions? Nothing dramatic like I have read about if you accidentally shorted + and - on a lifepo4 terminal which I read could cause a big explosion? Isn't that effectively what is happening if you shorted to the metal of the chassis?Oh wait a second, you mention two leads but won't the negative connection be there already through the chasis ground, so as soon as you connect the positive lead it will be live?How would one attach the fuse to the leads? I looked the prices up on the first round of research and remember that buying the right thickness of lead yourself to run the whole length was considerably more than getting ready made leads of the same. Or maybe it was just because the latter were just jump leads and not rated for continuous current? Jump leads are only meant to be on for short high bursts of a few seconds to start the engine aren't they so can be of a lesser spec than continuous current of the same wire run length?

author: Gyro
author_rank: Super Contributor
timestamp: May 14, 2025, 01:59:33 pm
content: Whoa there, too many questions!My two jump lead suggestion was based on completely disconnecting the leisure battery and connecting the leads to the (now isolated) battery. If you are confident in your DC-DC charger input isolator switch and chassis connection, you can get away with one lead BUT it breaks the 'Cardinal rule of jump leads', no way around it other than the in-line fuse and beingreallycareful. I can't 100% guarantee that just driving the output of the DC-DC charger (in parallel with the leisure battery) will be ok. It is clearly designed to have other charging sources, solar, house power etc. connected in this mode. A faulty, intermittent alternator could put some very nasty voltage spikes though, I just can't answer potential damage question.QuoteElectrocution, no; fire, definitely yes; explosion, if you get really lucky. A chunky Lead acid battery has enough output current capacity to turn a large spanner dropped across the terminals red hot. I vaguely remember the old AA book of the road (probably 60 years ago) suggesting doing exactly this to limit the damage to car wiring in the case of a serious electrical fault. Any cable you short it with, other than very chunky welding cable will smoke = fire. That includes the chassis return wiring. A lot of cars have a big fuse right at the positive terminal of the battery these days to reduce the risk.QuoteYou can (or used to be able to) buy in-line fuse holders that take an ordinary 11/4" fuse. [Edit, most of them take blade fuses these days and most top out at around 30A...https://cpc.farnell.com/c/electronic-electrical-components/fuses-circuit-protection/fuses-fuse-accessories/fuse-holders?st=in%20line%20fuse%20holder] I was thinking of just a length of automotive rated 30-50A single core wire. Yes, jump leads are ridiculously cheap and crappy, designed for very intermittent use. You're talking about lower current (maybe higher than we think) for heaven knows how many miles. Copper is expensive these days. I've also no idea whether an off the shelf jump lead would be long enough for your situation.

author: SteveThackery
author_rank: Super Contributor
timestamp: May 14, 2025, 02:19:13 pm
content: I was surprised to find that you could connect two lead acid batteries in parallel even when one is fully charged and the other discharged. Only a modest current flows between the two. In fact, my car has two lead acid batteries - starter, and ancillaries - and it always connects them in parallel to start the engine.My concern is when connecting a lead acid and an LiFePO4. Has anyone tried it?

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: May 15, 2025, 06:28:15 am
content: Hi, yes my mind does go 'a mile a minute' when learning new things which puts a lot of people off.Quote from: Gyro on May 14, 2025, 01:59:33 pmHmm well the chasis wire is 170a rated. Is that not going to be enough? I was thinking is it the amp rating of both batteries combined that may discharge (105 + 72) or not? If the starter battery is the one drained isn't it only going to be the max discharge rate of the leisure battery due to the theory of resistance that the energy will flow to the starter and vice versa should the starter ever be higher than the leisure - but the latter can be assumed would not ever be an issue for direct parallel connecting in this case? I remembered this yesterday and looked it up and the maximum discharge rate of the leisure battery is only 100a which is stated on the bms max discharge rate. You can see on the technical data sheet for the battery I linked earlier. 105a max battery discharge, 100a bms max discharge.QuoteWell I do have a 100a fuse for the link between the starter battery and the dc-dc charger. Wondering if I can utilize this circuit with the dc bypass method already suggested.QuoteYea I know about inline fuses, just thinking I would have to chop apart the ready made jumper leads if doing that. What do you mean by lower current? How much is it expected to be when connected? I am a bit confused as to what the max amps I could expect in order to size the wiring. Should it be the full additive amps of the two batteries (105 + 72) + 20% safety margin or should it be 100a max discharge of the leisure battery, as I have suggested? Why do you suggest it will be lower current (only 30-50a) rather than the figures I just proposed?I agree it is of course better to err on the side of caution with the charger and the whole system and if there is a risk as you imply there is then I would look to isolate either manually via taking the terminals off physically or by isolator switches. As it is only a once in a blue moon thing just physically taking them off is not really a problem.I suppose my ideas are 'all over the place' because it is a moving target to me at the moment. One of the main issues is I have to know what current I should be gauging for.

author: Psi
author_rank: Super Contributor
timestamp: May 15, 2025, 06:49:11 am
content: I've not read the thread, but a system to do a direct connection is usually not required simply to start the car if the main battery is flat.All you just need a buck+boost battery charger that can push like 20A from the 2nd battery to the main battery.Because to start the car you need at most 1000A for 3 seconds and that is probably overkill.So that would be the same as 20A for 2.5 minutes, and it's never a problem waiting 3 minutes to start the car. (Ok maybe if an axe murderer is chasing you and you need to get away. But in all normal situations waiting 3min is fine)The only time you might want a direct connection is if your main battery is not just flat but failed and you want to run the entire car on the 2nd battery.But that requires the 2nd battery to handle the current draw of the starter motor. If it cannot then consider adding some supercaps somewhere to solve that issue. Assuming you don't want to replace the battery.

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: May 15, 2025, 07:14:32 am
content: Quote from: Psi on May 15, 2025, 06:49:11 amThat is the problem then.I am not asking about just starting but continuous running in case of alternator failure.

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: May 15, 2025, 07:18:27 am
content: I have just been looking up jumper cables again and see a lot of the safety measures are already in place for these such as fully insulated clips except in the very inner most parts which will get rid of most safety concerns of touch metal won't it?Only thing with jumper cables is their ratings are for peak amp loads not continuous and they don't seem to list their continuous current rating that I can see so far, so would it just be a case of buy largest peak I could find that was affordable, and fuse them and wait and see?Even if there was an inline fuse though they could still melt before the fuse went if they were rubbish couldn't they if you don't know what amps they can handle? Would take a lot of trial and error of smallest fuses and going up and up until one didn't blow while protecting the wires?EDIT: Ah ok, I just hadn't looked enough. They do rate continuous current on some.If using the upper figure, still waiting for others to confirm that rating, of the additive value of 72 + 105 + 20% = 211. Looks like I would want around 250a continuous. They will probably do in 50a steps I guess.EDIT2: Ahtheselook like what I would be looking for eh!? Pure copper, 240a continuous rating. Not cheap at £81.84 but the sort of price I would expect for the quality to sustain that kind of current.Should I still get an inline fuse and snip the positive and connect it there for extra safety?

author: SteveThackery
author_rank: Super Contributor
timestamp: May 15, 2025, 10:17:12 am
content: Quote from: electroniclearner820327 on May 15, 2025, 06:28:15 amYou are mixing up amps and amp-hours. The battery ratings you quote are their amp-hour (Ah) ratings, not amp ratings.Amps is a measure of current flow; amp-hours is a measure of the storage capacity of a battery.Most lead acid batteries will provide hundreds of amps for a short while. Even the 12Ah batteries in motorcycles will deliver 150 amps for tens of seconds.

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: May 16, 2025, 02:25:10 pm
content: Quote from: SteveThackery on May 15, 2025, 10:17:12 amOk so what rating should I make the wire then and what is the calculation to get there?I got a reply from their support at a very popular auto electric company asking the same question and the answer doesn't sound right at all! It has got me wondering if it was an ai answer.Quote

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: May 16, 2025, 02:52:16 pm
content: Quote from: SteveThackery on May 15, 2025, 10:17:12 amI was advised elsewhere it would be better to connect them and just not attempt to start the engine and then I can still allow the solar to top up the starter battery and then disconnect after before attempting to start. That is actually what I had in mind in the beginning it is just the comment earlier about the gentleman having 2 batteries linked with one on the back seat whilst driving that turned my attention elsewhere, but in that case I guess they would have been 2 starter batteries so not the same issue of risking overloading a different type of battery as in my case.Again though, how would I determine what amount of current would flow from solar/the leisure battery to the starter? Would that be dependent on the solar charge controller's maximum output?

author: jpanhalt
author_rank: Super Contributor
timestamp: May 16, 2025, 04:20:23 pm
content: Quote from: electroniclearner820327 on May 13, 2025, 05:48:11 pmI assume by "on the road" you motoring along. What are some causes for that: no/contaminated fuel, mechanical failure (e.g., swallow an exhaust valve in a 1965 VW), plugged injectors, carburetor, totally dead alternator, shorted battery.Of that small list, the only items solved by a starter battery are the last two. If you've got a shorted battery, you need to remove it from the circuit, start and run from the auxiliary source.. If it's the alternator, you'll need to charge the battery considerably to get anywhere. If time is not an issue, then charging and going is a solution; otherwise, I'd just jumper and go to a service area ASAP. For me, the best service area is either home or shop, which was only 9 miles away when my Honda died.

author: johansen
author_rank: Super Contributor
timestamp: May 16, 2025, 04:43:28 pm
content: Most RV's just had a dual diode block aka battery isolator where the alternator feeds two diodes. yes, you can connect the two batteries together.Quote from: SteveThackery on May 14, 2025, 02:19:13 pmcar batteries when dead are rather high impedance and may only suck up 20 amps when the terminals are held at 13 volts, and a dead lead acid is still 12 volts. (10 or less under any real load) . its around 14 volts that they start pulling a lot of amps when "dead"your lifepo battery will discharge to 3.2/12.8 volts rather quickly and so you will not be able to melt down the car battery until you have a 5th cell in series and try to charge it from 16 volts.

author: J-R
author_rank: Super Contributor
timestamp: May 22, 2025, 12:08:20 am
content: Quote from: Gyro on May 13, 2025, 06:29:14 pmActually, most are capable of outputting a tremendous amount of current, easily hundreds of amps and can start a car with a completely dead (0V) battery:

author: Psi
author_rank: Super Contributor
timestamp: May 22, 2025, 10:15:04 am
content: Quote from: J-R on May 22, 2025, 12:08:20 amyes, there's 140C lipo packs.eg 5Ah/140C 3S2P or 4S2P about the size of two TV remotes stacked on top of each other that can dump 700Amps no problem (10kW or so) and that's rated continuous discharge, they can do much more if its only for 3 seconds.But there's also a huge number of crappy starter packs with low C batteries that cant supply much current.The issue is the shear number of crappy starter pack manufacturers creating the reputation that all starter packs are crap.It's not an issue with the battery technology being unable to supply enough current. Good batteries aren't cheap and the public don't really care enough to pay the extra cost so the crappy versions tend to win out and take most of the market share.

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: May 30, 2025, 04:22:59 pm
content: Quote from: Psi on May 22, 2025, 10:15:04 amAs mentioned somewhere in the thread I believe I was looking into getting a more powerful one but then I realized it was pointless since I would be charging it from the lifepo4 battery anyway so might as well but out the middleman and do the direct connection, hence the post.To update the battery light has not come on again now since a couple of little other blips so I lost the impetus to do this. hehe. I would like to know it though just in case but the need is not pressing as it was before.No one answered my question, what is the max amps it would draw to size the wire accordingly. 270a continuous seemed way more than needed it seems it will be much lower than that? As long as I fused what would be the lowest gauge wire that would be safe? Well I mean more that lowest that could handle continuous and not blow the fuse.

author: jpanhalt
author_rank: Super Contributor
timestamp: May 30, 2025, 06:00:42 pm
content: Quote from: electroniclearner820327 on May 30, 2025, 04:22:59 pmWhat cable to get can't be answered with the information available to far. However, "ampacity" tables for various cable sizes are readily available. Here's one source and a snippet from it: (https://www.directwire.com/resources/welding-cable-ampacity-chart/#jumptoampacitychart). Note the effect of length as voltage drop is also considered. Also, those are effectively continuous use. Intermittent use ampacity is higher.Assuming your cable is less than 50ft. #4 welding will be overkill for just starting. #6 (where available) is rated for 75A and is not very bulky. My cables are #2 and about 20' each, but were used for heavy duty starting. You might even go to #8 or #10, which may be a lot harder to find in a fine wire with rubber insulation version so it doesn't get too stiff.

author: johansen
author_rank: Super Contributor
timestamp: May 31, 2025, 03:44:44 am
content: Quote from: electroniclearner820327 on May 30, 2025, 04:22:59 pmif you search using google for my username and transformer design spreadsheet... It includes the degrees celcius per second at which the windings warm up.Chat gpt is probably better now.But from personal experience, 90% of the resistance of jumper cables is the connections.