What's the correct way to series/parallel connect a large battery bank? - Page 1
Forum section: electronics / general

author: Freesurfer
author_rank: Regular Contributor
timestamp: October 20, 2020, 02:17:44 am
content: I have twelve 12V batteries that I want to connect in a combination of series and parallel for 48V in an off-grid setup with an inverter and MPPT controller. But it is very hard to find any good information on how to do it for a large battery bank that consists of more than 2 batteries in parallel or more than 8 batteries in total. There is also a lot of misinformation out there which doesn't take even loading or safety into consideration. So what could be so hard? Series and parallel connections is Electronics 101, isn't it?Well, there are 4 problems that need to be overcome:1. Connecting the batteries in such a way that they are all evenly loaded under load/charge, when cable resistance and high DC currents are taken into account2. Being able to easilly even out any unbalance with a battery balancer/balance charging.3. Safety - where to put fuses to protect the battery bank from an external short, as well as internal faults in individual batteries. I'm not a fan of paralleling batteries without a fuse between the 2.4. The practicality of the actual connections. Drawing a line on paper is easy enough, making the actual connections is a bit different. The cables are thick, and fuses can be expensive.Things I have considered are (see drawing):Method 1: Paralleling each row, then connect in series with diagonal takeoffs at each intermediate step(I have this setup for my current 7P, 4S battery bank)Tidy setup, works well for 2 batteries in parallel, however diagonal takeoffs don't load the batteries evenly when there are more than 2 batteries (the 2 outer batteries get more stress than the middle one). Fusing between parallel batteries like this will be messy. The advantage is that it is easy enough to balance the batteries.Method 2: First connecting each string in series, then in parallel at the ends with same cable length to a common bonding point.Somewhat tidy, and all batteries get loaded evenly. With no internal parallel connections, each series string can simply be fused with a fuse at the positive terminals before the bonding point, and any internal fault will disconnect that string. But a real P.I.T.A. to keep each battery balanced as every battery needs individual balancing.Method 3: Bonding at every intermediate step with cables of same length.Very messy, but combines the advantages of perfectly balanced load/charge, and easy balancing. Needs lots and lots of fuses in parallel and the setup will look like a complete mess. Not very practical. But in theory this may be the best option.Method 4: Making as many connections in both series and parallel as possible to keep the resistance low.Might seem like the obvious thing to do, but has no advantage at all. It combines the problems of batteries being loaded unevenly in method 1, a need for a lot of fuses, and a messy look.So how do people deal with large lead acid battery banks?

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 20, 2020, 03:28:32 am
content: Do method 2 with a modification - add thinner wires at each level for balancing. Something like 18 AWG would be sufficient - the balancing currents are very small.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 20, 2020, 03:30:32 am
content: Are these VRLA, and what capacity do you refer to as 'large' ?I'd suggest that any industrial application would only use method 2, along with isolation/over-current devices, to allow individual strings to have maintenance done both now and well in to the future when concerns rapidly escalate (that could cover a whole range of activities depending on your expertise and experience with batteries and the history of the batteries you have). That does need an awareness that any maintenance that varies the SoC of a string needs to be managed either before reconnection or after. It also allows off-line maintenance to get down to individual 12V monoblock level, which could be quite beneficial depending on what charging and discharging accessory equipment is available. It can also allow simpler electrical voltage monitoring, such as rotating down to individual string level monitoring of just one string at a time, if your data acquisition capability is limited. It also allows finer definition of monoblock current, as the current of each monobloc is available for measurement using three shunts/sensors, and so maintenance assessment of float and intermittent charging can be available. It may also allow each string to be better co-located to minimise temperature differences between monoblocs within a string.

author: f4eru
author_rank: Super Contributor
timestamp: October 20, 2020, 05:13:45 am
content: Quote from: NiHaoMike on October 20, 2020, 03:28:32 amNo sorry. That is a very very bad idea. when one cell fails for any reason (and it will happen, latest @end of life), then the "balancing" currents in those small wires will become huge, and they pose a significant fire risk!Each and every big Pb battery install I saw uses method 1.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 20, 2020, 07:22:14 am
content: The battery monoblocs can't be too large for starters as the OP is using 12V monoblocs, not 6V, or 2V cells. If they are VRLA 12V monoblocks with front access for racking, then likely not much larger than 300Ah, as that is sort of a max footprint for weight and racking depth, although absolyte's go up to 600Ah.

author: Freesurfer
author_rank: Regular Contributor
timestamp: October 20, 2020, 01:18:33 pm
content: Correct, they are 12V/86Ah AGM ones with front terminals for M6 screws. They are not massive, but the concept is the same as for larger batteries, and for setups with longer series strings, or more batteries in parallel. I may be able to get hold of even more of these batteries later, which I'll want to add into the array. I'll be hooking up an inverter/charger combo unit capable of 3000VA nominal / 5500W 2-minute peak, and 35A of DC charging current. As well as an MPPT controller with about 240W of PV.It should be noted that these batteries are used, and have been in different locations as UPS batteries. They were all deployed at about the same time and have mostly been sitting there float charged and only ever discharged for an occasional power failure, but I'd still expect some variety in the performance of them.I have considered the idea of method 1 with thinner wires as balance wires, but through a fuse (which then becomes method 4) so the fuse will blow in the event of a battery failure or if the balance wires begin to take the main current of the load, but I am not too sure if it is the most elegant solution. If 48V battery balancers aren't too expensive I can live with buying one for each string and going with method 2.But I am not sure if they are that easy to get hold of. If someone can suggest one I'd like to know.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 21, 2020, 03:26:03 am
content: I'd recommend you go through the design process of fault protection and settle on what you can implement - either removeable fuses or CBs - and use devices that have a DC fault rating for your application no matter where for example a bolted short occurs (such as parallel strings feeding in to the fault), and relate to your monobloc worst-case fault current capability.Unless you want to take manual voltage measurements ad nauseum, I'd recommend organising some form of monobloc voltage monitor, especially as you are starting with a mixture of monoblocs that won't nicely float in a string, and may show widely different EODV levels, and in operation you really need to actively and automatically end any discharge on the weakest monobloc not the total string voltage. Based on what you observe, using one or more 12V specific chargers, and loads, will allow you to properly equalise at the 12V monobloc level, as well as capacity check to a set EODV, from which you may be able to get individual strings with monoblocs that float and discharge without too much variance.12V balancing bypass loads can be fairly easily made using a FET and series 10W resistors on a heatsink with a zener controlled turn-on, if you find one or so monoblocs taking too much voltage and causing the other string monoblocs to not charge adequately.

author: bdunham7
author_rank: Super Contributor
timestamp: October 21, 2020, 03:31:47 am
content: For flooded batteries you'd want to use method 2 in order to be able to maintain them with an equalizing charge. Some AGM manufacturers claim you can equalize, but I think it is a bit dicey. So, unless the wires are thin, methods 1,3 and 4 look functionally the same. The battery interconnects should be thick enough to really minimize any voltage differences. So I vote for method 1 with 2AWG cable to keep things simple.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 21, 2020, 03:46:18 am
content: As per 2nd last post, they are AGM VRLA - not flooded. The battery manufacturers datasheet/guide would identify what should or shouldn't be done - hopefully the batteries have enough 'quality' that they have a service/datasheet guideline.

author: Freesurfer
author_rank: Regular Contributor
timestamp: October 21, 2020, 10:02:33 am
content: They do have a datasheet. Can be found here (I have the M12V90FT model):https://www.batterie-solaire.com/Files/25346/Batterie-GNB_Marathon_M-FT.pdfBut it doesn't say much about charging. The most useful info is that they will recombine 99% of the gassing, which hopefully means they can take "some" overcharging abuse.I currently have them wired with method 2. But as expected they are a bit out of balance. In 2 of the strings, 2 of the batteries reached 14.40V while charging, while the others were still at around 14.00, at which point I stopped the charging to do some investigation. It should be possible to use something like a 12V inverter connected to a 12V charger, and use power from the most charged batteries to charge the most empty ones and equalise like that. even across the strings. But I don't want to constantly have to balance manually, I want something set up once which then sits there and maintains itself. Maybe I should just accept that the charger will cook the most charged batteries a little bit until they are equalised and that the gas should hopefully recombine, and then hopefully equalise for the next cycle. However something like a lead acid BMS would be nice. Most of the "proper" balancers by reputable manufacturers like Victron are only for 24V (2x12V), but there are some chinese ones here:https://capeverde.desertcart.com/products/159256672-battery-equalizer-48-v-solar-system-battery-balancer-equalizer-for-lead-acid-batteries-ha-02-chargerThe same ones can be found on ebay, alibaba etc. No idea about the quality.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 21, 2020, 11:01:46 am
content: Operating info at:https://www.exide.com/sites/default/files/2017-01/Marathon%2C%20Sprinter%2C%20Sunlyte%20I%26O%20Manual.pdfIf possible, I'd suggest doing a laborious round of individual monobloc charge/discharge/charge tests to benchmark what you've got, and then sort them in to strings. Imho, if you don't know what you've got, and don't have voltage monitoring/logging, and don't have low voltage disconnect, you can easily muddle along not knowing what to expect and possibly losing confidence. Using a 12V charger to sit each monobloc at 14.4V for 12hrs, and then discharge to some EODV consistent with discharge rate to get a benchmark, then recover at 14.4 for 24hrs (given some max charge current capability - the datasheet doesn't identify a max charge current sadly) should indicate capacity spread, which is a good start to grouping strings.Given daily PV charging, do you have constant loading, or are the batteries only discharged for some infrequent application? Does your PV charger allow just a set charge voltage, and does it have temp comp?It is often practical to PV charge a monobloc at 14.4V, but as you have a 48V string and voltage may not share really well then eg. set string charging at 14.4 x 4 = 58V minus say 1V for imperfect sharing. For infrequent charging as per PV, there is a compromise between getting charge in whilst not staying at 14.4 too long - the charge rationale is very different than float conditions 24/7.Quality usually starts with an operating manual that describes what actually happens and what parameters can be set.

author: mc172
author_rank: Frequent Contributor
timestamp: October 21, 2020, 11:20:20 am
content: Quote from: f4eru on October 20, 2020, 05:13:45 amThis is true in method 1.Also, you don't implement balancing circuits that can balance at hundreds of Amps, a few hundred mA at most is usually enough, so this scenario will never happen.

author: Freesurfer
author_rank: Regular Contributor
timestamp: October 21, 2020, 05:33:39 pm
content: @trobbins : I had a look at the operating instructions. There is some useful info there. About paralleling it says:QuoteI guess that part translates into "method 2" in my diagram (which is what I currently have). "High rate applications" shouldn't apply to me, so I guess I am free to extend with even more strings if I want.It states the max charge current to be 35A per 100Ah (so that means 30A for each of my 86Ah batteries). My charger/inverter combo (Victron Multiplus II 48/3000/35-32) is capable of 35A DC charging, so divided evenly on each string that's about 11.6A per string. I can even limit the charge current to 30A to be sure in the worst case. I currently don't have the proprietary USB cable needed to configure the Multiplus charger/inverter so I am not sure what options it gives me with the charger, but I know it has settings for different chemistries (and even custom charge profiles if I am not mistaken) and should give me some flexibility.The battery manual is a bit unclear about what to do with unequalised batteries.QuoteSection 9.3 only says:QuoteSo I am not sure where I am supposed to find this "higher than normal voltage". It clearly says not to exceed 2.40VPC (=14.4V per battery)These batteries are not really intended for PV or deep cycle purposes, they are meant to be used in UPS applications mostly sitting at float voltage.I'll be using them in a off-grid cabin which is visited just for a few weekends every now and then, where hopefully the solar will top them up between each visit, but I also have a Honda EU20i inverter (1600W continous) which I can use to top up if the batteries run low, or before leaving if they are very depleted. Chances are the batteries won't last forever in this environment, but can't complain when I got them for free. I am not too worried about draining them flat, I'll be monitoring that and keeping them within a safe range and recharge with generator if needed.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 22, 2020, 01:08:32 am
content: Do you know the largest loading, and if you can manage that to be as low as possible, or at least under 32A? If so, then it seems that each string could use a 32A CB, such that if ever needed, charging could be up to 32A (ie. only 1 string connected to charger combo), as well as max discharging. But I strongly recommend you assess the max fault current aspect, and look at what CBs or fuses you can scrounge/purchase that are properly rated for such a fault - skimping on that protective rating is dangerous. Similarly the cable you use for connections should be rated for the 1 sec fault current capability as well - that gets in to cable design, but well worth your effort and safety to identify what generic able cross sectional area and insulation type is 'within ratings".If there is no loading between visits, then yes back off the daily max voltage that full strings could sit at, as the total number of hours per day sitting at that level could get large. But I would suggest switching to 14.4V when at site when loading is occurring, as the time at 14.4V would likely be significantly diminished, and so not a long term degradation issue.If at all possible, don't leave the site with the batteries at low SOC unless you are at the start of a sunny day - perhaps at least 20-30% SOC would be a reasonable minimum level to leave them overnight or if the weather is unlikely to raise SOC for many days. But of course it will be any weak monobloc that catches you out - free batteries become a hassle when you need to rely on them but can't adequately manage the weak ones.

author: Freesurfer
author_rank: Regular Contributor
timestamp: October 22, 2020, 10:26:12 pm
content: I currently got them fused with some 100A ANL fuses - one fuse for each string between the positive terminal and the positive bonding point. So the battery bank can deliver a total of 300A. The batteries have short circuit currents in the 1400A range so should blow the fuses rather quickly and hopefully not melt the wires (I haven't checked the short circuit rating of the cables and compared with fuse blow rates but I think it should be ok). The interconnects are 16mm2 copper cable up to the bonding points, then from there and into the inverter I have used 35mm2 copper. (16mm2 may sound little but remember they only take a third of the load)If a short happens from something like a loose battery cable that touches a battery terminal next to it, (making it a 36V string) or an intenal battery short it should blow a fuse and disconnect the faulty string from the other strings and stop any short circuit current. The chance of a cable coming loose and shorting to another string through the unfused negative bonding point is very unlikely as the cables are too short to reach.Regarding normal operation, I also did some worst case calculations where I assume one string takes the entire 120A load (max the inverter will draw) and decided even that would be safe. I think I calculated the total loss of the battery bank under full load (120A total / 40A per string) to be 10W. And for typical loads of max 250-300W only a negligible 25mW.Today I picked up another 7 of these batteries (I guess someone snatched one out of the 8 so I am one short) so I will make it 4 strings now and maybe 5 if I eventually get hold of yet another one. Then they won't discharge as deeply. I am also considering getting some more PV, perhaps 3x300W panels.The MPPT controller that I plan to get is a Victron SolarSmart MPPT 150V / 35A one which has quite a lot of options for specifying charge parameters, and allows battery equalization and so on. It also logs quite a lot of data like charge power and voltages over time, so it will be easy to track the trends and work out any issues.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 23, 2020, 01:43:48 am
content: It's good to see you aren't shying aware from doing some basic design leg-work. Batteries like that can be a serious safety hazard, and I guess as the years roll on when someone other than you may have access, it is in your interests to have relevant hazard signage and operational procedures and restricted access.As I read the battery datasheet, each string has a max fault level of 2.44kA, and an example location for a fault such as on the common bus or within a string could now extend to 10kA with 4 strings. So a Bussman ANL does not have sufficient interrupt capability (the fuse has a 2.7kA capability).You are unlikely to be able to use 35mm2 for 120A (which extends to your aim of using one string to supply 120A if needed) - you really should identify what cable/insulation you have and what its max current rating is for how you are running the cable (eg. touching or spaced, and in free air, and max ambient temp), and similarly the cross-section of any interlinks you may want to use between front access terminals.Once you know your cable, and your fault current protective device, then you really should take a deep dive in to ensuring the cable will withstand the fault current that the device can clear, which goes to the clearing time of the device and the temp rise in the cable - that design is bread and butter for any industrial power system, but likely your first exposure.

author: Freesurfer
author_rank: Regular Contributor
timestamp: October 25, 2020, 11:21:20 am
content: The insulation is PVC, and the cable is a very standard, "battery cable" of fine stranded copper (the black and red type) that you get for automotive/RV/marine purposes. They don't have any datasheet with all the specifics available (maybe they can provide on request, but meh...) It was bought from a consumers hardware store.35mm2 is what Victron suggested in the inverter manual. Trust me, it is a beefy cable. The fuses may not be ideal. They are only really rated for 35V DC, but I threw them in there as a "better than nothing" solution.I have some good news too, I headed over to the place again this weekend and decided to top them up some more after last visit, and the voltages seem to have balanced better this time. The charger switched from bulk to absorption before the most topped up batteries reached 14.40V, keeping them in the safe voltage range. I also brought a DC clamp amp meter. At first the middle string didn't seem to perform as well and was only delivering and receiving about half of the current of the other 2 strings, but eventually it evened out and they are now about identical in performance for both charging and discharging. Maybe the batteries just needed a discharge/recharge cycle to refresh the performance.From what I can tell so far the health of the batteries seems to be good. They seem to be from a 2013 batch, and they have a 12 year design life under ideal conditions with various factors like temperature, float voltage etc affecting it, but from what I can tell these batteries have mostly been sitting under their ideal conditions, float charged in environments clean environments with proper airconditioning.I don't know exactly how to determine the health of SLA batteries, or "how" they degrade. If it is just that the overall capacity will degrade while still performing as if it was a smaller battery due to less area being available due to sulfation, or if there are other problems like self discharging, high ESR (heating up on charge/discharge) and so on. There is little info about it online other than "when your car cranks poorly on a cold day, the battery may be busted"I haven't got my PV stuff connected to the battery bank yet, I will see if I order the MPPT controller next week. And maybe some extra solar panels too. Solar panels have gotten so much more affordable over the last 3 years.

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: October 28, 2020, 12:36:25 pm
content: Victron have a lot of great app notes for stuff like this.Wiring battery banks is covered in this one:https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdfThey mention it in that pdf, but they sell a not very expensive battery balancer that will keep your mid-point voltages where they should be. You can configure 3 of them to work with a 48v system.https://www.victronenergy.com/batteries/battery-balancerI'd also recommend downloading a copy of 'Energy Unlimited' there's tonnes of good information in it:https://www.victronenergy.com/support-and-downloads/technical-information(I don't work for Victron, I just like their stuff)

author: Freesurfer
author_rank: Regular Contributor
timestamp: November 02, 2020, 03:44:21 am
content: I think I'll need to follow the advice and do some individual benchmark after all. Or at least charge every battery to full charge individually before putting them back into the array (I know I really should have done this to begin with).After consuming power from the battery bank for a while I noticed that the overall voltage was much lower than expected (around 44V) so I went and did some measurements.The first string was ok, with voltages within 10mV of each other. I also noticed with the ammeter that this string was delivering most of the current. So I think that string is healthy enough. In the second string, one of the batteries had discharged to around 9.5V while the 3 others were still at a higher and equal voltage. And the last string had one that had dropped to 10.5V. The problem with not parallelling the midpoints and no balancing is that each string only performs as good as the worst battery in the string. From memory I think that the most deeply discharged ones, were the ones that reached 14.4V first while charging. So they have most likely lost their capacity. But I still have spares that I can swap out.Once I get my new solar panels it will be much easier to play around and do some performance testing. Sadly there is no quick and easy way to check the health of SLA/VRLA batteries.In some other company I worked for, a colleague had some battery tester that would do a rough estimation on capacity on a fully charged battery. It was used for those small AGMs that sit in fire alarms to determine whether they needed replacement or not. It gave a reading in amp-hours. It's probably a battery ESR meter with some fancy math to work out an estimated Ah. I don't know what tool that would have been, or how consistent that method is.

author: trobbins
author_rank: Frequent Contributor
timestamp: November 02, 2020, 04:57:21 am
content: Capacity estimation, or impedance measurement, tools are risky to apply for one off measurements, and are only likely to indicate VRLA monoblocs that are 'different' from others. So it could be usefull to indicate whether a charged monobloc is similar to the rest of the batch, or 'different'.Monoblocks that show poor capacity may benefit from a full cycle or two, but that takes effort, and no guarantee of outcome, and is best done at the monobloc voltage level (ie. 12V discharge and charge cycles).No matter how you connect the monoblocs, I doubt you would get more energy out of the total population connected for a random batch of monoblocks, and would have to sort poor monoblocks in to their own string, or spread out in parallel with good monoblocks if paralleling.

author: geggi1
author_rank: Frequent Contributor
timestamp: November 02, 2020, 08:57:58 am
content: I have done a few replacements on larger UPS battery banks and the common way of installing the batteries is to run with paralell strings fused at each end. In your case with 12V batteries and a required voltage of 48V you will end up with 3 strings of 4 batteries.For fuses you should use melting fuses not electromechanic breakers. The reason for this is that the fusing device must he able to break the full short current ( Cell voltage divided on internal resistance). The full short current on your batteries is probably somewhere in the range of 10-15kA so Bussman fuses or something similar is probably OK.Becuse this is used cells you should also do a load test on the batteries. Fully charge the batteries and put on a load for 10 to 15 minutes compare the voltages on each cell before and after the load test. If there is a substantiation voltage drop on the battery after load test its a bad one. It is probably simplest to load test one battery a time. Charge the batteries in the standard configuration and remove the jumpers to do one battery at time. It is also a good idea to have a voltmeter connected over the battery poles when you do the load test.The simplest loadbank is a few 12V 55W auto bulbes in paralell.There are standards for doing load test on Pb batteries and they can be found on google.