Trailer power project - Page 1
Forum section: electronics / projects

author: phaseform
author_rank: Regular Contributor
timestamp: October 11, 2014, 02:42:34 pm
content: I've run a 12v line through my cars trailer plug connector to the back of my enclosed trailer. I want to avoid drawing too much current through this cable to the battery in the trailer, but I'm not sure the best method to go about this. I'm thinking a current limiter could be used to keep the current draw below 15A, which is the fuse I put in line with the trailer plug, although I'm not really sure if and how I could do this cheaply.Here's a blog page where I've detailed the build:Trailer Power ProjectAlso the trailer already has a small 12v solar panel for charging its battery, which I think is this:http://www.jaycar.com.au/productView.asp?ID=MB3501(it supplies 15V at around 100-120mA)I opened the solar chargers cigarette lighter plug and there doesn't seem to be really much going on, i just noted a diode and a LED, although the panel only has a small current I'm concerned it may over time over charge the battery.I'm not looking to use a high current power line to the trailer, I think 15A would be enough at this stage.Since I have a bunch of TI launchpads laying around I figured I'd use one for managing the whole thing, unless it's complete over kill. Could someone recommend if a method of current limiting would be suitable also if some kind of management should be used for the solar charger.Thanks!

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 11, 2014, 03:38:31 pm
content: Such a low current would just do a slow equalizing charge, so no real issue. To limit the current, you can use PTCs or a self resetting circuit breaker.

author: mc
author_rank: Regular Contributor
timestamp: October 11, 2014, 03:54:36 pm
content: From what I've seen dealing with such things over the years, the wire itself is usually good enough to limit current, and given you've got a diode, you're going to get an even bigger voltage drop anyway. Worst case scenario, the fuse blows, so maybe add some form of indicator to let you know it things are charging ok.The usual basic implementation is a standard 12V relay controlled via the alternator warning lamp terminal, to provide a basic split charge system, so the external battery only gets charged when the alternator is charging.More advanced will use a voltage sensitive relay, or you can go the expense of a full battery charge management system, which is usually overkill for most applications.As for the solar panel, as others have said, it doesn't sound like it will cause overcharging issues. Simplest way to test, will be charge the battery, then leave the solar panel connected up and monitor the battery voltage for a few days to see if it is staying within acceptable limits.

author: SeanB
author_rank: Super Contributor
timestamp: October 11, 2014, 05:58:29 pm
content: Use the relay connected to the alternator, and use a high current shottky diode connected across the relay terminals so that the car battery can be charged back via the diode somewhat if the trailer battery gets fully charged. Use a basic solar charge controller on the panel and you will never overcharge the battery.

author: G7PSK
author_rank: Super Contributor
timestamp: October 12, 2014, 05:48:15 am
content: Use a charge splitterhttp://www.the12voltshop.co.uk/Shop/about-split-charge-systems.htmlAs for the solar panel you have I have one like it I got here in the UK and it just about produces enough power to light its blue led trying to use a charge controller on it overloads it before the battery is even connected.

author: phaseform
author_rank: Regular Contributor
timestamp: October 12, 2014, 07:48:20 am
content: Thanks for the replies, since I can't figure out with confidence how I would create a current limiter using a 20A transistor, and also how much loss I would get by doing so, I've just opted for a much simpler design where I switch the power between trailer or car battery with a SPDT switch. This means the car can't charge the trailer battery, but I can live with that.I broke the cigarette lighter jack for the solar panel today, so I'll run some fig8 out of it to the battery, through a .5A fuse.I think this should be plenty suitable for my purposes.Also I just had a quick check on ebay for an inverter and saw a 1kW pure sine wave inverter for $150:http://www.ebay.com.au/itm/Pure-Sine-Wave-Power-Inverter-1000w-2000w-12V-240V-Car-Boat-Caravan-Camping-/270859893555?pt=AU_Boat_Parts_Accessories&hash=item3f1081fb33which apparently has all the required approvals. Initially looks good considering it's a quarter the price of whats available from jaycar.

author: richard.cs
author_rank: Super Contributor
timestamp: October 12, 2014, 08:45:31 am
content: I have always done this with resistive limiting. A flat battery will very quickly come up to about 12V and then sit there drawing loads of current untill it reaches the alternator voltage of about 14V. You need to drop 2V at 15A so you need 130 milliohms which will dissipate 30W. If you select your cable to be of that resistance then you're set - the power will be dissipated down it's whole length and it'll get slightly warm. Better than dissipating it all in one place with single resistor that'll run hot.This method means that when first connecting a very flat battery there will be an a substantial inrush current but is is normally of short duration (tens of seconds to minutes) and both the cable and the fuses will cope with that just fine. You should fuse both ends as if the middle of the cable gets shorted both batteries can supply fault current.The other cheap way is to use a parallel bank of filament lamps as a PTC element. Inrush will be less but all the dissipation will be in one place.

author: phaseform
author_rank: Regular Contributor
timestamp: October 13, 2014, 01:55:11 am
content: Ahh cool thanks richard.cs, I think I just learned something. I don't think I'll be able to measure the resistance of the cable due to it being below my multimeter minimum and not having a constant current source. Anyway I'll just add a 12v 100W lamp between the two positive battery terminals (=1.44 Ohms, =7.1A @14V short circuit and 1.4A at 2V drop), with my limited knowledge hopefully that will be able to absorb any excess current.Also I'd like to hook up a 'voltage sensitive relay' so I'll mess around with a horn relay I have sitting around and see if I can get it to close the contacts only when the alternator is running.

author: richard.cs
author_rank: Super Contributor
timestamp: October 13, 2014, 08:44:18 am
content: There are lots of tables and calculators in the internet where you can lookup the answer to "my cable is X long and I want Y Ohms so I need Z cable size". You then need to check that the calculated cable size won't run too warm, a very short length of thin cable would have the same resistance as a long length of thick cable but run much hotter. If you keep with the 100W lamp your charging will be very, very slow, there's no need to design to limit the short-circuit current with the lamps. With a 1.4A charge current (and falling as the volts climb to perhaps 1A) you'd need to run the engine continuously for 3-5 days to charge a typical deep-cycle battery from flat, I guess that's a few weeks worth of normal driving. Ok for just a trickle-charge top-up but not much use for anything else.On many vehicles you can simply connect the relay coil between the alternators pilot-lamp terminal and ground, or to the W terminal via a rectifier if you have it (mostly on diesel vehicles). Both of these are much simpler than the voltage sensing approach.

author: mc
author_rank: Regular Contributor
timestamp: October 13, 2014, 10:38:52 pm
content: Personally, I'd avoid voltage sensitive relays for a trailer application.I've dealt with various split charge systems for on vehicle systems, and voltage sensitive systems give the most issues, closely followed by any that involve diode blocks and modified alternators.The big issue with voltage sensitive systems, is you get a bigger surge when it reaches switch on voltage, so are far more likely to blow fuses.A conventional alternator controlled relay will connect the battery as soon as the alternator starts charging, which means the vehicle battery voltage will still be reasonably low when the batteries get connected, and both battery voltages will rise from a similar voltage.With a voltage sensitive system, the batteries only get connected once the vehicle battery has risen to a set level, typically 13.8V, so you get a surge as the second battery at low voltage gets connected.

author: phaseform
author_rank: Regular Contributor
timestamp: October 14, 2014, 02:03:28 pm
content: Trying to work this out on the fly is more difficult than I'd expected! Trying to keep this simple, with less trips to jaycar, so I've run ground through as much of the trailer frame as possible, also straight from the cars trailer plug to the car frame. The lead I've run from the 12v supply in the back of the car through the trailer connector to the trailer battery is stranded ~1.3mm core diameter (not exactly heavy gauge I know, but hopefully enough for battery charging and secondary supply voltage). So looking here at a wire calculatorhttp://www.bulkwire.com/wireresistance.aspI figure ~.18 Ohms wire resistance given 12m cable run from car battery to trailer battery. Now if the alternator is outputting 14.5V and my trailer battery is at 11.8 volts (ie today with the battery I'm trying to rejuvenate) then my understanding is that I need to lose 2.7V over the cable initially while the lower voltage comes up. So 2.7V and 0.18 Ohms with Ohms law gives 15A. The fuse for the 12V socket in the car I'm connecting to is 20A. I can't actually measure the current as I don't have a current shunt for over 10A but I think I should be ok removing the 100W lamp, worst case I blow a 20A fuse, so I'll test this tomorrow and see how much the cable heats up over 10-15 mins. Also the current seems within these boundshttp://www.powerstream.com/Wire_Size.htmThanks again for the input, it's making this increasingly complicated circuit more simple.. and I consider myself an electronics nerd!As for the voltage sensitive relay, I'm just concerned about leaving the car radio on or something and draining the battery too much. Today I was playing with a darlington pair to switch a relay in the trailer (not a store bought voltage sensitive relay, wheres the fun in that). Yer looks like this may not be an option after all, if I were to trigger the relay in the trailer at say 12.8 volts, then in the example above, with a flat trailer battery, drawing 15A my trailer voltage would drop down to 11.8 volts and turn straight back off. So that would suck... might have been ok with the voltage sensor behind a current limiting globe, but I'm removing that... I really cant be bothered putting a relay up the front connected to the alternator, as I don't have a service manual and I don't know the alternator pinout (3 wires) nor where the rear 12v accessory cable runs under the bonnet. Maybe if I unscrewed the fuse box I could find the rear acc cable, or perhaps run a cable from the alternator indicator lamp back to the rear 12 socket and put a relay there.

author: richard.cs
author_rank: Super Contributor
timestamp: October 14, 2014, 02:34:20 pm
content: What you've got so far seems reasonable without the 100W lamp, as you say the worst you'll do is blow the fuse. One thing to check is that there aren't any long runs of your cable embedded in insulation as it may not be so happy with 15A then.Alternator pinouts are fairly easy to figure out. You probably have a fat ground cable bolted to it's body, a fat battery cable for its output and one thin wire which is pilot lamp. A second thin wire on a diesel vehicle might be a rev counter connection. A manual for your car might be useful just to check that you've not got some CANBUS-controlled whatsit but I've never actually seen one in the wild. What's the car?

author: Muttley Snickers
author_rank: Unknown
timestamp: October 14, 2014, 02:52:01 pm
content: To phaseformI may have missed it in the previous posts but I will ask anyway, will you be adding an isolation or kill switch to the feed in case a mate borrows your vehicle and plugs in another trailer with an incompatible wiring configuration.RegardsMuttleyNote to self:Always check the schematics first, (found the switch)

author: phaseform
author_rank: Regular Contributor
timestamp: October 15, 2014, 02:15:21 pm
content: The car is an old 97 Toyota Prado, haven't been able to find an online service manual which could be the required motivation to add a relay for the trailer power supply. If anyone was interested I've updated the install blog with the current working build.http://projects.hascomp.net/Trailer_Power/Cheers