Charging 12V from 12V, but limiting the current

Ok, I'm not sure if this is the right place to ask, but similar other posts are here.

I'm a little rusty with my electronics (I used to tinker back in school, but not so much now)

So here's the deal, this is an automotive application.

I have my truck which has a 1100CCA 12V battery and the charging system so the voltage can range from 11V to 14.4-14.8V.

I have a trailer I pull behind said truck. The trailer has a small 12V battery (no idea as to the capacity) that is charged by the "system" for the trailer brakes in a break away situation.

Now what I want to do is add a much larger battery, 12V 750CCA, to power a winch. A lot of people just charge this battery with a traditional 120V car battery charger.. which would get annoying (It's what I do now and I have to pull the battery out, bring it in the house, charge it.. then forget to bring it back out). What I would like to do it tap into the system that charges the small brake battery and have it charge the large battery, now this in itself isn't that hard to do.

The problem is that when I'm using the winch I want to limit the current that can be pulled over the charging circut. Personally I foresee trying to pull 100A+ over a 20ish foot long 16Ga wire could be an issue. So I'm looking for a way to limit the current to say 5A max, 2A or 3A would be fine as well. I only use the winch every once in a while and when I do use it, it probably drains the battery by approx 25-30% so a slow or "trickle" charge is fine as I do the trailer a lot.

If anybody has any ideas that would be great. I've done a lot of googling and looking on this forum I found some stuff that is close to what I want, but not quite enough that it'll work.

Thank you


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Safest way would be just a double switch that connects the trailer battery to the winch and disconnects it from the truck (charging circuit)
I thought of that right as I posted, but I did a little more reading and while that works in theroy I saw 2 issues.

1. If that switch is "on" allowing the battery to charge what's to stop the truck from using it to crank/start off of, other than the other battery is closer has bigger wire/less resistance. Now if the front battery is in tip top shape all should be good, but what if it's not one day (this last week we hit -50 degrees). Ok fine, so I could put a battery isolator in...

Which led to issue #2 when I was reading about the different type of isolators and how they worked, etc it was mentioned a battery will charge as fast as the Alt can dish out the juice.. so being I have a 136A Alt(underrated, probably puts out closer to 150A @ 2000rpm) so remove a few amps for the ignition system and lights, etc.. say that's 75A left could be closer to 100A that I could potentially have attempting to flow down that 16Ga wire again. Unless the wire itself will limit the current, but somehow I'm guessing not.

Thanks again
Considering the resistance already in the long length of wire from the trailer battery to the truck battery I doubt that drawing too much current in either direction would be an issue without a dead short. If you really want to isolate it, put a resistor and a diode in line. Also don't forget to fuse the line if it isn't already.

Actually you probably have these things in the charging circuit you have. In which case you just replace the battery with a larger one.

I was concerned about it trying to draw too much current and heat/melt the wiring. A diode, ok works kinda like an isolator, well sorta... resistor... I'm guessing I'd need a pretty big one I think I have a 6OHM 50W resistor kicking around...

Your second point.. I'll have to check and see just what all is in that little box that holds the trailer brake battery,, maybe you're right.
Google SEPIC
Bob S
A diode will prevent back flow when your truck is starting. The resistor does not need to be that large it just needs to limit the current flow to maintain a trickle charge only.

As for a SEPIC converter, I don't know why you would need to change voltages since everything is at the correct voltage already.
I'm getting confused... wouldn't a larger resistor limit the current more than a small one?
I would just use a 12v automotive filament lamp.
really? they're about 26W or so...

I'm still not sure how adding a smallish resistor will reduce/limit the current flowing other than with say the bulb method acting almost like a fuse...
A bulb is a great idea.

The bulb does not act as a fuse. You can wire it straight between the positive and negative terminals of the battery because it has a certain resistance, and as such will only carry so much current at a set voltage. V=IR.
hmm would I not wire it in series like a resistor?

I know for testing I used a 1157 bulb as a "resistor" for testing to get my HID headlights to work with my DRLs..
I don't see why you don't simply parallel the winch battery with the truck battery, charging the winch battery with the alternator under the hood.
Personally, I dont see a problem with leaving the 16awg wire for charging as I dont think it will present a problem, If your winch battery runs down enough to pull significant current through it, you wouldnt be using the winch anyways (there wouldn't be enough current to run it). A good idea though, would be to put a fuseholder inline and use an automotive circuit breaker, instead of a fuse. Ive used them before and when they trip, they automatically reset in approx 5 mins. That way, if current got excessive, it would protect your wiring and save your truck battery from being drained.
Phrak, I could but I don't want to have to run a 4ga or 2ga wire from the batt down the truck, find a safe way to disconnect when the trailer isn't in use then run it to the tool box. The problem is the wiring that is there I think might not handle the load. From what I can tell the pos feed to the trailer is fused @ 20A, and the brake feed is 40A.. I'm not sure yet, which one charges the small battery
The light bulb method is an old trick used by a lot of ham radio operators to float charge a lead acid battery from a 13.8v regulated power supply. It should work equally well in this application, if I'm understanding it correctly.
I thought about solar... I have a smaller 5W panel.. but wasn't sure if it would be enough, the one you posted is physically to large to mount to the tool box in a safe location.

Ivan Seeking

Staff Emeritus
Science Advisor
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I thought about solar... I have a smaller 5W panel.. but wasn't sure if it would be enough, the one you posted is physically to large to mount to the tool box in a safe location.
What you really need is a simple current-limiting transistor circuit with diode protection for the reverse load condition. I would bet that someone like US Battery or Delco could direct you to a current-limiting trickle charger for parallel battery configurations. This may get back to the SEPIC technology mentioned earlier. You wouldn't want to cobble something like this together as the transient voltage spikes in an auto system can be pretty serious. Not to mention that this is for the emergency brake system.

I suggest that you call the support engineer at US Battery, in Corona, California. He will know what to do [almost certainly].
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hmmm wow could be harder/more expensive than I hoped

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