Charging 12V from 12V, but limiting the current

In summary, the author is looking for a way to charge a large battery using the charging circuit for the small battery on the trailer. He is concerned about the current draw and possible heat/melt of the wiring. A diode, resistor, and filament may be the solution.
  • #1
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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  • #2
Safest way would be just a double switch that connects the trailer battery to the winch and disconnects it from the truck (charging circuit)
  • #3
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
  • #4
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.
  • #5

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.
  • #6
Google SEPIC
Bob S
  • #7
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.
  • #8
I'm getting confused... wouldn't a larger resistor limit the current more than a small one?
  • #9
I would just use a 12v automotive filament lamp.
  • #10
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...
  • #11
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.
  • #12
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..
  • #13
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.
  • #14
Personally, I don't see a problem with leaving the 16awg wire for charging as I don't think it will present a problem, If your winch battery runs down enough to pull significant current through it, you wouldn't 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. I've 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.
  • #15
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
  • #16
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.
  • #17
Maybe you could consider losing the charge wire altogether
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  • #18
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.
  • #19
Gold5th said:
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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  • #20
hmmm wow could be harder/more expensive than I hoped
  • #21
Gold5th said:
hmmm wow could be harder/more expensive than I hoped

Maybe not. If someone makes a device like the one I imagine, and I would think that someone does, it could be fairly cheap and easy to install. I know that I could make what you need for probably ten bucks in parts, so it is possible that you can find a production device for under $100; maybe well under that.

I mentioned the engineer at US Battery because I have used him several times in recent years. In my line of work, good product-support engineers are worth their weight in gold. He is quite knowlegable and very helpful.
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  • #22
Here's your circuit:


The biggest problem with this as it's currently done is that the main battery would be charging the other! There are commercial dual-batter isolation electronics available for fairly cheap which solve this problem. If you use them, then don't use this.

Couple of notes: By using the lightbulb, you're limiting the current to a trickle, as was previously mentioned. Thus, you don't need super heavy-duty wire from the engine to the trailer for the trickle charge line. You'll still need heavy cable between the battery and the winch. And if the winch has it's own switch (most do), disregard the one in the picture.

You'll still have to solve for the right lightbulb wattage which would allow for a 24-hour recharge of the 750CCA battery. What is its amp-hour rating? You'll need that. Once you solve for that, you'll have the current and can then select the right gauge wire.
  • #23
This is getting a bit complicated. I design high performance circuits for a living, and I concurr, that the lamp was a great idea. Just place an incandescent lamp in series between the connection to the two batterys. The resistance of the lamps is fairly low for lower voltage drops and increases should the drop get higher. (hotter filiment = higher resistance)

Given that your lamp isn't wrapped up somewhere to cause a fire, the only thing I see going wrong is that you choose too small a lamp and it can't keep the battery charged.

A tail lamp will likely give you some tenths of an ohm when cold and at worst, would give a dim glow during wenching. They're also available with mounts for trailers.

- Mike
  • #24
I agree that the light bulb method is the best "cheap" way of doing it. But mugaliens has a good point that when the alternator is not running the 1100cca battery will charge the 750cca battery. How well it works depends on how it is used. If the winch is used a lot and pulls the 750cca battery down considerably, and if the alternator is only run for a short period of time, then the 1100cca battery would be pulled down by charging the 750cca battery (but over a longer period of time). I would choose the highest wattage bulb(s) possible without exceeding the current rating of the wire.
  • #25
Hmm very good ideas..

I don't winch very often sometimes once a week, sometimes once a month...

But if I could keep that battery charged I might start using it for "work lamps" and "don't run me over" strobes

As far as a light. I think I would mount it either external in a proper housing, or inside as a "toolbox" light.

The winch has it's own controls so the switches are built in.. I would most likely add a switch inline with the bulb so if the truck/trailer are going to sit connected over night I can "stop" the charging/light causing a drain on the whole system.

Thanks a lot for everybodies time. I was really concerned about melting the trailer wiring harness.
  • #26
Mike_In_Plano said:
This is getting a bit complicated. I design high performance circuits for a living, and I concurr, that the lamp was a great idea.

At best it verges on being reckless and irresponsible to jerryrig an emergency brake system with a lightbulb, professionally speaking.

Golf5th, I suggest checking with your insurance agent or the highway patrol and see how much they like they idea.
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  • #27
Well the "plan" is to borrow power before it gets to that system. So it should be fine.. in worst case should provide extra power to that system.

What I'm really hoping for is that in that box there's a 12V not related to the brake system... but the above light thing still applies to keep me from melting the whole harness...
  • #28
I don't see where this has anything to do with the brake system? That would be weird and crazy.

All I see are two battery's conjoined by the same sort of bulb that is commonly used in tail lamps. Said bulb being safely enclosed and connectored in that most common and readily obtainable housing - the replacement tail light. The entirety of the matter being safely mounted in some unobtrusive and inconspicuous region of the trailer.

Given that the trailer battery would experience a large, but short term, load, I don't perceive a need for any further additions, such as a switch. That is, assuming the load didn't consume so much energy as to deplete the vehicle's battery.

I guess another note is in order regarding the charge rate to the trailer battery. A common 1157 tail lamp looks to have about 1.5 ohms (on the brake filament) when lightly loaded. That means that the trailer battery won't experience anywhere near as fast a charging as the vehicle battery receives. Perhaps it will get .5 to 1 amp. That's not very much for a large battery, and I've heard recommendations against trickle - only charging.

For this reason, it's probably best to substitute an 1154 lamp (6 volt version). This will reduce your resistance to about a fourth. These lamps are still available, though you may have to order it. A good price is $3-$5.

- Mike
  • #29
I think the comment is talking about splicing into the emergency brake/disconnect brake system and possibly inadvertantly rendering it useless.. thus if the trailer brakes away from my truck.. it'll keep going til it hits something rather than jamming on the brakes.

Don't worry.. I know enough to make sure I don't do something that dumb...

Related to Charging 12V from 12V, but limiting the current

1. What is the purpose of charging 12V from 12V, but limiting the current?

The purpose of charging 12V from 12V, but limiting the current is to protect the battery from overcharging. When a battery is overcharged, it can cause damage to the internal components and shorten its lifespan. By limiting the current, the charging process is controlled and ensures that the battery is charged safely and efficiently.

2. How does limiting the current affect the charging process?

Limiting the current can slow down the charging process, but it is a necessary measure to protect the battery. It allows the battery to charge at a safe and controlled rate, preventing any potential damage or overheating. It also helps to prolong the overall lifespan of the battery.

3. What is the recommended current limit for charging a 12V battery?

The recommended current limit for charging a 12V battery will vary depending on the type and capacity of the battery. As a general rule, the current limit should not exceed 25% of the battery's capacity. For example, a 12V battery with a capacity of 100Ah should have a current limit of 25A.

4. Can limiting the current cause the battery to not fully charge?

Yes, limiting the current can result in the battery not fully charging. However, this is a necessary trade-off to protect the battery from overcharging and potential damage. It is important to find the right balance between charging speed and battery health when setting the current limit.

5. Are there any risks associated with charging 12V from 12V with a current limit?

There are minimal risks associated with charging 12V from 12V with a current limit. As long as the current limit is set correctly and the battery is monitored during the charging process, the risk of overcharging or damaging the battery is significantly reduced. It is important to follow proper charging procedures and safety precautions to minimize any potential risks.

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