trailer battery charging

madmaths

Observer
Hi! the more I read on this subject the more I'm confuse about this! And I should know the answer I'm a electromecanic and a industrial electrician... any way I'm not 100% sure so I'm looking for some good answer please. Some say its very dangerous to but not the same rating, type.... battery in parallel, some say use a isolator other say don't because they are diode and they have voltage drop , some say use a 12v to 12v charger other say use a solenoid....... Jesus!!! I would like to have you 2cents on this subject please! I just read here on adventurervctr.com a article and I'm very not sure if I agree here a copy and past :

The first four segments will deal with electrical TRUTHS and MYTHS in nature.

When towing my trailer down the road my tow vehicle will "charge" my trailer battery.
MYTH: Actually, your tow vehicle doesn't know there's another battery in your trailer only that there is a 12V load of some sort. Look under your tow vehicles hood and find the charge line from your tow vehicle's alternator to it's battery. It is a very heavy cable capable of handling very high amperage, 60A or more. The so called"charge line" from your vehicles battery to the 7 pin connector at the back is in relation a very small wire, carrying only a few amps. Your tow vehicle will supply 12V at low amps to the trailer should the trailer battery be dead to help put the slide in for example or to help supplement the 12V usage on board the trailer when traveling as in the case of running your refrigerator on 12V. The tow vehicle sees this "charge line" as just another 12V outlet, similar to a cigarette lighter. There are in fact only two effect ways to charge your battery on board your trailer. That is by either plugging the trailer power cord into a 110Voutlet so that your on-board converter chargers the battery or by removing the battery and putting it on a true "battery charger". The latter is the quickest but your converter will do a good job as well. It will just take longer.
 

DaveInDenver

Middle Income Semi-Redneck
It's basically right. The normal way trailers are connected may not charge the battery from a deep discharged state, at least not quickly. The +12V in the harness is primarily there to operate loads like brakes.

To guarantee a remote battery charges you need to use very heavy cable, basically running something like a 2 or 1 or even 1/0 AWG for both +12V and ground (remember it's a loop resistance). It has to look truly like a parallel battery to the charging system. You'll sometimes see people use Andersen SB175 for this, to provide a quick connect for a remote trailer battery.

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The other reliable way besides large cables is to use a DC-to-DC charger so you completely decouple the two batteries and the voltage on one side has no impact on the other. Then the wire size only needs to be large enough to handle the max input current of the charger. It won't charge initially as fast as a large cable could but it will be predictable. This is because at some point it starts boosting the voltage above the charging system to keep the trailer battery charging when the main battery has already told the alternator it's fine and it can roll back.

If you run a stand alone charger on the trailer batteries from shore power or solar, then the trailer harness likely has little net effect either way. Normally you'll fall somewhere in the "maybe" region depending on the state of discharge and the charging system voltage. Wires in the 12AWG-to-8AWG range will be in effect giving you variable trickle charge.

So the difference here is with the converter you can say from 75% state of charge it will take 4 hours no matter what. With the battery connected to the +12V and trickle charging it might take 3 or might take 8. Depends on how fast the main battery recovers, if you use the headlights or not, high engine RPM or low cruising RPM, etc.
 
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madmaths

Observer
Can't be more clear than this!! Thanks man! Any recomandation on a good b2b charger? And on a dual battery set up the best is a continuous duty selenoid? ( cheap and easy)

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DaveInDenver

Middle Income Semi-Redneck
Can't be more clear than this!! Thanks man! Any recomandation on a good b2b charger? And on a dual battery set up the best is a continuous duty selenoid? ( cheap and easy)
You shouldn't need an isolator with a DC-to-DC charger. It probably will have a voltage or ignition sense that turns it on when the engine is running. I'm only familiar with the Sterling Power. They aren't cheap.

https://sterling-power.com/collections/battery-to-battery-chargers

You'd really only need the isolator if you use the heavy cable option. Then that's like any other dual battery scenario when you want the house/aux isolated when the charging system isn't running and the batteries kept separate when cranking.

I suspect the cranking case is why someone would suggest a diode. That would prevent the trailer battery from trying to assist, which it won't be very good at doing anyway. But that has another issue, it further adds to the voltage drop and will make the trailer battery charge even worse.
 

madmaths

Observer
Yep I iderstand the no need of the isolatore with a b2b charge...i was asking for another set up ( a friend dual battery in is jeep)
Sorry bud
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DaveInDenver

Middle Income Semi-Redneck
Yeah, there's lots of good recommendations all over the forum about isolators. A continuous duty solenoid rated to handle the alternator rating is the least expensive way that works, much better than a diode isolator. Search here and elsewhere for reasons why you might want to spend more than $50 but suffice to say it just comes down to quality and features.
 

DaveInDenver

Middle Income Semi-Redneck
If relay is energised only after the motor is running will mitigate this. I think alot of modern cars have circuits what are only live whilst running but not cranking.
It would indeed, but my thought was if you did nothing else but connect a battery to the trailer harness. A diode would be basically half an isolator.

As an aside,
I always wondered if its worthwhile connecting a regulator voltage sensing wire at the rear where a trailer connects to.
That would absolutely work, assuming the drop isn't so much as to exceed the alternator's high side. The problem would be frying the main system potentially. If you ran a dedicated alternator you could run the sense from where ever you wanted.
 

DiploStrat

Expedition Leader
Couple of Comments

A B2B might work very well in a trailer situation, but remember, amps is amps - so if you want 40A of charge, you need to wire for 40A. Sorry - no free lunch.

I use a REDARC BCDC 40A charger on my truck. (
https://redarcelectronics.com/products/dual_input_40a_in-vehicle_dc_battery_charger) It has several nice features:

-- In my case, the first essential is that it is very voltage tolerant - 9 to 32v. (In my case, I use 24v to charge a 12v camper battery.) This means that it is a perfect fit with various Toyotas and others that have low charging voltage and, yes, it will compensate for minor voltage drops due to long wires and connectors. (Just remember, 40A of charge requires that you wire for at least 40A - and REDARC suggest that you fuse for 60A.)

-- It is ruggedized for under-hood mounting. So it is sealed, heat tolerant, and needs no fan.

-- It is physically very small, but still puts out 40A or more. Twice the output of the CTEK D250S and comparable with the larger Sterling units.

-- Like the CTEK, it also includes an MPPT solar controller. Since I consider solar to be just about essential for the longevity of lead acid batteries, I think this is a nice feature. In my case, it allowed me to add another 160w panel even though my main solar controller was maxed out.

-- Has four profiles, including LiFePO4.

-- These things are dead easy to install, but RECARC do a great job of hand holding, should you ask.

For your consideration.
 

madmaths

Observer
Thanks DiploStrat for jumping in! i've read a lot of you post! The proof pics down hehe thanks for all the info!

In my case I all ready have a solar panel and a solar charge controler. So i was look to add a way to safely charge my batt when driving. And like I said in my first post I work in electrical so my friends are asking me a ton of question and I wasn't 100%sure now its better thanks! 20180303_102113.jpg
 

DiploStrat

Expedition Leader
Thanks for the kind words - I hope my materials were useful.

A trailer is really no different than a really long vehicle with a disconnect. I am normally the champion of the you-don't-need-no-sinkin'-B2B school, but, in the case of a trailer, there may be some good justification as long as the B2B is close to the batteries.
 

TroySmith80

Adventurer
I see how installing a DC/DC charger would be advantageous. I don't get why just installing larger wires from starting battery to house battery is helpful.

There is some confusing stuff early on in this thread. Maybe somebody can enlighten me. I'll explain things the way i've been taught. Tell me if you all think i've got something wrong.

Most electrical loads pull however much current they want, and if it's too much for the gauge of wire they're using, it'll melt the wire, that's (part of the reason) why fuses and breakers are a thing. My understanding is that the power is basically pulled, not pushed, it's determined by the load, not the source. And the pull is in watts, not amps. Also, if the wire is too small/long there will be a voltage loss, and the load will pull more amps in order to get the wattage it wants (because V=IR, P=IV). I get lost when we start talking about reactive and inductive loads, i think with DC stuff we're usually talking about 'simple' resistive loads though, correct?

So if you have an appliance on your trailer that wants 1,000 watts, and it's getting 12 volts, it'll pull 83.3 amps. If your wire is too small to carry 83 amps, it'll get hot and melt. If it has a breaker or fuse like it should, the fuse will blow before the wire melts. What won't happen (my understanding) is that the appliance will just pull 20 amps because that's all the small wire can supply. The wire won't limit the power delivery. It will either work, or melt (or blow fuse). Is that all correct?

Now, perhaps battery charging is a little different than other kinds of "normal" electrical loads. According to the logic above, the wire size would not limit the ability of your tow vehicle to charge your house battery. I don't know how much power a discharged house battery wants to pull. I'm sure if you have a bank of 400Ah it will want to pull a lot more charging current than if you have a single 80Ah battery, so that is a consideration. I would think though that your batteries will either get the charging current they want, or else blow your fuse. If they're not blowing your fuse, then there shouldn't be any gains to be had by installing larger wire, right?

What I can understand making a difference would be installing a DC/DC charger because it can boost the voltage. If it can show your house batteries an extra volt higher than what's coming from your trailer pin, then that will probably make the batteries want to pull more charging power and charge more quickly.

I also don't have a good grasp on how the different phases of a good charger work, in terms of constant-current phase, constant-voltage phase, bulk, peak, float etc. So maybe that plays a role.
 

dreadlocks

Well-known member
With a battery, its being pushed.. not pulled in your thinking.. The reason your wires will melt with overload conditions pulling is because lets say your 1000W load at 12v is on too small of a cable, the voltage will drop to 10V, the load is still 1000W, so now the current is 100A, not 83A it was @ 12V.. which then exceeds the cabling, which causes more drop in voltage, and more amps and the next thing you know you either have popped fuses or melted wires.. its a runaway condition once it overloads as the voltage plummets and the amps skyrocket, thus fuses being required to prevent fires.

Your battery wont take a voltage lower than its voltage as a charge source, if its @ 12V DC and you give it 11.9V DC.. nothing is flowing into battery, not even a watt.. its not a fixed 1000W load, the load increases the as the voltage goes higher than the battery and decreases as it gets closer to the battery voltage... Its basically inverse logic from a static load.

A larger wire, means less voltage drop.. which means more current can charge the battery, which requires a bigger cable.. If you want to completely charge a trailer battery from the tow vehicle, and at a rate thats better than a few amp trickle.. larger gauge wiring is the only way to accomplish it.. so at the desired amperage, the voltage in your trailer is as close to the alternator output as you can get it.
 
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Buddha.

Finally in expo white.
I'm thinking about what it's going to take to ditch the generator. We part time "work camp" and like to boondock between destinations, no more than a week between hookups. So far we haven't had any problems going 2 days but I don't know how long we could have gone because I had no battery monitoring.

I have 220ah of 6v GC FLA. Our biggest power user is the furnace blower. Otherwise it's device charging; laptop via small inverter, phones, nothing big. I'm thinking about buying a 12volt TV so we can get free antenna channels here and there off grid, also been thinking about ditching the giant 120v home theater style tuner and getting a 12v automotive style head unit to power the existing speakers.

My truck has a 160 amp alternator and voltage stays a little over 13.5v. If I run a 20' positive back to the trailer and a 20' ground back up to the truck battery the loop is 40'. Is 20amps a reasonable max charge rate for 210ah bank? If so 40' at 20 amps with 1 AWG wire gets me .73% voltage drop to the bank if I'm doing the calculator correctly.
I cant figure out what gauge wire I'd use with a DC/DC charger. 50' of 1 awg cable is 150$. It doesn't seem cost effective to buy the DC/DC charger in able to run smaller wire.

Is it work trying to do this at all considering I'm looking to add solar? I'm thinking ~400 watts for our 210ah, is that a good amount?
 
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dreadlocks

Well-known member
20A is about minimum acceptable charge rate for that IMO.. typically I found anything less than 10A per 100AH not really worth the trouble.. anything much more than 20A per 100AH is unlikely to yield much in return for the extra costs as it'll spend so little time above that, if it ever does.

With driving you'll want to bulk it up mostly and finish w/solar or something.. it takes a good 6-8h of charge to refill a battery, unless your driving that far every few days then invest in trying to bulk it up with engine/genset and finish the long absorb charge w/solar at a much lower rate..

Get some monitoring up stat.. it will help greatly in designing a system that can achieve good results.. and help prevent it from going tits up on you out in the field.
 
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