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Advice on charging (2) 81ah FLA dump trailer batteries


New Member
Mar 10, 2022
I run a tree service and recently bought a F350 flatbed dually truck to tow a 20' dump trailer for hauling tree debris. The hydraulic dump is powered by (2) 81ah FLA 12v batteries wired in parallel. It came with a measly 5A AC charger I can plug into a 110v extension cord, but I'd rather charge the batteries on the go with the truck's alternators. The truck is heavy duty, so it has (2) FLA batteries under the hood and (2) alternators: primary=175A, secondary=157A.
From what I found so far, using the 12v accessory pin on the 7pin trailer lights plug will be insufficient as there will be too much voltage drop. Solar might be an option, but I'd rather charge it with the alternators because I'm concerned the panel could get broken with the tree work.
That leaves 2 viable options I have found:
1) DC-DC charger (probably a REDARC unit)
2) Anderson PowerPole plugs on large gauge wire Setup (possibly involving an isolator), this is the more economical option

How many Amp charger for DC-DC would you recommend? REDARC makes a 12A, it's cheapest, will that suffice?
Can I use the 12v accessory pin on the 7pin connection even though the wire is smaller gauge because the DC-DC charger will adjust the voltage? Or will I need to use Anderson plugs anyways so I can incorporate a larger gauge wire from the truck's battery?

If I just use Anderson plugs with sufficient gauge wire, do I need an isolator?
Do the alternators work at the same time or how does that work?
Will the alternators be able to support charging a total of (4) FLA batteries and keep them all topped off?
From what I understand, with this method I'll be wiring the 2 trailer batteries in parallel with the 2 truck batteries through an Anderson plug and adding a breaker on the trailer side, that's all. Does this wiring present problems I'm not considering or is it safe and effective?

Here's a link to a company that supplies Anderson plugs wire kit that seemingly solves this problem for dump trailer owners, no isolator, spoke with owner and said no isolator needed.
PolePalUSA Kit

Thank you for any input!
Whats the voltage on the 12V accessory pin when engine's running? It should be sufficient. IF not then you can use a DC-DC charger on that pin and it'll boost the voltage, just check the amperage and make sure to get a very small one. Even a cheap DC converter that'll output a set voltage would work.

They make small 12V solar battery chargers like this Yes it could easily get broken but you can just wire it up and be good to go. I used to have one on my trailer to charge the lead acid battery for interior lights/electric jack but didn't secure it and it flew off.

Do both and then the batteries will be topped off when driving but also topped off when sitting in the lot.
It looks like I'm going with the wiring kit. The DC-DC charger is more expensive and seems unnecessary if I can get it done without the charger. I posted this post in general discussion, so they finally got back to me. I might still add the solar if it seems like the alternators aren't doing it.
$20 and 5 minutes to install right on the trailer side of the 7 pin. Problem solved.

KNACRO 15V 8A 120W DC-DC Converter DC 12V (9-14V) Step Up to 15V 8A 120W Power Supply (in DC 9V-14V, Out 15V 8A 120W)

Add the $70 solar panel and it'll charge when sitting by itself.

SUNAPEX Solar Battery Trickle Charger Maintainer 30W 12V Waterproof Solar Panel Trickle Charger for Car Boat RV Truck Motorcycle Marine Tractor Battery

The kit is better but unnecessary. I asked a client who has over 20 dump trailers and they just use the 7 pin with zero mods. In winter they plug in. Trailers are all used 5 days a week

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