Is that trailer battery getting charged?

A few weeks ago I did some umbilical wiring repairs on my fifth wheel camper. When was checking everything out, I discovered the trailer battery wasn't getting charged by my tow vehicle. It turned out there's a five dollar 30 amp fuse in the truck to protect that circuit, and it was blown. There's also a 30 amp circuit breaker on the trailer side, but the fuse blew first.

Since this discovery, I've added another check to my normal lighting checks: After hooking everything up but before starting the tow vehicle, I check the trailer battery's voltage. Then I start my truck and check it again, making sure that the voltage is higher after starting the engine. The voltage will only rise a couple of tenths, but it's charging.

I suspect the blown fuse happened when I ran the trailer battery completely flat trying to operate the camper's furnace during cold nights at Yellowstone. I now carry a spare fuse, but haven't needed it. We just finished a 1200 mile trip around Lake Michigan and had no electrical problems during the trip.

My dump trailer uses the 12v battery that dumps the trailer for the electric brakes in the event the trailer breaks away from the truck.
Does your 5th wheel have a separate battery if it comes off the truck?

> Does your 5th wheel have a separate battery if it comes off the truck?

Nope. So if you run the battery dead while dry camping, there may not be any juice left to operate the brakes. Of course, the likelihood of losing a fifth-wheel trailer is very low, and if you do it's not going to go far once the landing gear hits the pavement. But just the same, the breakaway brakes are required to work, so it's important to keep that battery charged.

Just a note that the Chev truck i have 2015 and probably newer ? don't charge the trailer battery
unless the headlights are on or the tow/haul mode is selected. Just like the manual says

(quoted from post at 09:04:48 07/29/23) > Does your 5th wheel have a separate battery if it comes off the truck?

Nope. So if you run the battery dead while dry camping, there may not be any juice left to operate the brakes. Of course, the likelihood of losing a fifth-wheel trailer is very low, and if you do it's not going to go far once the landing gear hits the pavement. But just the same, the breakaway brakes are required to work, so it's important to keep that battery charged.

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If it comes loose surly the tail gate will stop it, Ha

Considering the low cost and the recommendation of an engineer I respect. I am thinking about the cheapest solar charger I can find . Low chance of overcharging and always ready to go.

My dump trailer battery is supposed to charge off my Truck. I'm not impressed. I connect a battery charger to the battery and top it off each time I use it.

> Just a note that the Chev truck i have 2015 and probably newer ? don't charge the trailer battery unless the headlights are on or the tow/haul mode is selected. Just like the manual says

I found no such instruction in the manual for my '17 Silverado. But how do you NOT charge the trailer battery? It's not like there's a switch anywhere for the trailer battery circuit. I think it's more likely that your truck WON'T charge the battery under some conditions because the alternator voltage is too low; switching on the lights or hitting tow mode causes the alternator voltage to increase enough to charge the trailer battery.

> My dump trailer battery is supposed to charge off my Truck. I'm not impressed. I connect a battery charger to the battery and top it off each time I use it.

George, the next time you hook up your trailer, check out how much voltage is actually making its way to the trailer battery. To use your words, I was [i:654c4848f0]not impressed[/i:654c4848f0] when I checked mine. More of a trickle than a charge.

(quoted from post at 02:43:31 07/30/23) > My dump trailer battery is supposed to charge off my Truck. I'm not impressed. I connect a battery charger to the battery and top it off each time I use it.

George, the next time you hook up your trailer, check out how much voltage is actually making its way to the trailer battery. To use your words, I was [i:1f8163ab57]not impressed[/i:1f8163ab57] when I checked mine. More of a trickle than a charge.

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It's only a 10ga wire, and a 20' run or longer. You won't be impressed.

If you want a high charge rate you need to run a dedicated 2ga feed and ground to the trailer battery.

> If you want a high charge rate you need to run a dedicated 2ga feed and ground to the trailer battery.

The circuit is fused at 30 amps on both truck and trailer, so AWG 10 wire is appropriate. But the OPEN CIRCUIT voltage at the trailer connector is only about 13 volts, possibly due to a diode in the circuit, so it's not just the wire gauge that's limiting the charge rate.

Page 338 in the 2017 owners manual for the half ton.

Let me change/expand the wording a bit...the trailer battery will charge to the trucks battery voltage level...to add to the conversation this truck (Canadian version) also cuts back the charge voltage to a 12.5 +? level when the battery is fully charged and operating voltage is as needed, not needed to charge itself or an other. Got caught thinking alternator had failed on the road after an hour or more, even the test meter showed 12.5vdc, got home /shut it off and re-started and high charge came back, factory built in feature.
Anyway back to the manual statement.. trailer section 98..page 89 vehicles equipped with Heavy Duty Trailer Wiring Harness, describes the operation of the charging system as mentioned, as for your 2017, i guess it depends on what trailer package you have ?
All good, just keeping the stick on the ice

i no longer watch any scripted episodic television. i blame Steve Smith for setting the bar too high. keep yer stick on the ice

> Page 338 in the 2017 owners manual for the half ton.

You're right. The manual for my 2500 is identical. And it pretty much confirms what I said earlier: Going to tow mode or turning on the headlights will kick up the alternator output voltage and ensure the trailer battery gets charged.

> Let me change/expand the wording a bit...the trailer battery will charge to the trucks battery voltage level...to add to the conversation this truck (Canadian version) also cuts back the charge voltage to a 12.5 +? level when the battery is fully charged and operating voltage is as needed, not needed to charge itself or an other. Got caught thinking alternator had failed on the road after an hour or more, even the test meter showed 12.5vdc, got home /shut it off and re-started and high charge came back, factory built in feature.

Thanks for the clarification, Phil. Newer GM vehicles have a feature that reduces the alternator output once the battery is charged in order to reduce the alternator load and squeeze out a tiny bit better fuel economy. I'm not sure my Duramax has this feature, but the gassers do. Also, I'm pretty sure there's a diode somewhere (maybe in the trailer brake control module) to prevent the trailer battery from backfeeding when you hit the starter. Diodes introduce a voltage drop of about 0.7 volts. The result is there's not a lot of voltage getting to the trailer battery under the best of conditions, and probably none when the alternator output is reduced.

The next time I hook up my trailer I'm going to check the battery voltage with and without tow mode turned on.