Batteries are not charging when connected to my truck

[Diesel Greg]

I have a 2020 GMC Sierra 3500hd. I run my 12 volt fridge when towing when I get to my campsite my batteries are low I have my main power switch on up by the tongue and I am towing a 2023 2109s Mini Lite. It doesn’t seem like the truck is sending 12 volt power to the trailer. I know the solar panels are charging the batteries because I can see it on the monitor. Shouldn’t my truck’s alternator keep the trailers batteries up. Any advise would be appreciated

 

[Bob K4TAX]

I have a 2020 GMC Sierra 3500hd. I run my 12 volt fridge when towing when I get to my campsite my batteries are low I have my main power switch on up by the tongue and I am towing a 2023 2109s Mini Lite. It doesn’t seem like the truck is sending 12 volt power to the trailer. I know the solar panels are charging the batteries because I can see it on the monitor. Shouldn’t my truck’s alternator keep the trailers batteries up. Any advise would be appreciated

There is a fuse under the hood in the power/relay box. This fuse is in line with the 7-pin connector.

You will see the pin assignment on the following. Be sure you measure 12V+ and ground at the connector.

Bob

 

[bikendan]

Moved thread from the Rockwood sub-forum to the Towing and Tow Vehicles sub-forum since the issue is probably the tow vehicle and not the trailer.
Have seen many such issues, with newer GM trucks.

 

[Aussieguy]

What battery you got in the trailer?
Lead acid or Lifepo4?
How much solar have you got?
How are you measuring "Low" ?

Alternator.... the wiring for the truck to the trailer is much of the time very limited in how much power (amps) it sends to the trailer

a) make sure your wiring / connectors are clean and tight, any corrosion will slow down the charging further .

Use a good multimeter to measure the voltage where it connects to the trailer BATTERY .... Disconnect the battery before measuring the voltage in the wire. If the Wire voltage is NOT at least 1 volt higher than your battery ..... very little to no charging will occur..
it will power the FRIDGE but not charge the battery...

Solar....
treat solar same as a battery (power source) all connections and wiring needs to be clean and tight
measure the voltage at the panels .. then the controller INPUT and then at the battery
Must be at least 1 volt more than battery

pay particular attention to the NEGATIVE connection from the truck to the trailer frame
and from Trailer battery to the frame often overlooked but very important.

 

[Diesel Greg]

Thank you this helps a lot, do you know if the trailer connector is labeled in the relay box and is it a resettable fuse.

 

[Aussieguy]

just follow the fat wire loom 7pin to the trailer and you'll find where the junction box is

turn OFF all the lights in the truck so the only power will be the 12v aux
you can measure the voltage there ...
under the frame ... where the 7 pin wiring connects to trailer wiring

when testing voltage
1) check if any volts with engine turned OFF... most trucks now days only send power when the motor is running... stops the trailer from draining your starting battery
2) measure voltage at battery of the truck ... with motor running
for a base reading should be close to 13.8v

at the trailer junction box you should be getting close to 13.8v as well

If you got a lithium battery... and it is almost fully charged it will not be drawing a lot of current as the battery is probably 13.6 or higher .... current will flow better for testing if you discharge the trailer battery to around 50-60%

 

[NJKris]

Thank you this helps a lot, do you know if the trailer connector is labeled in the relay box and is it a resettable fuse.

+1 for what Aussie and the others have said. I do not know about the relay box. But for clarity sake, there is no such thing as a fuse reset. Fuses blow, breakers can trip and be reset. All your 12v dc stuff in camper you'll see fuses for in the load center. All the AC stuff will be on breakers. There are also 12vdc breakers, which I think are what you are referring to, in the area of the tongue, sometimes hidden within a junction box.

 

[jlankford]

Thank you this helps a lot, do you know if the trailer connector is labeled in the relay box and is it a resettable fuse.

As helpful as this forum is, sometimes Google is still your friend.
Fuse #63 in the main engine compartment fuse block is labelled "trailer battery"

That's from a random webpage via Google search. I would confirm it using the fuse tables in your own truck's use manual.

 

[Aussieguy]

I just replaced my trailer 7 pin wiring
HINT.... do not let 7 pin cord drag on road for 200 miles.. does it no good


I did notice in the junction box
the 12v aux wire was pretty fat 10 awg it went into the junction box with the rest of the wires

Was joined to a 6g wire inside the junction box via a wire nut (not great)
THEN... 6g went straight out of the junction box to a resetable 12v breaker

kinda crazy... but didn't have any 'connectors' to simply run the 10g straight out to the breaker next time I work under there will fix that. (Mrs AG ...."Yeah right I bet you will")

 

[Aussieguy]

Finding the truck fuse is only half the solution

measuring voltage at the connections will tell you is power is getting past all of the joints etc
if you only get 11-12v at the trailer it indicates poor wiring.

 

[Diesel Greg]

Thanks Bob 30 amp fuse was blown under the hood in the relay/fusebox

 

[NJKris]

Uh oh. Now the question is, why did it blow? You're half way there.