Auxiliary Battery won't Charge

Hi Out there! I have a 2004 Toyota Tundra that I installed an extra auxiliary Deep cycle 12 Volt battery in so that I run lights and computers when camping. Under the hood I installed a Battery Relay Isolator and Relay so that the main battery would lose charge. This set up worked for a couple years but now I can get the Auxiliary battery to charge from my Alternator any longer. I can charge the auxiliary battery with a battery charger.

My alternator is charging my main starter battery with no problem. I the “Stinger” batter relay isolator only engages the Auxilary battery when the ignition is on. When The car is running I read with my volt meter 13,9 volts on all positive posts of the stinger. The Auxiliary battery only reads 12 volts at the battery. When the ignition is turned off I am reading getting only 12 volts on the Battery Relay at the Auxiliary battery line. The other posts that is connected to my starter battery reads around 12.9. It’s strange that the Auxiliary battery is getting 14 volts from the alternator but it won’t charge. and I can charge the battery fine with a battery charger. Does anyone out there have any ideas on what could solve this? thanks very much for taking the time to help. cheers

have you checked to see if you relay is not bad?

Good ground on the aux battery?

I’m not exactly sure how to Check that. but It definitely lets power through it when turn on the ignition.

How to Test a 3, 4 or 5 Pin Relay - With or Without a Diagram - YouTube

I think the ground on the battery is good. The negative terminal on the Auxillary battery is going a bolt on the trucks floor which I think goes into the frame. How can I check that ground is good?

I just watched that but I’m not sure that I understand it. the replay I have is Stinger SGP32 200 AMP Battery Relay Isolator and Relay

sorry I thought you had a separate relay. this should show you how to test it…
How to Troubleshoot a Dual Battery Isolator (
I would also follow the ground wire down to the chassis and make sure there is no breaks and clean the connection. its probably really rusty if not broken off.

How to Use a Test Light on Positive and Ground Wires - CAR FROM JAPAN

ok thanks for this. I will check the ground and relay and report back.

Make sure the other battery is good, could be draining the spare.

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If you have 13.9 volts on the output post of the isolator and 12.0 volts at the aux. battery there is a poor connection (open circuit) in the cable from the isolator to the aux. battery.

Examine that cable for loose crimps of faulty solder joints. You are using a cable of 4 gauge or thicker? A 16 gauge wire is insufficient.

I hooked up the negative terminals with a jumper cable to see if that would ground and now I am reading 13.6 volts everywhere. I guess that must mean where I have my ground wire on my auxiliary battery is not grounding good. I’m using 4 gauge wire and have the ground wire bolt to a bolt in the cabs floor. I guess that isn’t good enough ground.

That would be a good place for dirt and rust to be take it apart and clean it the area and retighten.

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Agree with @Renegade and @Nevada_545 rusty ground bolt will be a poor ground. Unbolt and wire brush it bright, reconnect. With the engine running, the voltmeter across the aux battery posts should read 13.9v or so.

If you disconnect the negative cable, you should still have 13.6 volts at the positive post. For diagnostic procedures you should connect the negative lead of the meter to body ground, then measure the voltage at each post.

With this method if you have a poor ground, the positive post would read 13.6 volts and the negative 1.6 volts. If there is more than 0.1 volts at the negative post, there is a poor ground.

Thanks Everyone! I think it’s fixed. As you all suggested it was the ground. I moved it to a better spot and the aux battery is now charging! I really appreciate it! Car talk rules!

Glad we could help. Happy camping.