Dual Battery Troubles


New Member
Hey y'all!
I'm trying to install a dual battery system in my truck so I can run a fan and have a couple outlets in my camper set up.
I'm using the dual battery and isolator kit from Keyline Chargers (linked bellow if you'd like to look at it).
The second battery is a dual purpose AGM duracell platinum battery. I also installed a couple of 150 amp fuses close to both batteries as an extra safety precaution.
Everything is set up and seems like it should be working properly. The second battery ground is solid. I turn my truck on, the red light on the isolator comes on (which indicates both batteries should be charging from the alternator) but when I measure the voltage on the second battery it doesn't read above 12.5ish volts. When I take the positive cable off and measure the voltage to that (positive probe of the voltage meter touching the lug at the end of the positive cable, negative probe on the negative terminal of the battery) it reads above 13 volts. So it seems like the proper voltage is getting to the cable but for some reason it's not charging the battery when it's attached.
I have not attached a power convertor or any accessories to the second battery yet because I wanted to make sure it was working first.
I thought maybe the charge was just happening slowly, but I drove my truck around for about an hour today and it still didn't read above 12.5ish volts. I can't find any similar issues on any forums anywhere else. And the customer support person at Keyline is as stumped as I am!
What do you think?
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Well-Known Member
You're checking the voltage with the engine running, not just ignition to 'ON'?
The little earth lead from the solenoid is to a good earth? All inputs and outputs from the solenoid on the correct terminals?
You have taken the insulated caps off the battery terminals :oops: (I've seen it done before)?

What vehicle have you fitted it to?


New Member
Yes, engine running when checking the voltages (not just ignition on, but I see where I worded that poorly). I drove around for about an hour and checked all the voltages again with the engine still running and the second battery was still at 12.5ish volts.
Yes, I've taken the insulated caps off the battery. Ha.
I double checked that the wires were set up on the right posts inside the isolator.
I'm thinking now that maybe the ground for the isolator might not be good? But would the red light come on if it wasn't?
I'm doing this on an 81 Toyota Hilux.


New Member
I haven't gone through and checked voltages of the positive wire at each fuse connection because I figured if the lug at the end of the positive wire connecting to the second battery is reading above 13 volts when it's not connected, that all the connections before it should be good. So would that mean a bad ground somewhere?


Well-Known Member
Yeah, its odd.

On an old Hilux it isn't going to be any CANBUS or computer messing with everything.

How healthy is your alternator? What is the voltage at the starter battery when it is running? It should be around the 14V mark, so maybe its the alternator on the way out?


Well-Known Member
What’s the voltage of your 2nd battery when it’s dissconnected from the supply ? Maybe you have a faulty cell. Also what size cable have you run and how far ? Is your aux battery grounded to the chassis or back to the start battery ? If it’s to the tray is the tray properly electrical bonded to the rest of the vehicle ? Is the ground connection from the chassis to the engine block and cab in good condition ? And same for the chassis and engine bond from you start battery. People have been known to put rubber under the tray mounts on old utes to stop the rattles.


Well-Known Member
Try another aux battery if you can or get your aux checked. If the problem persists then you know to look elsewhere.


measure across each fuse unit and the battery isolator with 2nd battery connected and engine running.
You should read 0v.
You should also be seeing more like 13.8 to 14 v at you main battery with engine running.


Well-Known Member
I was going to say it could be an issue with one of these modern smart alternators but that wouldn't be the case on an 81 Lux.
What is the cranking battery being charged at??

How long/far away from the cranking battery is the 2nd battery??
If its to far away you can be losing voltage in that way?