Batteries voltage?

[prototype561 10+ year member]

I just hooked up a second battery in the back of my car, its agm 140 ah battery, its connected to the stock battery, when the car is on the front battery sits at 13.8-13.9 and the back battery sits 13.3-13.4. I thought both batteries are suppose to be sitting at the same voltage? Nothing is connected to the 2nd battery in the back. Is the common or is there something wrong with the 2nd battery or the wiring of my 2nd battery? Your thoughts

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[MakeshiftAudio 10+ year member]

The standard charge on a lead acid battery, is 12.8V.

Need to let the back battery charge for a while, it's fairly big.

 

[prototype561 10+ year member]

oh ok, thought it was something i did wrong, that makes sense though

 

[prototype561 10+ year member]

well i have another problem, when i turn the car on the back battery doesnt hardly charge, it sits at 12.7 with the car off and when the car turns on it doesnt budge, it was going up into the mid to high 13's, could it be that the power connector is loose or something

 

[JuicyJeff 10+ year member]

Rev your engine

 

[loudmofo 5,000+ posts]

its a bad back ground thats causing this problem try to ground it to the frame

 

[SQcrewcab 10+ year member]

You could also try to run your - off the rear battery directly to the - of the front battery.

 

[loudmofo 5,000+ posts]

when the back battery isnt grounded the battery voltage doesnt go up it stays the same try the ground 1st i bet its the problem in this case.

 

[prototype561 10+ year member]

its a bad back ground thats causing this problem try to ground it to the frame

yeah thats what i was thinking, i will go and change it and see if that works, to the other guy i did rev the engine up and nothing change, so im hoping its a bad back ground, thanks guys for the quick responses

 

[adas Pro Seller]

both batteries should be at the same exact voltage if your power wire and ground are hooked up right.

 

[prototype561 10+ year member]

I got it, it was a bad ground in the back, now both batteries sit at 14.3 volts, thanks guys, now i wonder how many watts i can put on a 140 ah battery without it dropping too low?

 

[18inchjeepin 10+ year member]

4k?

glad we helped

 

[prototype561 10+ year member]

dang 4k, well that sounds about right because hc3800 is 135 ah, i dont think i will ever put 4k in my suv, maybe 2k-3k.//content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif

 

[smoka 10+ year member]

I just hooked up a second battery in the back of my car, its agm 140 ah battery, its connected to the stock battery, when the car is on the front battery sits at 13.8-13.9 and the back battery sits 13.3-13.4. I thought both batteries are suppose to be sitting at the same voltage? Nothing is connected to the 2nd battery in the back. Is the common or is there something wrong with the 2nd battery or the wiring of my 2nd battery? Your thoughts

When the truck is off, both batts will read the same. But when its running, you'll probably see around a 0.3v drop in the back battery because of voltage drop from the longer power/ground. fyi