Rear battery discharging much faster than front battery, why?

[akuma4u]

hey

i got 2 agm batteries,. both around same size, 34.. different brand however, one under hood and one in rear. both connected together and grounded each to the chassis. The rear battery ground is not the best spot, will redo it to stronger thicker metal like the seatbelt bolt under the rear seats (is that a good spot? please share a common and good rear battery ground point)

anyhew.. heres my problem.. my voltage use to be in mid 14s and for the past couple of months its been at MOST 14.. usually around 13.6 to 13.8 while driving

so i got my battery charger and would connect it to the front battery and sometimes it would take days to fully charge and i would end up giving up OR it would reach 100% within an hour.. but still.. voltage around 14 at most..

i tried many times to get to my old voltage of 14.5 but could never obtain it..

today instead of connecting to the front battery,. i connected to the rear battery and it was only showing around 30%.. so i found the problem.. and i am currently charging that battery.

but my question is this: Since BOTH batteries are connected together,. why is only the front battery (the one i would always have the charger connected to) being fully charged and not the rear battery? it seems the charge is not being passed onto the rear battery and isnt being charged along with the front with the charger is hooked up. Could it be because of the possible bad ground the rear battery currently has? or is it something else?

i always thought once u connect 2 batteries together they will act like 1 and you just have to connect charger to one and ,albeit may take longer, both will eventually get fully charged..

 

 

[WalledSonic]

Seat bolts are sometimes nutted to the car in a capture mechanism, and arent always a great ground point. Drill a hole in the heaviest part of the chassis, frame, etc and go from there. Use a backnut or tap into the steel if its thick enough. 

[bcbrassard]

Do you have the big 3 upgrade? Stock alt? topping off batteries is usually a good idea to do every couple months and try to do it where your just charging 1 battery at a time. You could install a battery tender and plug into that. 

Different brands could mean one is made cheaper than the other. But if battery is that low and wont come back you could weaken your starter battery.

[akuma4u]

6 minutes ago, bcbrassard said:

Do you have the big 3 upgrade? Stock alt? topping off batteries is usually a good idea to do every couple months and try to do it where your just charging 1 battery at a time. You could install a battery tender and plug into that. 

Different brands could mean one is made cheaper than the other. But if battery is that low and wont come back you could weaken your starter battery.

I only did the BIG 1 lmao, i changed the front battery to chassis wire to 0g and thats it,.. will be doing the other big 2 upgrades in the next couple of weeks

yes stock alt 120amp i believe, no one makes a h/o alt for my car. im running about 2250rms

heres the thing i dont understand though, if both batteries are connected why arent they both being charged up simultaneously?

i will look into a battery tender..

different brands yes but both high quality trusted brands.

what it seems like to me is that my system was only running off rear battery.. it feels like the 2 batteries arent connected at all because of one being so discharged vs the other. Could a bad ground on the rear  battery be causing this issue? 

i went out to check the rear batt and its 75% charged up now..

 

thanks for your help

[slowfkncar]

It's normal for the voltage to sag a bit when you add extra batteries on a 120A stock alt without big 3.

 You said it used to be at 14.4+, what batteries were you running then?

[akuma4u]

4 minutes ago, slowfkncar said:

It's normal for the voltage to sag a bit when you add extra batteries on a 120A stock alt without big 3.

 You said it used to be at 14.4+, what batteries were you running then?

i was using the same set up, same batteries.. come to think of it.. back then i was only running <1200rms .. so ive doubled the rms so it makes sense why voltage would dip more,. but still i dont understand why one battery is getting so much more discharged than the other.

when i wired up both batteries,. i did not run a wire between the 2 negative posts , i just wired to chassis. Would i have better results if i were to run a wire between both neg posts or would it not matter?

 

 

[Ahmed Johnson]

is there a difference when the car is off

[snafu]

You really can't expect the rear battery to charge properly if it's not installed properly.

[akuma4u]

4 hours ago, Ahmed Johnson said:

is there a difference when the car is off

just checked and with car off im getting 13.5 to 13.6v

is that normal?

[akuma4u]

5 minutes ago, snafu said:

You really can't expect the rear battery to charge properly if it's not installed properly.

true. I will have all fuses and alt/batt and wiring checked out tmrw.

Will also choose a new ground point for rear batt. if that fails i have no choice and will have to run a 0g between the 2 batts for ground. But is it ok to use one brand 0g for the + to + and another brand 0g for the - to - ?