Why second battery won't start vehicle

[stipek]

Just looking for some technical clarification on why this didn't work.

Midsize SUV has "local" brand AGM under hood and 2 pos runs to xs d7500 in back.....no isolator....singer ho alt.

Ran some errands around town with a few short trips, later went to work 15 min drive, went to go home 8-9 hours later and it didn't even turn over, dome lights way dim, door chime even sounded funny.

Got a jump, started right up, next morning took out battery under hood, put meter on xs battery and it was resting at 12.86. I never tested battery under hood assuming it was bad.

So....it is possible the starting battery was bad and the d7500 is still good? Also if the xs was sitting at 12.86 why couldn't it start the car?

[Dangrebel]

when you have a bad battery, it acts as a sponge that is dry. every time you try to pull a load on it and another battery is present, the good battery's amperage gets soaked up by the bad battery then it goes to the other electronics. so it effectively absorbs the amperage and turns it into heat. then your starter gets voltage but hardly any current. Autozone or a local parts store can load test batts for free.

[Jessica]

if the rear battery is charged and front is dead, check your fuses on your power runs to the back

[stipek]

Thanks for the quick reply is. Fuses are good, was also thinking the voltage drop due to the distance to the front may have been enough to not turn it over also

[stipek]

Do you think I need to put an isolator in....I don't bump it that much that it's even pulling off the d7500 often, just don't want to ruin it, it was just some insurance for the amp basically to be safe

[Jessica]

have you checked the front battery voltage?

[boometro]

double check the battery. just because it has the volts does not mean it still has the amps to turn over the engine.

[frogcase2002]

That just happened to me . Front batt was messed up and rear wass good. Wouldnt start . Then I took the front batt out and starts fine now

[stipek]

Never checked the voltage on the hood battery, just assumed it was dead and replaced it. Probably should've put it on the charger and seen what happened but I didn't have time for that, needed the car.