Battery question

Could a battery be so low that my alternator wouldn't charge it.

Left lights on-killed battery-jumped it and let it run for at least an hour but not getting very good voltage from alternator-after 1 hour still at 9 volts-hooked it up to other car and charged it to 13 volts and now alternator is charging just fine at 14 volts

puzzled as to why it started charging when the battery got back to reasonable level.

 

A lot depends on the capacity size of the battery verses the amp output capability of the alt. It's like trying to fast charge a 200 amp battery with a 2 amp charger. Will take many hours of charging to get the battery charged.

 

This reminds me of something I recently read in (4wheel&off road?) mag.

It said something about putting a second battery on the charger with the dead one to get the current flowing. I assume the second battery would have to be good but slightly low on charge. It's a way to charge an old battery that won't take a charge otherwise.

I haven't done this myself but I am interested in how it would work.

 

Strange you have this problem. My jeep actually is overcharging my battery some how. Yesterday it rained and i turned on my lights and gauge lights and wipers. i blew out like 3 fuses... and my battery gauge reads over 18volts...no good it drained the acid right out of the battery.. Keep water levels in battery up to pare! thats wat iv atleast learned. Im swaping the alternater after this happend.

 

If the battery has a bad/dead cell it won't charge.

 

I just re read your original post, I would think the fact you jumped the vehicle would have delivered the initional voltage needed to start the chargong. It may be as Walt states it may take a long time to fully charge a dead battery with the alternator.

 

it is pretty strange, on its own I could only charge up to about 9 volts, and I wasn't getting much of a reading off of the alternator. I hooked it to my other car with both running and immediately my alternator starting putting a nice 14 volt charge to the battery, and shortly, my battery got up to 13 volts.

pretty odd

 

A bad battery can be the root to many pesky electrical issues. But the charging system as a whole needs to be working properly also. I'm not an electrical genus by any means, but as a marine mechanic charging issues are plentiful. In my case the batteries are to blame most of the time (marine batteries get abused, mainly from non use and getting fully discharged). A quick down and dirty check is put a volt meter on the battery before starting, lets say 11.5 volts, start the engine and the voltage at the battery should be more than 11.5. A good charge to the battery will be 13 -14 volts. This just tells you if the alternator is working.

 

that's the thing though. engine runnning with battery at 9 volts or so, and alternator hooked up correctly(new alternator and battery in this case too) and then I hooked it to my other car and the battery quickly jumped to about 13.6 volts with both engines running. as soon as I disconnected the other car from mine after it ran for about 25 minutes or so, my battery jumped to about 14.16/14.17 volts steady off of my own alternator.

 

I would question the alternator. Charge battery with a charger. Make sure the battery will hold charge. (after fully charged let sit over night all hooked up). Meter the battery, should still be fully charged, 12 volts. If the battery did not hold charge take it back to where you bought it and have them load test it. Now the battery is good or its bad. If good look at the alternator, if the system has 12 volts, start. Volts should jump to 13-14 volts. If still at 12 or less I would question the alternator. Is it internally or externally regulated? If externally regulated you could have issue with the regulator. I would also question the battery leads, clean and tight, good ground, all other wire conditions and connections tight. It sounds like for what ever reason the addition of external voltage (jumper cables) wakes up your alternator.