Battery issue, my analysis correct?

[brijn]

Hi,


Our newly acquired bus probably has some unhappy batteries. Observations:
- After sitting for a few days, battery voltage at 12+ V (so looks OK)
- As soon as contact is turned on, it drops to 10.9V (bad!)
- Bus starts OK (ish I guess)
- The 160A alternator will push very high current (to the point the electrical issue light came on) for 1-2 minutes and then quickly drops down to maybe 10a. Levels out to near zero after some driving
- Charging voltage goes to 13.8V as soon as I start (this is actually during the high current phase already)


My analysis:
- Batteries are very unhappy
- Probably not a resistance issue since the alternator can push lots of current for a while
- Until I can test the batteries properly, it can be just a single battery that is causing the issue
- Voltage going so high early in charge cycle suggests surface charge on the plates?


Maybe a deep discharge with a aggressive charge cycle might help with some recovery. The bus sat for a while and I wonder if it sat with batteries empty


Bas

[Dead pirate]

Could be a positive drain. Mine did that too. Registers 12v then dropped to 5v under load. A positive wire was touching.

[TJones]

You don't want to deeply discharge the batteries. When working on batteries where safety glasses, long sleeves and rubber gloves, battery acid is nasty. If it is a non sealed battery check the water level in all the cells if low refill with distilled water. Disconnect batteries from bus and clean the terminal and the top of the battery with battery terminal cleaner. ( negative - first) This eliminates the possibility of discharge from the bus electrical or gunk on top of the battery. Charge the batteries one at a time with at least a 20 amp charger. If it isn't an automatic digital charger don't leave it on too long. You can blow the battery up if it is internally shorted (been there done that). Pick up a battery load tester and test the batteries once they are fully charged. If they pass retest them after sitting overnight. If one fails replace them all. You do not want to mix old and new batteries the old one will kill the new ones faster. If you have a good used battery laying around use it. If the batteries load test as good reinstall the battery cables and see if they stay charged. If they don't you have a load somewhere.

Ted

[john61ct]

99% likely need to buy a new set.

Thoroughly test to ensure proper care of the new bank.

[JDOnTheGo]

Howdy Bas,

Before buying new batteries, I'd suggest disconnecting and fully charging them (minding Ted's warning). Let them set for a day and then check the voltage (fully charged and resting for a day, voltage should be about 12.7 if in 'good' condition), or better, the electrolyte with a hydrometer. Another test, after charging, is to take them to one of the chain auto part stores and have them perform a load test. All of these will provide some reasonable data. If they checkout, I'd bet you have a load/draw (yes, even with the bus 'off') that is discharging them. Of course, if they are very old (like 5 years), they probably just need replaced.

[john61ct]

Better load test: give a Full charge as per spec.

Discharge at 5A per 100AH should take 20 hours to get from Full to 10.5V when new. If now takes under 14hours should be replaced.