Testing Batteries

Chui · June 2009

Is there a way to test individual batteries without special equipment?

I have a 1000Ah bank made up of 5 batteries and I know that at least one of them is bad.

When the sun went down yesterday my bank was at 12.4v I took the suspect battery off the bank and it dropped to 11.2 in about one minute. Two hours later, it was at 10.5v all by itself with no load.

Is that the best way to test a battery? What would be the lower limit voltage to know if a battery is good or bad?

Thanks

AntronX · June 2009

Re: Testing Batteries

Did the suspect battery drop to 10.5V or the rest of your bank? Sounds like one cell in it has internal short and was also discharging your other batteries. If it drops voltage like that all by itself, then it's dead.

niel · June 2009

Re: Testing Batteries

you may want to recheck the battery voltages of all of them individually (disconnecting them from one another) and keep track of what battery had what voltage. this is best done after they've been separated for at least a half an hour or so so they may settle into their own voltage rather than the battery bank's collective voltage. report back your findings on the remaining batteries. in the meantime do you mind telling us what it is you charge them with and the specs?

mike95490 · June 2009

Re: Testing Batteries

Try an EQ charge on your "suspect" Maybe it has a low cell, and just needs an EQ ?

jacobs · June 2009

Re: Testing Batteries

Chui wrote: »

Is that the best way to test a battery? What would be the lower limit voltage to know if a battery is good or bad?

When using batteries, a hydrometer is a must. It will tell you very quickly which cell(s) are shorted or weak. Hydrometers are very inexpensive.

Chui · June 2009

Re: Testing Batteries

The battery that went to 10.5 was just one battery, not the whole bank (that would have been bad!). That battery is no longer part of my bank, but I am still not having the best luck with the rest of my bank.

Chui · June 2009

Re: Testing Batteries

I have checked three of the 4 batteries individually over the past week. I would disconnect them after the sun went down and then check them in the morning. these are the voltages I have 11.9v, 12.1v, & 12.06v. I have yet to check the last battery, but it is significantly newer than the others.

I am charging the batteries with four 250 watt solar panels. I get about 9-10 hours of sun a day... pretty consistant, we live in the desert, so it is pretty warm most of the time (upper 90's to low 100's most days). We are right on the equater so the sun time is almost always the same. We are also in a valley which cuts our sun time from 12 hours to 9-10.

Chui · June 2009

Re: Testing Batteries

I EQ the bank once a month automatically, it was done last week.

niel · June 2009

Re: Testing Batteries

first of all, we never recommend mixing batteries of different age, capacity, or type. this creates problems and drags down the whole battery bank to the worst of the bunch. that 11.9v is significantly lower than the rest for a battery bank that had just recently been eqed and may have problems. what of the last battery's voltage even if just pulled for a half an hour to see it's voltage? i foresee a possible near future need to replace all of the batteries for as each one fails it stresses the remainder if left in line and even if out of line as fewer batteries carry the loads. mixing new ones with these will cause them premature failure too as their lifespan will be shortened.

RCinFLA · June 2009

Re: Testing Batteries

Sounds like you may have been overcharging the batteries.

Fully charged should be at 12.6 to 12.7 vdc after several hours of disconnected 'rest' state. Unloaded, open circuited they should not drop for days. Bulk charge is 14.2-14.5v, float charge is 13.2-13.8 vdc.

You did not mention the type of battery. Lead-calcium will go weeks without dropping open circuit voltage because they have very low leakage. Lead-antimony have higher leakage and will self discharge more on their own.

If they are open flooded type with ability to do an hygrometer test, besides specific gravity reading, also look at electrolyte clarity. If it it not clear and looks like it has ground graphite particles floating around it is a sign of overcharging.

I hope you are using a charge controller and not connecting the panels directly to the battery.

Make sure you have confidence in the accuracy of your voltmeter. For about $30 you can also get a clip-on DC amp meter which will quickly allow you to check how the available charging current is flowing to each of the five batteries. If you are getting 75 amps from PV's, ideally like to see 15 amps going into each battery. This is not likely unless they are all new batteries. Having a mismatch of lowest getting 10 amps and highest getting 20 amps would be acceptable. You will typically find one starts out getting more current but as charging time goes on it will reduce and one of the other battery's current will rise. For 200 A-H battery should also make sure that no battery gets more then about 50 amps of charge current.

Chui · June 2009

Re: Testing Batteries

My bulk and float rates are in line. I bulk to 14.0. The batteries are lead acid. the acid we get here in Kenya is a bit weak. The voltage is pretty accurate, my hand held volt meter, my Trace charge controller and my Outback Mate all concure.

I will do a hydro check and I never thought to use my clip on amp meter to see how much is gonig into each batterie, I will check that also. Thanks

Chui · June 2009

Re: Testing Batteries

I was able to find two ways to check the batteries easily. First I charged the batteries until they were full. then took them off the bank one by one and using jumper cables, I attached them to my car. Then turned on the lights and checked the voltage. Unfortunatly, the voltage on my batteries dropped like a rock on all of them.

I also have heard from other sourses that just taking the battery off the bank and let them sit overnight. According to an engineer I talked to any 12v battery that has been fully charged should not drop below 12.6 at rest.

I hope that is helpful.

niel · June 2009

Re: Testing Batteries

that sharp drop off in voltage means a lack of capacity and if the acid is weak (that is a specific gravity not where it's supposed to be) then this can be a cause. odds are they are just too old and used up. be sure the electrolyte levels haven't dropped as exposing the plates will make short order the battery lifespan.

Testing Batteries

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