Testing Rechargeable Batteries with a Multimeter

[mwieder]

I have several large 12V 7.5AH rechargeable batteries that were left to me to power the automation system in the house. Problem is, the sytem says the battery is low, and while I know one of the batteries is a new one and therefore good, I don't know which one. I have a multimeter and I'd like to use it to test the batteries to see which one is the best to use in the system. Excuse my ignorance, but I don't know what setting on the multimeter to set it to to test the battery. Additionally, since the problem may be with the charger itself and the batteries may be fine, the batteries may not be charged at all. Is there some way to test to see if the batteries are good even if they aren't charged? thanks!

[hallerb]

you need to test not only for voltage but current capacity.

set meter at something above 15 volts dc and check disconnected battery. shouuld get at least 12 volts or thereabouts

now connect battery plug in device, check voltage again, should be around 13.8, the voltage the battery charges at.

now let us know what you find and feel free to e mail me [e-mail]

batterys last only a few years at best, yours are probably both bad

[George E. Cawthon]

What kind of batteries? Lead acid, nicad, NiMH or?? A fully charged lead acid battery is 12.65 volts. If it is plugged into a charging system you need to let it sit unconnected for at least 1 hour and 3 hours would be better to get an accurate voltage reading. Any reading less than

12.4V would indicate battery capacity is reduced. BTW, the charging system should be putting out about 13.4V.

Set your multimeter to DC voltage at what ever range is the closest too but greater than 13V.

The answer to your question is NO. You can't tell anything about a battery if it isn't charged. However if a 12 battery (of any kind) reads below 10V it is probably ruined and below 9V severe damage is almost certain.

[Bob]

I have several batteries for my rechargeable drill. After several years they started loosing the charge faster and faster. My son told me that the instructions with his DeWalt said that if this happens, to leave the battery in the charger for a few days after the charge light went out. This seems to have solved my rapid discharging problem.

[mm]

Power it when there is a power failure, right? Normally it runs off of house current, right?

The one that is lowest is probably the bad one. Disconnect each one and use the scale that is 15 volts or higher, but as close to 15 as possible, on DC, direct current. The good ones will probably be within 0.1 or 2 or 3 or maybe 0.4 of each other. The bad ones significantly lower usually. But if they were bought at the same time, they are likely to fail at similar times, even if they have never been used (there were no power failures, and they weren't connected to anything when they were moved to your house.)

I doubt if all the chargers, or any of them, are bad. Even more than th ebatteries, they should give voltage readings (again on DC) that are close to each other. But if one is substantially lower than the others it may well be bad., Again, for the testing, the batteries have to be disconnected, from the chargers and either the AC current must be connected and powering your system, or the chargers must be disconnected from the system as well as from the battery. Preferably the second one. This is a pain in the neck, so I would just replace the battery, give it time to charge, a day, and expect the Low Battery Indication to go away. If it doesn't go away, then you can worry about that.

Remove NOPSAM to email me. Please let me know if you have posted also.