Test Lead-acid Battery. - Page 1
Forum section: electronics / general

author: SmurfKiller
author_rank: Contributor
timestamp: September 26, 2018, 09:38:12 am
content: I have a UPS that is telling me that the battery is dead, but when i stick my multimeter on the battery, it shows 26.67v. given that it is 2 lead acid batteries, i am guessing in series. i would think that it is charged.I replaced the battery and the UPS is back up and running. But this has left me a little confused, if the battery is dead, shouldn't the voltage be below 24v?are there other tests that i can do to test the battery?

author: Berni
author_rank: Super Contributor
timestamp: September 26, 2018, 10:21:01 am
content: Old batteries develop a high internal resistance. This means that when you take amps out of the battery the volts drop. On really dead batteries even just putting a small light bulb across them could cause the voltage to plummet to only a few volts.There are automotive battery testers made for testing exactly this. New ones come with fancy micro controllers in them, but the oldschool ones use the same principle. They simply have a voltmeter and a giant power resistor with a switch. When you push the switch it connects the huge resistor across the battery to draw a huge current (>100A), as you hold the switch for a few seconds you watch the voltage on the voltmeter. If the battery goes from 12.8V no load to 10V at 100A then its a good healthy battery. If you apply the load and it goes from 12.8V to 1V and comes back to 12.8V when you let go then you have a dead battery due to high internal resistance.

author: SmurfKiller
author_rank: Contributor
timestamp: September 26, 2018, 10:56:19 am
content: Cool, thanks for the reply... So i don't really want to go out and buy a battery tester that cost more than a replacement battery, but i want to see if i can still get some life out of this battery, so if i grab a 6.8ohm 100w power resistor (what i found on ebay), and if i check the voltage across that at set intervals say with an arduino i should be able to use that to calculate the capacity remaining in the battery correct?

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 26, 2018, 11:05:47 am
content: Just take spare light bulb from car headlight 12V 60WThey are great for low voltage load test, can withstand load for "infinite" time and have visual indication side effect

author: Gyro
author_rank: Super Contributor
timestamp: September 26, 2018, 11:13:01 am
content: QuoteNote though that the connection tabs at the back get alarmingly hot (from personal experience!) compared to the rest of the base metalwork. Don't use your favorite clip leads on them.

author: Berni
author_rank: Super Contributor
timestamp: September 26, 2018, 11:18:13 am
content: No you can simply take a multimeter and stick it across the terminals to monitor the voltage and then touch the wires of a heavy load to the battery. If the voltage stays reasonably close to 12V then the battery is still good enough for that sort of load.The load can be anything really. Resistors are a good load because they consume a predicable amount of current at a given voltage. But offten 12V car light bulbs are used as loads because the headlights typically are around 70W. For 24V truck ones can be used or two car lamps wired in series.For really heavy loads there is a cheep trick of wire in a bucket of water. A few meters of thin wire can work as a load up to a few kW, but because that makes the wire get hot really quickly and might melt it. To prevent that you simply coil the wire up some and dunk it in a bucket full of tap water. The wire is more conductive than the water so the current will generally stay in the wire but the water cools the wire. Eventually after a few minutes the water would also get hot and might start boiling, but for testing batteries like this you only need it to work for 1 to 10 seconds. (Also this is more appropriate for very big batteries that weigh as much as a person)As for trying to get the batteries working again, it is possible sometimes, but even if they do recover they will never reach full capacity again. Sometimes giving it a discharge and charge cycle helps them recover, other times sending huge pulses of current trough it can desulfate the cells and help them work again. But if the batteries are already really old its not worth bothering, at some point they simple run out of lifetime and die completely.

author: ogden
author_rank: Super Contributor
timestamp: September 26, 2018, 11:26:42 am
content: Quote from: SmurfKiller on September 26, 2018, 09:38:12 amVoltage does not define capacity. You can have tiny 12 voltA23 remote batteryand comparably big 12 volt UPS battery, both having 12 volts, yet one can deliver tiny amount of energy till it's fully discharged while another can run significant load for a long time.QuoteUnless you have specific instrument designed to test powerful lead-acid batteries, nothing much you can do except running UPS runtime test. Yet most likely it was done automatically and in result UPS determined that battery CAPACITY (not voltage) is lost.