battery voltage how and when to measure

wilson ind

Well-known Member
Some may recall while back I posted that putting chargers on a timer seemed to work well for me.Now I want to test batteries for
loss and capacity. How long after charging a battery for a couple of hours should I let it rest before measuring voltage?
Thinking 4 hours then again 24 hours.May measure after one week? These are mostly standard harbor freight batteries. Maybe I
should check charger output? What should an eighteen and a twelve volt charger measure? What are your thoughts.
 
Bill, the real test isn't the voltage of battery, but how long does it take your charger to re charge the battery. A lithium battery doesn't self discharge. To prove that, let a Li battery sit for days, then put in a charger, it won't take a charge. Put any Nickle battery in charger a few days later and see how much time it takes for charger to shut off. From what I've, read all nickle batteries can lose up to 20% of charge in the first 24 hrs. Then about 1% per day.

If I discharge my 18v Ni-Mh battery with flash light to where the bulb is dim, 12v, it will take my dewalt fast charger 50 minutes to put a full charge in battery. So I estimate each minute of charge time is about 2% of full charge. 10 minutes of charge time after sitting on warm shelf for 24 hrs means your battery lost 20% of charge. It's that simple, no meters necessary.

If you don't have a fast charger that shuts off when battery is fully charged, the second best way is to load test battery is with a tool that needs a lot of current, see how long it lasts under load. I have a circular saw that will zap a sick battery trying to cut one 2x4. Grinder and reciprocating saw use less current, followed by 1/2 drill, 1/4 inch impact, then grease gun.

I have little faith in battery voltages while battery is sitting on shelf and not under any load.
 
all 18 volt have 20 plus volts at 2 hours 12 volt dewalt have 14 same time going to check more time then one week. I will mark batteries under load as I all marked . One new one came with saw had date now . Thanks buddy
 

Open circuit battery voltage doesn't mean a whole lot. Voltage measured while the battery is flowing current through a load is worth something.
 
Funny finding this topic. I just charged a D battery and right out of the charger it had 5.4 volts on it. Within the hour though the voltage was down to 1.49 volts. It was a HF Ni-Mh battery.
 
Hello Wilson ind,

Next time you charge another pack when it comes off the charger take a voltage reading, then put it back in the charger. It will start charging again!. When it stops charging the second time. take a second voltage reading. Post back and tell us what you find. You can put the pack back in the charger as many time as you wish, no harm done. The voltage of a fully charged pack should be closer to 21 volts. It will peak at 22V and settle to 21V. Your pack at 20V it is NOT fully charged.................

Guido.
 

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