How to test battery after a charge

[TotalMadness]

I want to use a 12v lead acid battery to charge 4 AA, AAA, C, or D size batteries. What I do not know is how to test the charged batteries to see if they are fully charged. Any pointers as to the direction I should go?

Thanks

 

[crutschow]

There only way to determine if NiMH, NiCd, or Lithium batteries are fully charged is to monitor their temperature and voltage as they charge. You cannot determine it by just measuring the voltage after they are charged. Only lead acid batteries give an indication of their charge status from their open-circuit voltage.

 

[TotalMadness]

Ok, thanks.

 

[TotalMadness]

What would be the best way to discharge AA, AAA, C, or D size batteries as deeply as possible in the fastest time?

 

[crutschow]

What would be the best way to discharge AA, AAA, C, or D size batteries as deeply as possible in the fastest time?

Connect an incandescent light bulb across their terminals rated to draw a high current( perhaps a couple amps depending upon the battery) at the rated battery voltage. The bulb resistance will decrease as the battery voltage drops, reducing the normal drop in current from this, thus minimizing the discharge time.

For anything faster than that you would need to go to an active load regulated by some circuits to control the current and shut it off when the battery is discharged.

 

[MrAl]

Hi,

They do make battery analyzers/chargers that also make tests and do discharging, but i think they are kind of pricey.
The way i do it is connect my batteries to a special volt meter that connects to the computer, and apply a known resistive load. The resistive load allows you to know the current as well as the voltage by only having to monitor the terminal voltage. The software can then know when the battery is fully run down and also how much current was flowing over the total time it took to discharge, and it therefore can calculate the ampere hour capacity of the cells.
To do this without a special volt meter isnt too hard either, but requires logging values by hand every 10 minutes or so. You would connect a resistive load while monitoring the voltage, and knowing the resistance you therefore know the current, so as you log that every 10 minutes you get a list of values that you then add up and you can calculate the ampere hour capacity of the cell. It's not hard to do either.
Once you know the ampere hour capacity you can then monitor the charge of the cell and estimate the capacity that way too. You have to be able to measure the current for that procedure though, and log those values vs time.

The hardware and software i designed does all this automatically, but as i said you can do it manually and it's still very interesting. I've found many cells that dont live up to their ratings, and some that do. For example, i tested some Kodac low self discharge cells 6 months after they were charged and they did hold their charge pretty well, just as advertised. I did this with a complete discharge test. I did the same with 'regular' cells and found them to be almost discharged or totally discharged after that long. The Eneloops are supposed to be good too, but i havent gotten to test any of those yet.

 

[kiriakos-gr]

There is only one portable high quality battery tester with load, out there.
And this is the " Ansmann Energy check "

Its not cheap, but it is second to none.
I just love it ..

 

[3v0]

To answer these questions you need to specify a battery chemistry.

For example NiMh cells should not be fully discharged. By the time they read 1.0V the vast majority of useful charge is gone. Below 0.8V they are in danger of reversing polarity.

 

[TotalMadness]

Thanks for the replies. I learnt a lot.

The batteries would mostly be Ni-MH, but 12v lead acid batteries occasionally too.

 

[TotalMadness]

If I were to charge 4 type C NiMH batteries in series, what should the current draw be set to? The batteries are 1.2v 2200 mAH. I have a Duracel charger that will charge AA and AAA batteries to 1.43v before cutting out. So I assume the voltage can be set to 5.72. My guess is that if I wanted to charge the 4 batteries in series to be fully charged within 8 hours the current would need to be 1.1amps.

2200 / 8 = 275
275 * 4 = 1100mA

Is my thinking correct?

 

[colin55]

To charge 2,200mAH cells in 8 hours requires 2200/8 = 275mA plus 20% = 330mA
If your charger charges the cells in parallel, you need 330mA x 4 = 1.32A
This is assuming the cells are completely discharged.

 

[TotalMadness]

Thanks for the reply. To sum up, if the batteries are to be charged in series, the mAH for one battery is used to calculate the total current for all batteries in the series. If the batteries are to be charged in parallel, the mAH for one battery is used to calculate the charge current and then multiplied by the number of batteries being charged in parallel.

 

[colin55]

Correct.