# How can I measure the charge of a rechargeable cell?

I'm using a bluetooth keyboard which need four AAA batteries. After the cells are charged (at least according to the charger), the keyboard still doesn't work properly; there is a green LED that should stay lit for a few seconds, but it's red and blinking instead.

I'd like to know if the cells are actually charged. The battery specification is: AAA, 1.2V, 1000mAh. Once charged, my voltmeter reads 1.43V on each cell.

How can I measure the charge of a rechargeable cell?
I'd like to know if the cells are actually charged.
The battery specification is: AAA, 1.2V, 1000mAh.
Once charged, my voltmeter reads 1.43V on each cell.

You cells are fully charged.
I assume that they are NimH (Nickel metal Hydride) cells but that voltage would also indicate full charge for NiCd cells.
I use a nominal figure of 1.45 V / cell at 25 degrees C as end point when actually charging. If they measure 1.43V/cell when off charge that is even higher than I'd usually expect. If that is an on-charge figures at end of charge it indicates full charge.

A system intended to operate from NimH cells should work down to 1.1V/cell in all cases and ideally down to 1V cell. When lightly loaded (say C/10 load or less) NimH cells will operate at about 1.2 V across the major part of their discharge cycle. The keyboard should load them to far less than C/10.

Likely options in order of decreasing probability are

• Your bluetooth keyboard is faulty

• The red light indicates some other problem such as a lack of link connection from the "dongle" (aka receiver) that it connects via.

• The keyboard needs Alkaline cells
(but it should work on well below that voltage even with alkaline cells).

Thoughts:

I assume that the keyboard has a USB connected "dongle" which plugs into a USB port on your PC.

• You do not say if this worked previously.
If it was working and now isn't then a fault somewhere seems likely.

• If you have not rebooted the system try doing so.

• Try to minimise connection of other equipment which may cause interactions.

• Sometimes other equipment plugged into the same USB hub causes problems.

• Sometimes even equipment plugged into another USB port on the PC can cause problems .

• On some occasions a device will "enumerate" on one USB port and then not operate on any other port.

• Did the device came with drivers and did you load them?

• Try it on another PC.

• Is there a channel setting option which has been accidentally altered (most unlikely with Bluetooth).

• Do you have other Bluetooth or wireless based equipment in use which may be interfering?

• +1 for the mention of interfering wireless. This happened to me and took me quite some time until I noticed that my keyboard went rogue only when I wrote to my NAS... – 0x6d64 Jun 22 '12 at 11:21
• Thanks for all your advices. I think that the problem what not related to the batteries, and I know now how to test them (see other answers). It was related to the USB hub which is currently under heavy load. I cannot unplug the drives which are connected to this hub right now, so I'm still not 100% sure. The red light seemed to indicate a lack of link connection from the dongle. Unfortunately the Macbook Pro I'm using has only 2 USB ports. – alecail Jun 22 '12 at 13:11

There is a possibility that the cells are damaged or just had to many charging cycles, so their internal resistance has become to large. This means that your multimeter still reads a voltage of 1.4XV after charging, but any significant load current leads to a breakdown of that voltage.

You could check that by measuring the cell voltage under load (say: about 100mA). The resulting voltage drop gives you an estimate of the internal resistance (this page talks about 0.17 ohms for a new, charged AA NiMH cell).

Sounds like the cells are probably okay. Measure the voltage with the batteries plugged and the keyboard switched on to make sure. Or just use a 10-50 ohm resistor across the battery and test the voltage. If it's still good then there's something else wrong.