Resistor

[HOTDOGHARRY]

HI ALL
DOES ANYONE KNOW WHAT SIZE RESISTOR I SHOULD USE FOR TESTING AA OR AAA ALKALINE BATTERIES ie Ohms and Watts WITH A MULTIMETER PLEASE

 

[Lucien Nunes]

For testing the state of remaining charge, I'd load the AA with 200 - 500 mA corresponding to 7.5 ohms 0.5W and 3 ohms 1W respectively. For AAAs a lower current of perhaps 100mA requiring 15 ohms 0.25W.

For testing the performance of a new battery e.g. to compare brands, I would perhaps use a higher load to represent an application such as a camera, as the rate at which useful capacity drops off with increasing load is an important factor.

With alkalines, you can get a reasonable idea of the state of charge even with an unloaded voltage test. Some types have remarkably stable and predictable OCV, and the rise of internal resistance with use is not so important as it was with zinc-carbon types.

 

[HOTDOGHARRY]

For testing the state of remaining charge, I'd load the AA with 200 - 500 mA corresponding to 7.5 ohms 0.5W and 3 ohms 1W respectively. For AAAs a lower current of perhaps 100mA requiring 15 ohms 0.25W.

Thanks for info, how does the wattage affect the readings and why different Ohm ratings between AA and AAA as both 1.5V, is that to do with the current? Also why lower Ohms (resistance) for AA over AAA is that to do with Ohms Law?
Sorry but find electronics very confusing
Thanks again

 

[Richard Burns]

You are loading the battery but do not want to take the battery to maximum drain.
AA batteries can supply a greater current than AAA batteries so to load them equally in terms of their capacity you would use a higher value resistor on AAA so there is a lower current.
e.g an AAA battery may have a 900 mAh rating and an AA a 2500 mAh rating so you would want to load the AA about three times more than the AAA to give comparable measurements.

The wattage of the resistors will not affect the measurement readings but using P=VI you can see that
200 mA on 1.5 V is 0.3 W so you need a resistor that can carry over 0.3 W therefore 0.5 W standard size resistor
500 mA on 1.5 V is 0.75 W so you need a resistor that can carry over 0.75 W therefore 1 W standard size resistor
100 mA on 1.5 V is 0.15 W so you need a resistor that can carry over 0.15 W therefore 0.25 W standard size resistor

 

[HOTDOGHARRY]

Many thanks for your reply
So would i be right in saying if i used a say 30 Ohm resistor on either an AA or AAA battery it would apply a load of only 50mA on batteries which would not be enough to test batteries
Regards

 

[Richard Burns]

It would test them somewhat but would not give a true indication of how they perform under load, you would be getting close to measuring the unloaded voltage which is an indication of whether that battery is at all functional but not if it can actually supply current.