Hello All,

Question about an 18V drill battery. Both are full charged as per the VOM.
One battery works great, the other does nothing, but tests as fully charged.
Temp sensor tests good. No loose or corroded connections.

Any ideas would be helpful

Thanks.
PetroniusPius

 

I suspect one cell is "open". That is, internal wire is open but the chemistry of the cell conducts just enough current to allow voltage measurements across it. So, it may read 18V, and a charger may say it is full based on voltage, but as soon as you put any load on it, it cannot conduct any current across the open cell and "voltage" drops to zero (no current).
You can test this by simply putting a small load on the battery, like a 50 ohm resistor and measure the voltage across the resistor.

 

I'll try that thanks.

 

I'll try that thanks.

Make sure you use a power resistor.
\(\large P = \frac{V^2}{R} = \frac{18^2}{50} = 6.48W\)

A weak/dead cell could cause the pack to have high internal resistance. Putting a load on the battery will cause the internal resistance to drop voltage.

 

You can use a car headlight bulb as a test to see if the battery is duff.

 

May have one around I can use. Thanks.

 

You can use a car headlight bulb as a test to see if the battery is duff.

Unless the battery really does provide 18V, in which the light will be bright but brief as the bulb blows.