# Drill Battery Issue

#### PetroniusPius

Joined Sep 16, 2022
3
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.

Thanks.
PetroniusPius

#### sagor

Joined Mar 10, 2019
754
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.

#### PetroniusPius

Joined Sep 16, 2022
3
I'll try that thanks.

#### dl324

Joined Mar 30, 2015
15,113
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.

#### Dodgydave

Joined Jun 22, 2012
10,498
You can use a car headlight bulb as a test to see if the battery is duff.

#### PetroniusPius

Joined Sep 16, 2022
3
May have one around I can use. Thanks.

#### AlbertHall

Joined Jun 4, 2014
12,157
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.