Drill's battery not working.

[ThomasWheeler]

Hi, I have a 7.2V Ni-cd battery for my Bosch wireless drill. When I used it with the drill, the drill wouldn't spin. So I checked the voltage when I'm pressing the drill switch and the voltage drop from 7.2V to 1V. But when I hooked up the drill with my power supply, it works and the voltage only drop to 6V. I took apart the battery and check the voltage one cell at a time. There are 6 cells, they all have 1.2V which is good but why isn't the battery working? Please help me.

 

[73's de Edd]

Your manner of testing should have had a 10 ohm resistor across the meter terminals / leads . Do that and go back and do a quick test of each cell and see what your readings are being this time . . . multiple weak cells , or just one ?

73's de Edd

 

[davenn]

took apart the battery and check the voltage one cell at a time. There are 6 cells, they all have 1.2V which is good but why isn't the battery working?

As Edd hinted at
you cannot test a cell just by putting a meter across it, it needs to be tested under load
obviously one or more of the cells are failed

 

[kellys_eye]

Is the battery charged? By that I mean - are you sure the charger is working?

The 1.2V/cell is 'good' news but they don't seem to be holding a charge which could well point to the charger providing 'voltage but not current'..... and charger faults are quite common.

 

[segunodu]

Also, if the battery pack has an internal circuitry like that of cellphone batteries, the circuit may be faulty.

 

[Alec_t]

they don't seem to be holding a charge which could well point to the charger providing 'voltage but not current'.

Could also point to the 'memory effect', for which NiCd cells are well known.

 

[ThomasWheeler]

Thanks, I'll check the cells again, but how can I check if my charger is falty?

 

[ThomasWheeler]

As Edd hinted at
you cannot test a cell just by putting a meter across it, it needs to be tested under load
obviously one or more of the cells are failed

I'm sorry for the late reply. I have been very busy on my last week of school. About the batteries. I checked the voltage of each cell under load and they're all 0.01V. So I soldered the cells back together in series and charged the battery. After five minutes the battery was 7.2 V, but when I tested the battery under load the voltage dropped down to 1 V. Also, while charging the amperage dropped from 160mA at 0V to 20mA at 7.4V. Why did the battery charge up so quickly? That doesn't seem right to me.

 

[duke37]

A battery will have a voltage depending on the chemistry. This voltage will not change if the chemistry does not change. Thus a battery may show the normal voltage with only a very limited charge, it doesn't say that there is any significant amount of energy stored in the battery.

The charging and discharging voltage will be affected by ion concentration variation through the electrolyte which will even out during a rest period. Batteries are very complicated.

 

[Alec_t]

I checked the voltage of each cell under load and they're all 0.01V.

What was the load? Whatever, the batteries are dead.

 

[duke37]

Charge the batteries for a day and see if they hold the charge. NiCads can develop an internal short which will discharge any energy put in.

 

[ThomasWheeler]

Right now I'm charging my battery with an ac - dc adapter. Yesterday I used 7V to charge the battery for about 5 hours. The amperage was 150mA at 1V and dropped to 40mA at 7V. When I checked the battery under a 50Ω load the voltage was 7V. I tried using it with my drill. The drill spun for 5 seconds then stopped. When I checked the voltage under load again it was 6.4V. Today I'm using 8V to charge the battery. The amperage is 150mA at 6.4V and is still dropping. I think the battery is holding some charge so I will continue charging it for a while. How can I know if the battery is fully charge. I think when it's fully charge the volt will be a little bit more than 7.2V and the current will be low about 10mA, am I right? Should I use 8V to charge a 7.2V battery? Thank you very much.

 

[ThomasWheeler]

I charged the battery for 2 hours with 8V this morning. I measured the voltage of the battery and it was 8V so I tried to use it. The drill spun for 4 minutes then stopped. I measured the voltage and it was 6.2V. Now I want to completely discharge it before charging it again. I want to do this because of the memory affect a Ni-CD battery might have. How can I completely discharge the battery? Do I just leave the battery in the drill and press on the switch or can I short the battery with a resistor?