NICAD -> NiMH

[RickH]

I have a hair trimmer ... which is rechargeable, but has a life of about 1-2 mins max.
Removed the power pack and it is fitted with 2 x AA 600mAh NiCAD batteries

I have some new NiMH AAA batteries that have a larger capacity (850mAh)

Anybody had experience of changing NiCAD to NiMh.
The power module connects directly to DC plug in lead .... so would need to keep the same charging method. Charger is a pregnant plug type.

As far as I can figure it out, they are both 1.2V cells, and they can be charged off the same charger, full charge detection is what might not work, as there is no 'charged' indicator. I could just leave on charge overnight, and then not leave it plugged in.
It looks very basic circuitry .... pic attached.

 

[jj4091]

I use the same charger for ni-cad and ni-mh without problem and the charged idicator works ok. I wouldn't try to do lithium batteries with it though. I would suggest you put them on charge early in the morning and monitor the charger is not getting too hot as they charge.

 

[JohnD]

I have a smart charger that in some way identifies what kind of rechargeable battery you have connected, and adjusts the voltage and charge rate accordingly,

Older ones will only charge one type safely.

I suppose you could time it to reach correct voltage, and then take it off to prevent overcharging,

 

[RickH]

I use the same charger for ni-cad and ni-mh without problem and the charged idicator works ok. I wouldn't try to do lithium batteries with it though. I would suggest you put them on charge early in the morning and monitor the charger is not getting too hot as they charge.

The new ones I could use are defn. NiMH

I'll try the swap.

 

[boringoldcodger]

Removed the power pack and it is fitted with 2 x AA 600mAh NiCAD batteries

I have some new NiMH AAA batteries that have a larger capacity (850mAh)

Do you have some sort of adaptor so you can plug AAA batteries into AA slots?

 

[RickH]

Nope, if you look at picture, they are soldered to the board, and all this slides into a plastic module - part of the handle.
Don't think any AAA holder would fit in space

 

[JohnD]

I think he has in mind that you spoke of removing AA cells and replacing them with AAA

I thought, probably a typing error.

 

[RickH]

I think he has in mind that you spoke of removing AA cells and replacing them with AAA

I thought, probably a typing error.

The existing Ni-Cad are AA
I have AAA which are higher capacity Ni-MH
I have just ordered a double AAA holder, maybe it will fit, worth trying.

 

[JohnD]

But why don't you fit new AA cells?

Same size as original but more power.

 

[ericmark]

The Ni-MH tend to have the charge controller built into the battery, so in the main old Ni-Cad chargers will work, but not all.

The Ni-Cad battery had an odd voltage characteristic, where with a consent current charger, the voltage would peak just before being fully charged, and the Δ charger could detect this peak, and stop charging when it detected it. This method was only used with top of the range chargers, most were not that clever.

If the charger says do not charge for more than 16 hours, it is likely OK.