charger voltage reduction

[beale]

I have a charger for Nicd, Nimh batteries that is supposed to charge cells from 4.8V to 9.6V. I bought 2 - 6V 1200mah Nimh batteries for my model car receiver and charged them on trickle 150mA for 3 hours then checked the voltage of the cells 1 was 10.2Vdc and the other 8.6Vdc. This makes me think the charger is outputting 10V all the time what I would like to do is regulate that voltage down to 6V so I don't cook the cells.
Can anyone suggest what resistance or something to do this with? don't want to buy another charger if not necessary

 

[MikeMl]

Nicads and NiMh require a constant-current charger; not a constant-voltage charger like some other battery chemistrys. Nicads and NiMh do not have a fixed voltage at which you can say they are "charged". With a cheap charger such as yours, the only way to prevent cooking (overheating) the batteries is to take them off the charger when they begin to feel noticably warm to the touch.

 

[RCinFLA]

The charger is likely a 7 vrms transformer with rectifier, filter cap and and ballast resistor. It looks somewhat as a current source. You did not state what the current rating of the charger was.

A NiMh cell will be 1.2 vdc at nominal discharge. The packs are five series connected cells producing 6 vdc. When charged the voltage will rise to about 1.45 vdc per cell or 7.25 vdc for a five cell pack.

If you are reading 10.2v and 8.6 vdc you are severely overcharging the cells and will destroy them. A 1200 mAH NiMh can take a near continous charge of 120 mA. If you really want to leave them on continuous charge the current should be dropped to about 60-70 mA.

If you are truly charging at 150 mA and the voltage reading are with the charger connected the 8.6 vdc is about right at 1.72 vdc per cell at 150 mA. The 10.2 vdc reading would indicate a bad cell in the pack. At those voltages the pack should be warm.

NiMH batteries have a high self discharge rate. Within a couple of weeks to a month a NiMH will completely self discharge left open circuited.

 

[crutschow]

NiMH batteries have a high self discharge rate. Within a couple of weeks to a month a NiMH will completely self discharge left open circuited.

Except for the new NiMH "pre-charged" type batteries. They have a much lower self-discharge rate, generally retaining most of their charge for a year or more.

 

[beale]

Yes I would expect the charger to increase the pack voltage by a volt or so but I am reading 10V when disconnected from the charger for at least 1/2 hour?
the charger has 2 facilities trickle 150ma and fast 5A I don't use the fast as I'm in no rush the pack lasts a few weeks when charged but I leant a mate the pack that went up to 10V and it screwed his radio gear and now reads 6.5V
MFA como drills make a 6 - 15vdc voltage regulator do you think this would work?

 

[MikeMl]

Even 150mA constant current will cook a new pack if left on continuosly for a week or two...

No simple constant-current or constant-voltage charger will do what you want. You need a "smart" charger that has a microprocessor in it.