Re: only Ni-Mh batteries, a better man than I

There are two methods of detecting terminal charge of Nickel batteries; voltage *slope* (dV/dt) and temperature. The change in voltage on a NiCd turns negative as it gets to its terminal charge. NiMH does similar but it's too small to detect reliably so this method of detecting terminal charge won't work. NiMH rapid chargers have to monitor cell temperature. These (cell temperature) chargers will charge either NiCd or NiMH, but a charger that senses dV/dt can only be used with NiCds.

It's the rate of change, not the voltage itself.

These chargers will NOT work on NiMH. Temperature sensing is used on these batteries. Temperature sensing may also be used with NiMH.

^^^^^^^^^^^

No one makes a NiMH only charger as NiMH chargers will charge either.

Ryobi certainly do. The NiCd only one is significantly cheaper.

Do you mean *current* drop? If not, I am surprised by what you say.

For decades I've routinely charged nicads with a home-made charger. Basically just using a resistor in series with a voltage source to mimic quasi-constant current, but allowing the current to fall as the battery voltage rises. I tend to monitor the voltage with a DVM or ye olde waggle meter as this happens.

I've recently started recharging a sets of NiMHs in the same way.

Can't say I've noticed any drop in battery *voltage* as either type reaches full charge level. (Which I associate with a rise to a high plateau voltage and perhaps slight warming of the cells.)

FWIW I do keep down the rate to well below C/10. Generally around 14 hours or more for a total charge from fairly low. So maybe I just don't see the effects you describe for that reason?

I thought that the fancy autosensing chargers for NiMH checked the dV between charging and not-charging to tell when the battery was 'full' (?) But I've not used or measured one since I'm happy with slow charging at present.

Slainte,

Jim

There are loads of NiMH only chargers on the market. They charge at far too high a current for 14 hr nicads.

NT

No I mean voltage drop.

Google 'delta peak'

No. This is fast charge territory.

No, they detect a falling voltage with time, on charge. Is a few mv per cell with NiMh IIRC, and 10-20mV with NiCd.

I used to charge NiCd at around 3C. or more . 12 minutes flat to full of beans.

Nimh are not so tolerant....

all nicads are suitable for 1 hour charge. Or even less.

Bullshit. most chargers I have used detect it relaibly and work.

NiMH rapid chargers have to monitor cell temperature. These (cell

Bullshit

Its both.

Bullshit.

All model aircraft nickel chargers today will charge either with no resetting at all.

And have done so for at least 5 years.

The only difference is that cheap s***ty NiMh from the supermarkets and sheds cant handle high charge or discharge rates, that NiCd could.

And also self discharge and ruin themselves in very short timescales.

RC modelers are known to treat batteries very poorly.

The dV/dt method of charge termination is not reliable when used for NiMH batteries.

Not very helpful, though, as such a sweeping generalisation.

Firstly some 'chargers' don't sense the voltage at all. I use one that doesn't.

Secondly because the way in which the sensed voltage is used may vary. So some may sense for dV, others for something else (e.g. the drop you speak about).

You said that before. I then asked about it. I can't say I've ever noticed it in many years of rechanging NiCads by monitoring the voltage with a DVM. Nor noticed it during the last few months when recharging NiMHs. Can you explain why my measurements haven't displayed what you say is the case?

Slainte,

Jim

Do you have a reference for this claim?

NT