Ni-Mh battery / Ni-Cd B&D charger

[Skink]

I recently purchased from a Ebay vendor 2 Ni-Mh batteries I raised the question to them.

Convo below.


Your previous message
Unfortunately My B&D tool batteries are NI-CD also my chargers are all NI-CD, I Have NO way to charge the NI-MH !

bossup2020:
don't worry,it can charge,please have a try.

Are You Sure?
I have 3 Ni-Cd only chargers,,, they are NOT the Ni-Cd / Ni-Mh combination chargers they are more than 25 years old but work fine...

New message from: bossup2020
Ni-Cd / Ni-Mh batteries charger are universal，thanks.

Is he right a simple google search says he in not and if it were permissible what are the negatives?

Thank You

 

[amberwolf]

They are very similar, but not exactly the same. There are some good pages out there that explain the differences in charging requirements.

charging nicd vs nimh - Google Search

One example

BU-408: Charging Nickel-metal-hydride

BU meta description needed...

batteryuniversity.com

If your chargers have temperature monitoring for the cells *and* implement delta-T charge cutoff, as well as monitoring voltage and implement delta-V charge cutoff, and do not trickle charge after full, *and* the packs have a compatible temperature sensor built in, you may be able to use them anyway, as long as they are for the same number of series cells as the packs you have.

If they are like a fair number of "dumb" NiCd chargers and just stick a voltage out there at a low current, they could damage the NiMH cells if you don't monitor the charge process yourself for temperature and voltage and manually disconnect charge. If voltage drops during charging, a cell or cells has reached full. Same for if temperature increases on a cell.

If your chargers don't monitor for this stuff and you don't monitor it either, the cells will charge but be degraded every cycle, and if left on a dumb charger could overheat and in the worst case even explode. :/

If you don't know what the charger does, you should only use it on the packs it was built to charge, and get the proper charger for the others.

If you charge a pack slowly enough, delta-V cutoff may not work right, depending on how it's implemented (manual monitoring will still show a drop you can see and turn off the charger for, but you have to be watching the voltmeter the *entire* time without looking away.

If you charge a hot pack, delta-T cutoff may not work as it can't detect the temperature change. (some better chargers wont' charge a hot pack).


Any particular reason to keep using NiMH? Or to not just get an NiMH charger for the packs?

 

[Skink]

Thank you for your speedy reply Amberwolf... Sorry I am late responding,,, busy in my Victory garden...

Any particular reason to keep using NiMH? Or to not just get an NiMH charger for the packs?

The only reason is I have 4 NiCD chargers,,, before posting here I put a NiMH charger in my cart but came here first for more info...

I did purchase the NiMH charger because the NiMH batteries have twice the power capacity...

 

[amberwolf]

The specific NiMH charger is almost certainly better than a minimal NiCd charger. Does it use both DeltaT and DeltaV charge termination? And does it actually check not only for the presence of the pack thermistor before starting charge (and that it is at a safe low temperature before starting), but also fault out and stop charging if the thermistor becomes disconnected or fails?

I ask because the heat generated in a NiXX pack during and after the "full" state has been reached is significant, and if the charger doesn't stop, it can cause a fire, or explode the cells.

This is because NiXX cells *drop* in voltage once they are full; this is how they "self balance". (the energy converts into heat, which is why cell temperature increases "suddenly" at that point).

This means that if the charger stays on, outputting the full voltage used to charge at, it is a higher voltage than the cells now even though they are full, and the cells continue to (over)charge, dissipating all this extra energy as heat. If the charge current is high enough, and it goes on long enough, especially in a sealed battery case, dramatic kinds of failures can occur.

 

[fechter]

NiCd and Nimh cells are the same voltage range so a charger should be able to do either. If the charge rate is slow, there won't be any significant heating and everything should be fine. At higher charge rates is where things get a little different but I wouldn't worry about it.