Sanity check 18V NiMH charge limit voltage?

[paulca]

Can't find the drill charger.  So I put the 18V NiMH battery pack on the bench PSU.

Battery read 18.22V empty.  Normal.

I set the current limit to 200mA (it's 1.5Ah pack, so a bit over 10%C). 

For the limit voltage, an 18V nominal pack is made from 15 1.2V cells.  Fully charged they should be around 1.4-1.5, giving 22.5V.

I have set it to 22.5V for now.  The reason I am asking is I'm sure I have let it go way higher before, like 24V at a trickle.  My thinking was that if I was charging a NiCAD or NiMH AA cell I would happily put a 1.6V limit on it.  You are aiming for 1.5V and the extra 100mV is just your "elastic", as soon as you take the 1.6V away the full charged cell will fall rapidly to 1.5V anyway.

EDIT:  Note, I am not debating how to detect the "actual" top of charge.  I know there are complicated ways, but I used the simple one.  200mA of heat will be generated with the pack is full.  At 200mA the pack will generate virtually no heat until it's full.  200mA is not enough heat to damage it over hours.

[BeBuLamar]

Is your battery pack NiMH? I thought most of the 18V packs are Li-Ion.

[paulca]

Is your battery pack NiMH? I thought most of the 18V packs are Li-Ion.

Good job I checked.  It's NiCAD not Nimh.

I bought a cheap drill.

[RoGeorge]

In theory it's about 1.1*Vnominal, so count how many cells are in series, and multiply 1.1*n*1.2V.  So, 1.32V/cell, maybe a little higher.  1.5V/cell seems too much.

18V/1.2V = 15 series cells, but double check if they really are 15 cells inside, sometimes cheap drills have lying labels.  I have found a battery pack from a toy RC car with a metal rod instead of one of the cells.

18V*1.1 = 19.1V
15*1.5V = 22.5V, might be too much as a stop voltage, though NiCd batteries can take more abuse than other chemistry types.

If your power supply indicates the voltage, a good time to stop is when the total voltage start to decrease (under a constant charging current).   At first the voltage goes up, then after the batteries are fully charged, the voltage decreases a little.  For 15 series cell you might see the total voltage slowly decreasing about 0.5V, maybe more when overcharged, but you don't have to wait that long.

The idea is to disconnect the charger when the voltage is starting to decrease.

[mariush]

Have a look at  https://media.digikey.com/pdf/Data%20Sheets/Panasonic%20Batteries%20PDFS/Ni%20Cd%20Catalog%202002-03%20v1.pdf

There's a whole chapter (pages 10 to 24) where they list several charging methods with pros and cons and all the theory explained.

[paulca]

That's why I have migrated everything I can to lithium.  I only use the drill like twice a year and it works.

I'll set it to 22V and let it sit and keep and eye on it.  It's a few hours in an already sitting at 21.42V.

Basically the charger that came with it is 180mA unregulated DC supply.  Their instructions are basically put in on and leave it overnight.