Changing NiCad Batteries

Kart31 · Feb 1, 2020

I have rebuild two NiCad battery packs. I have the cells from the old ones. I don't believe they are all dead, only one or two (or a few). I have a rig to test them and one to charge them. I can charge them individually (yeah, that will take a week) or charge them as a pack. Should they be charged in series as they are used, or can they be charged in parallel? Can they be charged in parallel (the charger has the ampacity to do so, so I can from that aspect). If one of the cells is bad and they are charged in series, will the good cells charge properly?

audioguru · Feb 1, 2020

Ni-Cad batteries are ancient and the cadmium is deadly. What are they used in?
Many years ago I replaced all my Ni-Cads with modern Ni-MH and lately many of my batteries are Li-PO.

Kart31 · Feb 1, 2020

The chargers and NiMh do not get along. The equipment and NiMh batteries do not get along. I tried both of those routes. I'm working to see if I can save some or hopefully most of the cells from each pack. Should they be charged in series as they are used or can they be charged in parallel?

audioguru · Feb 1, 2020

Ni-Cad battery cells frequently short. Then if a shorted one is in series and you charge them then the voltage could become too high unless the current limiter stops it but they will over-charge because the charging will never end.

if you charge a bunch of cells in parallel and one is shorted then it will explode from the high current.

I have used hundreds of Ni-MH batteries (their capacity is much higher than Ni-Cad) and never had a shorted one. My chargers are designed for Ni-MH.

Pommie · Feb 1, 2020

Kart31 said:
The equipment and NiMh batteries do not get along

How do they not get along?

Mike.

rjenkinsgb · Feb 2, 2020

With NiCd cells, you can charge them at 10 hour rate or below indefinitely.

Just connect them in series and use a resistor & supply voltage that sets the current at or below 1/10th the cell capacity value.
eg. for 1.2AH cells, a maximum of 120mA.

Leave them on charge like that for a couple of days then check all the cell voltages after they have been off charge for a couple of hours - any showing 1.3V or higher should be OK, any at 1.1V or less are dead.

Pommie said:
How do they not get along?

Some fast chargers made before NiMH were available detected full charge by looking for a significant reduction in cell voltage when the cells reach full charge. That's an oddity of NiCd chemistry & it does not happen the same extent, if at all, with NiMH cells.

NiMH/NiCd Switchmode Battery Charger Has | Maxim Integrated

Two ICs form a NiMH/NiCd switchmode battery charger that monitors dV/dt at the terminal voltage to determine charge termination.

www.maximintegrated.com

Pommie · Feb 2, 2020

rjenkinsgb said:
Some fast chargers made before NiMH were available detected full charge by looking for a significant reduction in cell voltage when the cells reach full charge. That's an oddity of NiCd chemistry & it does not happen the same extent, if at all, with NiMH cells.

I asked why the equipment didn't get along with NiMH, not the charger. I can see the charger being problematic but why would the equipment being powered "not get along".

Mike.

Kart31 · Feb 2, 2020

Pommie said:
I asked why the equipment didn't get along with NiMH, not the charger. I can see the charger being problematic but why would the equipment being powered "not get along".

Mike.

The NiMh only last minutes, instead of NiCads that last a couple of hours.

audioguru said:
Ni-Cad battery cells frequently short. Then if a shorted one is in series and you charge them then the voltage could become too high unless the current limiter stops it but they will over-charge because the charging will never end.

If you charge a bunch of cells in parallel and one is shorted then it will explode from the high current.

So what you are saying is they can't be charged in series or in parallel?

rjenkinsgb · Feb 2, 2020

You can charge them is series with a simple current limit resistor (or filament lamp) as I mentioned. Anything up to 10 hour rate (1/10th C) if fine for days at a time.
Trying unknown cells in a fast charger could cause problems.

There is no possible reason I can think of that properly charged NiMH cells would give worse performance than NiCd cells in any device, as long as the cells are properly rated?

Just check the capacity and use a known manufacturers type. Many larger body cells are sold in two different types - a genuine high capacity and an "economy" version, with a small cell inside a plastic sleeve.

eg. A real D cell is typically 7000 - 10000 mAH. Some economy ones have an AA or C in a sleeve with much lower capacity.
I'd not be at all surprised if even some AAs are being made like that, with a triple A and a sleeve...

Pommie · Feb 2, 2020

Kart31 said:
The NiMh only last minutes, instead of NiCads that last a couple of hours.

I suggest you had worn out or defective cells. Get new ones and give them another try. What size are the cells?

Mike.

Nigel Goodwin · Feb 2, 2020

Kart31 said:
So what you are saying is they can't be charged in series or in parallel?

It's another of Audioguru's delusions - almost all NiCd and NiMh batteries are charged in either series or parallel, or both - the majority though are charged in series, and the special charging IC's are specifically designed for this purpose.

audioguru · Feb 2, 2020

Kart31 said:
The NiMh only last minutes, instead of NiCads that last a couple of hours.

You got it backwards. A Ni-MH cell has much more capacity than a Ni-Cad cell the same size.
I guess people are not buying high capacity and high cost C and D size Ni-MH cells any more so Energizer has the same capacity for them as a little AA Ni-MH cell. I think a modern Ni-MH AA cell has the same capacity as an antique Ni-Cad C or D size cell. But I could not find spec's for an antique Ni-Cad cell to prove it.

In Google, I selected articles that show a Ni-MH cell beating a Ni-Cad cell but there are other articles saying that a Ni-Cad is better than Ni-MH when the current is extremely high, when the temperature is very cold or is very hot and when a cell stays discharged for a long time.

Kart31 · Feb 2, 2020

Pommie said:
I suggest you had worn out or defective cells. Get new ones and give them another try. What size are the cells?

Mike.

Worn out cells from a new purchase would serious stink. Defective, may be but there are multiple purchases from different vendors. Same result. All the cells are Sub-C.

Nigel Goodwin said:
It's another of Audioguru's delusions - almost all NiCd and NiMh batteries are charged in either series or parallel, or both - the majority though are charged in series, and the special charging IC's are specifically designed for this purpose.

I'm not sure about the special charging ICs for series charging. All the packs are series and the bulk of the OEM chargers have little to nothing (ie., no ICs) in them.

The question is not to determine if I can or should use NiMh batteries. All the question regarding NiMh are irrelevent to the OP. I know I can charge them in series. I can just use the OEM charger for that. The question on charging in series, is if one or more of the cells are bad how does that effect the charging of the other cells. Can they be charged in parallel without the (unknown) dead cells effecting the charge of the good cells?

Nigel Goodwin · Feb 2, 2020

Kart31 said:
Worn out cells from a new purchase would serious stink. Defective, may be but there are multiple purchases from different vendors. Same result. All the cells are Sub-C.

I'm not sure about the special charging ICs for series charging. All the packs are series and the bulk of the OEM chargers have little to nothing (ie., no ICs) in them.

That's because they are crap chargers - decent chargers use electronics to safely and reliably charge the batteries.
They aren't special IC's for 'series charging' they are special IC's for 'charging', series or parallel, you simply configure them accordingly, and they take care of everything. The most common one we come across is the Linear Technology LT4011, this allows up to 16 batteries in series.

The chips do fail, and we've had various of the FET's used fail as well - but mostly it's just a matter of rebuilding the battery pack.

The question is not to determine if I can or should use NiMh batteries. All the question regarding NiMh are irrelevent to the OP. I know I can charge them in series. I can just use the OEM charger for that. The question on charging in series, is if one or more of the cells are bad how does that effect the charging of the other cells. Can they be charged in parallel without the (unknown) dead cells effecting the charge of the good cells?

In either scenario it's bad news - battery packs should be built with brand new identical batteries, if they are in series or parallel - and should never be messed with.

When the battery has failed, one cell, two cells,......etc. you MUST replace all the batteries with more brand new identical ones - don't even attempt to mess with old ones.

audioguru · Feb 2, 2020

Ni-Cads frequently shorted with stalagmites. When I used them a long time ago and found a shorted cell I would ZAP the short from a high capacity capacitor charged to 12V.
If you charge a Ni-CAD battery that has series cells and one is shorted then the charging current and voltage become too high and depend on how complicated is the charger circuit.
If the Ni-Cad cells are in parallel and one is shorted then it explodes or gets very hot from all the current supposed to go to all the cells. Maybe you get lucky when the high current ZAPs the short instead of ZAPPING the power supply.

I remember that a charging Ni-Cad gets cooler but a charging Ni-MH gets warmer.
About 10 years ago cheap Chinese solar garden lights used a Ni-Cad battery cell but now they use Ni-MH maybe because the Cadmium is so deadly it has been banned in Europe for the last 10 years.

vu2nan · Feb 2, 2020

The recommended method is to charge the cells in series at 1/10 C amps (10 % of the AH figure), for a period of ~~10 hours~~ 14 to 16 hours, using a constant current source.

Regards,

Nandu.

Edited - the charging period has been corrected thanks to rjenkinsgb

rjenkinsgb · Feb 3, 2020

vu2nan said:
1/10 C amps (10 % of the AH figure), for a period of 10 hours,

No, not quite - the recommended time at 1/10 C is 14 to 16, hours to allow all cells to reach full charge and equalise.

They are not 100% efficient and 10 hours at 1/10C would not give anywhere near full charge.

More info:

How to charge nickel cadmium batteries in cells or packs

Charging methods for NiCad batteries including slow chargers, fast chargers and smart chargers.

www.powerstream.com

vu2nan · Feb 3, 2020

Hi Jenkins,

I stand corrected.

Many thanks.

Regards,

Nandu.

Kart31 · Feb 3, 2020

rjenkinsgb said:
No, not quite - the recommended time at 1/10 C is 14 to 16, hours to allow all cells to reach full charge and equalise.

They are not 100% efficient and 10 hours at 1/10C would not give anywhere near full charge.

That is for sure!

Changing NiCad Batteries

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