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I have an old NiCd RC battery pack. It was not used for at least ten years but when I tried to charge it with the old charger I was suprised to see it still had some capacity.

Now I just went and bought a new pack with a charger anyways, and it's NiMH.

Could I safely charge the old NiCd battery pack with that new charger? What could happen in the worst case?

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The following application note from Texas Instruments on battery charging is quite relevant.

It is about some charger controller chips, but the bulk of the document is focused on how to charge NiCd, NiMH and LiIon cells safely. It also details which compromises a designer faces when he trades longer battery life vs. faster charge and so on. Very interesting reading.

Note, in particular, that trickle charge is not an issue: NiCd cells are more rugged and tolerant than NiMH ones of the same capacity, therefore if the charger is made for NiMH cells it will work also for NiCd cells (in trickle charge mode).


Relevant excerpts (emphasis mine):

Slow Charge Rates

NI-CD: most Ni-Cd cells will easily tolerate a sustained charging current of c/10 (1/10 of the cell's A-hr rating) indefinitely with no damage to the cell. At this rate, a typical recharge time would be about 12 hours.

Some high-rate Ni-Cd cells (which are optimized for very fast charging) can tolerate continuous trickle charge currents as high as c/3. Applying c/3 would allow fully charging the battery in about 4 hours.

The ability to easily charge a Ni-Cd battery in less than 6 hours without any end-of-charge detection method is the primary reason they dominate cheap consumer products (such as toys, flashlights, soldering irons). A trickle charge circuit can be made using a cheap wall cube as the DC source, and a single power resistor to limit the current.

NI-MH: Ni-MH cells are not as tolerant of sustained charging: the maximum safe trickle charge rate will be specified by the manufacturer, and will probably be somewhere between c/40 and c/10. If continuous charging is to be used with Ni-MH (without end-of-charge termination), care must be taken not to exceed the maximum specified trickle charge rate.


Fast charge, on the other hand, may give you issues, because it needs an end-of-charge detection circuit that works differently between the two chemistries:


Fast Charge

Fast charge for Ni-Cd and Ni-MH is usually defined as a 1 hour recharge time, which corresponds to a charge rate of about 1.2c. The vast majority of applications where Ni-Cd and Ni-MH are used do not exceed this rate of charge.

It is important to note that fast charging can only be done safely if the cell temperature is within 10-40°C, and 25°C is typically considered optimal for charging. Fast charging at lower temperatures (10-20°C) must be done very carefully, as the pressure within a cold cell will rise more quickly during charging, which can cause the cell to release gas through the cell's internal pressure vent (which shortens the life of the battery).

The chemical reactions occurring within the Ni-Cd and Ni-MH battery during charge are quite different: The Ni-Cd charge reaction is endothermic (meaning it makes the cell get cooler), while the Ni-MH charge reaction is exothermic (it makes the cell heat up). The importance of this difference is that it is possible to safely force very high rates of charging current into a Ni-Cd cell, as long as it is not overcharged.

The factor which limits the maximum safe charging current for Ni-Cd is the internal impedance of the cell, as this causes power to be dissipated by P = I2R. The internal impedance is usually quite low for Ni-Cd, hence high charge rates are possible.

[...]

The exothermic nature of the Ni-MH charge reaction limits the maximum charging current that can safely be used, as the cell temperature rise must be limited.

[...]

Fast Charge: Possible Cell Damage

Caution: Both Ni-Cd and Ni-MH batteries present a user hazard if they are fast charged for an excessive length of time (subjected to abusive overcharge).

When the battery reaches full charge, the energy being supplied to the battery is no longer being consumed in the charge reaction, and must be dissipated as heat within the cell. This results in a very sharp increase in both cell temperature and internal pressure if high current charging is continued.

The cell contains a pressure-activated vent which should open if the pressure gets too great, allowing the release of gas (this is detrimental to the cell, as the gas that is lost can never be replaced). In the case of Ni-Cd, the gas released is oxygen. For Ni-MH cells, the gas released will be hydrogen, which will burn violently if ignited.

A severely overcharged cell can explode if the vent fails to open (due to deterioration with age or corrosion from chemical leakage). For this reason, batteries should never be overcharged until venting occurs.

In later sections, information is presented which will enable the designer to detect full charge and terminate the high-current charge cycle so that abusive overcharge will not occur.

Again, the NiCd cells are more rugged, so it is not intrinsically dangerous to charge them with a NiMH charger. But keep an eye on the time spent under charge! The end-of-charge circuit might not be able to detect that the NiCd cells have been fully charged and so it might overcharge them.

An overcharge that is relatively limited in time could simply shorten your NiCd cells' useful life, but if you overcharge them for hours and hours they could vent and be damaged badly!

See the section of that application note about the end-of-charge detection for NiCd vs. NiMH for further details. Here's a glimpse of its content:

End-of-Charge Detection for Ni-Cd/Ni-MH

Both Ni-Cd and Ni-MH batteries can be fast charged safely only if they are not overcharged.

By measuring battery voltage and/or temperature, it is possible to determine when the battery is fully charged.

Most high-performance charging systems employ at least two detection schemes to terminate fast-charge: voltage or temperature is typically the primary method, with a timer as the back-up in case the primary method fails to correctly detect the full charge point.

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NiCd and NiMH have similar charge and discharge characteristics. If the pack voltage and capacities match, then it's worth a try.

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  • \$\begingroup\$ What could happen in the worst case? I don't want to burn the building down ;-) \$\endgroup\$ – André Stannek Mar 28 '18 at 14:53
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    \$\begingroup\$ Nothing much catastrophic will happen. Nicads aren't anywhere near as touchy as lithium batteries. Nicad in a NiMh charger should be fine. Trickle charge is fine as long as you keep an eye on the charge time, and fast chargers for NiMh will also detect a full Nicad and shut off the charge cycle. \$\endgroup\$ – JRE Mar 28 '18 at 15:09
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When fast charging, NiMH has to be stopped earlier than NiCd to prevent damage. This means your NiCds will be safe on a NiMH charger, just not quite 100% charged when fast charging stops.

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