NiMh and NiCd battery charger

[ADWSystems]

I know that LiPo batteries are not inter-chargeable (is that word) with NiMh and NiCd batteries. LiPo cells cannot be charged by a NiCd charger. With that said, can a NiMh battery be recharged by a charger that was originally designed for NiCd cells?

 

[MikeMl]

AFAICR, there is a subtle difference in end-of-charge detection in automatic chargers.

If just wanting to recharge either one using a brute-force method; a constant-current power supply delivering a 0.1C rate for 14hours works...

0.01C forever will overcome self-discharge without hurting the cells...

ps: go to this page, and follow the "How to charge NiMh and NiCd" links. The tech info on this site is very good.

 

[tcmtech]

In any device that has a half way decent charging system for NiCad yes it will handle NiMh just fine.

I have been gradually switching all of my NiCad battery powered cordless tools over to NiMh as I rebuild the batteries.

(Craftsman cordless, Good tools really s****y batteries.)

 

[audioguru]

Energizer and a Japanese Ni-MH battery manufacturer recommend a trickle charge current not higher than C/40, so C/10 which is safe for Ni-Cads will probably damage or reduce the life of the Ni-MH cells.
Some cheapo chargers simply use a certain current and a timer. But a Ni-MH battery cell has a much higher capacity than an old Ni-Cad cell so the Ni-MH cell will not be anywhere near fully charged when the timer times out.

 

[MikeMl]

Energizer and a Japanese Ni-MH battery manufacturer recommend a trickle charge current not higher than C/40, so C/10 which is safe for Ni-Cads will probably damage or reduce the life of the Ni-MH cells...

Here is what Dr. Mark W. Lund (CEO of PowerStream) says about overnight recharging:

The cheapest way to charge a nickel cadmium battery is to charge at C/10 (10% of the rated capacity per hour) for 16 hours.. So a 100 mAH battery would be charged at 10 mA for 16 hours. This method does not require an end-of-charge sensor and ensures a full charge. Cells can be charged at this rate no matter what the initial state of charge is. The minimum voltage you need to get a full charge varies with temperature--at least 1.41 volts per cell at 20 degrees C. The best charging practice is to use a timer to prevent overcharging to continue past 16 hours.

The cheapest way to charge a nickel metal hydride battery is to charge at C/10 or below (10% of the rated capacity per hour). So a 100 mAH battery would be charged at 10 mA for 15 hours. This method does not require an end-of-charge sensor and ensures a full charge. Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10. The minimum voltage you need to get a full charge varies with temperature--at least 1.41 volts per cell at 20 degrees C. Even though continued charging at C/10 does not cause venting, it does warm the battery slightly. To preserve battery life the best practice is to use a timer to prevent overcharging to continue past 13 to 15 hours.

Here is what he says regarding float charging:

In a standby mode you might want to keep a nickel cadmium battery topped up without damaging the battery. This can be done safely at a current of between 0.05 C and .06 C. The voltage required for this is dependent on temperature, so be sure to regulate the current in the charger.

In a standby mode you might want to keep a nickel metal hydride battery topped up without damaging the battery. This can be done safely at a current of between 0.03 C and .05 C. The voltage required for this is dependent on temperature, so be sure to regulate the current in the charger.

 

[audioguru]

I have never seen a PowerStream battery. I buy Energizer batteries and use their applications manuals.

 

[MikeMl]

I have never seen a PowerStream battery. I buy Energizer batteries and use their applications manuals.

Powerstream makes chargers, not batteries...
Does anything that Lund says conflict with the Battery Bible according to AudioGuru (as copied from Energizer)?

 

[audioguru]

Powerstream makes chargers, not batteries...
Does anything that Lund says conflict with the Battery Bible according to AudioGuru (as copied from Energizer)?

Lund says, "In a standby mode you might want to keep a nickel metal hydride battery topped up without damaging the battery. This can be done safely at a current of between 0.03 C and .05 C" but Energizer says,
"Finally a maintenance (or trickle) charge rate of less than 0.025 C (C/40) is recommended. The use of very small trickle charges is preferred to reduce the negative effects of overcharging".

 

[jpanhalt]

Don't know much abut the Bible or other religious texts, but BU seems to agree that NiMH are more sensitive than NiCd to charging and discharging, but except for very fast charging are very similar to NiCd (**broken link removed**)

John

 

[bountyhunter]

Don't know much abut the Bible or other religious texts, but BU seems to agree that NiMH are more sensitive than NiCd to charging and discharging, but except for very fast charging are very similar to NiCd (**broken link removed**)

John

No. The critical difference is in the tolerance of continuous overcharge (or trickle charge). NI-CD allows continuous trickle charging at rates of 0.1c or in some cases, as high as 0.25c for fast charge cells. NI-MH does not tolerate continuos charge well, the "safe" rate depends on whom you ask. gates energizer (who originated NI-MH) told us not to exceed c/40. I have seen some more modern NI-MH batteries that claim they tolerate c/10 but have not tested them. I suspect they claim that to make them "compatible" with NI-CD and accept the reduction in life as OK.

 

[jpanhalt]

How does that differ from what MikeML and I have said? NiMH are more sensitive to charging. (They also don't tolerate the high discharge rate of NiCd's, but that topic wasn't asked.) If you want the details, go to a reliable source, like PowerStream or BU.

 

[bountyhunter]

" The minimum voltage you need to get a full charge varies with temperature--at least 1.41 volts per cell at 20 degrees C."

On the topic of "trickle charging" NI-MH batteries to keep them topped up: I use a constant voltage charger with the final voltage set at 1.400V/cell. CV charging is typically not used, but it works well as long as the charge rate is controlled so the cells don't heat up during the charge cycle. I use a CV charger on my 150mA 9.8V (seven cell) batteries and they are presently about 8 years old and still work fine. IMO, using CV is better to float charge NI-MH because you don't keep dumping current into a fully charged cell.

 

[bountyhunter]

How does that differ from what MikeML and I have said?

Here is how:

"except for very fast charging are very similar to NiCd"

That statement is wrong. NI-MH and NI-CD are very different with respect to the slow charge "top off" or trickle charge to keep in standby.

NI-MH and NI-CD do also differ in fast charge characteristics, but that's not the only difference.

 

[bountyhunter]

Energizer and a Japanese Ni-MH battery manufacturer recommend a trickle charge current not higher than C/40, so C/10 which is safe for Ni-Cads will probably damage or reduce the life of the Ni-MH cells.

Exactly my point. Dropping NI-MH batteries into a Ni-CD charger MIGHT work but it might not. It may also "work" but shorten the life of the battery.

 

[bountyhunter]

How does that differ from what MikeML and I have said? NiMH are more sensitive to charging. (They also don't tolerate the high discharge rate of NiCd's, but that topic wasn't asked.) If you want the details, go to a reliable source, like PowerStream or BU.

Well, here is what Battery University says:

"Extended trickle charge on nickel-based batteries inflicts damage. NiCd and NiMH should not be left in the charger unattended for weeks and months."

"Because of its reduced ability to absorb over-charge, NiMH should not be charged on a slow charger."

Which is exactly what I and AG have been saying, except we correctly pointed out that the damage only happens if the continuous charge rate is too high which exceeds the cell's ability for the internal gas to recombine so that pressure doesn't build up enough to vent the cell. The BU statement is an oversimplification which needs more information but is essentially correct.

 

[audioguru]

A modern Ni-MH battery keeps its charge for one year (Panasonic-Sanyo-Eneloop and Energizer) so they do not need a continuous trickle charge like old cells did.
Did you know that a Ni-MH cell gets warm when charging but a Ni-Cad cell gets cooler? Try charging both at the same time like I did. Until they are fully charged then both become fairly warm.

 

[bountyhunter]

Did you know that a Ni-MH cell gets warm when charging but a Ni-Cad cell gets cooler? Try charging both at the same time like I did. Until they are fully charged then both become fairly warm.

True. I did the measurements and NI-CD cells actually drop a couple of degrees while charging at very high rates of charge because the chemical process is endothermic. The cell doesn't start to warm up until it gets near to full charge state. That makes end of charge cut off easy, all you have to do is watch for a temp rise.

NI-MH cells heat up all during charging because their charge reaction is exothermic. As they approach full charge, the temperature rises a little faster which is why they used temp change rate detectors to shut off fast charge on NI-MH.

Put a NI-MH into a fast charger built for Ni-CD batteries and you may get a pack blow up because the end of charge detector won't shut off the current.

 

[shokjok]

I put my trust in a solar panel to recharge NiCd and NiMH batteries. The trickle charge rate is dependent on the light source. In low light, the charge rate is below suggested levels. In bright sunlight with sheltered batteries their quality or charge capacity doesn't diminish. I haven't tried a low-voltage solar adapter for cellphone batteries.