NiMH Charging

[autir]

I have read a few articles regarding recharging methods for Ni-based rechargeable batteries (NiCd, NiMH). I have a few questions about the simple constant current method, that is without timers, dV/dt or dT/dt detectors, or anything else.

1) What do you think of the figures given here:
http://www.powerstream.com/NiMH.htm
Both in sections "Overnight Charging" and "Trickle Charging"? Do you like the C-based numbers?

2) Overcharging a NiMH battery can lead to the formation of small crystals of electrolyte on the plates, causing voltage depression. Is trickle charge harmful too?
I'm asking this because of this: (text from http://en.wikipedia.org/wiki/NiMH)

Some equipment manufacturers consider that NiMH can be safely charged in simple fixed (low) current chargers with or without timers, and that permanent over-charging is permissible with currents up to C/10 h. In fact, this is what happens in cheap cordless phone base stations and the cheapest battery chargers. Although this may be safe, it may not be good for the health of the battery. According to the Panasonic NiMH charging Manual (link below), permanent trickle charging (small current overcharging) can cause battery deterioration and the trickle charge rate should be limited to between 0.033×C per hour and 0.05×C per hour for a maximum of 20 hours to avoid damaging the batteries.

The bottomline is:
I have got a GP PowerBank charger (http://www.gppowerbank.co.uk/powerBankRange.html), which claims to be able to be "always plugged in", and whose specs are 2.8V @ 100mA. I have also got some brand new 2500Ah NiMH AAs. Given that the charger's current is 0.04C, what should I pay attention to when charging the batteries in order to ensure long battery life?

Thank you in advance.

 

[audioguru2]

Hi Autir,
I have a Ni-Cad powered cordless drill that has been "simple-charging" (about 0.08C) for many years without problems. I worked with wireless boardroom microphones that used series'd Ni-Cads that were always "simple-charging" and when their battery failed I asked the customer, "What happened?" and their answer was always, "We forgot to turn it off".

When a multi-cell battery of cells in series is over-discharged, the weakest cell gets completely discharged first, then the remaining cells continue discharging through it forcing the current in it to be backwards, causing crystals that short the cell.

Energizer recommends a 3-step charge for their Ni-MH cells: Fast-charge at 1C, intermediate-charge at 0.1C then a trickle charge at 0.025C.
Their applications manual is here:
http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf

Your charger takes "forever" (about 32 hours) for your cells to reach full charge, then slightly over-charges them for a trickle-charge. At least it doesn't grossly over-charge cells that weren't completely discharged. ;D

 

[autir]

Hi Autir,
I have a Ni-Cad powered cordless drill that has been "simple-charging" (about 0.08C) for many years without problems. I worked with wireless boardroom microphones that used series'd Ni-Cads that were always "simple-charging" and when their battery failed I asked the customer, "What happened?" and their answer was always, "We forgot to turn it off".

Are your two examples referring to devices that were always charging when not in use?

Your charger takes "forever" (about 32 hours) for your cells to reach full charge, then slightly over-charges them for a trickle-charge. At least it doesn't grossly over-charge cells that weren't completely discharged. ;D

2500mAh batteries in a 100mA charger require 25 hours to charge fully. Taking the typical 66% efficiency in consideration, we have 25*1.5=37.5 hours.
If this slight overcharging you have mentioned takes place for less than 20 hours, will it weaken the batteries - even slightly? Or there will be no damage at all? If I leave them for 40 or 48 hours, for example, what will happen in the long run?
Also what will happen if I just leave the cells in the charger forever? (e.g. 1 week).

Thank you.

 

[audioguru2]

Are your two examples referring to devices that were always charging when not in use?

Yes, but they are Ni-Cad, not Ni-MH.
In their Ni-Cad applications manual, Energizer shows the results of a test where their Ni-Cads were over-charged for 2 years. Their life and performance was hardly effected. They didn't make the same test on their Ni-MH cells.

2500mAh batteries in a 100mA charger require 25 hours to charge fully.

No.

Taking the typical 66% efficiency in consideration, we have 25*1.5=37.5 hours.

Yes, that is a very long time.

If this slight overcharging you have mentioned takes place for less than 20 hours, will it weaken the batteries - even slightly? Or there will be no damage at all? If I leave them for 40 or 48 hours, for example, what will happen in the long run?
Also what will happen if I just leave the cells in the charger forever? (e.g. 1 week).

An over-charge rate of 0.04C is very low, but not as low as the 0.025C rate recommended by Energizer, and not as low as the zero over-charge rate recommended by Panasonic.
I think if you left them over-charging at 0.04C continuously, they will still work fine when you want to replace them in the future with better cells, a few years from now. ;D

 

[autir]

Thank you for your reply

One final question:

When a multi-cell battery of cells in series is over-discharged, the weakest cell gets completely discharged first, then the remaining cells continue discharging through it forcing the current in it to be backwards, causing crystals that short the cell.

If we have some rechargable cells and we are not certain if they are all of the same charge level, can't we just connect them in parallel? So the full ones will charge the empty ones and everyone will be ok.
(I suppose that if the cells would have the same C it would be good, not to say needed  ???)

 

[audioguru2]

It is very dangerous to connect anything to a charged Ni-Cad or Ni-MH cell without current-limiting. They are capable of supplying many amps and a discharged cell will attempt to charge with many amps. They might explode!

For many years, two series strings of 6 AA Ni-Cad cells were in parallel in the amp I made for the beach. I connected the two strings in parallel when the cells were discharged to avoid an explosion. With them in parallel, the capacity was nearly doubled, but they never ran down completely so the 2nd set wasn't needed anyway.
Maybe they lasted so long because they never ran down completely so that the weakest cell never got completely discharged then charged in reverse (which would cause it to be shorted) while the strongest cells continued discharging.