remove batts early from charger?

I have a Maha MH-C9000 (OG01A) (recently replaced under warranty for one slot gone bad; thanks Maha for the good replacement service ! )

When using "break-in" to save/re-vive older AA NiMH batts not in often use, I have found that I don't have t go thru the whole cycle and if I'm at the charger when I see it fully charge to its full xxxx MAH rating, I remove the batts. and they seem to work fine and have full capacity

Questions:
- any harm done to charger?
- any disadvantage to using this method?

Hello Doug.S.
Welcome to CPF.

How do you know they are fully charged when removing them before C-9000 stops automaticly?

The charger put in more mAh than the cells could hold. If you take one cell out in your way, and let another charge with a proper termination you can see the difference in mAh, when you discharge both of them.


Anders

Doug.S said:

When using "break-in" to save/re-vive older AA NiMH batts not in often use, I have found that I don't have t go thru the whole cycle and if I'm at the charger when I see it fully charge to its full xxxx MAH rating, I remove the batts. and they seem to work fine and have full capacity

Questions:
- any harm done to charger?
- any disadvantage to using this method?

Click to expand...

What I think you're saying, for example, is that your batteries might be 2200mah batteries, so you remove them when the charger says that it has put in 2200mah. Is that right?

If so, then those batteries are NOT fully charged. All batteries need more energy put into them than just their rated capacity due to losses from internal resistance in the batteries, the charger itself, and heat losses.

If you want fully charged batteries, let the Maha do what it does best; detect the point at which the battery IS fully charged.

You're not damaging the charger nor the batteries, but you are not getting the full benefit of fully charged batteries. If you take them off when the charger shows the rated mah into them, you are likely stopping the charge at around 75 to 80% if you're lucky.

In the break-in mode, the charger is applying a 0.1C charge rate for 16 hours, discharging at 0.2C, and then applying the 0.1C/16 hour charge again.

Now the cells may indeed have reached a full state of charge after about 12 hours of the 0.1C charge, so you could remove them at this time and perhaps find them fully charged. However, the additional charge time up to 16 hours is not just to put charge into the cell, but rather to "form" the electrodes and electrolyte into a good state, returning the cell to its maximum charge holding capacity.

By removing the cells early, you are not allowing the break-in mode to do its job, and you may just as well use the regular charging mode.

Incidentally to what others have said, it is not unusual for the mAh in during charging to be less than the mAh out during discharge. This is because the true charge in a cell is not measured in mAh but in joules (or equivalently, mWh). Since power = volts x amps, and since the charging voltage is higher than the discharge voltage, you can put the same amount of energy into the cell at higher voltage and lower amps than when you take it out again.

Thanks for the replies. I now understand the input mAh is a poor measure and its the discharge rating that is valid.

I'll just let the C9000 do its job and be more patient to get a full capacity charge.

IF you totally know when its 98% charged , by knowing the cells that well, then pulling them before termination would actually be better for thier life.
knowing exactally when that is , that is the hard part

an advantage, it would never overcharge

Disadvantages,
the cells wouldnt be "balanced" through 100% charging, so series discharging in a device after, would more likly result in reverse charge
they might not have full potential capacity, so you lose a bit of runtime
there is a TOPPING off on most chargers that finishes up beyond the faster charging, sometimes this is done AFTER you get the "Full" signal.

Hello Doug,

I will echo a welcome to CPF.

If you want to shorten the Break-In time, watch for the rest at the end of the discharge and pull the cells then. You will go through your 0.1C charge for 16 hours, and hour rest, then a 5 hour discharge. At the end of this discharge, you can view the capacity discharged from the cell. You can remove the cells during this rest period, or continue with another 16 hour charge to the end of the Break-In cycle.

The amount of charge put into the cell is interesting, but what really counts is the amount removed from the cell.

Tom