NiMH Battery Question

Goeytex

Senior Member
Hi,

While unpacking some old boxes in storage, I came across 4 NiMH AA batteries in a charger. The batteries were quite dead, reading only about 100mv each. The batteries had been in storage well over 18 months. When installed in the charger the "charge" lamp would not come on. I assumed they were toast.

So out of curiosity, I put the batteries in a battery holder and connected them to the bench supply with the current limited to 100 ma. Then slowly raised the voltage until the current was 100ma. As the current began dropping I continued the raise the voltage and so on until the voltage reached ~ 4.5V. I then left them there for about 10 minutes until the current dropped to 20 ma, and then removed them from the battery holder and installed them in the charger. To my surprise the charge lamp came on. Then after several hours the Ready Lamp came on. The batteries were quite warm, but not hot. After this charge the voltage across the 4 batteries was ~5.52 V. I have been using them to power a breadboard ( 10 ma average) for a day now and the voltage is around 5.14v

Questions:
1. Did the storage and resultant discharge of these batteries significantly reduce their capacity?
2. What should I expect from these recovered batteries?

They seem to be working fine. The reason I ask is that I have never use NiMH batteries before and found about 40 more of these in another box in the same condition and wonder it they are worth recovering in the same manner.
 
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Dippy

Moderator
I can't comment on your AA but I have 'recovered' some (not all) old NiCds. Never as good as new but got an extra year out of them.


Also, my Bosch cordless drill batteries recently suffered a similar fate. That is NiMH 18V 2aH (I think).
Sat around for 3 years and wouldn't work on the Bosch charger.
I put my TTi Bench PSU on CLimit=200mA and kept increasing voltage (25V?) until, all of a sudden, it took charge.
The second battery, however, wouldn't do the same.
I can now use my drill (and Bosch charger) again. It doesn't seem to have the same oomph or operational life, but then for a 10 year old battery I'm not too shocked. Trouble is that the drill brushes smell now so may be time for a new expensive cordless.


I have heard various stories as to what the over-voltage zapping does. I'm sure there are more opinions around but it didn't work with all stubborn batteries so I guess it depends on what has caused the battery coma or death.
 

John West

Senior Member
Run load tests on a couple of the reconditioned batteries. That's about the only way to see what you have.

Most battery chargers check the initial charge voltage and won't try to charge a cell that has a high impedance, because the surface charge will already indicate a fully charged cell as soon as power is applied. A lab power supply will do the job though because you turn the voltage up until current is flowing into the cell. Once it gets a charge into the cell you just have to see how badly damaged the cell has become from long storage.

Cells kept in cool, dry places will store a lot longer than those that are not, no matter what battery chemistry they are.
 

Buzby

Senior Member
Some battery chargers won't accept a 'dead' cell, because the charger can't tell if the cell is the right way round or not.
Better not to charge at all than reverse charge !.
 

Armp

Senior Member
1. Did the storage and resultant discharge of these batteries significantly reduce their capacity?
2. What should I expect from these recovered batteries?
NiMHs are pretty robust, and unlikely IMO to be damaged by storage in a discharged state.
In fact many NiMHs made more than 2 or 3 years ago self-discharge at an alarming rate upto 3%/day.
What does kill them is reverse biasing, overcharging and high temps.
The suggested procedure for discharged cells is to slow charge (C/10) the cells to about 1.4V. Which is close to what you did.
Discard any oddballs.

NiMH do not have a 'memory' effect like NiCds, nor do they grow 'whiskers' so no point in zapping bad cells.
 

Dippy

Moderator
How do you explain my NiMH behaviour? (They have NiMH printed on them).
i.e. for my 18V cells, nothing until Vsupply was about 25V then, suddenly, current passing.
It wasn't gradual, it was sudden and I'm pretty certain all connections were good.
I'm genuinely interested to know as i can't reproduce the effect now that the battery is 'happy'.


PS. I've just been searching/reading - Lordy, there are as many opinions as there are experts..;)
 

g6ejd

Senior Member
Before I gave up on Nimh and switched to LiPo batteries for my model aircraft it was a common problem to find batteries had discharged to very low lens and chargers would not bring them back, however a quick high voltage spike would quickly get them back to operation, usually 12v on 1.2v cell. The widely held belief was that the cell plates became oxidised and the high voltage broke down the oxidisation layer and enabled the battery to start working again.

No such problems with LiPo technology.
 

nick12ab

Senior Member
How do you explain my NiMH behaviour? (They have NiMH printed on them).
i.e. for my 18V cells, nothing until Vsupply was about 25V then, suddenly, current passing.
It wasn't gradual, it was sudden and I'm pretty certain all connections were good.
I'm genuinely interested to know as i can't reproduce the effect now that the battery is 'happy'.
Is it a 'smart battery' with an IC that monitors voltages and refuses to allow the battery to charge on an inadequate charging voltage, or is it just cells?
 

premelec

Senior Member
Simple load test reminder [mentioned before] Take a mechanical 'quartz clock' that runs on a single cell and determine voltage it quits at - usually .8 to 1 volt. Put a resistor to draw suitable current in parallel with clock and discharge battery into this noting starting and ending time of clock. For Manuka a resistor voltage divider is recommended so the clock stops at 2.9 volts or better.... LiFePO4 AND watchout for overdischarge or put in 3 diodes in series with the LiFePO4 !. You can get a good idea of the milliamperehours with this method. [old clocks about $1-$2 in yard sales and thrift shops].
 

Goeytex

Senior Member
I just want to use these "Free" NiMH batteries for my daughter's XBox wireless game controller instead of continuing to buy alkiline AA's. Even if they have to be charged every day it will be a savings.

On another note I am developing several RF applications that will be battery powered and I think the 3.2v LiFePO4 will be ideal for operating both the Picaxe and the RF modules. My only reservation is how they behave when discharged to much.

Does anyone know of a battery charger IC that is taylored for LiFePO4. I want to be able to plug the RF device into a USB port for charging the battery.
 

manuka

Senior Member
There certainly are such ICs- check MicroChip's MCP73123- although simpler TL431 approaches may do too.

But surely it's the discharging voltage level that you're most concerned about? I've found a switched white LED across the cell handy,as if it's dim when tested the LiFePO4 needs charging (<3V). Failure to light at all means the recognised low level of 2.5V has been reached (or exceeded). Some makers even specify 2V as the critical limit in fact!

You could always get the PICAXE to periodically "CALIBADC" check it's own LiFePO4 supply voltage of course.

A quick charging hack may be to just build into your project a ~US$6 slim AA/AAA LiFePO4 USB charger!

Extra: I've been running outdoors a single AA LiFePO4 powered passive IR security lamp all NZ winter. Our winters are pretty sunny, but after some days of cloud & rain the lamp refuses to trigger,showing just it's white LED pilot lamp. Even a few hours sunshine subsequently restores to it's normal after dark behaviour.

Aside from it's fixed PV/sensor/lamp mounting limitation,I've been MOST impressed overall. It's performance is far superior to 3-4 x AA NiMH powered equivalents. NiMH cells may go wonky after a few 100 charge/discharges of course, but a traditional woe relates to the old outdoor bugbear of grime & corrosion on multiple cell contacts. A single LFP cell is automatically spared most of such irksome mischief.
 
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