Top-up unused NimH after a week?

I read that an unused NiMH call will lose about 10% of its charge after 24 hours and then discharge will slow down after that.

If an NiMH has lost that much charge, then after a week or so (or even after a few days) would the capacity of the cell have dropped sufficiently for the delta-V point to be picked up by a domestic microprocessor controlled charger such that is charging terminated before the cell is overcharged?

In other words can I top up the cell after a week or maybe after just a few days?

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Alex Coleman
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It does not mater. You can charge NiMH batteries after several days to a full state of charge. delta-V occurs when the positive is fully charged and goes to overcharged state. when this occurs your charger should identify this and stop charging.

M. Moorthi www.battery-c> I read that an unused NiMH call will lose about 10% of its charge

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The real problem is that self-discharge can vary between different cells in a pack. This was my key finding after a few months of reading about NiMH.

When you recharge a sealed multi-cell pack, either the cell with the highest remaining charge is overcharged and damaged, or the cell with the least remaining charge isn't fully charged, and so can go reverse polarity when discharged (effectively reverse charged), damaging that cell.

I saw a suggestion that to avoid cell imbalance due to different self-discharge levels, you should store the NiMH cells discharged (maybe 1.1 V?), and only recharge just before you want to use them, as there is less self-discharge at lower charge levels.

NiMH don't like being trickle charged, so trickle charge can't be used to ensure that the cells are all fully charged.

The last 4 x AA NiMH charger I bought terminates the charge of each cell individually (Energizer), and it's very surprising how much the recharge time can vary for each individual cell. After a few weeks storage, probably not discharged, I saw one cell taking what felt like ages more than the rest to recharge!

As the rule goes: recharge cells individually.


Reply to
Paul Taylor

I've been trickle charging the same set of NiMh for >4 years with no sign of trouble. They are in an AM/FM radio that gets daily use, and usually goes ~ 2 weeks between charges. NiMh needs to trickle charge at a different rate than NiCd if you choose to trickle, but if you do it right there's no problem. You can't leave either kind on a trickle forever, but you can trickle them and either taper or terminate when full voltage is reached. I think the warning applies if you put them on a trickle and leave them at the trickle rate for prolonged periods.


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You can leave all batts on trikkle charge , just don't allow temp to rise .

Batts need to get a very hi current at first ( Li-Ion in notebook PC

can so 2 amps sometimes) . Then tapper off so as not to create escaping gasses ... ever allow batts to get warm , charge in freezor if needed .

Never allow them to store discharged , it WILL reduce life .

NiMH are worse than Ni-cad but govt did not like Cadmium ,

so " Off with their heads , off with their heads ..."

A wet Nicad ( as Jet engine starter ) is the most amasing battery ever made , but stay far away , Cadmium causes cancer and you'll bark at the moon .....BS BS BS ... It's the heavy oxides of some metals that mutate cells ...

It has the lowest R of all , it can put out 1000 amps in a package

1/2 the size of your car battery ! And NO DAMAGE EVER ! They last for 10 years and cost a fortune .

Nicads never had memory effect .

Use a "buck regulator" to charge . A TO220 PNP driving a 300 microHenry coil to a 105 degree rated 1000 MF Cap ...... LM431 pulling down on a 300 ohm R , R is also across a small PNP to short EB of the large PNP driver . 1n5820 from PNP/coil junction to ground .

300 ohm needed to make 431 perfectly square gain ( 370 v/v ) in order to slam the big PNP off quickly . One more diode to the Batt to isolate the pow supply when it's OFF and the battery may try to drive backwards into P.S.

Wanna pulse charge it ? You allready have the circuit , simply one shot it ! This is the cheapest "switch mode" circuit and most reliable .

ehsjr wrote:

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