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If you're interested in the context, I want to replace 3 AA size NICD batteries with 3 NIMH batteries in a 30+ year old Hewlett-Packard calculator. The charger provides a constant 50mA.
Should be fine. Common rechargeable NIMH's are in the 1800mah range for capacity, and 50ma's is only 1/36th that capacity, usually you can safely trickle charge at 1/10th C assuming you have a 75% charge efficiency it's going to take two days to charge from empty.
Thanks both. I pulled the old corroded Nicads and replaced with fresh NIMH from Panny rated at 1500 mAh. Been charging about 15 hours and works great so far.
The old HP should have about 3x the original battery life now.
Energizer's AA Ni-MH cells are 2500mAh. On their website they have an applications manual for Ni-Cad and another for Ni-MH battery cells. They say that the trickle-charge current should be no more than 1/40th of the capacity of the Ni-MH cell for longest life. That is only 37.5mA but it is close enough to 50mA so it won't make any difference.
But he didn't buy new Energizer 2500mAh cells. He bought old no-name-brand (Chinese?) 1500mAh ones from a cheapo store. His 'new" battery might be older than him.
No I didn't buy the Everyready gigaamphour batteries, nor did I buy cheapo Chinese no-names.
I used Panasonic Type-13 HHR-P505A/1B if it's important, normally used for cordless phones. Sure it's only 1500 mAh but it has 3 AA batteries already soldered together which saves me a lot of time. And I don't have to worry about overheating the battery ends by soldering.
And the Panny's are made by some no-name Indonesian manufacturer. So there!
You said: "and replaced with fresh NIMH from Panny". Aren't they the no-name ones?
Most battery manufacturers make modern high capacity Ni-MH cells. Electronic equipment manufacturers like Panasonic don't.
Battery cells have plastic inside so soldering them is bad. The factory quickly spot-welds them.
They say that the trickle-charge current should be no more than 1/40th of the capacity of the Ni-MH cell for longest life. That is only 37.5mA but it is close enough to 50mA so it won't make any difference.
Just wondering what happens if you had a solar panel or something charging the battery, and at the same time, the battery had a small load on it (drawing less than the charge current) what would happen if the battery became fully charged?
Is there a way too tell if a battery has reached full charge (coming from a micro controller perspective)
Hi Gramo,
In Energizer's applications manual on Ni-NH battery cells, they show a graph of the slowly rising voltage as a cell is charging then as it reaches full charge its temperature rapidly increases, its internal pressure rapidly increases and its voltage drops a little.
A battery charger IC senses the increase in temperature and the small drop in voltage to stop the charging. A timer is used as a backup if the sensors don't detect full charge.
Actually for your application the higher the capacity the better. high capacity cells are only 'bad' when used in high discharge applications. They get the extra capacity by making the walls of the cell and internal structure as thin as possible which makes it prone to oxidizing (to an inert state) when used in high discharge applications.
That must be why my sub-C cells for my RC cars have a lower Amp-hours/volume than AA NiMH cells. Those things get over 70 amps from what I understand. Matched GP cells.
Definitly. NIMH's are better nowdays by far, but some people still use NiCad's because of their superior current supply ability. As a refrence I've heard that given equivilant cell construction a Lipoly can supply 2X it's rated capacity in current safely, a Nimh 5X and a NiCad 10X
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