i did a search on that topic, but found nothing. so please forgive me if this is common knowledge.
well, most of us use nimh cells from day to day to power their flashlights, digital cameras, mp3-players and so on... and most of us, at some point looong ago 😉, have fallen for the low prices these cells are going for over the net. at least that's how i got these nice aaa's:
as you can see, these are supposed to hold 700mAh of charge and were made in china. i got 8 of them and paid ~10 USD including shipping. after a couple of charge/discharge-cycles none of them made it past 500mAh on my charger, so i figured that this was a 10 dollars lesson learned. so, after another unsatisfactory discharge i pulled them and was decided on trashing them.
out of curiosity i checked one cell with a dmm: open circuit voltage was ~1,1V, slowly declining to 0,9V if put under load with a 50 mOhm resistor. that's the point, the charger will stop the discharge cycle and starts recharging the cell. so far so good. but if you let the cell rest for a couple of minutes, it would recover to 1,1V
.
that's where voltage depression comes into play. the cells still contained a fair amount of charge, but would not release it fast enough. this phenomenon is caused by Ni-use in rechargeables. NiCad's suffered double from that problem and when NiMHs were developed, we only got rid of one part of it. it has to do with the way the Ni is disposed in the cell and the way it will form crystals that won't break up as easily (btw - crystal forming will benefit from slow charging, so fast chargers are better suited for NiMH chemistry, if they reliably detect a full charge).
anyway, the charger would not let me discharge the cells completely, since the load it put on the cells would cause a voltage depression, after which the charger sensed an empty cell and started recharging. so i put the supposedly empty cell in an arc aaa and it would start out bright, but within 30 seconds it's output would decrease until completely dead in about two minutes. but turning it off and on again would work over and over - not a good way to deplete the cell...
so, what to do ?
individual NiCad cells can be discharged to 0V without ill effects. that's what the rc-guys do, if their battery-packs show signs of aging. just connect both poles with a resistor and let the cell sit for a couple of hours (that's for individual cells only, so for battery packs they used a device called "nagelbrett").
NiMHs will be harmed by deep-discharging them, so a simple resistor is not suitable to get rid of the lazy battery effect. diodes to the rescue:
that's a 2.7 Ohm resistor and a 1N4001 diode in series, connected to both poles of the cell (and a nice example of a bad soldering job - but it works nevertheless 😉 ). the 1N4001 has a breakdown voltage of ~0.7V. it will let current pass through, unless the voltage sags under 0.7V. cost of materials is about one dollar and assembly may take three minutes (if your skills with a soldering iron are as bad as mine
).
i let it sit for a day:
that brought open cicuit voltage down to 0.88V and following charge/discharge-cycle got 0.62Ah from that cell
. i will repeat the process a couple of times to see wether it will gain even more capacity. anyway, that's close enough to 700mAh for the price i paid.
so, before you trash old or supposedly crappy cells, it may be worth to give it a try... (i doubt it will work on suspiciously cheap 2.800mAh Tamiyas though 😉 )
chris
well, most of us use nimh cells from day to day to power their flashlights, digital cameras, mp3-players and so on... and most of us, at some point looong ago 😉, have fallen for the low prices these cells are going for over the net. at least that's how i got these nice aaa's:
as you can see, these are supposed to hold 700mAh of charge and were made in china. i got 8 of them and paid ~10 USD including shipping. after a couple of charge/discharge-cycles none of them made it past 500mAh on my charger, so i figured that this was a 10 dollars lesson learned. so, after another unsatisfactory discharge i pulled them and was decided on trashing them.
out of curiosity i checked one cell with a dmm: open circuit voltage was ~1,1V, slowly declining to 0,9V if put under load with a 50 mOhm resistor. that's the point, the charger will stop the discharge cycle and starts recharging the cell. so far so good. but if you let the cell rest for a couple of minutes, it would recover to 1,1V
that's where voltage depression comes into play. the cells still contained a fair amount of charge, but would not release it fast enough. this phenomenon is caused by Ni-use in rechargeables. NiCad's suffered double from that problem and when NiMHs were developed, we only got rid of one part of it. it has to do with the way the Ni is disposed in the cell and the way it will form crystals that won't break up as easily (btw - crystal forming will benefit from slow charging, so fast chargers are better suited for NiMH chemistry, if they reliably detect a full charge).
anyway, the charger would not let me discharge the cells completely, since the load it put on the cells would cause a voltage depression, after which the charger sensed an empty cell and started recharging. so i put the supposedly empty cell in an arc aaa and it would start out bright, but within 30 seconds it's output would decrease until completely dead in about two minutes. but turning it off and on again would work over and over - not a good way to deplete the cell...
individual NiCad cells can be discharged to 0V without ill effects. that's what the rc-guys do, if their battery-packs show signs of aging. just connect both poles with a resistor and let the cell sit for a couple of hours (that's for individual cells only, so for battery packs they used a device called "nagelbrett").
NiMHs will be harmed by deep-discharging them, so a simple resistor is not suitable to get rid of the lazy battery effect. diodes to the rescue:
that's a 2.7 Ohm resistor and a 1N4001 diode in series, connected to both poles of the cell (and a nice example of a bad soldering job - but it works nevertheless 😉 ). the 1N4001 has a breakdown voltage of ~0.7V. it will let current pass through, unless the voltage sags under 0.7V. cost of materials is about one dollar and assembly may take three minutes (if your skills with a soldering iron are as bad as mine
i let it sit for a day:
that brought open cicuit voltage down to 0.88V and following charge/discharge-cycle got 0.62Ah from that cell
so, before you trash old or supposedly crappy cells, it may be worth to give it a try... (i doubt it will work on suspiciously cheap 2.800mAh Tamiyas though 😉 )
chris
Last edited:
