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Photo Forum / Digital Photography / Digital Photo / August 2005NimH batteries
I get very confused about NiMh batteries. On the one hand we are told that you don't have to have them fully discharged before recharging because they don't have a memory. But on the other hand I understand that if you don't have an expensive charger that knows when full charge has been reached and automatically switches off, your batteries can overheat and deteriorate. I think I have hit on a simple solution. I remove the batteries before they are fully exhausted. Then I put them into a torch, switch the torch on and leave it on until the light dies. I then give the batteries a full timed charge as if for first charge. Is there anything wrong with this idea please? Thanks. > I get very confused about NiMh batteries. On the one hand we are told > that you don't have to have them fully discharged before recharging [quoted text clipped - 7 lines] > batteries a full timed charge as if for first charge. > Is there anything wrong with this idea please? Thanks. NiMH have way less memory effect than NiCd, say almost none. Commercials say they don't have it, but they do a bit, so it's good to discharge them before charging, if not every time, just at least once in a while. Sure, it's best to have some good charger. Discharging with torch is an issue, just don't let it to die completely, or at least don't leave them dead too much time (when you see it die, shut off), since it's not a good idea to empty them totally (to zero volts), as in this case reverse charge can occur and damage them. Mostly depends on which type of charger do you have - if it's 14 or 18 hour type it's not that bad if you don't empty them before charging, since charging is slow and most expensive automatic chargers after finished quick charging switch to trickle charging, with current of about 1/20th of capacity (sayabout 90 mA for 2000 mAh battery) and charging with such current is allowed to continue for days, weeks even., But emptying is reccomended if it's faster, say 5 or less cheap one, but in this case most chargers already have electronics built-in. If you have that 18 hour charger, then your application (camera...) will empty them enough so you can put them into charger. > I get very confused about NiMh batteries. On the one hand we are told that > you don't have to have them fully discharged before recharging because they [quoted text clipped - 6 lines] > charge as if for first charge. > Is there anything wrong with this idea please? Thanks. By the time the light in the torch (or flashlight) has died, the batteries are far over-discharged, and that will cause deterioration of the batteries as well - in fact, it's more risky to overdischarge them than to underdischarge them. Overall, you have to ask yourself if it's really worth all of that trouble for a set of batteries. To me, it's not - they're just not that expensive, and the decrease in lifespan isn't all that great. I just stick mine in the charger whenever they're dead, low, or even if I just want to be sure that they're full, and they still last two or three years for me. steve >I get very confused about NiMh batteries. On the one hand we are told that > you don't have to have them fully discharged before recharging because [quoted text clipped - 8 lines] > charge as if for first charge. > Is there anything wrong with this idea please? Thanks. www.batteryuniversity.com will tell you all you want to know .. full discharge will be indicated by a minimum cell voltage of approx. 0.9 V .. a lower discharge may damage the cells .. investing in an "intelligent" charger is worth every penny .. Thanks everyone for your helpful information. I found a cheap and safe way to discharge any NiMh or NiCd batteries down to 0.9V. You must be a little bit "electronically inclined". Needed parts: a battery holder, one NPN 2N3055 transistor, one 1K resistor, one Scottky diode like 1N5817 and one load resistor like 5 ohms or 10 ohms 1/2W. 1) battery (-) and transistor emitter to ground 2) transistor base to battery (+) through a 1K reistor and a Schottky diode 3) load resistor appropriate for the battery capacity from battery (+) to transistor collector The transistor will drive the load resistor until the battery voltage falls down to arround 1.0V. As the battery voltage "tries" to go lower, the transistor turns off and the load is removed. Only leak current remains which is negligeable. Jerry >>I get very confused about NiMh batteries. On the one hand we are told that >> you don't have to have them fully discharged before recharging because [quoted text clipped - 16 lines] > V .. a lower discharge may damage the cells .. > investing in an "intelligent" charger is worth every penny .. >I found a cheap and safe way to discharge any NiMh or NiCd batteries down >to 0.9V. You must be a little bit "electronically inclined". [quoted text clipped - 12 lines] > the transistor turns off and the load is removed. Only leak current > remains which is negligeable. funny you mention this :-) I was playing with a similar idea but ended up in building a full fledged hi-tech discharger using op-amp comparators :-) and a stabilised voltage reference (0.95V) .. just for the fun of it .. :-) |
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