NiMh discharging to 0?

[zmoz]

I have a Maha C204F charger...and sometimes I end up using 3 AA batteries. Since the 204F charges in pairs, this leaves me with one battery that I can't charge. Would it be ok to discharge this battery, by itself, and another to go with it, down to 0 volts? If so what size resistor should I use to do this?

 

[shiftd]

NiMH should not be over discharge down to 0V. I think the cut off allowec voltage is .8V, below which a reverse charging occured on the battery and it will reduce its performance greatly. In other words, using a battery down to less than .8V will be more likely kill your battery, sooner or later. My way of dealing with this kind of problem is by using 6 AA batteries. After you use the first 3, charge 2 of the batteries and keep one on a cool dry place /ubbthreads/images/graemlins/smile.gif . And then after you use the second 3, charge the 3 and the other 1 from the previous batch.

 

[zmoz]

[ QUOTE ]
shiftd said:
NiMH should not be over discharge down to 0V. I think the cut off allowec voltage is .8V, below which a reverse charging occured on the battery and it will reduce its performance greatly. In other words, using a battery down to less than .8V will be more likely kill your battery, sooner or later.

[/ QUOTE ]

But I thought that was just when they are used in series?

 

[LEDagent]

I use to have the C204F charger myself until i purchased the new C401FS charger that charges the cells individually.

I don't know exactly how the C204F chargers, but i think it senses a maximum total voltage for each indivudual battery before turning green, indicating a full charge, however, since it charges only in pairs, i think it will turn green once one cell is fully charged regardless if the other cell is not completely charged. But in trickle mode, i think there is enoegh current to continue charging the cells until they are topped off. If the voltage difference isn't too great between the two cells, it may be fully charged in about 8-16 hours. ....but i may be wrong.

I say you invest in the new C401FS charger if you constantly use odd sets of batteries. It became essential for me to have a charger that charges individual cells when i started using equipment that needed only 1 or 3 batteries at a time. I think it is worth it because i have found that some batteries that i DO use in pairs actually needed a longer charging time in the C401FS, indicating that the 204F was not giving a complete charge to all my batteries over a long period of time. I feel as i have extended the life of some of my old batteries.

Also, i don't know if it is bad to fully discharge an individual NiMH cell. Shiftd's theory is correct, but i think it only applies to the discharge characteristics of 2 or more batteries in series, that is why many manufacturers caution at fully discharging NiMH battery packs.

 

[shiftd]

Well, if they are used individually, then the batteries themselves won't be reverse charged. But, you were asking if it is ok to "discharge this battery... and another to go with it, down to 0 volts?" are you not? Well, if you are using other discharged battery to go along with another discharged battery, then isn't it the same case as using them in series (or paralel, does not make any big difference)?

 

[ElectroCaver]

NiMH's don't like to be discharged much below one volt. Definitely not good to pull them down to zero. If it's a good quality cell, doing it once or twice probably won't kill it, but it'll shorten it's cycle life. The size of resistor you use will depend on how fast you want to discharge it. Discharge rate is usually stated as a ratio of the mAH capacity of the cell. For example, a 1/5C discharge rate for a 2000mAH cell is 400mA's for 5 hours (2000mA / 5 = 0.4A). Use Ohm's law, R = V / I, to figure the ohm value of the resistor. The nominal voltage for MiMH's is 1.2 volts. So, for the 400mA 1/5C example, R = 1.2V / 0.4A or 3.0 ohms. Power (P) = V * A. Using our example, 1.2V * 0.4A = 0.48 Watts, so a half-watt (or bigger,) three-ohm resistor would work fine. It's best to use a resistor that will discharge the cell at roughly the same rate as the device you use it in, but if it's a LED light you might not want to wait that long. If you can find the spec sheet for the cells you're using, lookup the maximum recomended discharge rate and stay well below that. Use a voltmeter to monitor voltage during discharge and stop discharging when the voltage matches the other cell of the pair (which was measured using the same load resistor you use for discharging). Then charge the pair. It's probably best to forgo all of this and get a charger with independent (single cell) charging capability. If the device you're using the cells in automatically powers down when the voltage is too low, then you could use a multiple of three cells, say six, charge them up, run both groups of three cells through the device, and then charge the three pairs. This will work because the device will shutdown at a predetermined point, and all your cells would stay roughly matched. I use paint to color code my matched sets of NiMH's. But again, your best option is a charger that doesn't require cells be charged in pairs. I love my MAHA 401.

 

[batterystation]

1.05 volts is what Motorola considers DEAD for a nicad or metal hyride cell. Metal hydrides hate being run down at all, but to kill them (zero) is deadly to them. There is a good Rayovac charger that Wal Mart sells that charges 1 to 8 at a time. Great charger!!! About $20 or so.

 

[Lurker]

Getting a smart charger that can charge individual cells is definitely the way to go. There is also a $10 Rayovac smart charger at Walmart that charges 1-4 AA or AAA, so it is really just a question of how much you want to spend. I even saw it packeged with a pair of free batteries recently.