batteriesnimh
Is there a minimum voltage below which a NiMH battery shouldn't be discharged to?
One of the contributors on this website writes
800 mV is where you definitely want to stop to avoid damage. There is so little energy left at that point that there is no benefit in draining the cell further anyway.
Nevertheless, the website www.batteryuniversity.com claims that to "restore" a Nickel-based battery it should be discharged slowly to "about 0.4V/cell and lower".
I've built several products around different chemistries. I have found LiPo the easiest to use since there are already specialised charger IC's that do all the work however, like you said, having them in series is not that straight forward. I believe the NiMH is a better solution if you don't have a lot of experience with chargers or if you don't find a good solution for charging LiPo in series.
Just one note, do not trickle charge NiMH indefinitely, use a timer at least to avoid overcharging the battery too much(even when you trickle them, they overcharge). You could also use, like Pwocky suggested, an LM317 to charge at a constant current and monitor the change in temperature in the battery pack, once the delta T becomes big enough (the temperature changes more rapidly) the battery is charged. You could also mix this method with delta V and a safety charge timer. This is what I do and it works perfectly. I charge my batteries fast and never overcharge them.
There is a lot of info online with graphs showing how the temperature changes when charging a NiMH battery.
You have to estimate or use the discharge graph from the battery specification. Different manufacturers may provide different graphs.
The picture below is from a modern Sanyo 2700mAh Eneloop battery datasheet. The manufacturer gives us a capacity vs cell voltage graph:
It looks like 2000mAh should be available above 1.2V if you are discharging with 540mA. There is no 300mA discharge curve, but we can assume that will be similar or better than the curve for 540mA.
We can therefore estimate that 2000mAh/300mA = 6.6h.
For a GP NiMH battery that I randomly found, it gives a voltage vs discharge time graph which gives you the answer directly:
These are the datasheets I used for the pictures and calculations:
http://www.gpina.com/pdf/GP240AAH_DS.pdf http://www.eneloop.info/fileadmin/web_data/Data-Sheets/HR-3U_2700_.pdf
Note: Sometimes datasheets provide information about high discharge currents, but I don't think these very high currents are normal. You probably can't drain so much continuously from battery because of heat generation.
Best Answer
I read that link. First of all, when the authors say to discharge to 0.4V, they are talking about NiCd batteries which are uncommon nowadays. It is not clear from the article if they intend that advice to also apply to NiMh which is what Olin was discussing on this site.
Also note that this is suggested as a measure to be employed on a damaged cell. Not a normal lower limit for routine charge cycling. Olin was talking about routine limits, not limits for attempted revival of damaged cells.
But, to answer your question, since they say discharge "slowly," you can assume that the discharge rate is C/10 or something like that. Just in case you don't know, C/10 means capacity (in amp hours) / 10. The units don't really work out. But for a 1000 mAh battery, C = 1000 mAh. So C/10 = 1000/10 = 100 mA.
So the impedance would be whatever is needed to get a current of C/10. It would probably be OK to use a current anywhere from C/8 to C/20. Otherwise it really wouldn't be slow discharge.
I also agree with the other answer, which I upvoted. But I wanted to take a stab at explaining the discrepancy you highlighted in your question between what Olin said and what you read at Battery U.