# What is the minimum safe voltage for NiMH batteries, if at all?

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".

• Note, minium voltage UNDER LOAD is different than minimum voltage under no load. You need to know which it is you are looking at or you may damage the battery when you think you are in the safe zone. Apr 17, 2021 at 19:48
• @DKNguyen, Good comment, but this may arise the question: What is the impedance of the load? So when they talk of 0.4V/cell I assume that they mean the voltage under no load. Apr 18, 2021 at 8:56
• Impedance of the cell reduces the voltage seen on the outside the more current it needs to supply. It is just like welding a series resistor to the cell (or an ideal voltage source) and treating the combination as the cell. Apr 18, 2021 at 19:13
• @DKNguyen, My point exactly. The voltage drop of the internal resistance of the battery is proportional to the current the load draws. Apr 19, 2021 at 6:01

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.

• Thanks. So you think by a voltage of 0.4V they mean the voltage when the current is about C/10? Apr 19, 2021 at 6:13
• Yeah. I mean, I am guessing a bit because "slowly" is not very quantitative. But if it was C, then nobody would consider that to be slowly for a NiCd. C/10 is kind of overnight discharge rate. C/100 would be so slow it might take forever just to do it. So I am guessing C/10 or C/20 at the slowest. Apr 19, 2021 at 6:24

To avoid permanent damage from a single discharge cycle, a NiMH should not be discharged below 0 v. This should not be done routinely however, as it's likely to accelerate the normal wear and tear on the cell.

For a single cell, this is obviously trivial to achieve.

In a multi-cell pack, it's possible for the remaining cells to take the first cell to run down to a negative voltage, even with a positive output voltage on the pack. This is why most packs or equipment using them will have a warning or shut-off threshold around 1 v per cell, to reduce (but not completely eliminate) this possibility.

• I would say that ideally battery powered products should be designed to work correctly down to around 0.8V per cell (under load). 1.000V is probably OK. 1.1V is almost too high and might cause you to not get good battery life from NiMH. Apr 17, 2021 at 21:08
• Yes, the key message for OP is that 0.8 V is not a long way from 0 V. The difference in the state of charge is minimal, a cell at 0.8 V will quickly drop down to 0 after just a little bit of extra discharge. 0.8 V is thus a relatively arbitrary threshold adding a little bit of safety margin for cells in a battery. Apr 18, 2021 at 7:51