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I have a battery pack of NiMH batteries. It is ten cells with 1.2V, 4000mAh each, put together in series. So rated voltage is 12V. After charging, i.e. when the charging device says that it is finished, I measure 14.3V.

When I now discharge, at what voltage should I consider my battery as "empty"? Is this the lowest voltage that I can discharge it to without damaging it? Which voltage is that?

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NiMH cells start at about 1.5 V right when fully charged, drop to about 1.2 V most of their discharge life, and are pretty much empty at 900 mV and stopping there is usually safe. 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.

You may think therefore that your 10 cell pack can therefore be discharged to 9 or 8 V, but unfortunately not. There will always be some imballance between cells. If you can measure individual cells, you can go until the lowest cell hits 800 mV, but then stop right away. The cell with the least capacity will get there first. Once it does, it's internal resistance goes up and further current causes the voltage to drop rapidly, causing permanent damage.

10 NiMH cells really should not be put in series without a way to at least measure individual cells. If you designed the battery pack, then you need to fix this. If someone else did, then they are not trustworthy and it would be a good idea to dump them and find someone that knows what they are doing. With 10 cells, it's hard to pick a reasonable stopping point because possible imballance between cells could be significant, especially after a few charge/discharge cycles. Maybe use 1.1 V average per cell, but this is really not a good way to deal with a 10-cell pack.

You will have the same problem with charging. You will have to use relatively low charge current, like maybe C/4 until you think the first cell is near full, then maybe a C/10 or so to trickle charge for a couple of hours so the other cells hopefully catch up. Again, the right answer is to not get yourself into this mess in the first place. Packs with this many cells need to have individual cells measured at least, and the best way is to have some charge ballancing ciruitry. This shunts some charge current around the full cells so that they don't get overcharged while the less full cells catch up. Of course this requires measuring individual cells to know when to enable the shunts per cell.

Again, work with someone or a company that actually knows what they are doing. This sort of thing is a lot more complex than it appears at first glance.

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You got that right... The same thing with LED's they are simple and people run them in series and wonder what happens when you switch then off by shunt fast. The charge storage near zero V makes the high impedance one go negative voltage.. Same with batteries cells in series.. Beware of shorts or fast discharge cycles. YOu need to understand this to know what actually happens. Good points Olin... this is hard to explain with 174 characters left. –  Tony Stewart Jul 16 '12 at 22:47
    
Hmm, ok. I bought a pre-assembled battery pack with the ten cells in series. What you write does not sound very comforting. Unfortunately it is not possible to measure the cells individually, at least not without opening the whole assembly a bit. –  Till B Jul 17 '12 at 7:42
    
@TillB: Is this a custom pack you had this company make for you, or is it one of their off the shelf products? If the latter, give us a link to the datasheet. –  Olin Lathrop Jul 17 '12 at 10:55
    
It is off the shelf. I uploaded the datasheet into my dropbox: tinyurl.com/bpukjow but it is only for a single cell. From this I would read that I can discharge down to 10V (for the whole pack). –  Till B Jul 17 '12 at 12:56
    
@TillB: That datasheet talks only about single cells. I see nothing in there to suggest what a safe 10-cell pack discharge voltage is. Again, this whole thing is a bad idea. Find a manufacturer that knows what they are doing. –  Olin Lathrop Jul 17 '12 at 14:04
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NiMH cells have a value of 1.2V for most of their discharge cycle. Fully charged they are about 1.4V. Since you are getting slightly more I'd recommend checking that they are all at the same potential and slightly discharging the ones above 1.4V before your next charge. I'd recommend not discharging below 1V though I've gone lower a number of times by accident and there was no permanent damage.

For really low current applications you can probably go down to about 0.8, you do have to draw the line somewhere though and I'd suggest not lower than this when using multiple cells in series since the danger exists for some of the cells to be charged in reverse by the others. This damages the affected cells and future charge cycles become unreliable when using delta V charge termination.

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Q For NiMH .. When I discharge, at what voltage is "empty"?

A It depends on discharge rate indicated by a ratio of the capacity, C (Amp-hr) . Most manufacturers (Sanyo and Panasonic for example) measure capacity at a discharge rate of C/5, which is called the five hour discharge rate.

Q Which voltage is that?

A A cut-off voltage of 1.05V/cell is used when discharging at the C/5 rate and 0.9V/cell when discharging at the C rate.

You can use the formula:

V(cutoff/pack)= 1.2V * (N cells-1)

This gives a cut off of 0.9V per cell for a four (4) cell battery pack, 1.05V for an eight (8) cell battery pack.

Q Is this the lowest voltage that I can discharge it to without damaging it?

A For short term, NO, for long term , YES

You do not want to leave the batteries flat for extended periods of time otherwise a corrosion that insulates the plates starts and reversing the polarity from fast series cell discharges can cause damage.

So C discharge rate is aggressive and C/5 is safer. It depends how long you want it to last. as discharge depth x # of cycles is accumulates to the life expectancy under normal temperatures.

In addition lifetime can be reduced faster with temperature rise and thermal sensors to cut-out the charger are the cheap solution to chargers. Heat management and temperature monitor as well as V "empty" thresholds are the features of intelligent chargers.

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If you do not adhere to the above cut-out rules then you need the individual cell sensors and shunt controls, suggested by Olin below to regulate each cell to prevent inverse voltages on any cell that would make the pack fail. This design enhancement extends the range of storage without failure... neglecting individual cell voltage can lead to early failure –  Tony Stewart Jul 17 '12 at 3:57
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