[aos]

At what voltage is a standard NiMh 1.2V cell considered to be "empty"? I seem to recall that it keeps at 1.5V when full, quickly tapering off to somewhere closer to 1.2V and then (when empty) starts falling rapidly. I was thinking that 1.1V might be the point where you'd say that it's time to recharge. I know I should be looking at graphs but I am sure somebody can answer this.

 

[chillysalsa]

Most chargers that have a discharge feature to cycle the batteries & measure their true capacity, will use 1.0 V as the cut-off point before re-charging cycle begins. Discharge is usually under a 50 or 100 mA load, it varies and I'm not sure if there is a more formal standard for applied load.

 

[tangent]

Quote:

I seem to recall that it keeps at 1.5V when full

When a cell isn't being charged, the peak voltage should be 1.4V or a bit less. A cell can have about 1.55V across it while charging.

Quote:

quickly tapering off to somewhere closer to 1.2V

Probably 80% of the run time of a NiMH cell is over a near-linear slope between 1.3V and 1.2V. So, sometimes you'll see the average running voltage stated as 1.25V for NiMHs.

Quote:

I was thinking that 1.1V might be the point where you'd say that it's time to recharge.

The quasi-linear discharge slope continues to about 1.15V, where it starts dropping faster. By 1.1V, it's in a near-vertical dive.

The graphs I have stop below 1.0V. I've seen numbers as low as 0.8V as a reasonable minimum.

I suppose the question of where in the 1.1V to 0.8V range you call "minimum" depends on your discharge rate. This affects the rate at which the voltage drops, which tells you how useful it is to worry about those last few tenths of a volt. In a PPA, the last few tenths could go away in minutes. In a light-drain device, they could buy you a few more hours of run time.

Quote:

I know I should be looking at graphs

Yes, you should.

I recommend the Energizer and Panasonic links on this page: http://tangentsoft.net/audio/ppa/bb/links.html

 

[aos]

Ok, so my assumption that it will be a bit over 1.2V most of the time is correct, as is that 1.1V can be considered empty. What I had in mind would have moderate current draw so no much point in trying to squeeze out every last bit of juice from the battery. I'm basically trying to estimate what voltage to set a regulator to so that it's as high as possible while allowing for extraction of most of the energy from batteries. Say I have 7.2V nominal battery (6 cells of 1.2V) that I want to split in two and have a guaranteed 3.3V or rather 3.4V to allow for dropout on subssequent regulators. If I set it to 6.8V then the cells will be considered empty at 6.8 / 6 = 1.133V. Actually, most regulators at that voltage have lover dropout than 1V (some as low as 50mV) so 6.7V would probably be just as good.

Quote:

Yes, you should

The thing is that the information furnished by manufacturers may not be generic enough so I like to hear from people who have the actual experience, and since I knew there are some here... Besides, I did look at those diagrams before and my comments above were based on what memory I have of those reads. Nevertheless, I found the diagram you talked about, indeed at 1.1V those batteries are pretty much dead. Thanks!

 

[hadron]

For battery packs, it's best to never discharge below 1.0 V per cell, this is a reasonable value to prevent any of the cells from becoming overly discharged. The Maha charger (MH-C777) I have has a discharge mode that uses this 1.0V/cell figure for cutoff.

I've been playing with a 10 cell 1800 mAhr AA pack recently. It starts off at a bit over 14 V fresh off the charger, first few tens of mVs go pretty quick, then it's fairly linear for many hours (my personal data is incomplete here). With a 110 mA load (DacKit + several aux circuits), dropping from 12.0 to 11.0 takes about 80 min, 11.0 to 10.0 takes 15 min, around 10 it plummets.

 

[ITZBITZ]

When I raced electric R/C cars, it was common to discharge NIMH cells to 0.9V during a 5m race. This is at a 20A (yes, 20A) discharge rate though. A pack (6 cells) typically lasted right at 5 minutes, maybe 5:30.

I would think that is the bare minimum before I would cut-off the output to prevent deep discharges.

 

[ppl]

See this site for recomendations on NI-MH battery usage
http://www.sonicstudios.com/batsys98.htm#tips

http://www.myra-simon.com/bike/bike-bat.html

http://www.powerpage.org/cgi-bin/Web...y?newsID=11821

http://ridley.dyndns.org:1494/photo/batteries.html


http://zurich.csail.mit.edu/hypermai...6-08/0393.html

http://www.mtbreview.com/hotnews/products/ppr002.shtml

http://www.dvanhorn.org/NIMH/Index.php

http://pc1-archbo.bot.unibas.ch/~pas...l+charger.html

and last but not least a Battery Eng Speaks out
http://www.vencon.com/index.php?page=support_art3