Is there a way to use a voltmeter to tell how much charge is remaining in an AA Nimh battery?
A freshly charged 2000mah battery is about 1.35v. I often see battery discharge charts like below.
Then do I understand this correct:
When the voltage has dropped to 1.25, about 80% of the charge remains.
On a light with about a 1a draw (the purple line on the bottom chart) the battery has used up 1600ma when it drops below 1.20v , about 85% has been used of the total capacity of 1870ma.
First of all, you shouldn't steal pics from other people, but w/e. Second, those are graphs of Voltage under continuous load. Voltage recovers after load is removed.
By no intent do I claim these charts as my own... only including them for reference... Perhaps I should have only linked.
On the voltage recovery: they do recover but not completely. For example, batteries I've been draining for the past 12 hours now read at 1.20v. They were reading 1.18 when I took them off the load a few hours ago.
It is hard to get much information from a nickel cell when you only know the voltage. They tend to hold a steady voltage even under silly loads - I have sub-C cells that held their voltage constant at an 80amp discharge. At the end the voltage starts to fall like the proverbial dead fish from the seagull's beak. They come off the charger at around 1.4-1.5V but will settle to around 1.3V in a day or so. The voltages on eight Camelion LSD cells I am doing occasional tests on are.
1 Charged 4th December 2010 1.307V
2 Charged 2nd January 2011 1.275V
3 Charged 19th January 2011 1.326V
4 Charged 19th January 2011 1.297V
5 Charged 9th October 2010 1.309V
6 Charged 9th October 2010 1.301V
7 Charged 16th October 2010 1.302V
8 Charged 16th October 2010 1.294V
It is 23 weeks since cells 5 and 6 were discharged - they are due a discharge. The results from this long-term self-discharge test are posted here
http://budgetlightforum.cz.cc/node/382
There isn't really a correlation between voltage and usable charge left in the cell. You can see that the most recently charged cell (Which does not appear to be a weak cell) is actually the lowest voltage of all of them. These cells have never been exposed to real loads, just discharging in my Angeleyes chargers to get the numbers and recharging.