measuring capacity.

How do you measure the capacity of a cell? By draining it as you suggest. The capacity will depend somewhat on the rate at which you drain it. You can find lots of info with a Google search.

If it's a rechargeable, you should fully charge it and conduct the capacity test, and then you'll have an idea of the capacity for future discharges, for a while, though it will "wear out" with each discharge/ charge cycle.

If it's not rechargeable, you can test similar cells, or rely on the manufacturer's data.

With _some_ chemistries, you can get an idea of the state of charge from the voltage, and by monitoring the voltage versus the energy drawn (and likely the temperature, too, since voltage is generally dependent on temperature as well as state of charge), you can have an idea, at least, of the remaining capacity. However, with many chemistries, the voltage is pretty constant over the center of the amount-of-discharge curve, so you get limited info till the battery nears exhaustion.

** That problem ( ie a " fuel gauge " ) has been the bane of all sealed, rechargeable cells since they were invented.

There is NO electrical test that reveals a Ni-Cd or NiMH cells' state of charge - excepting if it is quite flat.

The best effort is to have a uP monitor the cell or pack and compute the mAh consumption since the last full charge and give an estimate of what is therefore likely to be left.

....... Phil

If you go with Lithium, and some research, Maxim-IC (Dalas) has IC's that will monitor the usage of various cell configurations. They also provide free samples of many of their IC's.

Full discharge of an electrochemical cell happens when the oxidation/reduction reaction runs out of reagent. So, it isn't an electrical question at all, but a measure-chemical-components question.

One way is to know the capacity and history of the cell and compute its remaining charge.

Another is to observe changes in the anode or cathode or electrolyte. A auto battery can be evaluated in terms of the specific gravity of the battery acid; the density of the battery acid goes down as the battery discharges. And some rechargeable cells change their output impedance according to charge state - you can draw a little current and look at the voltage drop. Another variation, is to note that charge current causes charging of battery until it reaches full capacity, THEN the charge current just causes heating. A thermometer in the battery pack senses the temperature rise and stops the charge cycle.

The "capacity" of a cell in mAh will vary with how and at what rate you discharge it, and to what voltage to choose to discharge it to. E.g. The advertised rated capacity of say a 2500mAh NiMH cell is only

2500mAh under the discharge load conditions the manufacturer specifies. With any other load and rate it can vary.

Charge gauge IC's are all the rage these days, they can measure the incoming charge (as you charge the cell from flat), and then measure the outgoing charge rate, so you'll have a pretty good idea of how much charge is left in the battery. Although it won't account for self discharge and other losses.

Dave.