# measuring capacity.

Discussion in 'Electronic Design' started by [email protected], Jun 13, 2007.

1. ### Guest

how do you measure the capacity of a cell? expressed in mAh. I suppose
you can drain it at a constant rate until it's empty, but what if i
want to know the remaining cap without draing the battery?

2. ### Tom BruhnsGuest

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.

3. ### Phil AllisonGuest

** 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

4. ### homermandohGuest

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.

5. ### whit3rdGuest

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.

6. ### David L. JonesGuest

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.

7. ### neon

1,325
0
Oct 21, 2006
you answer your own question discharge at a constant rate into a load EMPTY that is not a good idea the battery if old may never recover again.