When you buy lithium cells (most EVers use LiFePO4), you might get lucky. The vendor may have shipped 100% good cells,
all at the same state of charge, and all with the same self-discharge rate.
However, this has never happened to me. I have always found large variations in the initial state of charge (even
though the cell voltages are the same), and there have always been weak or defective cells in the batch.
So, it is only prudent to test them. You don't need expensive equipment; it only requires time. If the cells are going
to be stored for a while, for example while you complete your EV conversion, then you have lots of time!
Actually, you're only going to spend about 5 minutes per cell on this test. The rest of the time is just waiting
for the cell to discharge, and then waiting for it to recharge. All you have to do is check it now and then, and write
down the results at the beginning and end of the test.
- An adjustable regulated power supply that you can set for 3.6v. Anything from 1 to 10 amps will be fine. Get one with meters
that show volts and amps.
You'll find thousands of power supplies on eBay for $50 and less. I prefer older used name-brand supplies to
new, junky ones. Name brand supplies will be more accurate and will last longer. That's good, because you'll find
lots of uses for your supply once you have it.
- A load resistor. This can be anything that draws 1-10 amps at 3.3 volts. I like light bulbs, because they draw a roughly
constant current. A couple of 12v car headlights will work fine. They barely light at 3.3v, but that makes no difference.
- A SPDT relay, with contacts that have enough current capacity to switch the load and charger current, and a coil that
drops out at about 2.5v (the desired end-of-discharge voltage). A common 12v automotive relay will work. Use your
adjustable power supply to test its pull-on and drop-out voltages. It
will probably pull in at 6v-9v, and drop out at 1v-2v. Add cheap common diodes (1N4001 etc.) in series with the coil to
raise its dropout voltage (0.6v per diode) to 2.5v.
- A timer. The cheapest and easiest is an analog clock (the kind with hands) that's powered with a single 1.5v AA cell.
- A reed switch. It's a little glass tube with a wire on each end. They're sold at Radio Shack, and burglar alarm sales and
service stores. This is the gadget you attach to a door or window, with a magnet on the moving half. When a magnet (or
magnetic field) is near it, the switch closes. When there is no magnetic field, the switch is open.
- Connect your power supply to your load resistor. Write down the current it draws. This will be your discharge current.
Disconnect the load from the supply.
- Connect the cell positive to the relay's common contact.
- Connect the normally-closed relay contact to power supply positive.
- Connect the normally-open relay contact to the load resistor.
- Connect the negative of the power supply, load resistor, and cell all together.
- Connect the coil (and however many diodes you need to reach 2.5v) across the load resistor.
- Connect the reed switch in series with the AA cell in the clock. One way to do this: remove the AA cell, and tape one wire
from the reed switch to the AA cell's positive end. Hold the other wire from the reed switch against the positive contact
in the clock's battery holder. Now insert the AA cell. The tape on its positive end should hold the wires in place,
but keep them from shorting to each other. Test to make sure that the clock only runs when a magnet is held on the reed
- Wrap one of the wires to your load resistor around the reed switch a couple times. This creates a magnetic field when current
is flowing to the load, so the clock runs while current is flowing.
This setup automatically recharges the cell after a discharge test. It's also intrinsically safe for the cell, even if you forget
and leave it for days. It automatically turns off the load before the cell gets too deeply discharged. And because the charging
voltage is regulated to a value that's safe for an LiFePO4 cell (3.6v), it won't hurt the cell even if it's left on charge for
- Set the power supply for 3.6v (or whatever you want your fully charged voltage to be). The cell will charge at as
much current as the power supply can provide. Any value a normal supply can produce will be safe for the cell. If anything,
it's the power supply you should worry about. A cheap one, especially, may have trouble providing its maximum current for many
hours, and could burn out.
- Let the cell charge until the current falls to less than 1% of the cell's amp-hour capacity, expressed in amps (i.e. under 1a
for a 100ah cell, 0.1a for a 10ah cell, etc.) This is not critical, and may take a day or more. For example, a 2 amp power
supply will take 50 hours to fully charge a dead 100ah cell! So keep checking back once or twice a day, looking for a current
- When the cell is fully charged, manually push the relay contact closed with your finger. If you can't see the contact (i.e.
the relay case has a cover), either remove the cover, or momentarily touch a 9v transistor radio battery across its coil
to make it pull in.
When it pulls in, the relay
- disconnects the power supply (i.e. stops charging).
- powers its own coil, so it stays pulled in.
- connects the load.
- starts the clock.
- Set the clock's hands to noon. The clock will run as long as the relay is pulled in and the load is being powered. Thus,
it will record the elapsed time.
- Check on your test at least once every 12 hours. If it has shut off, write down the elapsed time. The cell's amp-hour
capacity is the elapsed time multiplied by the load current. For example: Your load draws 3 amps. The clock started at
noon and stopped at 10:00 (10 hours). Then the cell delivered 3a x 10h = 30 amphours.
- If the clock is still running when you check, write down the elapsed time so far, and set the clock back to noon. With a
big cell and a small load current, it could take more than 12 hours to fully discharge it. For example, it takes
3a x 20 hours to discharge a 60ah cell. So you might check and reset the clock twice (reset to noon at 11 hours,
then found it off with 9 more hours when you came back in another 10 hours). So the cell capacity is 3a x (11h + 9h) = 60ah.
This testing method is slow, but it's cheap! And in the end, you'll know what you really have, instead of having to rely
on the claims of battery salesmen.