evtlfp20 via EV wrote:
Third, if you were able to charge them, check their internal
resistance. It cannot be the "17-22 meg" that you said. Did you
mis-type, and it's really 17-22 milli-ohms (0.017 to 0.022 ohms)? How
did you measure it?
Hi evtlfp20,
A good battery's internal resistance is very small; tiny fractions of an
ohm. It takes special equipment, or special measurement techniques to
measure resistances this small.
A "milliohm" is a thousanth of an ohm -- 0.001 ohm. "Milli" is
abbreviated "m" (lower-case m). A "megohm" is a MILLON ohms -- 1,000,000
ohms. "Mega" is abbreviated "M" (capital M). So you can see my confusion
when you said your battery was "17-22 meg ohms".
using my icharger 306b it says 18m n omega sysmbol.
I don't know what an "icharger" is. But it has become fashionable (a
marketing gimmick) to put pretend internal resistance scales on battery
testers. Lots of meters sold to auto repair places claim to test
internal resistance as a quick-n-dirty good/bad battery test.
> A 30 amphour cell good for 10C would have to deliver 300 amps.
10 c this is burst current I hope they can do 2 or 3 c steady ..
can you show us the math how this nmc cell could not do this?
Sure!
"10C" means the battery is able to supply a current of 10 times its
amphour capacity ("C"). For example, a 30 amphour car starting battery
that can deliver 300 amps to crank an engine.
But to deliver 300 amps, the battery must have a very low internal
resistance. For example, suppose a 12v car starting battery sags to 11v
wit a 300 amp load for 10 seconds. Then its internal resistance is
(12v-11v)/300a = 0.0033 ohms = 3.3 milliohms. That's good.
Now suppose it sags to 6v under that load: (12v-6v)/300a = 0.02 ohm = 20
milliohms. That's bad! 6v isn't enough to crank an engine or power all
the computers a car needs to start.
PS: This tells you how you should measure internal resistance for yourself:
1. Measure the battery voltage: Preferably with a small load, like 1 amp
to remove the effect of any recent charging. This is the "unloaded"
voltage and current, Vlo and Ilo.
2. Connect a high current load: Preferably 1C, or whatever current you
expect to draw.
3. Wait 10 seconds. The voltage should fall quickly to a new level, and
then stabilize at a slow rate of decline.
4. Measure the voltage again. This is the "loaded" voltage and current,
Vhi and Ihi".
5. Calculate the internal resistance: R = (Vlo-Vhi) / (Ihi-Ilo).
For example, your battery is 12.5v at 1 amp, and 12.0v at 30 amps. Then
R = (12.5v-12.0v) / 30a-1a) = 0.017 ohms = 17 milliohms.
EV batteries generally need to supply high currents for long periods.
You can see that this 17 milliohm battery can't do it.
A good 18650 lithium cell (like you'd find in a laptop) will have an
internal resistance under 3 milliohms. A plain old 6v golf cart battery
is under 5 milliohms.
--
Fools ignore complexity. Pragmatists suffer it. The wise avoid it.
Geniuses remove it. -- Alan Perlis, "Epigrams on Programming"
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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