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Unread 18-12-2010, 12:43
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Ether Ether is offline
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Re: Warming up the batteries

Quote:
Originally Posted by Ether View Post

Updating previous post with new info:

Quote:
- Don suggested applying a 12 amp load for 15 seconds and measuring the voltage under load. Above 10 volts is Very Good. 29
It was apparently a typo. Don meant 0.01 ohm, not 1 ohm. [edit]Don did not mean 0.01 ohm. see his new post.[/edit]

But with 10 volts across a 0.01 external resistance, the load would be pulling 1000 amps... and that raises two problems:
1) Fifteen seconds of that could potentially damage the battery or lessen its life. It would also probably burn up or damage a 25 watt 0.01 ohm resistor.

2) The battery can't supply 1000 amps at 10 volts. Its internal resistance is too high*.
Perhaps the resistance of the copper wire is increasing due to heating. Let's say the wire gets boiling hot (100C). That's a dT of 80C, which would increase the wire's resistance from 0.01 ohm to 0.0132 ohm. At 10 volts, that would be 758 amps. Still too high.

Perhaps the test depends on additional external resistance, maybe from a switch that is used to complete the load circuit? (I hope someone is not just holding the wires and jabbing them into the battery terminals )

Don, do you guys have a clamp-on ammeter? Could you record the actual current draw next time you do this test and post the results?


Bottom line: Don's test is certainly severe enough to challenge even a brand-new battery. I worry a bit about safety, though, and battery damage. Do not attempt this without competent training.

Quote:
- Ether cited the battery discharge graph in the datasheet for the FRC battery which shows 12 volts after 16 minutes at 18 amps. 31

- Joe stated that the voltages in the datasheet graph are taken open circuit. 33
Still not 100% resolved, but I think this is one of those rare times that Joe is mistaken.

Quote:
- Al stated that he believes the datasheet graph is showing load curves 34, and that to his knowledge the CBA graphs are load curves too. 36

- Al's 7 amp graphs show a healthy battery still reading over 12 volts (under load) after 43 minutes at 7 amps. Even a very sick battery (blue line) still reads over 12 volts (under load) after 17 minutes at 7 amps. 30
All true, I think.


*the internal resistance of the battery drops as the temperature increases, but not enough to allow 1000 amps without damaging the battery.



Last edited by Ether : 18-12-2010 at 15:28.