Battery Life, 50A load discharge test method/results

[Dale(294engr]]

Re: Motors/Battery life

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Quote:
Originally Posted by sanddrag
Sometimes you really have to question what professionals tell you if it just doesn't seem right. The battery life probably would not be sufficient for a wheelchair but most definitely would be for an average FIRST robot. Also, the battery is rated as 18 Amp-Hours capacity, not 18 amps supply. Roughly roughly speaking, it means you can continuously pull one amp for 18 hours or 18 amps for one hour, etc. You should look at the spec sheet http://www2.usfirst.org/2005comp/Specs/batex.pdf for a more specific answer.

Plus, twice the motors doesn't automatically mean twice the current draw. What it does automatically mean, is almost half the loading each, which would mean approximately half the current draw (maybe a little more) each resulting in the total current draw of the drive system being only slightly higher than with only two motors. It is kind of the same concept as walking in snow with stilts or walking in snow with snowshoes. You weigh the same no matter what, but the snowshoes distribute the load better, because they have more area. For a robot, more motors, more load distribution. Less current draw per motor. Only slightly more current draw total.




I've performed continuous 50A load discharge FIRST KOP 18AH SLA Battery tests to 'exhaustion' (9.72v)

The discharge Vbatt Vs time characteristic for this years is different in shape with less time at a lower Vbatt for 2006 Exides Vs earlier years - perhaps slight different electrolyte implementation ? (requiring P/N change?) Ca Vs Pb?
(2003-5 Ca were 13.30v fully charged Vfloat Vs 13.00 to ~31.10 for 2006
else lot deviation on batts we got though BOTH of this years show this.

Specifically, with a 100% Midtronics charge
(achievable only by overnight charging i.e.
[fast charge to ~80% capacity+several hours capoff for remaining 20% TBD]
(100% not likely achieved during multi-chg-dischg during competition =~80%)

Tests results:
9 min life with Vbatt@term 13.10v noload, @50A load quickly down

11.61v then gradually ending at 9.72v at 9 min,

Vbatt@term drops roughly linear, ~2mV/sec

note: Rload is ~constant so current gradually decreased with time, I=V/R
R=11.61v/50A = .22ohm (note: add .5A @11.61v from fan load)

Equipment:

DMM direct to battery terminals,
Two each Harbor Freight 12v 100A Battery/Load testers in Series (~$15ea)
(50A load for longer test time and match typical 2005/6 robot drain)
Used large 12V .5A KOP fan over vent holes to extend test time over
mfr 10 sec limit
(keeps resistor element loads from burning bright red over 9 min test)

Vbatt@terminals taken ea 10 sec & imported to excel spread sheet & graphed

I don't have the data & graphs on this computer but will post, if requested.

BTW this method/data was presented at my FIRST Advanced Electronics 2005 Workshop at Calif State Univ Northridge.

Dale(engr294] (TRW / Northrop Grumman)
Dale.Hall@earthlink.net 310 374-8323