Your math is right, but not your physics. You can't use 100% of an SLA
battery's capacity ... you can barely use 50%. At 20 amps, the 7AH
battery plates are working very hard to push electrons and are being
pushed into inefficient operation. The inefficiency shows up as heat.
If you use larger batteries (say 16Ah), not only will the capacity
increase, but so will the efficiency, because the batteries have larger
plate surface areas. Basically, you're trying to suck molasses through
a narrow straw ... get a bigger straw.
If you switch from SLA batteries, to another type, such as NiCad, you'll
also see better performance. NiCad batteries can deliver almost 100% of
their capacity and, better still, they can deliver current at higher
levels. In fact, back when we ran model warships, we determined that an
SLA battery of X AH capacity could be effectively replaced by a NiCad
battery of X/2 AH capacity. Given that the density of NiCads is less
than SLAs, that's a double weight savings. NiCads do cost more, but
they are worth the extra expense in the long run.
There is also a new breed of SLAs coming on the market which use better
anodes to deliver almost the same performance levels as NiCads, while
keeping the cost close to standard SLAs. I haven't seen them sold in
the mass market yet, but they're definitely on their way.
Frank P.
On 5/31/2011 8:31 AM, Phil wrote:
I've been carrying out some tests on the Hetzer T057. I couldn't work
out why it would run for a short period then stop. I think it might
bge overheating batteries,. But do batteries overheat?I need some
advice on how to extend power.
I carried out some tests over the weekend. The Hetzer is powered by 2
no. 18V de Walt drill motors. Witreasonably charged-up 6V and 12V
batteriies (7.2Ah and 7Ah respectively) connected in series the thing
managed 4 laps of the garden (ie about 195 yards in total) at a lively
3-4 mph, slowing down on the last lap, at which point it stopped due
to lack of power. Motors were lightly warm, but batteries were
noticeably warm, like a hand-warmer. I left them for 10 mins or so and
then we got another 65 yard lap, but which time it was slowing down.
It would then do the same thing- 10 mins break followed by 1 lap- for
another couple of laps or so, by which time voltage was down under 17V
and the batteries were just flat.
I believe these motors are about 180W each, which at 18V means they
are drawing some 20 amps (there is no speed controller so the motor is
either "on" or "off"). Certainly the main fuse (30A) has never blown.
That means that I should get some 20 mins run time, or probably about
40 mins battling, as you're not moving all the time. In fact I'm
getting about 5-7 mins, though there's a lot of friction in the tracks
so I may "lose" the later part.
Can anyone advise on what I ought to do? When battling the tank should
stay cooler inside as the batteries are right next to the gas bottle,
which was warmed by the battieries during my test, but would be cold
when fired and therefore a cooling element in itself. Will cooling
help? I can try some experiments like using ice packs or leaving the
top off (to simulate adding a fan and cooling vents) but I'm not sure
if warm barrieries are (a) normal (b) mean defective batteries or (c)
are the probable cause of the problem.
Thanks guys
Phil
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