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Old Oct 29, 2014, 01:41 PM
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chickenLittle's Avatar
Joined Jul 2014
16 Posts
Question
LiPo voltage current controller??

Am planning to use some muscular LiPo batteries to power a small boat running
on one of those electric trolling motors. To be clear, this boat will move me
around. It's not a model.

Have six batteries, 22.2 V and 5 amp hours so I should have enough power.
But don't want to suck up too much current or drop the voltage down to the
point where I destroy the batteries

Can you suggest an off the shelf controller or a circuit that I can build to limit current draw and warn of impending low voltage?

Tanks,
DOC

Have robots. Will travel.
http://www.robot-one.com
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Old Oct 30, 2014, 04:05 PM
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Joined May 2012
102 Posts
Will you be connecting the batteries in series or parallel?

A LiPo Cell capacity is measured from full charge at 4.2V to full discharge at 3.0V. But prevailing wisdom is that you should not discharge to 3.0V, it accelerates wear out and the battery will not last as long as it might otherwise last. The right minimum cutoff voltage is a bit muddier. Typically, the number people quote is 3.7V/Cell. However, 3.7V is the nominal voltage of the Cell, meaning you've only used about 1/2 the capacity when you hit this threshold. I typically fly my models down to 3.4V/Cell. This gives me a few 10%s of more flight time.

A simple voltage comparitor is all you need. Maybe a yellow LED that lights up at 3.7V/Cell and a red one when your pack hits 3.4V, and a cut-out at 3.3V.

Limiting current is a different problem. But a 5A/H battery is probably at least 30C, which means you're OK pulling as much as 150A. I'm assuming you're going to rig the batteries in parallel, so you've actually got a 30A/H pack, which at 30C is good for 900A. I doubt you're going to pull that much amperage, so current limiting is probably not necessary. 900A * 22.2V = 19KW = 25HP. That trolling motor is probably 5HP...
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Old Nov 04, 2014, 03:57 PM
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chickenLittle's Avatar
Joined Jul 2014
16 Posts
Thanks for the suggestions.

I'll be using them in parallel. Prolly two banks of three batteries.

Easiest/cheapest idea is to use one of the lipo testers that cost five or
ten bucks.

Can't see me pulling more than bout 50 amps so just need to keep an eye the voltage.

FWIW, the plan is to use this setup on a small lake and run various RC boats and planes.
So if a plane crashes I "speed" over and retrieve it before it gets TOO soggy :-)

Water's getting a bit cold now and will freeze up soon. So am expecting most of this will only happen next summer.

.DOC
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