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[DPRG] charge and forget

Subject: [DPRG] charge and forget
From: Ed Paradis legomaniac at gmail.com
Date: Tue Mar 30 11:27:58 CDT 2010

And I guess I've come full circle.

My original idea was to build a small NiMH pack with an integrated
charger that'd keep me out of trouble for over charging.  (Can you
spot the "fire and forget" design requirement? :D )

It sounds like for these small LiPos, I can charge at a reasonable
rate with a temperature monitor on the battery for emergency shut off
and sleep well at night.

Really, the NiMH would charge the same way.

I suppose one of my reasons for moving away from NiMH was the long
charge time, but if I say "reasonable charge rate" with a LiPo, I've
lost the benefit of using LiPo, since NiMH has no memory effect and I
should be able to charge at will.

It sounds like I would benefit from concentrating on a generic charge
management system that supported enough brains to charge various
chemistries.  Ok, just NiMH and LiPo.

Here is the algorithm for LiPos: http://www.powerstream.com/li.htm
Requires current monitoring and voltage monitoring and some brains to
switch betwixt the two.

Here is the algorithm for NiMH: http://www.powerstream.com/NiMH.htm
Requires voltage monitoring but only current regulation and some
brains to shut things down.

Both require temperature monitoring for unattended charging.

Ah! One benefit of not using NiMH is cell balancing.  For a 3.3v
system, a single LiPo will suffice with one of the Pololu switching
regulators I love so much.

Right now I use three NiMH in series, and theoretically I'd want to
cell balance them.  You know and I know that no one does that and I'm
not looking at charging fast enough to cause troubles.  Temperature
monitoring would catch trouble if you were to mount the thermistor
such that the three NiMH cells surrounded it.

As far as Lead Acid batteries, my robot is significantly wussier than
yours, David.  With a lead battery I'm afraid I'll be tearing treads
off and putting a good deal of wear on the Tamiya gear box.

Beating my head against the same problem again,
Ed

On Tue, Mar 30, 2010 at 10:36 AM, David <dpa at mail.isem.smu.edu> wrote:
>  I spent a lot of time doing battery comparisons when we built
> jBot, and settled on NiMH (two 12 volt packs) as the best compromise.  They
> are safe and truly charge and forget.  They can be topped of (no "memory
> problems" like with NiCads) and provide all kinds of current when needed.

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