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PICList Thread
'[OT] lead acid vs niCad batteries'
2002\07\19@123828 by sam woolf

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I need to get some rechargeable batteries for powering an art exhibit. The
batteries are powering six constantly on sensor units which each draw 16 mA,
and which occasionally cause small relays to be triggered. The exhibit needs
to be able to operate without power problems for 8 to 10 hours, and can be
recharged at night. Not sure whether nicad or lead acid batteries or
some-other are the most suitable. Would appreciate an outside opinion.. Am
slightly worried about high current possible with lead acid batteries could
somehow fry my electronics, but am also worried that nicad batteries won't
last long enough, or that they will suffer from 'memory effect' and leave
the exhibit conked out in the middle of the day.
Any thoughts?


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2002\07\19@124429 by Lawrence Lile

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A good sealed lead acid battery (Check http://www.mouser.com) should meet your
requirements well.  Don't buy a car battery.

You are not limited by weight IIC.

Cost will be lower for sealed lead acid batteries

Charging a lead-acid battery is really simple and they are hard to destroy

Nicads are finicky, take special chargers, and can die unexpectely from not
charging correctly.  They are also expensive.

OTOH, they are lightweight and store more enegry per Kg.

--Lawrence


{Original Message removed}

2002\07\19@125655 by A.J. Tufgar

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I'd use a lead acid.  The high current capability of lead acids won't be
present because current doesn't change unless voltage or resistance
changes.  They are also usually cheaper.

A good source for them is TV stations, they have to pay to throw out
thier batteries, so after some grumbling from a lazy tech about howe
he's to busy for this, he'll happily hand them over.  These are also in
good condition as a tech once told me the batteries they throw out are
always good.  Because they are not going to risk using a bad battery in
a camera and miss a story.

As for capacity no idea without knowing more about the duty cycle of
your relay and it's current draw.  I'd I'm guessing a camera battery
should be sufficent.  If it isn't, go bigger.  :)  Or get two....

Aaron

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2002\07\19@134023 by Byron A Jeff

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On Fri, Jul 19, 2002 at 11:43:57AM -0500, Lawrence Lile wrote:

I just wanted to agree with Lawrence on all counts.

> A good sealed lead acid battery (Check http://www.mouser.com) should meet your
> requirements well.  Don't buy a car battery.

Agreed and agreed. The only real problem I see below is that the draw is so
light that it won't really cycle the battery at all. I'd advise float charging
it overnight.

The Amp-Hour rating is projected over a 20 hour draw. So most any battery would
have no problem with the 100 to 200 milliamps you're projecting.

>
> You are not limited by weight IIC.
>
> Cost will be lower for sealed lead acid batteries

Yup. There's a recent type called AGM that's extremely simple to use.


>
> Charging a lead-acid battery is really simple and they are hard to destroy

Right on the destroy. However I've been learning more about charging. In
deep cycle applications a three state charger does the best:

Stage 1: BULK CHARGING. Feed the battery as must of the rated current (usually
C/20) until it reaches the trigger voltage of 2.4V per cell (14.4V)

Stage 2: ABSORPTION CHARGING: Charge at the 14.4V rate until the battery is
drawing less than 1/10 of 1% of its rated charge.

That's it. It's now full.

Stage 3: FLOAT CHARGING: Charge at 2.3V/cell (13.8V) indefinitely.

But as I said, the current draw is so light you can probably just float charge
every night (or every other night) and be absolutely fine.

>
> Nicads are finicky, take special chargers, and can die unexpectely from not
> charging correctly.  They are also expensive.

Agreed on all counts.

>
> OTOH, they are lightweight and store more enegry per Kg.

But not worth the hassle here.

As for the high current, simply fuse the battery.

BAJ
>
> --Lawrence
>
>
> {Original Message removed}

2002\07\19@150935 by Rick C.

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Use gel cells and use a smart charger to keep the batteries topped off without
overcharging. Check out A&A Engineering for a good smart charger at:
http://www.a-aengineering.com
Click on "Smart Battery Chargers"
They have a 1 amp and 5 amp unit that will that will treat your batteries
right..
Rick

sam woolf wrote:

{Quote hidden}

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2002\07\19@151958 by Peter L. Peres

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Try NiMh. 1800mAh AA size cells cost $2 in bulk.

Peter

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