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Sizing a battery charger

G

G. Morgan

Jan 1, 1970
0
mike said:
Rechargeable lithiums in AA size are a product liability disaster waiting to
happen. Joe sixpack is gonna stick 'em in every AA socket he can find.

I was surprised too. Here is a shot of the battery next to a NICAD
Energizer "AA": http://i49.tinypic.com/npmdcw.jpg

Exact same dimensions, way different voltages!
 
M

mike

Jan 1, 1970
0
I was surprised too. Here is a shot of the battery next to a NICAD
Energizer "AA": http://i49.tinypic.com/npmdcw.jpg

Exact same dimensions, way different voltages!
You're right, they're all over EBAY.

I consider myself to be a knowledgeable, observant and careful
person. But it wouldn't be the first time I plugged the wrong thing
into a compatible socket and blowed something up. I can imagine
Miranda SixPack in a hurry to get that personal appliance back online.
"You've got second degree burns on your what???"

Let me repeat that the typical voltages I mentioned apply to
older lithium ion technology and do not apply to these.

Quick search suggests that LiFePO4 is even trickier to charge
properly than lithium ion.

I have a pile of 18650s from old laptops and a battery tab welder.
I have a computer controlled charge/discharge controller and have
done a bunch of experiments to characterize their behavior.

I still haven't found any application where the benefit of using them
outweighs the hassle of using them.

Still think you're betting on the wrong horse. Lead-Acid
is safer/easier/cheaper for what you're trying to do.
 
G

G. Morgan

Jan 1, 1970
0
mike said:
Still think you're betting on the wrong horse. Lead-Acid
is safer/easier/cheaper for what you're trying to do.

Yeah, I think a gel-cell is the way to go.
 
J

Jasen Betts

Jan 1, 1970
0
I was surprised too. Here is a shot of the battery next to a NICAD
Energizer "AA": http://i49.tinypic.com/npmdcw.jpg

Exact same dimensions, way different voltages!

It's called 14500 when it's a lithium cell, it's possibly 1% shorter
than a nominal AA

if there's a special term for the 3-cell NiCd variant that's about the same
size as an AA I don't know it.
 
J

josephkk

Jan 1, 1970
0
Martin Brown wrote:


These 3000 ma 18650's could do some damage, I agree. I just liked the
idea because they pack so much power into a small container. I thought
if I can keep them charged with cheap solar cells I'd always have a
high-capacity 12v source.
The idea is decent, the thinking it all through needed some help. Any
high capacity battery has some interesting and potentially dangerous
attributes. Not the least of which how it responds to fault currents. If
there is no protection against excessive fault currents you have a recipe
for fire or worse.

Different battery chemistries have different characteristics. Most
batteries dislike to hate complete discharge, but NiCd thrives on it. Few
battery chemistries like float service, but most lead-acid likes it.
Variations in chemistry and cell construction can have surprising effects,
know your cells well before building this.

?-)
 
G

G. Morgan

Jan 1, 1970
0
G. Morgan said:
What is a good formula for the needed minimum current (and over voltage)
for charging batteries? Use a 12V car battery for example, how much
'trickle charge' is needed to keep them alive when not being used?


Thanks for all the input guys. I will have to do more research before I
build this thing now, you've convinced me of a fire hazard. <g>

I may stay away from lithium's and just use a 7-10 Ah gel-cell. I'll
find out how many little panels I can stack to keep a trickle with these
cheap solar panels. It may turn out to be cheaper just to buy some
bigger 5W panels instead of messing with dozens of small ones.
 
M

mike

Jan 1, 1970
0
Thanks for all the input guys. I will have to do more research before I
build this thing now, you've convinced me of a fire hazard.<g>

I may stay away from lithium's and just use a 7-10 Ah gel-cell. I'll
find out how many little panels I can stack to keep a trickle with these
cheap solar panels. It may turn out to be cheaper just to buy some
bigger 5W panels instead of messing with dozens of small ones.
I'm a hobbyist. I've been cobbling together useless crap for half a
century.
I was mostly using stuff I had on hand. If you're gonna buy stuff,
you have more options.
Keep your objective in mind.
If cost has any bearing on the project, there is no cost effective
solar solution. And solar is not portable. You have to find
a location to mount it where it gets sun year round.
If it fell off the window shelf and your battery discharged, you're SOL.
Ditto if you mount it on the roof and the squirrels ate the wire.
Once you screw it to the wall, you lose any portability advantage.
And you'll get so little power in the aftermath of a hurricane,
that you won't miss the connection.

If you're after emergency power, do you really want a system
that has a battery and some wire and a fuse wrapped in duct tape
hooked to some socket wrapped in duct tape into which you plug
some adapter that gets a light working?
It's all about the packaging. I'd want something with a handle
that I could pick up with one hand, flashlight in the other.

I don't recommend this one, but it was easy to click the link:

http://www.harborfreight.com/12-volt-jump-start-and-power-supply-38391.html

I'd get one without the jumper cables but add a 120VAC inverter built-in.
Without the inverter, they can be had for not much more than the
cost of a new battery. And for light loads, you can get cheap
inverters that plug into a cigarette lighter socket; still meeting
the one-hand rule.

And you can charge 'em with something like this:

http://www.harborfreight.com/15-watt-solar-battery-charger-68692.html

Really doesn't make any difference whether it's one watt or five watts.
Neither will be of any real use in an emergency, and either can keep
the battery topped off.

Cobbling together a bunch of solar cells removed from other stuff is
ill-advised
unless you already have the other stuff.
The problem is not the circuit. The problem is getting them soldered
together without breaking 'em. And making sure the flux doesn't
corrode. And sealing them against weather using something that doesn't
block the sun today or turn yellow and block the sun next year.
And keeping the water out so they don't corrode and break the
connection. And finding a way to limit the voltage and current.
You want to be able to have some charge in the winter without
cooking the batteries in summer.

A wall wart solves all those problems. And many more not mentioned.

Designing an electrical system like this is trivial. The devil is
in the details: short circuit protection, connectors, charger, mechanical
details of the package...the one-hand rule.

FWIW, if you believe the graphs published on the battery sites,
you can get 4-ish amp hours out of a 7AH battery at a one-hour rate.
Your car battery can supply this amount of energy without breaking a
sweat...and a LOT more in short bursts. The system weighs a ton, but
it's on wheels ;-)

Looking back on all my hair-brained projects over the years, I wish
I had simplified my objectives, bought instead of built stuff
and spent more quality time with the (ex)wife.
 
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