bbs00068@uafcseg.uucp (Joel Kolstad) (12/08/90)
[I posted something similar to this several weeks ago, and didn't get any
responses... so here I go again... :-) ]
I'm currently making a temperature logger that will eventually float around
somewhere off the cost of Florida. It contains the analog temperature
sending circuitry, a microcontroller, and a radio transmitter. Every now
and then (at about 10 minute intervals), the microcontroller is turned on
and transmits a bunch of data back to shore. So far, so good.
This all works fine sitting on a bench here in Wisconsin powered by a nice
regulated power supply, but once it starts floating, it's only going to have
solar cells to power it. I'm trying to decide whether to to use NiCad or
GelCell batteries in the thign, but I really don't know that much about them.
Here's what I (think) I know:
NiCads -- Can be overcharged all you want. For the space, you get less a*H's
then with Gel Cells. Major problem: They form memories. In an
application like this, where I charge for awhile, drain (probably)
a very small amount of the capacity of the cell, and they recharge,
it seems to me that I'm almost trying to create a memory on
purpose. Since the reason NiCads are there to begin with is to keep
everything going on cloudy days, if they form memories, I'll be
dead in the water (no pun intended :-) ) if there ever IS a cloudly
day. Finally, NiCads are simple to charge... Solar Cells to diode
to resisitor to batteries. Don't mind being deep cycled.
GelCells -- Apparently partially damaged is overcharge too much or deep
cycles more than a few times. However, DON'T form memories. Hard
to charge properly -- require a regulated charging. Considering
that, I'd have to have a circuit powered by the batteries just to
recharge the batteries. :-( This means that if the batteries ever
get so low of a charge that I can no longer power the regulator
circuit, the batteries will probably stay discharged! Arghh...
That's about it. I'd love to hear from somebody who's designed this kind
of stuff before... 'cuz this is my first time! I'd love to hear somebody
tell me that NiCads don't form memories! (Because besides that, they're
just about perfect, it seems. Whereas getting GelCells into a workable
setup would seem to be a royal pain!)
Sitting right across the batteries is a CMOS timer that goes to a MOSFET.
this is what determines when power is applied to the microcontroller, etc.
The microcontroller is then responsible for shutting itself off. With this
setup, if I used NiCad batteries and they were totally drained for some
reason or another, the batteries would still have a good chance of being
recharged given enough sunny days.
For those interested, all of this stuff is fairly low power -- each time
the circuit is powered, is draws about 10maH (the microcontroller
squashes the data together and then burst transmits it -- so evyerthing is
only one for a matter or seconds). We're looking about batteries in the
1aH range.
Any help is appreciated. I really want to design a system that WORKS!
Please respond on the net, or e-mail me at kolstad@cae.wisc.edu. Thanks!
---Joel Kolstad
kolstad@cae.wisc.eduraoul@eplunix.UUCP (Nico Garcia) (12/10/90)
In article <5664@uafhp.uark.edu>, bbs00068@uafcseg.uucp (Joel Kolstad) writes: > GelCells -- Apparently partially damaged is overcharge too much or deep > cycles more than a few times. However, DON'T form memories. Hard > to charge properly -- require a regulated charging. Considering > that, I'd have to have a circuit powered by the batteries just to > recharge the batteries. :-( This means that if the batteries ever > get so low of a charge that I can no longer power the regulator > circuit, the batteries will probably stay discharged! Arghh... Hmmm. I won't address your NiCad questions, since I've never worked with those directly. However, for a Gel-Cell charger, use a LM317 rated for your maximum voltage from your power supply to provide a regulated charge voltage, and drop a resistor on the output of it to limit the current to your batteries. You only need a trickle charge anyway, and a 12 Volt supply, even if you put the battery in backwards, will only put a 24 Volt drain on it. Put a 1 K resistor there, and a diode to stop discharges, and you win. -- Nico Garcia Designs by Geniuses for use by Idiots eplunix!cirl!raoul@eddie.mit.edu
raoul@eplunix.UUCP (Nico Garcia) (12/10/90)
In article <985@eplunix.UUCP>, raoul@eplunix.UUCP (Nico Garcia) writes: > In article <5664@uafhp.uark.edu>, bbs00068@uafcseg.uucp (Joel Kolstad) writes: > >[questions about gel cells] > Hmmm. I won't address your NiCad questions, since I've never worked with > those directly. However, for a Gel-Cell charger, use a LM317 rated for > your maximum voltage from your power supply to provide a regulated > charge voltage, and drop a resistor on the output of it to limit the current > to your batteries. You only need a trickle charge anyway, and a 12 Volt > supply, even if you put the battery in backwards, will only put a 24 Volt > drain on it. Put a 1 K resistor there, and a diode to stop discharges, > and you win. Sorry, accidentally sent this off before checking your power demands. For a 12 Volt supply, with a 10 mAH drain once per hour, you want a recharge of oh, say, 100 mA to charge quickly: and 1/10 your maximum AH capacity as recharge current is 100 mA for 1 AH gel-cells. So, figure a mimimum 1 Volt differential, and a 10 Amp resistor is needed. 12 Volts for a completely discharged battery, and you get almost 15 watts heat dissipation. Gaah, a bit large. You could use a second LM317 as a current limiter. The National books have good application notes on this. Depends on your size and circuitry constraints. Don't forget a 3-Volt drop and minimum currents for good regulation. I've had very good luck with gel-cells left on trickle charge. The Yuasa 12-Volt 0.8 AH have behaved very well, even if their connectors are a bit weird. -- Nico Garcia Designs by Geniuses for use by Idiots eplunix!cirl!raoul@eddie.mit.edu
myers@hpfcdj.HP.COM (Bob Myers) (12/11/90)
>NiCads -- Can be overcharged all you want. For the space, you get less a*H's > then with Gel Cells. Major problem: They form memories. In an > application like this, where I charge for awhile, drain (probably) > a very small amount of the capacity of the cell, and they recharge, Nope. Exactly backwards, I'm sorry to say, but this is the common misunderstanding regarding NiCds. I've posted some articles with more detail (and will re-post or e-mail if there's enough demand), but the basic story is that the problems of NiCd "memory" are vastly exaggerated; a true "memory" problem requires some very specific and unusual circumstances, and very, very, rarely happens in real-world applications. (So seldom that one is tempted to use the word "never.") The real problem with NiCds, and what usually gets labelled "memory", is actually called "voltage depression" and occurs when the cell has been overcharged. When overcharged, the cell voltage will drop sharply fairly early in the next discharge cycle, causing the *appearance* of a loss in cell capacity (especially in equipment which monitors cell voltage to trigger a "low battery" light or some such). The cell will recover undamaged if it is properly charged in the next cycle. Unfortunately, the vast majority of NiCd chargers, such as for consumer rechargeable equipment, are too simple to detect the completion of a charge cycle, and will happily continue to fill the cell well into overcharge. Thus, if the user puts the cell in a charger after only a partial discharge, the cell is very likely to go into overcharge and exhibit voltage depression on the next discharge cycle - giving rise to the notion that it has developed a "memory", even though the depth of discharge is NOT really the problem. (This has, in turn, led to people constructing "deep dischargers" and other silly things to attempt to combat a non-existant problem, when in reality all they're doing is wasting a charge.) What you really want to say about NiCds is that they can be left on "trickle charge" forever, but only if the charger is smart enough (or the trickle current low enough) so as to keep the cell from overcharging. Sealed lead-acids also trickle-charge nicely, and aren't quite as finicky about a slight overcharge. (You do not, of course, want to overcharge them severely either.) Gel-cells would probably be my choice for such an application as the one described, but in EITHER case I'd definitely want to invest a little effort on the charging system. Bob Myers KC0EW HP Graphics Tech. Div.| Opinions expressed here are not Ft. Collins, Colorado | those of my employer or any other myers@fc.hp.com | sentient life-form on this planet.
scott@hpcvca.CV.HP.COM (Scott Linn) (12/13/90)
/ hpcvca:sci.electronics / myers@hpfcdj.HP.COM (Bob Myers) / 11:51 am Dec 10, 1990 / >Sealed lead-acids also trickle-charge nicely, and aren't quite as finicky about >a slight overcharge. (You do not, of course, want to overcharge them >severely either.) Gel-cells would probably be my choice for such an >application as the one described, but in EITHER case I'd definitely want to >invest a little effort on the charging system. My experience exactly. Also, it has been much easier for me to find correctly designed gel-cell chargers than correct nicad chargers. I have been using gel-cells during cave exploring for 5 years now, and they work great. I can leave them on my charger continuously between trips, without any problems (I have been using 3 packs for 5 years now, 2-3 packs per trip). Scott Linn
wb8foz@mthvax.cs.miami.edu (David Lesher) (12/13/90)
{Gel-Cell vs Ni-cad vs whatever else}
Note there are two different families of Gel-Cells:
float and cycle
It's worth getting the data sheets from {Globe? I'm still
asleep...} and ordering the right ones for your job.
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