Solar power & battery charging

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#1
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One of the projects with which I have been tasked needs to run on solar power. The average power draw would be negligible. However, it would infrequently see brief periods (1-5 seconds) of high-current draw, between 1-4 amps.

The first thing that popped into my mind was 4 AA nicads, hooked (with a diode) to a panel which would provide about 1/30th to 1/50th of the "C" rating of the battery - just enough to make up for the brief power bursts and self-discharge, and not enough to damage the battery.

Now, my question: Will trickle-charging on that sort of level - day in, day out - degrade the batteries noticeably over the normal degredation from time and cycling? A battery replacement schedule of once per year would be acceptable - but that should be a preventative measure, not due to failure.

If that is, indeed, sufficiently kind to the cells, then I will jump with it and run due to low cost and component count. But if not, then I guess I'll have to add some charge-control circuitry.

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A maxwell boostcap is 450 farads, size of a D-cell, costs about $20. It takes a few minutes to charge up on the lab supply at 3 amps. That'll run it! Only 2.5V though.... need to wire 6 PV wafers in series... .45V ea to match the boostcap

Imagecraft compiler user

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Wow... 450 farads. Crazy! It is too bad that the voltage isn't high enough. :(

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Why not use a small Pb accu?
They're cheap, Ri is small, no memory effect and
for charging you can use a simple voltage regulator.

Regards
Sebastian

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Problem with NiCd batteries is memory effect and early failure from over-charging.

Many home solar applications are now using Iron-Nickel batteries. Very tolerant of over charge. Electrolyte is sodium hydroxide. Problem is there is only one source, and that is in China. There are multiple US distributors, I believe. They come in packages like motor cycle batteries and up.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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I've considered a lead-acid, but between weight and (to a lesser extent) a few other factors, a set of AAs would be preferable if they'll work. If they won't, then an SLA would be the fallback.

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AA batteries should work but you will need to be careful about charging. Overcharge can result in serious life reduction. The mode you describe is also an almost perfect prescription for inducing "memory". NiCd batteries need to be fully discharged on a regular basis and don't like to be topped up all the time with a trickle charge. They will rapidly loose there storage capacity.

There may be some NiCd charger ICs out there, but solar input adds a whole new dimension to it that most chargers are not designed for. I would be really tempted to do my own PWM charge controller driven by the micro that is already in the system. Sense the battery voltage and the solar voltage and do some charging only when the conditions are correct.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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The memory effect is much weaker in NiMH accus. But they don't like it cold.

Regards
Sebastian

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Neither hot. And are much less tolerant to overcharge.

Anyway, solar cells are really a new dimension to the system, and definitively should be controlled to match the charge when they offer the required power.

Perhaps two big capacitors, supercap or similar, sized to one or few farads, in series, would be my best choice. Less wear out, easier to charge, no need to control (except maximum voltage, perhaps), and less problems to account for when preventive maintenance. High temperature is also a negative thing, but nothing comparable to any battery chemistry.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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Quote:
Many home solar applications are now using Iron-Nickel batteries. Very tolerant of over charge. Electrolyte is sodium hydroxide. Problem is there is only one source, and that is in China. There are multiple US distributors, I believe. They come in packages like motor cycle batteries and up.

In my youth (last millenium) the milk- and bread-vans (and our lab supply) used to run on NiFe (knife) cells, but they were large, heavy and considered dangerous because of the NaOH. Has anyone used the new ones? Has anyone a link? (Google wasn't very helpful)

C. H.
-------------------------------------------------------------------
It's only waste if you don't use it!

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Quote:
I've considered a lead-acid, but between weight and (to a lesser extent) a few other factors, a set of AAs would be preferable if they'll work. If they won't, then an SLA would be the fallback.

Can you say a bit more about your requirements? The SLA was my first thought. Very simple, and robust, and heavy :-). Good at low temperatures too, if capacity is not an issue. What are your design constraints, since you have obviously considered this avenue?

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Maybe something like the Hawker Cyclon batteries? Good energy density and lead-acid technology in 'D' cell size package.

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Those D-sized SLAs are pretty cool. Once when I was in the battery store, I noticed in the recycling pile that someone had brought in a pack built of no fewer than 96(!) of them in series.

Due to the environment, there is a small chance that some of the units will eventually sustain serious physical damage, and lead gets all the bad rap in the environment. Not that it's undeserved... I'm not sure that nicads are all that good, either, but at least they fly under the radar a bit more.

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FWIW, the solar charged buoys that I programmed some years back certainly used SLA batteries. The lead has to be somewhere, might as well be sealed, less chance of children eating it.

Four legs good, two legs bad, three legs stable.