Re: Request comments on simple solar charger
- From: Bob Monsen <rcsurname@xxxxxxxxxxx>
- Date: Sun, 24 Jul 2005 17:52:58 -0700
larwe@xxxxxxxxx wrote:
I've got an application where I would like to use 12V 7Ah SLA batteries such as Powersonic PS-1270, with a solar charger. My device needs 24V and it is desirable for the system to be able to monitor (coarsely) the state of individual batteries as well as check the availability of solar energy.
(Detail: There are actually two sets of batteries and solar cells, and the system can switch them in and out according to their state of charge).
Anyway: I've sketched a preliminary idea at http://www.larwe.com/solardemo.png. The regulator shown is a placeholder for any old LDO, the idea being to set the output for around 13.6V and let the battery decide how much current to suck.
This isn't an ideal charger by any means, I realize, but it only has to keep the batteries within 70% of nominal capacity for a year. After that, it can die and I don't care.
Does my system look workable? Note that I don't care about discerning the condition "solar good, battery bad" - I only want to know "battery good/bad (solar cell offline)" and "solar cell Vout=good". Also not shown, hysteresis resistors on the comparators, that's a minor detail.
A switcher seems like the way to go. Since you are only using 300C a day, that is 10mA for 8 hours. Thus, you need about 24*10mA / E to run the thing. Assume E = 50%, then that is about 500mW. Assume 1/3 of the days have sun, and you need 1.5W (ie, one day will charge enough for 3 usages, on average.) If you use the cheapo 500mA panels you posted, that means you need 3V to supply this. Since a switcher isn't going to run very well at 3V, buy a few more (they are cheap) to get it up to 5.4V. Assume 10. Then, you have 5.4V at 500mA, or 2.5W, which is more than enough to keep your batteries charged, assuming a bit of self-discharge.
SLA batteries like CCCV charging, which simply means that you don't want to dump a bunch of current into them initially. I've heard 1/10 C as a reasonable figure, so assume 700mA is the max charge they'll take. However, if you charge them at 50mA, you can basically just leave them on forever. You may want to cut the charge when they get to 27.2V (which is the float value). Your call. I think that the cells will last longer if you do, but 50mA is a fairly tiny trickle, so it may not even make a difference.
So, use 10 panels at 0.54V, 500mA each, a switcher chip to get you 24V at 50mA from 5V, and stack the two 12V SLA batteries.
An LT1373 from linear could work nicely, or you could use a microcontroller to build you own with current limiting.
-- Regards, Bob Monsen
If a little knowledge is dangerous, where is the man who has
so much as to be out of danger?
Thomas Henry Huxley, 1877
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