Charge battery from solar panel

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I've got some 10W solar panels with a Voc of 21.5V and i wish to charge a 6V12Ah lead acid battery.

I've looked at a load of battery charge ICs, but these all use a voltage source, which the solar panel certainly isn't.

Any ideas of how to deal with this?

Last Edited: Tue. Jun 1, 2010 - 06:54 PM
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lagger wrote:
I've got some 10W solar panels with a Voc of 21.5V and i wish to charge a 6V12A lead acid battery.

I've looked at a load of battery charge ICs, but these all use a voltage source, which the solar panel certainly isn't.

I think a solar panel qualifies as a current-limited voltage source.

Terminology aside, the simplest solution would be to charge three of those batteries in series for a total of 18V which would not be very far from the maximum power point of your panel. A simple protection diode for the panel and you'd be good to go I think.

If three batteries in series is not an option, then you probably want to look into a step down DC-DC converter and take it from there. I know Maxim and a couple of others have app notes out on this, you might want to check that out.

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"Dare to be naïve." - Buckminster Fuller

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Achilles wrote:
I think a solar panel qualifies as a current-limited voltage source.
To be precise this is one SPICE model of series photovoltaic cells (a current source):
d1 n001 neg dpv
i1 neg n001 0.9
r1 n001 pos 0.5
r2 neg pos 1000000
.model dpv d(is=1e-011 11 n=85.6993033770364)
c1 neg 0 100n rpar=10meg
.end
This was copied from LTspice user group (may have to be a member to read these files):
http://tech.groups.yahoo.com/group/LTspice/files/%20Lib/PVcell/
http://tech.groups.yahoo.com/group/LTspice/files/%20Lib/Solar%20Cell%20Module/

"Dare to be naïve." - Buckminster Fuller

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gchapman wrote:
Achilles wrote:
I think a solar panel qualifies as a current-limited voltage source.
To be precise this is one SPICE model of series photovoltaic cells (a current source):
d1 n001 neg dpv
i1 neg n001 0.9
r1 n001 pos 0.5
r2 neg pos 1000000
.model dpv d(is=1e-011 11 n=85.6993033770364)
c1 neg 0 100n rpar=10meg
.end
This was copied from LTspice user group (may have to be a member to read these files):
http://tech.groups.yahoo.com/group/LTspice/files/%20Lib/PVcell/
http://tech.groups.yahoo.com/group/LTspice/files/%20Lib/Solar%20Cell%20Module/

But that's not precise. The model is not a current source, it's a current source and a anti-parallel diode (d1) -- the voltage across the latter being the reason why a solar cell behaves as a current-limited voltage source.

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Nice! The LT3652 fits the bill perfectly.

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Is it a flooded or sealed lead acid battery? Will it be cycled frequently? By 6V12A I assume you mean C=12 amp-hour capacity.

Flooded cells can be continuously charged at rates up to a couple C without damage as long as you as add water as needed to keep the electrolyte levels above the plates. SLAs will not lose water if the charge rate is less than C/3, 4 amps in your case, see e.g. http://www.powerstream.com/SLA.htm.

Those panels would put out the maximum 10 watt power around 18 volts, or ~550 milliamps, so that is the maximum current you would get with a direct connection to a 6 (or 12!) volt battery. For charging, looks like you could have up to 7 panels in parallel without any kind of charge controller.

But if you ever get to the full charge the voltage will keep rising and once above 7 volts (temperature dependent, see the link) the plates will corrode. If you don't want to monitor the voltage yourself then at a minimum you will want a charger that clamps at a lower "float" voltage.

Of course you would be wasting half the panel wattage with a direct connection, more efficient would be a buck converter to trade the excess voltage for more current. Most commercial ones will do a daily absorption charge before dropping to float, not so great if the battery is rarely discharged. ATTinyx5's driving a FET and bucking coil make great custom charge controllers.

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Maybe this one (german though) , but code & schematics are "universal"
A Tiny13 + 2 Fet's

http://www.mikrocontroller.net/t...

/Bingo

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Bingo600 wrote:
Maybe this one (german though) , but code & schematics are "universal"
A Tiny13 + 2 Fet's

http://www.mikrocontroller.net/t...

/Bingo

Google Translate from ze Germans to English:

http://translate.google.com/translate?js=y&prev=_t&hl=en&ie=UTF-8&layout=1&eotf=1&u=http%3A%2F%2Fwww.mikrocontroller.net%2Ftopic%2F46903&sl=de&tl=en

Also, gchapman, I hope I didn't misinterpret your last comment. I wasn't intentionally aiming for The Pedantic Asshole of the Day award.

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dak664 wrote:
Is it a flooded or sealed lead acid battery? Will it be cycled frequently? By 6V12A I assume you mean C=12 amp-hour capacity.

Yep, I meant 6V12Ah. Battery type is AGM. I edited the OP to reflect that.

It is important that the charger is effective and won't degrade the lifetime of the battery. So a cc/cv switch mode such as the linear chip above is a very good candidate.

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Well, if effective means efficient I think those are incompatible requirements. AGM batteries can last for 15 years if never discharged, but once the voltage goes below float you want to recharge as soon as possible to prevent capacity loss through sulfation. For solar charging that usually means by noon, after which the rest of the day's PV production is wasted.

I'd look at 2 15AH LiFePO4 cells in series charged to 6.9 volts. Costs less, capacity the same, chemistry just as safe, and letting the charge level hover around 50-90% over days and weeks will actually increase lifetime, compared to a full charge by noon and wasting the rest.

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yes, I meant efficient :(

And no, i don't think that those are incompatible requirements, since that means i can put in fewer solar panels to keep the batteries charged.

My google-fu says the 2x 15Ah LiFePO4 cells are $90, or >3 times the price of the lead acid battery. But otherwise a very interesting battery type, do you have any preferred vendors for these?

In northern Europe on a site with solar power you will have ~9 months, where the solar charger does a little trickle charge, and waste the rest. It's the remaining 3 months that matter, because the amount of power you get is a tiny fraction of what you get during the summer. In this period, you actually expect little to no charge at all, and depend on the battery capacity to do the rest.

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Well I was thinking of 15AH Headway cells but I can't find a current source for them, the factory made some production runs in February but things have been quiet since then. I got their 10AH cells for US$17 each a year ago and am very pleased with their performance, wouldn't hesitate to try them in an AVR application where they were recharged once a year.

If it is possible to use the battery for 3 months and then recharge over 9 months, efficiency would prefer a low self-discharge rate. With that requirement, first would be lithium ion, next rechargeable alkaline, then lead-acid in third place.

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Achilles wrote:
Also, gchapman, I hope I didn't misinterpret your last comment. I wasn't intentionally aiming for ...
No problem. I have a thick skin and a forgiving demeanor.

"Dare to be naïve." - Buckminster Fuller

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lagger wrote:
My google-fu says the 2x 15Ah LiFePO4 cells are $90, or >3 times the price of the lead acid battery. But otherwise a very interesting battery type, do you have any preferred vendors for these?
LiFePO4 6.4v 12Ah for 70USD:
http://www.batteryspace.com/6.4-LiFePO4-Battery-Packs.aspx

"Dare to be naïve." - Buckminster Fuller

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Dear Lagger

Please look at my site here it will help you:
http://www.avdweb.nl/index.php

I am making a solar powered bicycle and the batteries will be charged with a PV panel also. With restrictions, you can connect a solar panel directly to a battery, but you need a BMS to protect your batteries.
Here is information about simulating circuits with solar cells:
http://www.avdweb.nl/solar-bike/...

Here you can read why MPPT is not always better:
http://www.intusoft.com/nlhtm/nl...

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Hi.
I did a charger for my portable power source from 10W solar panel. It charges 6V 4.5AH sealed lead acid battery, when battery is charged - it stops charging and resumes charging when battery is discharged a bit.
Here is schematic and outlook. If you're interested, I can explain further.

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