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I'm trying to figure out how to build a DIY micro irrigation system for my small greenhouse. A part of that task is to figure out how I can charge 8 AA batteries using solar panels.

I've found a set of 4 solar panels with the specification 6V 130mAh. However, if I've understood right I need to have the panels to deliver 12V and 260 mAh if I've got 8 x 1.5V 2600mAh AA batteries. I've read that 10% of 2600mAh is a safe charge rate.

How can I turn these 4 6V 130mAh solar panels into a single array which is capable of delivering 12V 260mAh?

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    \$\begingroup\$ FYI mAh is milliamperehour, unit of capacity (energy). Solar cells have no capacity (they can't store energy). They have maximum current, which is measured in A or mA (amperes or milliamperes). When 1mA flows thru battery for 1 hour it charges it (or discharges) by 1mAh. \$\endgroup\$
    – Kamil
    Apr 22, 2014 at 20:57
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    \$\begingroup\$ Please take a look at this answer that was for a similar project. It covers some things you may want to consider when designing your solar/battery/load system. The application isn't exactly the same, but the info is still useful :) \$\endgroup\$
    – bitsmack
    Apr 22, 2014 at 21:26

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Sure, theoretically you can get 12V 260mA from 4 solar cells with 6V 130mA (not mAh!).

Just like that:

schematic

simulate this circuit – Schematic created using CircuitLab

However:

  1. Solar cells rating is maximum rating, at full direct sun.
  2. You need some voltage and current regulation for safe battery charging - 8x NiMH is 8x1.2V = 9.6V nominal, and 11,2V fully charged (8x1.4V). 12V charging is too much for uncontrolled charging.
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Simple answer: two in series make 12v, two in parallel doubles the current.

The more involved answer is that for the cost of 8x AA nimh cells you could probably get 1 small(5ah or so) lead acid battery, which are the usual choice for solar power storage. This would probably last longer, because they tend to do better under constant standby charge from what I hear.

Better still would be lifepo4, but those need a current and voltage limited charge that needs to be very accurate, and multi cell packs need to have a balancer, or the cells will almost certainly be destroyed.

What I would actually do is to get a switching regulator module from eBay for a few bucks, put all 4 panels in series, and use the eBay regulator to drop that down to 13.5v(the lead acid float voltage) and feed that into a lead acid battery through a shottky diode.

Or possibly even get a commercial solar lead acid charger panel if I found one for the right price. Something like this 30$ one that claims to come with a charge controller and give you 10 to 20 watt hours a day. http://www.amazon.com/Solar-Panel-Charging-Kit-battery/dp/B00EB2VGTW/ref=cm_cd_ql_qh_dp_t combine that with a 12$ battery like this one: http://www.amazon.com/Volt-Alarm-Battery-12v5ah-Hour/dp/B0010Z4MDK/ref=sr_1_2?ie=UTF8&qid=1398201863&sr=8-2&keywords=5ah and you have a lot more power available than you would get from 8 AAs. And you can choose any sized lead acid you want.

But then that's me and I really hate batteries wearing out.

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  • \$\begingroup\$ Thanks for your answer. I'll probably build it myself and take the DIY path - just for the fun of it. I'll put the panels in series and get a lead acid battery. However, can I replace the regulator with a "Solar Charge Controller" (ebay.co.uk/itm/…)? \$\endgroup\$
    – sbrattla
    Apr 23, 2014 at 17:32
  • \$\begingroup\$ You absolutely can, solar charge controllers were made for exactly this kind of thing. I'm not sure how much power they need to "get going", but I'd imagine a 5 to 10 watt panel to be enough. Watch out for solar panel prices though. Sometimes a 50w panel will be almost the same price as a 10w panel and in that case you might as well get the bigger one if you have room. Just be sure not to charge faster than the battery can handle, \$\endgroup\$
    – EternityForest
    Apr 23, 2014 at 18:05
  • \$\begingroup\$ Ah, so two small 2W solar panels probably won't be enough for a controller? I do see that the controller specification says "for 120W Solar Panel". If I just use a voltage regulator and no controller, how can I be sure that I do not overcharge the battery (and something breakes/explodes)? I'm looking at two of this panel : ebay.co.uk/itm/231187065786 \$\endgroup\$
    – sbrattla
    Apr 23, 2014 at 18:17
  • \$\begingroup\$ Two 2w panels might be enough but it depends on how much sun there is where you live and what controller you get. I've never actually used a charge controller because I mostly use lifepo4 batteries and charging those is a lot different than lead acid. But from a quick look around the net it doesn't look like any of the solar charge controllers specify their minimum wattage. \$\endgroup\$
    – EternityForest
    Apr 23, 2014 at 19:10
  • \$\begingroup\$ Another thing to keep in mind is how much power demand there will actually be. If something is running 24/7, even just a single watt, the power adds up pretty fast. Remember solar panels are rated by their output in full sunlight. In the summer, you might get the equivalent of 6 hours of direct sun per day. Every region has charts of how much sunlight there is per year. If you run 24/7 as is common for timers and automation, during the day you need to not only power it but have enough left over for all night, and the battery must be big enough for the night. \$\endgroup\$
    – EternityForest
    Apr 23, 2014 at 19:17

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