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Solar Powered Battery Recharger

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Tony Tonysin

New Member
I have been thinking of making a portable power socket for when the power goes out. I am planning on buying the inverter for the project which will run off of a 12 volt battery but the solar pannels for charging the battery are going to be used from a dozen or so solar yard lights. There are a few things that I need to know for this project before I start:

For the solar array:

.What is an acceptable voltage for charging the 12 volt battery since it needs to be above 12 volts for it to charge while in parallel to the solar array, eg: is there a certain point where excess voltage can bamage the battery say 20 volts.

.What is an acceptable amount of amps to use while charging the battery.

.Should I put in diodes when constructing the solar array if I need to wire some of them in parallel to stop them from discharging through the other solar cells which could damage them. What are the pros and cons of using the diodes, eg voltage drop

For the battery:

.Is there a way for the battery to stop charging when it is full even if the sun is still shining on the solar cells.

.Could I use the power socket while the solar array is charging the battery, eg: can you discharge a battery while you are charging it because if I was doing this what would stop the solar cells power from going into the inverter instead of the battery and charging it.

Thanks in advance and I can get the specs for the cells by measuring them but I didnt think there would be a point because there power output can vary depending on the time of the day.

MikeMl

Well-Known Member
There have been many threads on these forums about recharging lead-acid and Sealed-Lead-Acid (SLA & AGM) batteries. Rather than rehash it all again, do some searching. The BatteryUniversity is a good reference.

Note that battery recharging is independent of where the current comes from, i.e. the wall socket or a solar panel.

After you understand how to charge the battery, you will be in a position to gather the requirements for a solar array, and a "solar charge controller". Googling the latter will give you a lot of hits.

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audioguru

Well-Known Member
Solar garden lights use a solar panel that produces about 40mA at 3V at noon in summer. If you connect six in series to make one string then two strings in parallel you will have a charging current of only 80mA at noon in summer at 18V that will be reduced by the battery being charged. The average current during a day in summer might be only 50mA which will charge a tiny 12V battery.

I have one pretty big 1.8W 12V solar panel that charges a car battery in about 2 months in summer.

microtexan

New Member
solar batery charger

"I have one pretty big 1.8W 12V solar panel that charges a car battery in about 2 months in summer. "

AG, you don't use that car much, do you?

audioguru

Well-Known Member
There seems to be a lot of people in 3rd world countries where "the power goes out".
I live in civilized Canada where the power is reliable and we cheer and play in the dark if it ever "goes out".

The last time I lost mains power was about 6 months ago when it turned off for about 2 seconds. I don't know why it failed, maybe from a lightning storm. Then I needed to reset a couple of clocks. My pc simply turned off.

Tony Tonysin

New Member
I should have rephrased that part. Its not that im expecting the power to go out at any moment (the power is quite reliable here in New Zealand) but I was going to use it as more of a substitute to using the mains power or to be used in the unlikely event that the mains power does go out.

HarveyH42

Banned
Unless you have a bunch of those solar cells from the garden lights already, I'd suggest looking into some that are more productive. Even 40 would take a week or so to charge a small SLA from a UPS (7 Ah). I've got several of those 1.8 Watt panels mentioned earlier, and they don't provide much useful power. I've got a 5 watt panel that does a fair job keeping a 12 Ah battery charged, but sometimes takes a few days if the weather isn't clear.

I kind of started playing with solar, much like you are now. A 100 watt inverter, will light a 14 watt CFL bulb about 6 hours, off the 12 Ah battery. Those small inverters are dirt cheap (under $10). Would also look into a charge controller, have seen several over the years, under$20. It protects you battery from being over charged, or over discharged. Also have the diodes built in, to prevent night discharge through the panels. Batteries are expensive, but will last quite a while if you treat them kindly. The circuits I've seen, are kind of complex, and likely to cost about the same as off the shelf. Depends on if you are in it for the challenge.

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