Ex Archive: Solar Panel Battery Charging

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Ex Archive: Solar Panel Battery Charging

Post by Carrot Cruncher »

Solar Panel Battery Charging
by Almach » Thu Apr 22, 2010 11:18 pm

Hanfgeist wrote:
Initially, I'm going to start off by aquiring one car battery to run some of my projects off like the HHO reactor but will also want it as a back up supply to run the laptop and possibly the wireless broadband off or the TV, if the grid goes down, though I have a hand powered drill, could also be useful to run the electric one off of it for small jobs .....
Hi Hanfgeist,

This is how I do my calculations for solar battery charging applications:

Battery Capacity:
A typical car battery will be rated at 30 or 40 Ah which is the absolute best storage capacity that it will have at room temperature, however this degrades down to only 80% of that capacity if it is stored at temperatures of lower than 5 degrees C or so.

So using the power equation, P=VxI, a 12V battery rated at 30 Ah, can store 12V x 30 Ah = 360 Wh of energy which means that it can theoretically supply power to something like a 36W bulb for 10 hours before the battery is drained. However this is simplistic and assumes no losses. Also, as a battery ages over say five or ten years or so, the more this Ah figure will slowly degrade until it falls to practically zero.

A typical Lead acid battery should never be discharged lower than about 11.0 Volts minimum, and should be recharged as soon as possible after this to ensure maximum life expectancy of the battery.

Solar Panel Size:If you had something like a 40W solar panel which by using a crude rule of thumb, will recharge 40W of power back into a 12V Lead Acid battery during an average 24 hour period of UK sunlight.

However, again this is simplistic and assumes no losses, when in fact there is a loss factor in battery charging where only about 75% of the power you put in, can you get back out.

So in reality, you could only take about 30W out of the battery each day if you want it to fully recharge for the next day. 30W will allow you take 30W / 12V = 2.5 Ah of power per day if you wanted it to be fully recharged by the next day.

Also a Solar Shunt regulator like the this Maplin 5 Amp one will be needed to prevent over charging of you battery, otherwise the battery will be useless in a matter of months.

Current Consumption of Appliances:
From above, the 2.5 Ah of power will allow you to take 2.5 Amps out the battery for 1 hour, or 1 Amp for 2.5 hours, or 0.5 Amps for 5.0 hours and so on. Look at the 12V appliances you intend to use and read the current consumption of the device, or measure it with an amp-meter. Generally the appliances that take the most current are the ones that generate the most heat, but I would expect a small 12V telly to take perhaps 0.3 Amps and a laptop to take about 4 Amps.

Power Factor Losses:
Also, you must remember that any voltage conversion from say 12V DC to 240V AC or vice versa produces further power loss factors of typically 20%.

Basically, this means that the initial 40W of the solar panel loses 25% by storing in the battery, which will lose 20% if this feeds a 12V DC to 240V AC mains invertor, which will lose another 20% if this goes into a standard laptop mains to DC psu.

So 40W of the solar panel minus the 25% loss storing in the battery leaves 30W.
30W from the battery panel minus a 25% loss converting to 240V AC by means of an invertor leaves 22.5W.
22.5W from the invertor minus a 25% loss from the laptops typical 19V DC PSU conversion leaves 16.9W.

So you actually only get 42% of what the panel initially generates because of all the losses in the voltage conversions.

However, if you are lucky enough to have only 12 to 14V DC appliances, then you can cut out the last 2 conversions.

Bottom Line Bullet Points:

A car battery can typically provide 360 Wh of electricity before it is empty.
A 40 Watt panel will typically charge 40 Wh per day into a lead acid battery.
40 Wh per day taken from a lead acid battery will drive a 40 Watt bulb for one hour per day.
The above three points are simplistic and don't include the typical 20% loss factor for each stage.
12 to 14V DC appliances driven straight from the battery are much more efficient because the losses caused by each conversion stage are reduced to one, charging and discharging the battery.
Suitably rated fuses must be used to protect the battery from short circuit at all times.
Finally, if you chose not to use a shunt regulator, and you accidentally disconnect your 12V battery from the circuit while your appliances are still connected to the Solar Panel, you will more than likely blow up all your low current consumption appliances with the 27V at 1.3 Amps produced by the panel on a sunny day.

Safety Note: - Remember this is only some information on how I do it so if you don't have a good understanding of what is going on you should get someone experienced with this equipment to confirm that what you are doing is correct before you connect the battery or any appliances because a short circuit car battery is very dangerous and very frequently is the cause of car fires. And... if you blow up your telly, It's not my fault!

Al
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Re: Ex Archive: Solar Panel Battery Charging

Post by hobo »

Good stuff. This is very useful. Thanks!

Hobo