Panel for charging lipo batteries

[Collectors]

I am looking at the pros & cons of buying extra lipo batteries or setting up a solar charging system for my electric model RC planes that i carry on an 8x4 trailer, so room for a solar set up is not a problem. Now the lipos i use for the RC planes are 5000mah 14.8 volts at a cost of around £45.00 each & i have a lipo charger that uses 12v x 120watt to charge a single battery or 240 watt to charge 2.
If i take a fully charge 110amp 12-volt battery with me "fully charged from home" what sort of solar panel size would help to top this up. Or is it a case of the bigger the better.
I also have a 40amp solar charger/controller. Also, there is no electric were we go & generators are not allowed.

 

[Ampster]

How many fights per day do you contemplate? Are you there during max insolation times for your location?

 

[svetz]

Hi Collectors!

Welcome to the forums!

...i have a lipo charger that uses 12v x 120watt to charge a single battery ...

Interesting problem. So, Ampster's questions about the number of flights and time of day are important as the number of flights is the amount of power you consume, and the insolation for the time of day is the amount of solar energy available from the sun. You can find insolation maps for your area by googling for them, but they'll be daily averages.

Rather than wait for that though, I'll just show you the math and you should be able to work it out for yourself.

Let's start with Watts = volts x amps; and watt-hours = volts x amp-hours

So, we know that your 12V charger consumes 120W. The next thing you need to know is how long it takes to charge the battery. You said the batteries were 5 ah @ 14.8V...

So, working backwards the RC battery holds: 5 ah x 14.8V = 74 watt hours. Since the charger can provide 120 watts, it should take roughly 74/120 = 37 minutes to charge (probably longer for inefficiencies, so let's call it 40 minutes).

A 110 Ah 12V battery has 110 x 12 = 1320 watt hours. Each RC battery holds 74 watt hours, so your 110 Ah battery should be able to charge 1320/74= 17 flights. If you need more flights than that in outing, you'd want some solar to charge your battery. Read up on DoD in the Battery FAQ.

At Solar noon on a clear day cool day you'd probably see about 80% of the rated wattage of the solar panel. But earlier and later in the day you'll see some percentage less as the light must go through more of the atmosphere where it loses energy. The table to the right will help you figure that out. Each hour is 15 degrees of sun altitude. Let's say you have a 100 watt panel. 10 AM is is two hours from solar noon, or 2x15=30 degrees. 90-30 - 60, so the solar intensity from the chart at 60 is about 85%. So if at solar noon your panel typically yields 90W, then you could expect 90 x .85 = 76.5 watts. In one hour, the panel would produce just enough power for 1 flight (74 watts).

 

[svetz]

...I also have a 40amp solar charger/controller.

A 40 amp SCC should be plenty, for more, see:

• What does it mean to have solar panels in parallel and series?
• How to figure out how much Solar and inverter wattage is needed to charge a battery
• Figuring out how many panels in series and parallel based on your MPPT