Trickle charging

My project involves using an Arduino Uno to raise and lower a small door by activating a 12VDC motor. I would like to power it all with a small (5 Ah) 12V battery that is kept charged using a 1.5W solar trickle charger. I am estimating that the total daily current draw (assuming the board itself draws 45 mA) from the system will be around 1.2 Ah. Is this enough reserve power and charging capacity? Thanks.

Assuming that 1.5w is the "nameplate" output of the solar panel it will only produce that when aimed directly at the sun when it is not obscured by any cloud. You have not said where you live but here in the UK in winter that is a rare situation. This JRC web page can help you estimate the actual average output from your panel. Remember it is an average and plan for an extended period with maybe only 50% of the average.

I.5w at 12v is about 0.125 amps. If you got (say) 10% of that for 6 hours of a day in winter that would amount to about 0.075aH - a lot less than your Arduino consumes. (Hoping my maths is correct).


Your calculation for the need of amp. hours looks good provided that the amount of motor time running is well estimated. A factor of 4 is good.

@railroader -- the motor is expected to run twice a day, and for about 1 min. each time. I'm not sure what the current draw is under load, but the no-load draw is 250 mA; I assumed twice that for the estimation.

I guess the motor consumes no current at all while standing still. Get started!

@robin2 -- I think you're right - it would take 10 hours of sunshine daily to keep up. A 5W charger would bring that down to 4 hrs. That's more likely but a margin for safety would probably be a good idea, I think.

The balance of charging and discharging is important. Regardless of the capacity of the battory You need to charge more than You discharge.

I forget the magic number - but had to use it many years ago - but a standard lead acid batter can take a certain percentage of it's rating without fear of over charging or baking it - I would get a panel that is about that size and use it - then you don't need any charging regulator

I did that on a van that I did not drive much several years ago to keep the battery from going flat. Remember in you calculations, batteries are not 100% efficient and they will also loose charge just sitting there, so those losses also need to be calculated if you want to do fancy engineering

I think I’m going to go with a 7.6W charger – two hours of direct sunlight (or the equivalent) each day should produce about the same power as is used in 24 hours. I think on average we get quite a bit more than that. If that proves to not be enough, I can always add a second panel.

Another thing to keep in mind is that the life of a lead acid battery is seriously shortened if it is not very fully charged about once per week or once per fortnight at worst.

Solar energy comes free after you have bought the solar panel so get a big one. If necessary get a charge-controller to prevent the battery being overcharged on bright summer days. A bigger battery is less likely to be overcharged.


And don't discharge a lead battory totally. Some of them does not handle that well. They might loose quite some capacity being emptied totally. Monitoring the battory and cutting off operation at too low voltage is a possibility

Railroader: Monitoring the battery and cutting off operation at too low voltage is a possibility

Not a "possibility" but an absolute necessity!

Don't charge your sealed lead-acid battery the same as you do for your flooded lead-acid batter in your vehicle. They are not charged the same, no do they have the same voltage.


The Battery University website has all the info about batteries and charging.