# Solar panel, accumulator and Arduino

Hi.

I would like to have solar panel, accumulator and Arduino in one circuit. I am having a solar panel of 13,71V/10W and a gel accumulator rated 12V/7.2Ah (this solar panel is dedicated to this accumulator) so there is that. I can easily charge this accumualtor with this solar panel as there is a regulator attached to solar panel (voltage is fixed - it is always 13,71V). Additionally I would have 12V->9V regulator just for an Arduino but that does not matter here.

Here I am wondering, how can I put Arduino in the same circuit. As long as I know, I can not charge and discharge accumulators at the same time. How could I make this work so I could have all of them in one circuit? Would I need a special solar charge controller? Could I overcharge my accumulator without additional circuit between solar panel and accumulator?

You can charge and discharge accumulators simulteneously, there is no problem with that. Every car does it while engine is on: it charges and discharges its battery at the same time.

You can connect some Arduino directly to the battery. Just check the specs. For example, Arduino UNO employs NCP1117 regulator, it can handle input voltages up to 20V.

The NCP1117 specs make it pretty clear that you will only get a fully regulated 5V output with inputs well under 20V with a 5V output. http://www.onsemi.com/pub_link/Collateral/NCP1117-D.PDF

Look at the input conditons for the different characteristics:

5.0 V (Vin = 7.0 V, Iout = 10 mA, TA = 25 °C) (Vin = 6.5 V to 12 V, Iout = 0 mA to 800 mA) (Note 4)

5.0 V (Vin = 6.5 V to 15 V, Iout = 0 mA)

5.0 V (Iout = 0 mA to 800 mA, Vin = 6.5 V)

Output Current Limit (Vin−Vout = 5.0 V, TA = 25°C, Note 6)

5.0 V (Vin = 15 V)

Above 15V, regulation will be out of spec, and the part will be hotter than heck with any kind of current load.

Above 15V, regulation will be out of spec, and the part will be hotter than heck with any kind of current load.

Temperature rise can be estimated because thermal resistance is known; junction-to-ambient is 160C/W for SOT-223 with minimum size pad. For instance, lets assume that Arduino consumes 30 mA of current. Also lets assume voltage drop across input diode to be 0.6V.

• when input voltage is 9V power dissipation is 102mW, expected junction temperature rise is 16.3C
• when input voltage is 13.7V power dissipation is 243mW, expected junction temperature rise is 38.9C
• when input voltage is 20V power dissipation is 432mW, expected junction temperature rise is 69.1C

For ambient temperature as high as 40C junction temperature at 20V would be 109C, that is still well below the maximum rated 150C. With input voltage of 13.7V Arduino UNO itself should work without problems.

So we're in agreement than that 20V input voltage is not good for Arduino?

CrossRoads: So we're in agreement than that 20V input voltage is not good for Arduino?

Err . . I think we've drifted a bit off topic. If the Arduino is powered directly from the battery, with that charge regulator, it will never get more than 13.7 V so no problem.

Russell

akouz: You can charge and discharge accumulators simulteneously, there is no problem with that. Every car does it while engine is on: it charges and discharges its battery at the same time.

You can connect some Arduino directly to the battery. Just check the specs. For example, Arduino UNO employs NCP1117 regulator, it can handle input voltages up to 20V.

So why are there Solar Panel regulators like those from eBay? I mean it is for to not overcharge your accumulator but then don't I need it aswell?

whoru: So why are there Solar Panel regulators like those from eBay? I mean it is for to not overcharge your accumulator but then don't I need it aswell?

Lead acid and NiCad batteries are typically charged using a constant voltage source. The solar panel regulator presumably limits the voltage presented to the battery terminals to something appropriate for the batteries (13.7 V) independent of the voltage at the output of the solar panel. The current into the battery is limited by the smaller of the current available from your source solar panels or the rate at which the battery accepts charge. The latter will drop to essentially zero as the battery reaches full charge. If the voltage presented to the battery is too high, it will eventually damage the batteries.