# 12v versus 5v for solar powered circuit

I need expert advice on finishing my circuit board. I have designed it such that it needs 5v and 4 amps to function. The project will not be making use of mains power supply but will strictly draw power from batteries. I need to squeeze every drop of efficiency on the batteries to increase battery life. I am considering powering it out of two options, kindly advice Which would be best;

(a) 12 volt 7 ahmp battery connected to adjustable power supply on circuit board, which in turn is fed by a 12v solar charger. (b) 5v battery fed by 5v solar charger.

which of these options will give me my desired result?

• 12V, a cheap solar charge controller (protects the battery, mostly available for lead-acid) and a buck convertor to 5V. – Brian Drummond Mar 28 '18 at 9:04

The answer depends a lot on the exact components you use, and what the parameters are of your circuit.

First, lets consider battery voltage. You can't get a rechargeable battery in an arbitrary voltage. NiCd are 1.2V nominal, and Lithium-Ion are 3.7V nominal. Neither of these gives you exactly 5V, and the voltage will vary significantly over the charge/discharge cycle. So, first question is, does your circuit need EXACTLY 5V, or is 4-6ish V OK? If you need exactly 5V, the 5V battery solution probably isn't for you.

Next, let's think about the 12V to 5V converter. It'll probably be between 90% and 95% efficient, so that's one point in favor of the 5V battery solution (which might not need a DC-DC converter).

And, finally, what do your solar cells look like? How much control do you have over the number of cells in series? A single solar cell is maybe 0.5V, but most solar panels will have several cells in series. Is your solar panel closer to 12V output, or 5V output? If your 5V solar charger incorporates a 12V to 5V converter, it will have 90-95% efficiency just like the one above, and negate that point in favor of the 5V battery. If your solar charger uses direct energy transfer (no DC-DC converter) it will have higher efficiency - but may have other disadvantages (too complex to get in to here).

In the end, the optimal solution depends on a lot of variables, and you will need to take each into account to calculate the overall system power efficiency.

• The circuit needs EXACTLY 5V so i guess your answer automatically rules out the 5V battery option for me. More so because i believe the 12V battery will give a longer run time without charge compared to any 5V battery, right?. my solar cell outputs a strong and steady 12V. – Allen Jakpa Mar 28 '18 at 7:56
• you can get 5v from a boost+buck, which also allows 12v input, or about anything really... – dandavis Mar 28 '18 at 10:19
• Since you need regulated 5V, you need a converter between the battery and the circuit regardless. dandavis is correct that you could use a 5V to 5V buck-boost, but since you need to get down from 12V somewhere anyway, you have fewer conversions with a 12V battery. Whether a 12V battery or 5V battery runs longer... hard to say, depends on the batteries. It's all about energy density and internal resistance. – Selvek Mar 28 '18 at 12:53