# 5v solar batterypack to 12v output

This is just a concept I was thinking about. I have an arduino project with some dc motors that needs to run outdoors for a long time. The DC motors themselves do not need to run all the time, usually a 2-3 minute burst and at most for 30 mins at a time. They are 12v motors, drawing 0.4A at peak. I expect the overall circuit to draw around 2A during load.

I was looking at these solar battery packs, meant for charging phones with 5v output (has 3000mAh battery). I was wondering if I use a DC/DC booster and boost it to 12v to drive the motors, is it going to work well? I expect the battery will just drain faster, but since the motors are not used all the time, the solar panels can top up the battery when the system is resting for rest of the day.

DC/DC step up regulator: http://www.robotshop.com/ca/productinfo.aspx?pc=RB-Dfr-121&lang=en-US

That could almost work. The thing to consider with DC/DC voltage converters is that they can change voltage but they can not create or destroy energy, thus the power demand from your energy source is the same as the power demand from the load, plus inefficiencies in the converter. Power is the product of voltage and current, so if your 12V motors draw at most 2A, then the power (at most) is:

$12V \cdot 2A = 24W$

Knowing that the solar battery you referenced is approximately a 5V voltage source, we can calculate what the current must be:

$24W = 5V \cdot I$

$24W / 5V = I = 4.8A$

That's assuming 100% conversion efficiency. The website for your DC/DC converter says the efficiency is 90%, but that seems overly optimistic to me. Let's say we need 6 amps to allow for some inefficiency.

The question then is this: can your battery pack provide 6 amps? It doesn't look like this one can. So, either you need to find one that can, or reduce your motor load, or store the energy in a 12V battery that can supply 6 amps at least long enough to run your motors (and then recharge it slowly).

• Hmm, I understand. I actually found some models with 5v, 2.1A output. This still isn't enough, but what if I treat them like batteries and put 2 of them in parallel (+ve to +ve and -ve to -ve)? Is it going to damage the solar charger? The ones with 12V output are meant for laptop charging, and I think the price and weight of those are an overkill for my project. And since weight is a concern, I'm trying to stay away from SLA batteries. – Tauseef Latif May 27 '13 at 20:56
• @TauseefLatif putting voltage sources in parallel is usually a bad idea. If you put them in series, you will still double the available power. As a bonus, you will also decrease the current required, which will reduce your total resistive losses in the wire and converter. – Phil Frost May 27 '13 at 23:55

In theory the idea is sound. However, those particular components won't work. The battery pack is rated for 5 watts. Your circuit uses 24 watts. The regulator has 90% efficiency, which means you actually require 27 watts, and the regulator is only rated for 15 watts. You'd also need to take leakage current into account, to make sure your battery actually has time to charge.