What is the most efficient way to power a solar computer (5V) from a 12V lead acid battery?

My project: Power a Beaglebone Black (BBB) entirely from solar energy. Obviously energy efficiency is crucial here. My setup so far: PV panel connected to a deep-cycle lead acid battery. Now I need to get from the battery's 12V to 5V (2A) for the BBB.

Most similar projects suggest the use of a UBEC (Universal Battery Elimination Circuit), like this one. However, I am not sure if this is the right thing for me. The efficiency curve of a typical UBEC drops pretty badly for low currents. I expect the BBB to spent most of the time idle, i.e. draw around 150 mA, which means that efficiency would be pretty low.

Is an UBEC the best solution or are there more efficient ways to supply 5V/2A(max) for my purpose? Since my soldering arts are rather limited, I would prefer a ready-made solution.

• SLA C-ratings scale poorly. How long would you have to draw from it for? Commented Nov 19, 2013 at 13:12
• 24/7 :) The SLA is quite large (100Ah) Commented Nov 19, 2013 at 15:09
• You can find DC-DC modules on eBay for cheap, that only require 4 solder points. Commented Nov 19, 2013 at 15:16
• Thank you! Which type should I use (when I mostly draw very small currents)? Commented Nov 19, 2013 at 15:25
• Can't remember the part number. Just look at the module details and pull up the datasheet for the regulator. Commented Nov 19, 2013 at 15:27

3 Answers

In terms of efficiency, this is about as good as it will get. The most efficient method of transforming the voltage from 12V to 5V is to use a buck converter as opposed to a linear regulator. The UBEC is a buck converter. For power levels in this range, an efficiency of over 90% is considered good.

If you were willing to search around for a custom buck controller IC and do some PCB design, you could potentially eke out up to 96% by selecting an IC that includes a technology called synchronous rectification. An example of such an IC is https://www.fairchildsemi.com/ds/FA/FAN2001.pdf (however note that the input voltage on this IC only goes up to 5.5V)

With synchronous rectification, you can still achieve a good efficiency at low current levels (see page 5 in the pdf). For 100mA you can still get well over 90% efficiency.

• OP's question is about a solution with 90\$ for low, 100mA draws on average. They know it can do 90%, simply that the chosen device needs ~300mA before it will have that efficiency... Commented Nov 28, 2013 at 21:34
• @Passerby Please don't downvote if you don't take time to read the answer properly. Take a look at the pdf of the datasheet and you will see such a solution can offer good efficiency at both nominal and low loads. Commented Nov 29, 2013 at 11:04

The output voltage is stable when you connect your beaglebone to the normal DC power supplies. While the output of solar output need to be regulated by Maximum Power Point Tracking (MPPT) algorithms to keep stable. Normal buck IC like UBEC or FAN2002 do not provide MPPT capability.

As most solar converters aim to generate large power, there are rare ICs designed for portable power level applications. But there does exist some, such as LT3652 from Linear. And due to the same reason, a simplified version of MPPT algos is used here which resulted lower MPPT efficiency compared large power counterparts. It claims:

When powered by a solar panel, the LT3652 implements MPPT operation by simply programming the minimum input voltage level to that panel’s peak power voltage, VMP. The desired peak-power volt- age is programmed via a resistor divider.

The accurate efficiency hasn't stated, but I guess it should be around 80~90%. This is already the state-of-the-art solution.

• Maybe I did not fully understand your answer. I already have a solar charger and would like to power the beaglebone from the SLA battery, not the PV module. So input will be more or less stable at 12V. UBECs, as the one I referred to, do have broad input ranges (6V-23V). Commented Dec 2, 2013 at 19:59

I don't know if your still looking for an answer, but here is a buck converter claiming up to 96% efficiency:

http://dx.com/p/hzdz-adjustable-step-down-buck-module-blue-3a-280240#.UxGbJ_ldWSo

It would be a good idea to contact them for a data sheet.

Or this one:

http://dx.com/p/dc-4-5-35v-to-1-25-30v-adjustable-voltage-step-down-module-red-156808#.UxGblPldWSo

Both are only a few dollars!