Advice needed: backup 12v UPS w/ charging


I'm trying to figure out a circuit to meet the following requirements: - take 12VDC external input; and back when power resumes - directly supply 12VDC to Arduino (or 5VDC), and 5VDC to BeagleBoard - fail over between 12VDC external and battery without interruption (and back again) - allow the arduino to detect state; whether running on battery or external - trickle charge and manage a LiPo battery (avoid overcharging and unnecessary draw), as large as poss without being too expensive (e.g. - be as power efficient as possible (I don't want lots of power being sucked by the detection circuit etc) - include protection - e.g. diodes, and against surges/spikes, but the upstream 12VDC supply will be fused - not be a product - I want to integrate this onto my own circuit board once I've proven on a breadboard

I can only find partial coversations on this forum and no compelling 'answer'. This is the closest: ... but doesn't appear to have been tested or actually charge a battery.

Maybe I'm using the wrong search terms; this must be a common need.

I don't mind figuring this out myself, but would appreciate a few pointers...



Thanks for the reply.

It is not clear how a 3.7V LiPo battery is going to substitute for a 12V source?

I thought a step-up transformer could do that? Otherwise I could connect batteries in series, and could supply 5V to the Arduino instead of 12V. Or find an alternate battery.

Is there some reason why the traditional gel-cell 12V battery with a float charger wouldn't do everything you need? The telephone company has been using this scheme before your grandparents were born and they continue to use this method to this day for battery backup for people with phone service on optical fiber optics, etc.

Well, not if I was 85:) I am trying to create a modular unit that is self-contained and will swap in/out (similar to an industrial PLC brain). I want to keep size/weight down; hence LiPo.

You never mentioned any current load or backup time specifications. Those would appear to be critical factors in any design decision.

I know it's lazy, but the answer is 'as much as possible.

In terms of load, it's a modular system and I don't know exactly what modules I will be powering directly from this circuit. However, the most demanding appear to be the arduino mega (would like to allow 500mA), beagleboard xm (which will be powered down when switched to battery), and a 4D Systems display (about 150mA when active, but I have a 'sleep' timeout which takes down to 4mA).

Time is as long as possible (while not sacrificing reasonable weight/time goals) to allow time for the failure to be resolved.

Hi Richard,

It sounds like a more "holistic" approach to power sourcing, control, conversion, backup, and standby/sleep/backup modes is in order. It feels like you are looking at only part of the picture and you could end up with a less than optimal solution. Of at least you have mentioned only part of the details here (which is all we have to go on.) WHY do you need backup? Is there no viable recovery mode from power loss? Do you need some limited functionality during power loss?

Yes, I need limited functionality during power loss. I'm satisfied that if I can meet the requirements set out above (which I just clarified to include detecting which power supply is active), then I will have an optimal solution. So, any ideas for the circuit I described?

In case I can't get an answer here, and because this isn't an arduino specific question; is there a more general circuit design forum ppl can recommend? I think I'll need that in addition to this forum - which is tres helpful.

Googling yields a lot but I'm yet to find a great one...

Maybe I'm using the wrong search terms; this must be a common need.

I thought that wrong search terms might be the answer... but if it is, I'm not using the right terms either.

I've added a page to the playground for information about ways of powering Arduino projects....

I've added a page to the playground for information about ways of powering Arduino projects....

Wow! Good job. I'll read through that and see if I can find anything I can apply to solve this problem.

This is the closest thing I've found:

It is not quite a direct solution; e.g. it uses a 12VAC supply, and may need to be modified to support other types of batteries, and I'm not sure if there are better ICs etc that could be used, but it's a start. I'll try and pick it apart; should help me learn about circuit design.

"Mini UPS" seems to yield a few results.

Here's the best product option I have found, but needs a SLA battery...