How might you use a LiPo battery as backup for mains power loss?

Question

Many people seem to run their projects off the mains (e.g. with a wall wart) or from a battery, but there doesn't seem to be too much discussion about how to run a device off the mains, charging a battery, but then switching over to the battery in the event of a power outage.

I imagine that with NiCD/NiMH batteries you could just hook them up in parallel with the power supply (more or less), however since LiPo batteries hold more power for their size I would like to use them instead in a project like this - but they don't seem quite so simple to use in this way!

Can anyone offer some pointers as to how LiPo batteries might be used for backup power? I imagine you would need some logic to power a charger and switch over to the batteries when the mains disappears, but I'm not sure how this might work. I imagine you'd also need a large capacitor to avoid a momentary dropout during the switch over as well. Or perhaps there's some cheap IC that can do all this for me?

Thanks for any suggestions!

Answer 1

There are a number of ICs that will do these functions. I would go to the Maxim and Linear Technology sites. The downside is that most of these devices is that they are only in surface mount packages and a lot of them come in packages without leads (QFN, LGA).

A part like the LTC4088 is nice. It has an input for the battery, an input for the USB (or wall adapter) and a switching regulator.

You could also use separate ICs (but you are probably better off with a single). There is a device that Linear Tech calls a PowerPath controller (LTC4412) that performs the switchover function. You could add a MAX1555 Li-ion charger for a low cost charger and add the LTC4412. Also there are devices called "Ideal Diodes" that will perform switchover. These devices are FETs so you get a much lower voltage drop than a Schottky.

The best solution depends on a couple of things you haven't mentioned -- system current, battery capacity and recharge rates.

Answer 2

There are many ways to do this. One way you may want to consider is a dedicated battery management / battery backup IC. This link to Mouser should give you many options:

http://mouser.com/Search/Refine.aspx?Keyword=battery+management+backup&FS=True

You should look for the "Backup for LP Processor" ICs. Read the data sheets closely, but they should provide sufficient specs to give you what you need. The downside is that I didn't see any through-hole chips (though I didn't give an exhaustive search).

To do this, you will either need to get a proper LiPoly charging circuit to keep the battery properly charged while power is applied to the circuit or find one of these chips that can handle charging a LiPoly as well as perform fail-over functions. I think I saw one on Digikey that does everything for $20, but that seemed like it was an unnecessarily expensive solution.