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!
Best Answer
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.