Computing stuff tied to the physical world

Bad LiPo charge chip

In Hardware on Jan 26, 2013 at 00:01

(Not planned this way, but still a nice follow-up on yesterday’s Junk USB post…)

While thinking about some minor tweaks for the JeeNode USB board, I wanted to try out a different LiPo charger chip – mostly to reduce costs, given than not everyone using the JN USB would be interested in the LiPo charge capability (it’s also fine as a JeeNode-with-built-in-USB-BUB after all).

So I had a look at the MCP73832T – in fact, Paul Badger and I went ahead and had a new board made with it:

DSC 3220

The good news: as a LiPo charger, it works absolutely fine.

The bad news: without an attached LiPo battery, it’s not usable.

It turns out that this chip uses some sort of charge/discharge cycle. This is what happens without LiPo attached:

SCR34

IOW, it’s delivering 4.2V for a while, and then dropping the voltage to see whether the LiPo itself will fill in the gaps. A pretty clever way to figure out the state of the attached battery, if you ask me.

One way to use the chip without attached LiPo would be to bypass the Vin and Vout pins of the chip, i.e. just disable it altogether via a (solder-) jumper. Drawbacks: 1) you have to remember this, and act accordingly, 2) this means PWR would be 4.2V with LiPo attached, and 5V without, and 3) when bypassed, there would be no over-current protection for the USB port.

Especially that 3rd issue is bad – JeeNodes are about tinkering with stuff, and JN USB’s are about tinkering while attached to a computer USB port. Without over-current protection, tinkering can damage your computer – scary!

There is one more way to solve this, but it’s not very practical: add a big electrolytic cap which sort of takes the place of a battery. I used a 6800 µF (which pulls too much current on power-up, BTW). The result:

SCR36

A voltage on the PWR pin which carries 3.8 .. 4.2V, with a 10 Hz ripple. Not great, but good enough to make the JeeNode’s internal regulator work. Except that a 6800 µF capacitor is huge and highly impractical, of course!

Sooo… back to the MAX1555 it is. That chip works differently: it senses the charge current and the output voltage.

Note to self: don’t replace chips without testing all essential modes they’ll be used in.

And good bye Mr. Murphy, how considerate of you to drop by once in a while…