How do energy harvesting ICs like LTC3106 and LTC3105 safely charge lipos?

Question

I'm designing a simple device with a single cell LiPo that I want to be chargeable from a single solar power cell. As I start to look through the data sheets, I'm seeing that the battery charging is typically done just by providing a constant voltage output to the battery.

For instance, LTC3106, page 19:

Or LTC3105, front page:

Note position of batteries in line with the load??

These circuits seem to be designed to charge the batteries in question by providing a constant 4.2V power to a single cell battery. Is this really safe? Is this equivalent to trickle charging? Everywhere I've read that a LiPo needs a very specific type of charging discipline, such as MCP73831 (which I use in all of my projects without issue.) For instance, is it actually safe to charge a single cell lipo with a constant-voltage, variable current source and no cut-off?

Can someone more familiar with battery chemistries and charging process help enlighten me? (PS: I am not EE-trained, I am a home hobbyist.)

Answer 1

These 4.2V are unloaded, i.e. with battery disconnected.

The important bit with LiPos is that you control the charging current, and the limit is usually relative to the capacity of the battery.

The charging current is relative to the voltage difference between the charger output and the battery, and vice versa. High-powered chargers can provide more current than that, so they have to reduce their output voltage so the charging current stays in range. Trickle chargers cannot provide a lot of current, so their output voltage is the battery voltage plus the difference given by the available current.

Answer 2

No, you are correct, you must not trickle charge LiPos.

If these converters do not have a way to stop charging, then they must be used with something that can.

It doesn't look like the 3106 is intended to charge its spare source (quick read, might be wrong). The 3105 doesn't mention LiPos inside the data sheet, showing supercaps and NimH cells, but that front page is very naughty.

Answer 3

These parts do have a way to stop charging - it stops when the desired terminal voltage is reached. The details depend on the strength of the source and the Amp-Hr rating of the battery, which you didn't give, but in general, these are low power converters (and usually low power sources, such as a small solar panel) that can safely charge a "typical" battery due to the low charge current. However, it's best to calculate the maximum output current the converter can provide at your expected Vin with a 4.2V output (or whatever you program Vout for). You can find this information in the datasheet curves. Compare this current to the rated charge current of your battery.