Electrical – Schematic review LiPo charge load sharing with booster

Coin cells will not work well with your needs. While it varies by brand and size, most coin cells have low capacity, and an internal resistance that limits current output. A CR2032 has a nominal 250ma capacity (and that varies based on current load btw), and an internal resistance (Equivalent Series Resistance or ESR) of 18 to 30ohms, again based on current load). Depending on the speed of the current draw, you are going to get voltage drops, and quickly drain the coin cell. If you can get that much current out of it at a given time. 70mA is pushing it even for high quality CR2032, and 220mA is improbable, frankly, impossible. That is why your CR2032 boosted to 3.3v isn't working. The current draw of the regulator is causing the cell's voltage to drop.

That is, without having multiple in parallel (or using high capacity caps). That's the second part you asked. Batteries in series are a sum of the voltages, with the same amount of current. Batteries in parallel are a sum of the current capacities, with the same voltage.

As for your options, one or two AAA batteries with a boost converter would suit you better. It gives you relatively high capacity while still keeping space to a minimal. The TPS61200 you mention would work, if you can deal with a tiny (3x3mm leadless package. The TPS6107x family would do the same, same minimal components, in a sot6 package. You only need 1 battery for either of these. An AA gives 2400mAh nominal, an AAA gives 1200mAh nominal.

1. Do I need to limit the current progressively as the charge progresses? Yes. My understanding is that the battery will 'accept' lower and lower current as it nears the end of charge. You will need to be able to recognize that and then choose a value at which the battery is full. For example when charging current becomes 10% of original charging current then battery is full. For a faster charge, some people might choose a larger percentage as a cutoff.

2. Do I cutoff charging when voltage reaches 25.2 volts? As you can tell from the answer to number 1, no. To charge the lithium chemistry you start with a constant CURRENT until the battery reaches final voltage (25.2) then you charge at constant VOLTAGE allowing the current to slow down as the battery fills up, terminating charge as described in question 1.

3. Any IC to build charger around? Yes, lots. You do need to know how to build a charger around the IC but look here for an example. http://www.ti.com/product/bq24616. I am sending this via my phone so please excuse me if this link doesn't work. Just go to the TI.com website and search under power management and you will find around 200 to choose from.

Finally, the BMS that you are going to use could technically control the termination of the charging of the battery and it will in fact provide the all important balance charging that your battery will require, but most people recommend using it only as a backup to a proper battery charger and not to rely on the BMS as a battery charger.

Good luck.