Hi Everyone,
I'm developing a low power electronics device that I'm hoping to last a significant amount of time (1 year +).
I'm looking to use two 18650 cells as the are pretty ubiquitous, low cost and power dense.
The batteries will be removable from the unit for charging/replacement - also justifying the use of 18650s as chargers are common.
The radio in the unit can be powered directly from a LiPO cell (3.0 - 4.2 V), the rest of the circuitry is 3V3 and requires a regulated supply.
During TX, the radio can draw bursts of up to 800mA.
I am unsure whether to run the batteries in a 2S or 2P configuration.
Running them in a 2P gives me a voltage that is correct for the radio and switching regs that produce 3V3 from a 1S voltage are common.
However, as the batteries are charged out of the unit, I'm worried about cross charging (as they will be different voltages) when they are installed back into the unit and unequal draw from the batteries.
Cross charging can be solved with a Schottky diode in series with each cell, however, this will drop the cell voltage 0.3~V reducing the operational life of the batteries and wasting power through the diode.
Running them in 2S means I'll need a much grunter regulator to supply a regulated voltage to the radio and remaining circuity.
I'll also have to integrate a more complex BMS chipset.
These devices will need to get certification to work in Europe, so I'm trying to do this the "right" way.
If you made it through all that - well done, I have the following questions for the hive mind:
Am I missing anything?
What would you do?
Any other resources, datasheets, idioms or random thoughts that might help me?
I am personally leaning towards the 2P approach as it makes the power regulation stage simpler and lower loss.
Thanks in advance.