Electrical – Is it possible to only fill a Li-ion BMS partially

It protects against over-charge, over-discharge, and over-current. Another name for this function is Protection Circuit Module (PCM). According its specifications that module is suitable for 18650's.

It should cut when any cell voltage goes too high or low, but it can't equalize the cell voltages. This isn't a problem if the cells are initially balanced and have very low self-discharge rates.

However it might be worth providing a facility for manually checking and balancing the cells. On Lipos this is commonly done with a 'balance connector' which taps into the connections between the cells.

Depends.

What do you want the BMS to do?

When I design a BMS, the first thing it does is safety:

• Temperature monitor for charge/discharge
• Over current events (with a software fuse)
• Charge control
• stopping over discharge events

The only other tasks a BMS will do are things like:

• state of charge
• some basic functionality (if you're doing something very basic, complicated functionality should be taken out of the BMS to make sure the safety tasks are taken care of)
• cell balancing (if you have a series pack of cells)

So, as you have your charger outside the BMS, the question is how safe do you want you pack to be? If you're selling it, then you need a BMS as safety is (should be) paramount. If it's just for you, you know what you're doing, and you can control the environment, charge and discharge patterns and all that, then there is no reason to have any BMS at all.

Personally, I'd say yes you do. But if I'm quickly throwing something together, I have been known to go without.