I have a device that runs on a single cell Li-ion / LiPo. The physical case for that device allows me to put in one to three 18650 cells. Unfortunately given the disposition of the cells, I can't buy a 2 or 3 pack already assembled.
The device already has a charging circuit. The maximum load will be 300 mA and the maximum charge current will be 500 mA.
Given the above, if I connect 3 cells in parallel (1S3P), do I need a BMS (Battery Management System) or any additional protection circuit?
I'll take care to have them at the same voltage level before connecting them together to avoid current flow between the cells.
TL;DR; Does a 1S3P Li-ion 18650 pack need a BMS?
Answer
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.
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