Suitable battery protection modules are readily available on ebay, amazon & aliexpress etc.
They are often labelled as "BMS" boards, battery management system - which is rather inaccurate, they are definitely not a complete management system!
For a single cell voltage, either one cell or two or more parallel so still the same voltage, you need under / over voltage protection and short circuit protection.
For two or more cells in series, it
must also include cell balance, to avoid the cell voltages becoming uneven, as that is one of the common causes of battery failure or fire; if any one cell has a higher charge than the others, that can overcharge and fail before the overall battery voltage reaches the charge cut-off point.
Search eg. ebay for Lithium BMS and you will find a vast selection; a couple of example listings:
It also features over current and short circuit protection. Model 3S 12.6v 20A lithium battery protection plate (with Recovery function -AUTO Recovery). Module:1S Li-ion Lithium Battery Protection Board.
www.ebay.co.uk
Note:The balancing board must need BMS to use together,and the BMS is without balancing function.
www.ebay.co.uk
1S / 2S / 3S / 4S / 5S / 6S / 7S / 10S / 13S 18650 Lithium Battery Protection Board BMS PCB Specifications: Fine workmanship and durable. For the rechargeable (generally refers to the lithium battery) from the protection of the integrated circuit board. Lithium battery (rechargeable type) so...
www.ebay.co.uk
To find ones for a specific number of series cells, add eg. 1S, 2S, 4S or whatever the cell count is, to the search.
Make sure the module is rated at far higher current than the motor rating you will be using (presumably for a flywheel gun?)
Dc motors take many times the rated current when starting or if loaded so the speed drops down a lot.
And, be sure the cells are rated for an even higher discharge current! Too high current can overheat the cells and cause failure.
As well as the battery protection, you still need a proper lithium cell or battery charger. The voltage regulation is critical, and it must have a current limit within the cell charge current ratings.
An alternative approach for multi-cell batteries, as used with such as radio controlled flight systems, is have the balance part in the charger rather than the battery.
In addition to the main power connections, wires are brought out from the pack end and each cell to cell junction, with the charger having both power and multi-way connectors to suit.
Chargers for this system are generally fully programmable for cell type, count and charging current.
Example:
Find many great new & used options and get the best deals for Turnigy Accucell 6 50w 6A Balance Charger 1-6S LiPo LiHv LiFe NiMh NiCd PB+LEAD at the best online prices at eBay! Free delivery for many products.
www.ebay.co.uk
Another advantage with those is that they can often operate either from such as a laptop power brick, or 12V from a vehicle.
(Or use standard size cells such as 18650 type, in holders, & remove / exchange them for charging).