Charging 2x 3.7 Lipo but using the full 7.4v for project

Question

Hello, I'm trying to power a 15kg servo with 2x 3.7 600 mAh Lipo batteries (in series to use 7.4v), and I'd like to create a system to charge those batteries. I bought TP4056 but I realised that, although charging one or even two in parallel is pretty straight forward, I wouldn't know how to then connect the rest of the system to the full 7.4V without blowing anything. Pretty sure this is a noob question and apologise for that Thanks in advance for your help 
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the-dubster · Accepted Answer

You need a Battery Management System (BMS) board - for 2S in this instance (2 cells in series).

The charger balance charges them properly, protects them against over charge, over discharge and over current if you get a reasonable one.

The only real consideration is the required output current from the cells (what the project you're powering demands), that will decide the power requirements of the BMS, try to draw more than it supplies and it will (should) shut the output down.

example board on eBay:

https://www.ebay.co.uk/itm/2S-8A-7-4V-8-4V-Protection-PCB-Boards-For-Lithium-LiPo-Li-ion-BMS-18650-Battery/164322871242?…

Example wiring: (from that link)

This is not a recommendation, just an example, others are available out there is you search.

the-dubster · Answer

A BMS has a Battery +ve connection, a Battery -ve connection and a Balance connection (as shown in the linked diagram).

Measuring across B+ and BM will give the voltage of 1 cell, if you measure across BM and B- it gives the voltage of the 2nd call, and finally across B+ and B- you get both cells in series - full pack voltage.

The charging also uses the same connections and 'balances' the cells to ensure they are both charged to the same voltage.

Cheaper options with only B+ and B- are available, and will work to a degree, if the cells manage to maintain balance, or you remove them periodically and charge them individually. The only problem I have is during discharge, if 1 cell drops lower than the other then you can run into problems if it over discharges (fire is a real possibility here - that cannot be overstated).

Having a proper balanced charge/discharge circuit alleviates that issue, and the Aliexpress image you show doesn't appear to have that function from the connection pads I see.

Also, with an input of max 8v and an output of up to 27v - this is a boost converter too - not required if you just need the voltage supplied by a 2S setup - if it's to drive servos (as you state) then I guess you need more than the 7.2 - 8.4 from a pair of LiPo packs.

Be aware, depending on the project, Buck (step down) and Boost (step up) regulators can be electrically VERY noisy as I'm sure some folks here can attest - the project you intend to run will decide whether noisy (pretty high ripple) DC is acceptable - you're not just using it to power the servos here, it wouldn't be advisable to just connect your project directly to the battery contacts and use vout for the servo - there is zero protection for the cells - UNLESS, the cells themselves are protected . . . . .

EDIT:Looking at the cells in the picture, they DO have protection onboard for overcharge (and discharge I would expect), I'd personally still advise balanced charging.

Charging 2x 3.7 Lipo but using the full 7.4v for project

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