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

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.

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.

Thanks for your idea. I've searched and found that this is actually a great solution for other projects that I have.

For this project I'm trying to go for a more all in one solution, with LED indicators for charging status and mini or micro USB input.

The output current will be around 700mAh.

I found this on Aliexpress:

Specifications:

Input voltage: 4.5-8V DC

Output voltage: 4.3-27V DC (Continuously adjustable)

Charging voltage: 4.2V DC

Charging current: Max. 1A

Discharging current: Max. 2A

I think it will charge both batteries plus step up the voltage for the servo. Would you think is a good solution or am I missing something here?

Thanks again

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.

Thank you so much for your very in dept explanation. I have now a pretty good notion of the BMS potential.

I did have a lot problems with buck converters in a previous project where I created a FPV system. Drove me crazy to identify what was the culprit of the image interference. Never managed to get completely rid of it in the end.

I think that the noise might not be that relevant in this system but, have to try first.

Thanks again for your time