Just finished a prototype of a simple multicell LiPo charger which is part of a larger project. Might be of some use to others and the DC-DC 1W modules are worth a look for other projects too. There is certainly lots of alternate solutions that work some of which I own but this design was because I had the modules in stock. Also the cost of bits are around $10 AUD for a 4 cell charger.
There is no circuit diagram for it at this stage nor do I really want to rehash BS of BMS boards it is a low current charger that is all!
However for those who are not aware due to the normal series connection of the cells for charging some form of isolation or floating ground is needed on the charging circuit to enable cells to be charged individually and maintain less than 4.2V/cell (puff voltage) as the others finish topping up.
If you want a Rapid charger for multicell packs then there is plenty on the market already this is a 'float' style charger for circuits under 50mA draw.
Basic Specs.Maximum charge current 180mA with other steps of 120, 60 and 30mA. The 180mA max is governed by the 1W DC-DC.
Input voltage range of 4.5-5.5V DC.
Basic Blocks and Components.5x7cm Protoboard (I had a stack of rubbish ones already)
B0505S 1W Mornsun 5V DC-DC isolated converter.
http://www.mornsun-power.com/uploads/pdf/B_D-1W.pdfTP 4056 based single cell Lithium charger board (cutdown).
https://dlnmh9ip6v2uc.cloudfront.net/datasheets/Prototyping/TP4056.pdf3 Pole DIP switch
6.8K, 3.3K, 10k and 20K 1% 1206 Resistors but through hole would be fine.
Stash of wire, connectors etc as per the photos.
Construction Notes.Take the evilbay TP4056 board and chop it as per the first photo also remove the current set resistor. The board is double sided with the lower surface being a ground plane for most of it. One side of the 0.4 ohm resistor is Vcc and In- and Bat- are common. Bat+ is obvious. Using some cutdown angled header pins I effectively staked the board to the 5x7 board. If you use double sided protoboard raising the board a touch or some kapton tape would be in order.
The surface mount resistors fitted nicely between the bent over DIP switch pins and while it won't win any beauty contests it works nicely. The switches short or bypass the resistors to limit current and the 6.8K is unswitched. The formula for calculating the resistors is in the datasheet above but 6.8k = 180mA, +3.3K = 120mA, +10k = 60mA and +20K = 30mA
The pad the normal current set resistor goes to runs through the resistor chain to ground so 6.8k to 40k max.
Output wiring/header loops or shorts the Bat+ of the first cell to the Bat- of the second cell. In the case of the shown 3 cell Bat+ of the second board loops to Bat- on the third board.
Minor Issues!Output Ripple of the Mornsun modules varies a lot so some playing with Caps to improve that. Output Ripple on the chargers in use is virtually Nil.
The Mornsun Modules generate a little heat so standing vertically will help dissipation. 70% efficiency but still up to 0.3W.
If anyone is really keen I can throw a wiring diagram together but the App notes above and photos should be ok to have a go for most.
Only 7 or 8 more boards to go