Building a charging circuit for a 2 cell Li-ion battery

Thread Starter

toomanylegos

Joined Feb 21, 2020
3
Hi, I am in the process of designing a battery to use with a pair of FPV goggles for drone racing, the goggles require a 2 cell battery, but I don't have a 2 cell battery charger. I do have a bunch of TP4056 charging boards, and I was looking to design an all in one unit that would hold 2 batteries, the two charging circuits, and some sort of switching device in order to safely charge the two batteries. My plan is to use something like the pic attached, where the circuit would automatically detect a USB input, and disconnect the batteries from each other and the output, and reconnect them with the appropriate charging circuits. I don't know what type of electronic switch to use, so I just labeled them as relays. If anyone has ideas on what type of switch to use, or just flaws in this circuit, please let me know! (also sorry for the sloppy diagram, it's my first time drawing a circuit.)Charging circuit.PNG
 

Alec_t

Joined Sep 17, 2013
11,746
Welcome to AAC!
Your drawing is fine and the scheme looks workable. However, I suspect the cost of implementing all the switching with either relays or transistors could outweigh the cost of using an off-the-shelf charging module designed for a 2-cell battery. Have you considered/costed one of those?
 

jpanhalt

Joined Jan 18, 2008
10,518
With individual charging, why would you need a balancing circuit?
The title calls it a 2-cell battery, and the voltage is shown as 7.4V. The implication is a two-cell pack (i.e., a 2S battery pack). If the author really meant two, one-cell batteries, then balancing is not needed.
 

Thread Starter

toomanylegos

Joined Feb 21, 2020
3
Yeah, looking at the complexity of this I think it would be better to use something like this. Do you guys think there is any downsides of integrating this charger inside the battery? Essentially just leaving the charger plugged into the battery at all times?
 

jpanhalt

Joined Jan 18, 2008
10,518
It's described as a balance charger, and it does have the type of connector typically needed for a 2-cell (2S) pack. Does your pack have the appropriate female plug? The LED indicators are pretty typical. Of course, you have to depend that the settings are what you want.
 

Thread Starter

toomanylegos

Joined Feb 21, 2020
3
It's described as a balance charger, and it does have the type of connector typically needed for a 2-cell (2S) pack. Does your pack have the appropriate female plug? The LED indicators are pretty typical. Of course, you have to depend that the settings are what you want.
I’m going to build my own battery out of 2 4000mah batteries that I harvested out of a portable charger, and I think I’ll end up chopping up the charger and directly soldering it to the pack. I’m going to remove the USB input and replace it with a female micro USB, and maybe design a 3d printed enclosure for the whole thing. 800ma rate is pretty slow for a 4000mah pack though.
 
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