how to prevent a connected charger from draining battery when not charging?

[flat-earther]

I have an RC radio handset(Taranis QX7) which doesn't have internal battery charger which means I had to remove the battery every time I needed to charge it.
So to improve it, I added a 15W USB-C charger board(link) inside and soldered it to the same connector on the PCB where the battery plugs into, great, now I can simply charge it with a USB-C cable.

but the problem I found is that after the radio sat for two weeks without being used the 2S 18650 battery that I'm using was at 4.2V!
So the battery is backfeeding the charger and being drained.

How can I prevent the battery from being drained?
Maybe some kind of diode between the charger and PCB?
If not then is it possible to add a mosfet between the charger and the PCB, and somehow make the mosfet turn on while charging but then turn off otherwise so no current can flow from the battery to charger?
I don't know if those ideas are possible because I'm not knowledgeable enough about electronics, what do you guys suggest?

[stj]

the BMS 3wire bit means a diode or fet wont work.
maybe a tiny 2pole relay to only connect the board to the cells when it has 5v going in will be a good method.

[flat-earther]

the BMS 3wire bit means a diode or fet wont work.
maybe a tiny 2pole relay to only connect the board to the cells when it has 5v going in will be a good method.

thanks stj,
so two of these 5V relays would work? SRD-05VDC-SL-C https://www.aliexpress.com/item/1005007905038217.html
is that the type of relay you mean?
I'd have to see if there's room to fit those.

I just wonder how other radios which have built in internal charging deal with not draining the battery? they don't have mechanical relays.

[Unspun01]

The 3-wire battery management system uses B+ and B- with a common wire. Perhaps you only need to disconnect the common wire. If you mean to accomplish this with a relay, I think disconnecting the common wire when 5V not applied is simple to try.

[stj]

maybe the bms can be disabled,
can you get a datasheet for the chip?

[flat-earther]

The 3-wire battery management system uses B+ and B- with a common wire. Perhaps you only need to disconnect the common wire. If you mean to accomplish this with a relay, I think disconnecting the common wire when 5V not applied is simple to try.

Thanks, so I may try disconnecting the common wire and letting it sit and see if the battery still gets drained.

[flat-earther]

maybe the bms can be disabled,
can you get a datasheet for the chip?

What do you mean by disabling the BMS?
The charging chip is marked IP2326 and I found a datasheet for it. https://www.chipsourcetek.com/DataSheet/IP2326_V1.2.pdf

[lotas]

BMS - Battery Management System.

[stj]

if the module lets you disconnect pins23 & 24 it will disable 3wire mode and not try to balance the cells

[flat-earther]

But I'm using a 2 cell battery, obviously I need to keep the balancing feature enabled.

Anyway I have removed the BM middle wire and left the battery connected to the radio for 23hours, at the start of the test the battery was at 7.27V, after 23h it's at 7.16V.
So the battery is still being drained even with the balancing wire disconnected.

So the only option is to install two of those relays right? one on B+ and one on B-

[stj]

you can get a relay with 2 sets of contacts in it.

[flat-earther]

you can get a relay with 2 sets of contacts in it.

can you share link of such 2 pole relay and I'll see if I can get any, otherwise I'll get those that I found, I think there's enough space to make two fit.
the relay I linked above was like the only 5V one I found and it's electromechanical which is what I should get right?
Since electromechanical physically disconnects the contacts, I can be sure the battery won't be drained.
There are also solid state relays, but I worry maybe they have some resistance even when off and can still drain battery?

[stj]

https://www.aliexpress.com/w/wholesa...pdt-relay.html

[flat-earther]

https://www.aliexpress.com/w/wholesa...pdt-relay.html

Thanks, that looks good HK19F-DC5V-SHG https://www.aliexpress.com/item/33007295607.html

I read somewhere about needing to add a diode on a relay coil.
is that so? do I also need to buy some diode?

[stj]

a 1N4148 diode will do it - they are used on just about everything so you can probably recover one from some junk.
they are only worth a couple of pennies btw.