I bought 4 of those overvoltage relays(4 dollar each)and could not get it to work on my 4s battery. I had the same idea, it will work with a 1s though, as long as you have a seperate power supply. Now I use these as overvoltage protection relays, they didnt go to waste. On these the minus of dc input is the same as minus that records the dc inputvoltage. Ialso tried adifferent relay that went up to 99 volts and that one also had the minus (ground) connected together. If the minus groundwas isolated those would be perfect for what you want to do.
If you want something to bleed balance, they sell the babybloc balancer those bleed almost 2 amps and have over/under alarms, they start bleeding at 4.15 volts but the person that sells them will adjust the voltage to what you require, cost 13 dollars per balancer. I decided to use active balancers but for a 4s thats in the 100 dollar range.
I found out on a battery that isn't too much out of balance, if you charge at 2 amps or less you get a very good charge, at higher amps the BMS will always cut out too early. With a battery that has balance issues you need these high amp bleed or activebalancers. The built-in bms balancers at 60ma is useless to keep a batterypack balanced.
babybloc balancer