I have 2 series Li-ion batteries which are connected to a 2s battery protector. I want to indicate battery level only when a button is pressed. The problem starts when I measure battery level while charging. If I connect the indicator across the P+ and P- it will assume the adapter's voltage as battery voltage thus always give fully charged result. That's why I want to connect the battery level indicator across B+ and B- as below:

new connection

Is it something I should avoid or is it okay to bypass the protector?

The indicator:


The protector:


The charger adapter is 9V/1A

  • \$\begingroup\$ I would put a few ohm in series just in case. \$\endgroup\$
    – winny
    Commented Aug 26, 2022 at 13:54
  • \$\begingroup\$ In case of a short? Well yeah, I can put a fuse for example, if anything else is okay. \$\endgroup\$ Commented Aug 26, 2022 at 13:59
  • \$\begingroup\$ Sure, but a resistor is cheaper :-) \$\endgroup\$
    – winny
    Commented Aug 26, 2022 at 14:04
  • \$\begingroup\$ A Li-Ion battery is NEVER charged directly from a power supply. It is charged from a charging circuit that limits the charging current which reduces the charging voltage. When the charging battery reaches its maximum of 4.2V per cell it is still charging. When the 2s discharging battery voltage drops to 6V it is just about dead and should have its load disconnected. \$\endgroup\$
    – Audioguru
    Commented Aug 6, 2023 at 19:33

1 Answer 1


You'd lose the safety that your battery protection circuitry is supposed to give you against shorting. So, don't.

Your li-ion charger needs to keep track of the charge state itself, so it knows the charging state, which is effectively impossible to read just from the voltage over the battery during charging, anyways, since that always incorporates temperature changes and time-dependent behaviour.

  • \$\begingroup\$ There are only 5 Leds on the indicator thus the resolution is really low. So slight wrong measurements via voltage won't matter I guess. Also, can you give me a better way to measure the battery level? Thanks. \$\endgroup\$ Commented Aug 26, 2022 at 13:42
  • \$\begingroup\$ "impossible to read" does not mean "more accurate than 20%", it does mean "useless for estimating the charge state". I gave you a better way: ask the charge controller. A 5-segment display seems pretty good. Getting any more precise is going to be relatively hard without counting ampereseconds. \$\endgroup\$ Commented Aug 26, 2022 at 13:43
  • \$\begingroup\$ How can I extract the battery level information from the charge controller? Should I tap into some pads of the ICs? \$\endgroup\$ Commented Aug 26, 2022 at 13:54
  • \$\begingroup\$ I don't know the charge controller, so I can't tell you that. You bought the charge controller, so you should have its datasheet! \$\endgroup\$ Commented Aug 26, 2022 at 14:02

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