I’ve not tried it but you should be able to disable the battery percentage entity that’s provided by the device, create your own helper template getting the correct voltage to percentage you want, remove the battery note for the device and re-add it just to be safe and it should only find your helper one.
Oh, I’ve just re-read it, you are not getting the voltage so disabling the sensor won’t give you anything to base your template on.
You’re not going to be able to get an adjusted battery % unfortunately with this method, but you can still use the low threshold, just set to higher than normal, to throw events that it’s low.