I suspect that you have voltage drop between the charger and the battery.
with a good volt meter measure the voltage at the bat connections on the Charger and then measure the voltage at the batteries. They should be the same or very close.
if there is a difference then you have a voltage drop mainly due to wthe wires being to small.
1 install bigger cables
2. install a smart bluetooth battery sense unit (the 150/70 must be a smart unit or you would also need a bluetooth connection dongle)
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