I have designed & assembled a Tri-Quad (less than $25.00) to monitor the battery pack on my electric motorcycle conversion
It uses an Amp/Volt meter (~$15.00 delivered) to monitor the energy consumption & battery pack voltage "capacity"
(Amp: displays the amperage drawn by the motor & Voltage: displays the "total" battery pack voltage)
…& (2) individual Volt meters (~$3.00 ea. delivered) is to monitor the balance of the battery pack
(One will display the voltage reading of 1/2 of the battery pack & the other will display the voltage reading in the other 1/2)
* As long as they both show the same reading, the battery is "in balance" or "balanced"