[Old Tele man]

...just a curious question: are you wanting to measure the quantity of current (AMPERES) going into/out of the battery, or the amount of electrical energy (CHARGE) remaining IN the battery?

...they're two different things: AMPERES is rate-of-charge (or discharge), like gallons-per-second, whereas CHARGE (COULOMBS or WATT-SECONDS) is like remaining volume, how much there is left in the tank/battery.

...some of the semiconductor companies actually make specialized IC-devices to measure battery CHARGE, although they're usually targeted towards laptop computers and cell-phones, but they CAN be scaled upward to handle 100's to 1,000's of times larger capacities.