Voltage is just a potential force, the real force comes from current drawn by a load from this potential and is limited by the resistance of the load.
Current is the rate of charges flowing per second.
Voltage is like Water Pressure or the potential to supply water, that may drop when too much current is demanded like water pressure drops from a larger valve to flow more current. THen when shut the pressure builds up again or the potential to supply voltage. This variation depends on the impedance or resistance of the voltage source or battery. Coin cells are very high R in the kilohms and LiPo packs are in the xx milliohm range.
like a bucket brigade of water and this potential which is generated by "something" may be alternating current, AC or direct current, DC flowing just in one direction and continuing to "return" in the return path.
In a car lead acid battery there are 6 2V cells in series with only 2 terminal posts +,- giving 12.5V at full charge.
The energy is converted from the chemical reactions into charges on the electrode plates. This is just a potential voltage of 2V per cell. If we had just a 2V cell Pb cell or an Alkaline 1.5V cell with a load resistance to wires across the battery, ....
... then the charges flow from the acid to the electrodes to the wires to the load and back thru return wire to V- on the battery and then to the -ve electrode thru the acid all around in a circle, until the load switch is opened anywhere in the loop of current. The heat or power generated by this current is greatest where the resistance is highest. So wires, connectors must be very low resistance.
The bigger the battery, the lower the inner electrode resistance ( in milliohms ) and the more current it can generate to a load following Ohm's Law.
Since "secondary cells" are rechargeable the charge flow can be reversed and put back into the acidic chemicals which can be measured by "specific gravity" of the acid relative to water.
Similar but different chemistry occurs in Lithium cells which have a different voltage potential per cell.