Using Voltage to Determine Battery Life / SOC

Overview

You may have noticed that as a battery discharges it's voltage goes down too. We can therefore, after making a few calculations and adjustments, use a battery's voltage to determine it's SOC (State of Charge) with decent accuracy. 

Calculation Considerations

A few factors can affect our voltage to battery % calculation. Among the largest are: load (how many amps the battery is discharging at that moment), cell being evaluated (which chemistry or model), temperature of the battery, and general health of the battery. 

Model and Load

Assuming the battery is at a moderate temperature (between 10C/50F to 30C/86F) and in good health (see full details below) then the load that the battery is currently under is the largest factor in our pack % to voltage consideration. All that's needed then is to find the model and it's voltage and you have your estimated battery % / SOC! Titan has analyzed a number of cells, here is a link to a spreadsheet with our current testing results.

Temperature and Battery Health

The colder a battery gets (temperatures lower than 10C/50F) the lower it's final capacity and it's operating voltage will be. The below graph will give you an idea of how much cold temperature affects lithium ion / polymer batteries. Hotter cells (over 30C/86F) when the discharge cycle begins won't affect cell voltage as much for the lower discharge rates, but for higher rates, the pack will reach it's thermal limit quicker (and therefore should stop discharging) than it would otherwise. Titan has planned to do this testing late 2018.

Battery Health

Finally, a battery's history can affect it's performance. Events like overcharging above 4.2V per cell, discharging at a higher than designed rate, long term storage at fully charged or very empty, being left in a very hot environment, etc. Can affect a pack's voltage and final capacity.