State of Charge

How to calculate the capacity or remaining power in your battery


State of charge or capacity or AH and voltage of a battery are all related. The chart below shows the correlation between the voltage of the battery and the State of Charge. This means the amount of power left in the battery. Another way of addressing this is to use Depth of Discharge but for now, let's cover off the State of Charge.

If you were to use the PC2150 as an example this battery has 100Ah capacity so if the battery is 12.50 Volts with a state of charge of 75% it would also have 75Ah capacity remaining. This voltage is assumed to of occurred due to self-discharge of a battery for either a duration of time (up to two years) but the self-discharge accelerates when stored in a temp above 200C. Either way lets refer to this state of charge as its rested state.

You'll also note that the voltage doesn't go down to zero volts. You are doing damage to your battery below 10.5 volts and should never be left at this voltage if you want to use your battery in the future as it will quickly become sulfated.

Voltmeter Reading State of Charge
12.84 Volts or higher 100%
12.50 Volts 75%
12.18 Volts 50%
11.88 Volts 25%



Battery Load Voltage vs. Depth of Discharge

When a battery is discharged the speed in which the current is drawn will affect the amount of energy the battery is able to provide before going flat at 10.50 volts. So the 1 hour rate on a PC2150 is 73Ah. The 20 hour rate however is 100Ah, so yes it can provide 100Ah over a 20 hour period or 73 amp hour over 1 hour.

Depending on the current draw and it being continuous it can be cross-referenced against a comparable equivalent. We will use the 20 hour rate (i.e. the PC2150 is a 100Ah @ 20 hour rate, so the 20 hour rate is 5 Amps) The 5 Amp discharge will have the effect in the chart matching the 20 hr. Rate and because we are discharging we will use the term depth of discharge or DOD%. (meaning amount of energy drawn from the battery). There are many different hourly rates used for different battery applications but we'll focus on the main three, 1 hour, 8 hour and 20 hour rate in the following chart.

Note these voltages are shown with the battery being under load. That means energy is still currently being drawn from the battery and the voltage a representation of the depth of discharge at a given rate at 250C.

DOD% 1 hr. Rate 8 hr. Rate 20 hr. Rate
10 12.23  12.60 12.65 
20 12.16  12.51  12.55 
30 12.07  12.39 12.42 
40 11.96  12.25  12.28 
50 11.83  12.11  12.15 
60 11.70  11.98  12.02 
70 11.55  11.79  11.83 
80 11.38  11.59  11.61 
90 11.15  11.32  11.34 
100 10.50  10.50  10.50 
Back to top