The State of Charge (SOC) Calculation Lead Acid batteries
Voltage only gives a rough estimate of State of Charge (SOC), the table below is just an example, the voltages can change significantly when the battery is under load, in most canal boat there is always something using the battery, our fridge while efficient is always on, so to measure this properly you need to have the battery disconnected and allowed to rest for up to a couple of hours (Not always the simplest thing to arrange).
| Capacity % | Resting Voltage** |
| 100% | 12.70 V |
| 90% | 12.50 V |
| 80% | 12.42 V |
| 70% | 12.32 V |
| 60% | 12.20 V |
| 50% | 12.06 V |
**Note: Example Only – typical lead acid battery. Best to check with your battery supplier if you use this method.
For your battery monitor to be effective you need to get the battery to a known state of charge to set a base/synchronisation point for future calculations. This is typically done when the battery is at 100% State of Charge (SOC).
You can tell when a battery is fully charged by looking at the battery charger. When you charge a battery, the charger will start at its maximum current; this is known as the boost phase, during this, the voltage of the battery will increase to near the gassing voltage.
What is Gassing and why is it so bad
Gassing is basically very bad, as gassing mean that the current applied to the battery is not being used to transform the plates back to their fully charged state but is breaking the water down to Oxygen and Hydrogen, which is a highly explosive mixture, try to remember back to school science lessons and electrolysis.
A smart battery charger will monitor the batteries internal resistance so then when it reaches near 80-85% charged it will switch over to the next phase known as the absorption phase. The last 15-20% will take a lot longer for a battery to absorb. It was explained to me much like the seats in a cinema, when the cinema is empty it is easy to find a seat, but as the cinema fills up the available spaces are harder and harder to find and fill.
The Float Phase
Once charging is complete the charger will switch to the float phase, where the voltage is kept at a point to maintain the battery capacity, this is often referred to as trickle charging.
State of Charge (SOC)
With the battery now at 100% charged, we have our known State of Charge (SOC). A battery monitor can use this to calculate what usable power is available while monitoring further charging and usage of the battery.