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ITGlossary · Batteries
State of Charge (SoC)
A battery's current charge level as a percentage of its capacity, from 0% (empty) to 100% (fully charged).
Technical level:Basic
SoC is to a battery what the fuel gauge is to a car. Together with DoD it adds up to 100%: a battery at 70% SoC has delivered 30% of its capacity.
In lead-acid batteries it can be estimated from the rest voltage, once the battery has stood for a few hours with no load and no charging. In LiFePO4 batteries the voltage stays almost flat over most of the discharge, so the BMS measures the current flowing in and out to calculate the SoC.
Do not leave a lead-acid battery at a low SoC for days. Hard lead sulphate crystals form, a process known as sulphation, and permanently reduce its capacity.
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Related terms
- Battery Charging CurrentThe amount of current supplied to charge a battery, regulated by the UPS system.
- Equalising ChargeA controlled charge at a higher than normal voltage that brings all cells of a lead-acid battery to the same state of charge.
- C-rateThe charge or discharge current expressed as a multiple of battery capacity, e.g. 1C for a 100 Ah battery means 100 A.
- Fast ChargingA charging method that restores battery capacity quickly without damage by controlling current, voltage and temperature.
- Ampere-hour (Ah)The unit of battery capacity, showing how much current a battery can deliver and for how long, e.g. 9 Ah means 0.45 A for 20 hours.
- Cycle LifeThe number of charge and discharge cycles a battery withstands until its capacity falls, usually, to 80% of the original.