What is #SOC mean in battery & how to determine SoC indirectly 
State of charge ( SoC) is the level of charge of an electric battery relative to its capacity. The units of SoC are percentage points (0% = empty; 100% = full). An alternative form of the same measure is the depth of discharge ( DoD), the inverse of SoC (100% = empty; 0% = full). SoC is normally used when discussing the current state of a battery in use, while DoD is most often seen when discussing the lifetime of the battery after repeated use.
In a #battery electric vehicle (#BEV), hybrid vehicle (HV), or plug­in hybrid electric vehicle (PHEV), SoC for the battery pack is the equivalent of a fuel gauge. It is important to mention that state of charge, presented as a gauge or percentual value at any vehicle dashboard, especially in plug­in hybrid vehicles, may not be representative for a real level of charge.
In that particular case, some noticeable amount of #energy stored in the electric battery is not shown at the dashboard, and it is reserved for hybrid­work operations. It permits a vehicle to accelerate with electric motors mainly using battery energy, while engine serves as a generator is used to recharge the battery to the minimum level needed for such operation. Examples of such cars are Mitsubishi Outlander PHEV, where 0% of the state of charge presented to the driver is a real 20­22% of charge level (assuming zero level as the lowest level of charge permitted by car producer). Another one is BMW i3 REX (Range Extender version), where about 6% of SOC is reserved for PHEV­alike operations. Teslas have stated that their SoC should be less than 95%, with some saying between 30%­80%. There is some data to back this up as well.
Usually, SoC cannot be measured directly but it can be estimated from direct measurement variables in two ways: offline and online. In offline techniques, the battery desires to be charged and discharged in constant rate such as Coulomb­counting. This method gives precise estimation of battery SoC, but they are protracted, costly, and interrupt main battery performance. Therefore, researchers are looking for some online techniques. In general there are five #methods to determine SoC indirectly:
◾ chemical
◾ voltage
◾ current integration
◾ Kalman filtering
◾ pressure