State of Charge

State of Charge is a calculation meant to help estimate remaining energy in the Battery.

Concepts

SoC is a porportional representation; a percentage (fraction), so it is only applicable with a known total or max.

Riders all have experienced different ranges under different temperatures, various environmental conditions etc.

So full is really a subjective term, like asking When is a balloon full? Before it pops? But if you fill it with cold air then move to a warm environment, it pops.

• Paraphrase of ElectricZen's explanation

SoC is most easily calibrated using the battery pack's voltage at rest or near rest.

Battery Voltage

This is a rough guide to correlating at rest battery voltage to the state of charge.

There is respectable unofficial guidance that this curve varies as the pack ages over its lifetime.

These figures are determined from some selection of owners with logs, with minimal experimental controls.

Warning:

Your mileage may literally vary!

% SoC vs Voltage

Voltage vs % SoC

Voltage per SoC At Rest

VPack	VCell	SoC
115.9…116.2	4.14…4.15	100
113.9…114.24	4.07…4.08	95
112.0…112.3	4.00…4.01	90
109.76…110.8	3.92…3.96	85
108.36	3.87	80
106.96	3.82	75
105.56	3.77	70
104.16	3.72	65
103.04	3.68	60
101.92	3.64	55
101.36	3.62	50
100.80	3.60	45
99.68	3.56	40
99.12	3.54	35
98.56	3.52	30
98.00	3.5	25
97.16	3.47	20
96.60	3.45	15
96.04	3.43	10
95.76	3.42	5
95.20	3.4	0

(Obsolete) SoC Indications per protomech

Before Zero removed pack capacity readout from the mobile app, there were some corroborating indications available:

• The app appeared to simply show kWh using Zero's max capacity rating, i.e. present voltage times present amp hours.
• The voltage and amp-hours were likely to be correct, but the kWh readout was a rough guide at best.
• The percentage reading adjusted to the estimated pack maximum over time, so as the pack aged, the reported Ah and kWh readout would decrease, but not the maximum percentage.

Variance across Battery Pack Years

• The sensor equipment measuring current and temperature for the battery pack improve over the years, impacting SoC indication.

  2017 model year bikes appear to have an increased range of current sensor calibration quality, improving that year's estimations.

• Cell behavior also varies over the years, generally improving in robustness.

  One can find multiple complaints of going into limp-home mode at 5-15% from 2014 SR owners, for example.

Variance by riding conditions

• Cell discharge is an electrochemical process that depends on the cells' physical conditions.
• The resistance of the cell (which should be low) increases with environmental factors like temperature.

  This results in reduced performance, since the resistance lowers the terminal voltage of the cells and the overall battery pack.

• At low SoC values, SoC might drop considerably once the BMS observes low terminal voltage as an indirect result of low temperatures or a cell imbalance.

  The lowest-performing cell becomes the limiting factor in pack performance at low voltage / SoC, and Zero does not provide indications to help you anticipate or diagnose this.

Other References

Inaccurate SoC Estimate

How OEMs calculate Battery Capacity, and why it's not totally their fault they are wrong by Brandon Nozaki Miller