4.1 Powering the system
The two bike batteries have different working voltage ranges:
the Dover from 21 to 30 V and the Velo-cité from 30 to 42 V. The
higher level represents the voltage level after a full charge. The
voltage must not drain below the minimum level or it will damage
the battery. SEMS connects to the e-bike battery in parallel, at a
place in the circuit before the e-bike’s drive system; this means it
can run when the bike is not switched on, but also means that it
needs to provide its own battery management system to prevent
over-draining. This management is provided by an under-voltage
lockout circuit, which cuts off power to the system below a preset
threshold, and restores it when the input voltage rises again. The
circuit is designed with a low-power LF442 op-amp to minimise
current drain. Both e-bike battery voltages are too high to power
the IOIO board which runs at a maximum of 15 V, so the voltage
is stepped down using an efficient LM2576 regulator. There are
two versions of SEMS, one for each model of bike. The two
versions are very similar, with small differences due to working
voltage ranges and assistance monitoring methods (see Section 4.4).