other things is based on the State Of
Charge (SOC) of the battery pack.
If the SOC is low, the management
system will recharge when
descending a hill, braking, or use any
surplus energy from the engine. If the
SOC is high, then the battery pack
will be used to drive the car at low
speed or to supplement the engine
when driving, climbing hills, or
overtaking. In practice, the SOC
is moving about the entire time,
dependent upon traffic and
I saw two main problems in
adding a large battery pack in parallel
with the existing battery. The first was
what would the reaction be from the
Toyota management system if — out
of the blue — the existing battery
started receiving charge from an
outside source — the second battery!
The second problem was how to
control this external charging source.
The control system needed to be
such that the existing batteries’ SOC
could be manipulated so that the
Toyota management system saw a
high SOC and used the battery instead
of the engine wherever possible.
The first problem was simple. I
connected my EV charger across the
Toyota battery pack and charged the
pack. The SOC increased up to fully
charged (about 80% SOC). The
battery manager took into account
the pack temperature and voltage,
and computed the SOC quite
happily. Solving the second problem
— transferring energy to the
Toyota’s battery — was the main
area of work.
I was lucky enough to have
acquired a set of 56 Thunder Sky
Li-Ion cells which I could use as a
second battery. These are connected
in series to give a resulting DC voltage
of around 210V and more than 50
Ah. The Toyota’s NiMh battery
produces around 240V DC, so I
knew that I would need an inverter
to allow the additional battery pack
to charge the Toyota’s own battery.
I also wanted to be able to recharge
the Li-Ion batteries overnight, so I
needed a recharge circuit. I needed a
circuit to control the flow of charge
into the Prius’ own battery, as well.
You can see the circuit in Figure 2.
■ FIGURE 2. Block schematic
of the system.
August 2008 57