23
CPi2 Power source switching and reaction to Mainbus voltage
The CPi2 has dedicated circuits to constantly monitor Mainbus voltage and will immediately and quickly switch over
its power source to the backup battery if the Mainbus is below 10.3 volts with a Pb main battery, or 11.7 volts with a
Lithium main battery. The CPi2 will also switch over the ignition coil +12v power to the backup battery at the same
time. Switching does not interrupt the CPi2 and it will continue running without any reset and the ignition system will
continue to run without misfire. Coil power is switched using the latest Mosfet technology with only 50 milliohms
resistance and result is very little power loss and very little heat in the switching circuits. There is some hysteresis
for the voltage switching and Mainbus must return to 11.8V(Pb) or 12.6V(Li) for the CPi2 to switch back to Mainbus
power. This hysteresis helps prevents oscillation or continuous switching back and forth from Mainbus to backup
battery power. If the battery voltage drops down too far though, the CPi2 will switch back to Mainbus voltage if
Mainbus is below 12.5, at this point there will be some oscillation back and forth and you would see the MAIN LED
flicker.
Power down of the CPi2.
Due to the ecu having its own backup battery, power down is unique on the CPi2. When you turn off Mainbus 12 volt
power to the CPi-2 it automatically switches over to its backup battery. If there is no engine rpms, and no keypad
keys are pressed, it will begin a power off countdown within 5 seconds, and after counting down from 10 to 0 it will
shut off unless Mainbus power is switched back on. So total power off time is approximately 15 seconds.
Power up of the CPi2
The CPi2 normally powers up when voltage is present on the purple wire going into the 14 pin main harness
connector. There is no supplied power on switch with the CPi2 system. The intention is for the installer to wire the
purple wire to their own panel mounted power switch. There is no way to power ON the CPi2 from its backup
battery.
Battery trickle charge
The CPi2 will send a trickle charge to the backup battery to keep it topped up. Normally this is enabled by default in
the CPi2. If you have some other charging system onboard the aircraft for your backup battery then you should
disable the charge feature on the CPi-2, this is discussed earlier in the manual in the Batt Charge section. Having
two different systems charging the battery could be bad for the battery, and may also confuse the CPi2. Charge
current is limited in the CPi2 circuitry so there is no damage to the battery. Charge current will be less than 100mA
typically on a partially run down battery. On Dual board ecu’s, the “B” backup ecu will not send any charge current to
the battery, as this would cause confusion in the system.
Do not connect a dead or really discharged battery to the CPi-2 because the CPi2 will not attempt to charge a
battery in a discharged condition. This is not practical with any backup battery because charge time would take a
very long time and the CPi2 needs to see a battery voltage of at least 10 volts for charge system to function, below
10V charging turns off. If Mainbus power goes higher than 19 volts then charging also shuts off.
When connecting an external charger to AGM or other batteries, make sure you have the proper charger for your
battery. The battery recommended to be used with the CPi2 is AGM, and these batteries require a charger that
limits current to avoid damage to the battery. You cannot just bridge 12 volts over from Mainbus especially if the
backup battery is in a very low state of charge as the charge current would exceed the batteries rating.
The Backup battery can remain connected to the CPi2 without any switch. Current leakage is only 1.6uA on a Single
board CPi-2 and 3.2uA on a Dual Board CPi2.
Choosing your own battery.
Thinking about a lighter weight or smaller size battery? The one
concern with a smaller battery is not simply the amp-hour rating but also the ability to deliver surge current required
to fire the ignition coils. If two coil packs are running, the peak surge current can be around 14 amps, so running a
smaller than recommended battery may be very risky as voltage will dip too low with the current surge. This can
cause weak spark and worse, possible reset or shut down of the CPi2 earlier than expected.
Crowbar
The CPi2 has a built in crowbar and if Mainbus voltage goes above approximately 24 volts for several milliseconds,
the crowbar will short circuit and blow the 2 amp fuse on the side of the ecu disconnecting the CPi2 from your
Mainbus power. Trip time varies for the crowbar depending on how high the mainbus voltage goes. Higher voltage
equals quicker trip time. The crowbar has filtering to prevent triggering from short voltage spikes.
