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VP-X Installation and Operating Manual
Rev. D (August 5, 2020)
3.7 Alternator Current Sensing (Shunt)
If you are not using a Vertical Power PPS, you must consider whether to
wire an ammeter (usually a shunt or hall effect sensor provided with the
engine monitor) on the wire connecting the alternator(s) to the main bus. The
ammeters indicate the amount of current the alternator is providing.
A shunt is not required to tell if the alternator is working. It is very easy to tell
if the alternator is working correctly by simply looking at voltage. If you see
14 (or so) volts with the engine running then it is working. If you see 12 (or
so) volts it is not working or not turned on or the devices are drawing more
current than the alternator can provide (note, engine must be running). If you
set your low voltage alarm on the EFIS at 13 volts, then you will get a low
voltage alarm if the alternator fails.
Since the VP-X provides basically the same information as a shunt installed
on the alternator b-lead, our position is that adding a shunt gives you no
additional meaningful information.
A shunt on the alternator b-lead shows the amount of current the alternator is
providing to power the devices and charge the battery. The VP-X total current
reading shows the total amount of current the devices attached to the VP-X
are using. The delta between the two is the battery charging current, which
goes to (basically) zero after re-charging any loss from starting the engine or
charging a run-down battery.
If the battery charging current is important to you, then you should install
a shunt. If not, then simplify your wiring and don’t install it. Your call. And
of course each builder’s needs are different so there is no absolutely right
answer.
The EFIS displays a VP-X page which shows individual device current as
well as total current through the VP-X. The EFIS also has an ‘Amps’ gauge
that is used to show the readings from the shunt. In some cases the EFIS
‘Amps’ gauge can be used to display total system current from the VP-X.
Please check with your EFIS manufacturer for details.
If you don’t install the shunt then the shunt wires on the engine monitor/EFIS
are not used.
3.8 Bench Testing the VP-X
To test on the bench, connect the VP-X to a power supply and connect either
J10-9 or J12-4 to ground. Be aware that short circuit protection may not work
properly in this case. You must have the VP-X connected to a battery with at
least 8 AWG wire and good solid connectors.