PBO electrics expert Duncan
Kent answers a multi-part
electrical installation query
from reader Dan Haas
Q
I’d like to add a
windlass battery, to my
34ft Westerly Falcon, but
due to space and weight
limitations the most
convenient location is 5m
forward of the two domestic
batteries and 2m from the
windlass. I’m looking at
buying three new 12V/120Ah
AGM batteries. Charging is
via a 100A engine alternator
working through a Digital
Voltage Sensitive Relay
(DVSR) to prioritise charge
to the starter battery. I am
also considering adding a
310W/33.0V solar panel
through a MPPT controller. I
have fi ve questions:
DUNCAN KENT REPLIES
- Will resistance of the 5m
cable between the domestic
bank and the windlass
battery mean it won’t charge
much until the other two are
near full capacity?
Providing the cables are the
same size as you’d use when
directly wiring between the
domestic and windlass
batteries the difference in
charge rates will be minimal.
However, if the starter,
domestics and windlass
batteries are connected
together in parallel they’ll all
discharge simultaneously
(albeit slightly unequally) when
diameter wire could be used
exclusively for charging an
isolated windlass battery, if all
three are connected in parallel
then the required power will be
drawn from both the windlass
and domestic batteries
(assisted by the engine
alternator if running). So, the
current fl owing through the
adjoining cable will be the
same as if it were coming
directly from the domestics
you run the windlass, unless
they’re separated when the
windlass is in use. If you
choose to separate them with
a relay, remember no power
from the alternator will get
through to the windlass when
operating, which isn’t ideal.
- Will the windlass battery
ever reach full charge?
Yes, provided suffi cient
charge is supplied for long
enough and the cables are
correctly sized. - What cable size should I
use for the 5m run from
domestic bank to the
windlass battery and on to
the windlass?
Although in theory smaller
Wiring diagram to power a windlass with a three-battery bank
Ask the experts
ELECTRICS
Powering a windlass
IN DEPTH
Charge
control
+-
+ +- +-
+
Windlass
Solar
Distribution
panel
Switch
EmergencySwitch
switch
Alternator
Circuit
breaker
Fuse Fuse Fuse
VSR 2
Domestic
battery
Windlass
battery
Start
battery
VSR 1
Key
Wire
Joined wire
Switch
DC supply
Earth
Fuse
+-
and should, therefore, be sized
exactly as if you wired it
directly from the domestic
bank to the windlass. That
would most likely be at least
50mm^2 cross-section for a
1,000W windlass.
- Should I fi t a separate
DVSR to prioritise the
charging of the windlass
battery over the domestics?
To prioritise the windlass
Patrick Roach
Dan Hass’s Westerly Falcon
Silent Falcon
Gekko Studios/Alamy