Practical Boat Owner – August 2019

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

1. 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.

2. Will the windlass battery
   ever reach full charge?
   Yes, provided suffi cient
   charge is supplied for long
   enough and the cables are
   correctly sized.


3. 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.

4. 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