Practical Boat Owner – June 2019

Q

I’m updating the
electrical system on my
1983 Ebbtide 33. The boat
has a common arrangement
of an engine battery and two
house batteries. What is
different to usual is that all
of the batteries are joined
together on the negative
side but the two house
batteries are not connected
together on the positive
side, ie all the batteries are
independent.
Each battery has its own
on/off switch and then has a
cable to its own busbar
which feeds a number of
instruments/items on the
boat. This means that if, say,
the battery feeding the auto
helm and nav lights runs flat
you can’t link any of the
other batteries to feed the
requirement and the flat
battery has to be charged
before the auto helm/nav
lights will work again.
I’m now in the process
connecting the house
batteries in parallel and
using a single bus bar to
feed the instruments etc.
My issue is that the engine
charging system runs
through what I presume is a
split charger with three
cables feeding the batteries,
one for each. When I join the
house batteries can I still
use this split charger?
If I need a new split

instead. VSR’s don’t have any

voltage drop issues that split

charge diodes tend to suffer

with. If there’s a 0.5V drop

across the diode itself, for

instance, that’s half a volt that

your batteries aren’t getting

and therefore will be

undercharged from the engine

• though in your case the
  shore power battery charger
  and wind generator, would
  ensure the batteries get their
  full charge.
  VSRs basically work by
  connecting the +ve of the
  service bank to the +ve of the
  start battery for charging and
  then disconnecting when the
  engine is stopped. VSRs are
  not expensive; so personally
  that’s what I would do.
  When wiring a VSR, the start
  battery is considered as the
  ‘main battery’, when starting
  the engine the relay contact
  remains open so the start
  battery can recharge first, until
  it reaches a pre-set level (this
  varies slightly from
  manufacturer to manufacturer)
  but usually closes at about
  13.3V to 13.7V. The service
  bank is then connected and
  will charge accordingly. After
  the engine is stopped, the
  relay contact will remain
  closed until the voltage drops
  to the pre-set ‘open’ point
  (about 12.65V) then opens so
  the start battery is now
  disconnected from the service
  bank (so will not flatten start
  battery). It will now remain
  open until the voltage on the
  start battery achieves the
  pre-set ‘close’ point again.

more sense to do what you

are proposing, effectively

having one service bank (at

presumably twice the capacity

of one of the single batteries,

assuming they’re both the

same capacity) feeding a

single supply switch panel.

Currently fitted is a ‘one in’

‘three out’ split charge diode,

all three outputs are the same

so you can connect any

output to any of the batteries.

In this case you can just

remove one of the cables so

you are only going to use two

of the outputs. However, the

split charge diode itself does

look a little ropey, so I’m

inclined to suggest you

remove this altogether and fit

a VSR (voltage sensitive relay)

charger, which type would

you recommend? (I also

have a Promatic AC charger

which feeds each battery

and a wind generator that

does the same...)

Robert Tuckwood

Cotgrave, Nottingham

ANDY HAINES REPLIES:

The way your Ebbtide 33 was

wired is pretty unconventional

for a small boat.

It would have made more

sense if the two separate

service batteries (via separate

switches) fed a single switch

panel, therefore if one battery

went low, you could switch to

the other battery and things

would still have worked.

That said, it makes much

...bus bars that feed three

separate battery banks

Alternator wiring first goes to... ...the split charge diodes...

...which have three outputs that

lead to...

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CRUISING

Stuart Carruthers

is the RYA Cruising

Manager and has

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coal and oil field

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Colin Brown runs

a marine survey and

consultancy company,

CB Marine Services

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Keith Colwell is

author of the

RYA Sea Survival

Handbook

SEALING AND

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Gareth Ross is

Sika UK’s Marine

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Unconventional battery wiring

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