T&T: Gen-Set Question (long reply)

[Candy Chapman and Gary Bell]

A couple of days ago Capt Joe posted:

My present power arrangement is a starting  27-series battery to the gen-set
that always needs charging because I do not use  the gen-set very often so
it's alternator never charges the battery.  I  have been going around this
issue
with 3 choices listed below in no particular  order:

1) Hard wire the gen-set starter to the house bank  which is always charged
by the Pro-Mariner 3 bank charger.
*****************
REPLY: When you hard wire the genny (or any other) battery in parallel with
the house bank, that battery bank effectively disappears, and it just becomes
another parallel element of the house bank.  This is a bad idea, and
potentially even dangerous.  When anchored, one wants to start the genset when
the house bank gets low -- just when the genny battery would be low as well.
And dangerous because a start battery is a very different kind of battery,
with numerous thin plates to give it lots of surge current for the starter
motor, whereas the house bank is deep cycle, with fewer and much thicker
plates, to avoid warping when the battery goes to a very low charge.  Your
starter battery will very soon warp its thin plates whenever the rest of the
house bank normally discharges.  When the start battery warps plates far
enough, they short together, and you convert your start battery to a very low
resistance short across your whole house bank.  Not good.  Also consider the
distance between most folks genset and house bank (I don't know about your KK,
but on my PDQ it would be 25 to 30 feet), and consider the size of cable
required (plus cost and weight) to carry the hundreds of amps the start motor
would need all the way from the house bank.  Also don't forget that deep cycle
batteries are poorly suited to providing huge power in short spurts.


2) Share the gen-set battery with a switched line  sharing the windlass
battery
(My Pro-Mariner has 3 outputs, 1 to the main  engine, 2nd to the house bank
and 3rd to the windlass  battery. )
****************
REPLY:  OK.  Don't wire two such batteries in parallel, unless they are right
next to each other.  If they are apart, cable length and heavy discharge at
one end, loose terminals, corrosion, internal shorts or whatever can cause
huge currents to flow between them and you could have a disaster.  If the
windlass and genset are close, they can simply be both directly wired to the
SAME starting type battery (works fine on my PDQ with the genset forward in
the port hull).  Much like the starter, the windlass will draw a lot of
current for a fairly short time, and you could size this start battery to have
enough left to reliably start the genset, and/or make it standard practice to
have the genset running whenever the windlass might be used (both to obviate
the need for genset start with a depleted battery and also to provide recharge
through the 120VAC charger).  If they are distant from each other, the wire
size to safely carry the large currents involved in either application would
be prohibitive.  See the following choice...

3) Buy a small dedicated hardwired battery charger for  the gen-set.
****************
REPLY:  I think this is by far the best choice, see above.  Use a dedicated
charger for each of the battery banks aboard, and never parallel batteries
with any difference in type, size, location or even age.  Use a number of
dedicated chargers each suitable for the battery and load system involved, and
mounted nearby to the battery, to provide the highest level of safety because
of freedom from paralleled batteries and to limit the need for long heavy runs
of expensive 12 volt cabling.  The chief factor in selecting here is that the
charger needs very modest wiring to carry the 120 volt input current, and even
reasonably sized  wiring to carry the 12 volt current because the charger
output is limited by the ratings of the charger.  A battery can and will
provide ENORMOUS output currents to any low resistance load, including start
motors, windlass motors, and particularly to a short from a chaffed wire or a
misplaced crowbar, as well as a failing battery wired in parallel.  Thankfully
these disasters are not too common, but they do happen, and the cost of the
subsequent fire or explosion can sometimes be reckoned in boats and even
lives.  The size, cost and weight of the cabling needed for long high current
runs of 12 volt DC discourages me as well.  Virtually ALL plus 12 volt DC high
current cabling needs to be fused as well, check out the fusable links used in
automotive systems (exception, the short cable feeding a big engine starter,
which is protected in other ways, and the negative or ground cables of
course).

I envy the folks who have a dedicated and completely isolated (except the
ground) 12 volt battery system for radios as well, and enjoy freedom from
electronic noise and the ability to use radios in emergencies where the main
system is compromised.  I am thinking of an AGM, sized to easilly accomodate
the transmit currents needed for my SSB.  A computer UPS (really a battery
with built in charger/inverter and sensing/switching circuits), or use of a
self powered laptop on a 120 VAC power supply would similarly isolate that
sensitive load.

So this approach allows you to use different kinds of batteries where
appropriate to their use and situation (open or sealed wet cell, gel cell and
AGM).  A single three output charger cannot accommodate this use of multiple
kinds.  These batteries should be matched to their loads accounting for their
output characteristics; their adaptability to fast or high current recharge;
their size and weight; their ability to mount be mounted in odd positions and
their freedom from corrosive fumes near electronics or other potential
corrosion risks (AGM and gel cell).  The chargers should also be matched in
this way to the battery kind, type and size as well as the characteristics of
the load and how long or often the battery is discharged, as well as the
nature and availability of recharge power.

In an example like (the way I want to set up) my boat, each of the six
discrete battery banks and their attendant chargers can be right near their
application.  First is one of the newfangled hand carried jump-start
batteries, to jump whichever dead battery is needed without jumper cables
(most often the genset for me), to power the air pump for the dinghy, the big
handheld searchlight, etc..  This is recharged from any 120 VAC source, or any
of the 12 VDC cigar plugs present in every hatch and door aboard. In our case
this is suplimented by a group 31 maintenance free wet cell in a portable
case, used with the trolling motor for stealth (bird watching) trips and the
other uses listed.  The two regular start batteries (twin engines) are sealed
'maintenance free' wet cell marine equivalents to the modern automotive
battery and each power their respective engine, it's bilge pump (catamaran), a
couple of cigar plug outlets on each engine for drop-light, oil change pump,
etc. use, and which are both connected in an emergency to each other and to
the wet cell house bank with an automatic combiner (or a manual switch) to
jump a dead starter.  The house bank can be a series-parallel set of giant
deep cycle wet cells, charged from a large inverter/charger, and through the
combiner from either engine.  The windlass/genset could be a gel cell or given
their higher discharge and recharge capability an AGM start battery -- with
its charger, a smallish echo or discrete smallish multistage unit.  A separate
battery for the radios will of course be a fair sized deep cycle AGM mounted
up in the flybridge with its charger, which is disabled whenever the radios
are powered. Again, only the grounds connect with the remainder of the boat,
so that each of the plus voltages float above that independently.  This avoids
a problem with floating radio grounds, where the radio equipment can
accumulate one heck of a charge relative to the rest of the boat.

Select discreet chargers to match the different characteristics of the size,
kind and type of battery; the size and timing of the load; and the nature of
the power available for recharging.  For the house bank the recharge current
will ideally be high (up to 35% of the bank's capacity per hour -- more only
increases outgassing), and we are interested in getting as fast a recharge
from the genset and/or motors as possible, particularly if they are fitted
with additional alternators/smart regulators.  Starting batteries can be far
more leisurely recharged, because they are refilling much smaller capacity
batteries which are discharged infrequently, and not all that deeply.  In
automobiles, the 'dumb' alternator regulator does the job nicely, which works
in boats too, so long as the motor is run enough.  If not (or in the case of a
genset, which often has no internal starter charging provision) a small output
charger with a float level sized to the self discharge of the battery should
work well (this float stage is another instance where a small solar charger
might fit well).  Given that most gensets are shrouded, additional alternators
are difficult to adapt.  Radio batteries would benefit from a rather small
charger that could be entirely disabled whenever the radio was turned on, thus
inflicting no charger noise.  Small solar panels are an interesting source of
float charging currents, particularly for the radio battery.  Sufficiently
large solar or wind systems can of course be used for any stage of battery
charging, with a suitable regulator, but the smaller ones seem particularly
suited to the float charge stage.

Whatever the charger's input, each charger (including extra alternators added
to motors or gensets) should automatically provide multiple output levels:
ideally the bulk, acceptance and float levels -- and for deep cycle batteries
which discharge far enough to have sulfation problems, an equalization level.
The 'nanopulse' sulfation suppression systems might be useful as well, but not
online with the radios because of the electrical noise involved.

The last issue involved in the selection of chargers and alternators is that
the high output voltages present in equalization stage can damage some
electronic equipment.  Always disconnect the entire load from a battery bank
if you do an equalization.  In a few cases the high voltage spikes generated
when switches carrying significant currents open or close can zap ya as well.
Protection for such problems in your 12 VDC system is not commercially
available, although the electro-geeks in the crowd can suggest a low voltage
equivalent of the surge suppressing circuit in the common power bar (a
suitably rated Metal Oxide Varistor, a capacitor or choke with a resistor,
and/or perhaps even a husky zener diode).  Such a 12 volt system spike eater
would probably end up having to cover the entire 12 VDC circuit, and so should
be spec'd to handle the anticipated voltages and currents.  Entirely separate
is the issue of even momentarilly removing the load from an alternator, which
usually zaps the diodes, disabling the alternator until you can get it
repaired/replaced, and disabling your wallet for a short time thereafter.  In
this case add 'zap protectors' to all alternators and use only
break-before-make switches, and/or special switches with separate alternator
field switch sections.

Low voltages, from shorts, or most often from deeply discharged batteries, can
wreak havoc when the low voltage causes the currents drawn by the different
loads rise enough to zorch some critical component.  For DC (resistive) loads
when the voltage goes down the load draws a correspondingly higher current.  I
lost my 12 VDC fluorescent lights to this when my house bank drained too low
accidentally.  Some motors, pumps, refrigeration systems and some electronic
systems can also be particularly susceptible.  Inexpensive low voltage
protection for the 12 VDC system is likewise not easily found, although it
might be fabricated with a voltage regulator or a zener driving a big ol'
relay with suitable filtration.  A solid state 12 volt DC to 12 volt DC power
'converter' or perhaps a solar regulator might do the job, but still at
considerable expense.  Arild, et al, please feel free to jump in here with
commercial or homebrew solutions I haven't found, or with corrections to my
theorizing...

Gotta get back to work,
Gary Bell, AKA that old drone Mr. Science