I just bought a 900 cca battery at Costco to use as my starting battery
($58,100mo). My plan is to put that battery in bank 1 and to purchase
two Trojan T105s for bank 2 (hooked together in series of course). I
have the standard "1,2,both" switch. My motor is a Yanmar 2QM15.

I have an alternator which was recently rebuilt, supposedly to put out
60amps, and according to the ampmeter puts out about 35 amps at max. I
have a shore power West Marine 5 amp charger. I'm hoping to not have to
upgrade anything else due to low finances, especially not an expensive
new alternator!

Can someone show me the figures as to what problems I will have if I
stubbornly refuse to upgrade anything else? How long will it take to
charge them with the motor or with the shore power?

Last question, Do I have to do something unusual to hook everything back
up? Can I just charge both banks at the same time as I did before or do
I have to charge the banks separately?

Thanks so much for any help!!

--
Stephen

-------

For any proposition there is always some sufficiently narrow
interpretation of its terms, such that it turns out true, and
some sufficiently wide interpretation such that it turns out
false...concept stretching will refute *any* statement, and will
leave no true statement whatsoever.
-- Imre Lakatos

Stephen Trapani wrote:
I just bought a 900 cca battery at Costco to use as my starting battery
($58,100mo). My plan is to put that battery in bank 1 and to purchase
two Trojan T105s for bank 2 (hooked together in series of course). I
have the standard "1,2,both" switch. My motor is a Yanmar 2QM15.

I have an alternator which was recently rebuilt, supposedly to put out
60amps, and according to the ampmeter puts out about 35 amps at max. I
have a shore power West Marine 5 amp charger. I'm hoping to not have to
upgrade anything else due to low finances, especially not an expensive
new alternator!



Can someone show me the figures as to what problems I will have if I
stubbornly refuse to upgrade anything else? How long will it take to
charge them with the motor or with the shore power?

Last question, Do I have to do something unusual to hook everything back
up? Can I just charge both banks at the same time as I did before or do
I have to charge the banks separately?

Thanks so much for any help!!


You've asked a lot of questions but still have to give a few details.
For instance, will you daysail, overnight, or hang out at anchor for
a few weeks? Do you need the system to be failsafe and idiot proof,
or can you accept that a little forgetfulness can lead to a dead
battery and possibly a SeaTow call?

First, the charge rates:
The output of the alternator will be a function of the batteries,
charge state, and the regulator. The T105's will accept about 50 Amps
so the alternator is sized right, but if the regulator is not set to
the right voltage it will take too much time, or overcharge. With a
modern 3-stage, it should take about 90 to 120 minutes to go from 50%
charge to 85%. However, if you have the stock Hitachi alternator with
a dumb regulator, it will take longer. The shore charger, being
small, will take about 24 hours (or more) to top off the bank from the
50% state. (Hopefully, the starting battery should not be discharged
more than a few percent.)

Underway:
Ideally, the starting battery should be connected directly to the
starter, and the house bank to the other power sinks. This means that
there is no "big red switch" needed except for emergency situations,
such as using the house bank to start the engine. My last two boats
are wired this way.

The problem this creates is that you want to be able to charge both
from a single source. The best way is to have either dual output
alternator and chargers, or a combiner (an automatic relay), or an
Echo Charge, (a small changer for the starter battery slaved off of
the primary charger). Without one of these automatic systems, you
have to manually select "both" on the big switch for charging, and
then hopefully remember to disconnect when you're discharging. This
can cause another problem, which is that the alternator can cook a
diode if the batteries are disconnected even for a split second.
(Newer switches avoid this with "make before break".)

The easy solution, without spending (much) more money, is to hardwire
the two banks to their respective sinks, and use the "both" setting
only for emergencies. The starting battery should last for a number
of starts, so if you have it well charged when you leave the dock, it
should be good for an overnight. Even this is a bit of a problem,
since the alternator output on the Yanmar is probably fed directly to
the starter. It will need its own cable back to the house bank; this
annoyance is implied by the concept of having a dedicated starting
battery. And of course you'll want to have some way to disconnect the
batteries.

If this seems complicated, just imagine that electricity on boats, and
battery powered appliances are wholly unnatural concepts! And doing
anything cheaply on a boat is problematical.