My current boat has two Optima starting bats plus a house bank with 4
6v Trojan T105's. Charging is with Xantrex 100 Amp, and a high output
alternator plus solar panels. The factory setup was a large 3 bank
combiner. I had to modify it when I added the high output alternator
because the alternator output had to feed the house bank, not a
starter battery, or else the regulator sensing gets very confused
(which can lead to a meltdown!).

Although the system works, I never like the "all or nothing" aspect of
the combiner, and occasionally it would cycle on and off for no
apparent reason. Finally I got an EchoCharge for about $100 and use
that to charge the starting bats, which are still on the combiner.
I've found no downside (so far) to this setup. The starting batteries
get an appropriate trickle charge whenever a charge source is
available, and everything is disconnected otherwise. The starting
batteries virtually never present a large load, so there is no
noticeable "kick in" of the EchoCharge.

I don't like the diode isolator solution, because of the voltage drop
which means you have to raise the voltage in the regulator to
compensate. And it seems you're just converting some of your juice to
heat. As for adding a "120 Amp alternator," your battery bank will
not accept that much power. Even with a smart regulator, it would
only accept about 50 Amps. On the other hand, your 55 Amp Yanmar
(actually probably Hitachi) alternator probably only puts out about
25-30 Amps, because it has a simple built in regulator. The new
alternator would want a smart regulator, so this is adding up to a
serious upgrade - if you want thoughtful advice on this you'll have to
tell us want you plan to power with this!

The Honda generator is not a bad way to go, I've been on the verge of
getting one for a few years. The output is much higher than 10 Amps,
because you can feed the AC output into your battery charger. Several
threads here discuss the efficiency losses. My problem is that I'm
not likely to equal the 100 Amps my alternator puts out, but if I had
a smaller system, this would be an attractive alternative. Most of
my sisterships (PDQ 36) have outboards rather than diesels, and many
use a Honda 2000 for charging.






GBM wrote:
I was thinking about adding a battery combiner to my boat's electrical
system, but I may have to rethink this.

Present System
------------------
Bank 1 - Starting battery
Bank 2 - House - 2x6V Trojans (225AH)
Shore power charger - 2 bank Truecharge 10 charger connected direct to
batteries with fused leads
Alternator - 55Amp on Yanmar 2QM15.

Presently, my alternator charges back via the starter motor cable. It leads
to the common terminal of the "red" 1-2-All switch. The batteries are
connected to the other two terminals in the usual way. So, I can chose to
charge either battery or both. House load also connects to common terminal
but also has it's own breaker switch.

1. Using Combiner
----------------------
I have looked at using the New Zealand made BEP VSR (Voltage Sensitive
Relay) as the combiner. But, in reading their website, they warn against
using such devices if the alternator and batteries are not matched. They say
that for the combiner to work, the alternator must be larger (80-90A) or the
batteries smaller (100-130AH). Otherwise, the unit will oscillate and not
work properly.
http://www.bepmarine.com/showproduct.cfm?productid=12
http://www.bepmarine.com/products/inst-710-100a-vsr(web).pdf

Another supplier - Sure Power talks of same problem but says that time delay
avoids chattering - But they don't provide details - Perhaps if alternator
is connected to house battery, it may work better?
http://www.surepower.com/separator.html

West Marine do not mention this problem in my older catalogue.

2. Echo-Charge
-------------------
Another possibility is to use a Xantrex Echo-Charge.
http://www.xantrex.com/web/id/103/p/.../5/product.asp

Seems alternator is connected to House bank. Then when voltage reaches 13v
this unit actuates and then charges starting battery. No mention of cycling
with undersize alternator. When unit kicks in, wouldn't voltage likely drop?

3. Diode isolator
--------------------
Would work, but voltage drop would likely reduce battery capacity and life.

4. Manual switching
-------------------------
That's what we do now! We have zap-Stop to protect alternator. But, it is
easy to forget to switch the batteries.

5. New Alternator
---------------------
Installing a larger alternator (say 120A) would be an option, but with
single belt and a real HP output of less than 10HP at normal rpm, this could
diminish engine output and perhaps put loads on engine that 'it's not
designed for. (The 55A is the optional alt. - normal is 35A!). Seems
overkill just to allow easier switching, but this would allow quicker
recharging of house batteries.

6. Portable Generator
--------------------------
This has passed my mind - Use a portable Honda to charge the house
batteries. But at 10A, this would require a long run!

Ideas, comments or condolences welcomed

GM