Sigh, it's time to buy new batteries again.  It seems like every other week, (but of course, that's not true, it just seems like it).  One of my two batteries has two bad cells.  The other is about the same age as the first.

I have a single bank of two group 31 batteries, no starting battery.  Also,. I have no room for four batteries.

I'm wondering what replacements to buy.

Gel Cells --  Early on Tarwathie, I bought gel cells.  They were a disaster, failing almost immediately after installation.  I went through 4 sets in 18 months.  Unfortunately, for West Marine, they replaced them four times under warranty.  I sold them to another cruiser, and he within a week he too got free replacements at West Marine after I provided him with my receipt.   I finally figured out that my engine compartment runs far too hot in summer (about 140 F) for gel cells.  My conscience felt bad about that, because the whole thing was my fault, not West Marine's.

AGM -- These batteries cost the most, and many people assume that they are the Cadillac of batteries simply because of their high price.   Actually, the primary advantage of AGM is that they can accept charge more rapidly.  But to take advantage of that I would have to double the capacity of our alternator, double the capacity of the shore power charger, install double pulleys on the engine for the alternator belts, buy an electronic charge controller, and install engine room ventilators;  a lot of stuff.   

The reality is that when we run the engine on the ICW, we run it 10-12 hours per day.  When not on the ICW, we don't use the engine for recharging, we use solar panels.   AGMs for Tarwathie would costs a big pile of money for little advantage.  Also, I know three or more cruising friends, who had horrible experiences with brand new AGM batteries.

Flooded Cells -- Conventional batteries are best for Tarwathie.  But recently I went to a battery seminar sponsored by Trojan Battery Company.  They made good arguments, as to why their premium brand was better.

Today, I went to the local battery store to shop.  First, I found that Trojan group 31 batteries cost $240 each, compared to $120 for regular brands.  Double!   The warranty on Trojans is the same one year as on the others.  Trojan claims that you might get 60% more life with their batteries compared to brand X, but they won't warrant it.  Sounds to me that double the price is hard to justify.

Then, I learned something new.  The dealer was selling dual-purpose batteries.   They are halfway between starting batteries and deep-discharge batteries.  I never heard of that before.   I run a single bank for both cranking and house loads. I used to have a small separate starting battery, but I junked it after nearly 5 years of disuse because it was so inconvenient using the battery switch to select the starting battery just to start.  

I'm skeptical of dual-purpose batteries.  What do Westsailors have to say?   What about using a single bank of deep discharge for both cranking and house power?   Normally, my engine starts with less less than 1 second cranking.

I did a lot of homework before putting the batteries we use into SUNDOWNER and I believe there is no place on a boat for "dual purpose" batteries.  They don't accomplish what a true deep cycle battery will and that is longevity and number of cycles.  Therefor they are not economical in the long run.  This is further compounded when you think about what it takes to start a small diesel engine... That is, not much.  Two 6volt true deep cycle batteries wired in series to form a 12v battery can easily provide the amps needed to crank one of our motors.  Please see this video:

https://www.youtube.com/watch?v=nrQXnqNOI9Q

Gel batteries are expensive and extremely temperamental and can fail due to temperature and overcharge.  Most people I've spoken to that used these have switched away due to expensive failure.

AGM batteries have higher acceptance rates but they must be treated right and that means 100% charge often.  If you don't do this it will kill them early.  They are also expensive. They have no real maintenance and won't leak, which is nice.  But like I said, if you don't have the solar or shore power to top them up often, they might not be a good choice.

Flooded lead acid batteries are the cheapest entry to the market, the most available, the most used, and they tolerate a LOT of abuse.  The down side is that they are heavy and need maintenance.  They were our choice.  We have a 440 amp hour battery bank that we bought for 400 bucks and can expect at least 5 years of service out of them provided we don't discharge them past 50% too often.  That means for for 400 bucks we have about 220 amp hours of use out of them reasonable.  One of the downsides is that they will drain down if not being maintained.

The newcomer to the scene that I don't believe Dick mentioned is LiFePO4 chemistry batteries also known as "lithium" batteries.  These are expensive, but not by much compared to AGM/Gel and they have the promise of being the longest lived provided you can treat them right.  They'll require custom programming of alternators/shore charger/solar charger etc, but in return you get a battery that can discharge lower, harder, longer and charges VERY fast.  They will hold a charge for a LONG time with no charge source. They have no maintenance and weigh a fraction of the of other types of batteries.  I was HIGHLY tempted to install these but the initial costs were just too high for our limited budget.  They promise to be the most economical in the long run if you have the setup for them.  They are the future.

Alright, how do we setup multiple battery banks in the most simple and hassle free configuration?  I will try to give a coherent explanation of our setup and why we did what we did. 

You can see a crude diagram of the 12v system of our boat here:
http://i1121.photobucket.com/albums/l518/svsundowner/120701MajorWiring21.png

The main things of note pertaining to this specific question are the use of two switches.  I have one ON/OFF switch, and one 1/2/B/OFF switch installed. 

NORMAL OPERATION:
The day to day life aboard works like this.  When we get on the boat, we turn the ON/OFF switch to ON.  This turns everything on the boat on.  When we leave we turn it to OFF, this turns everything off.  When we turn the engine key, the motor starts from the starting batteries.  The alternator and solar both charge the house bank.  A Xantrex Duo keeps the starting bank topped up by siphoning a small amount off the house bank. Note:  The motor will start even if the ON/OFF switch is OFF.

EMERGENCY START:
If the starting battery bank would fail for some reason, only then would we need to do something to start the boat off of the house bank.  I would change the 1/2/B switch to "2" which would then energize the starter from the house bank.  This is our "emergency start" procedure. The placard I made, "If engine will not start, turn emergency switch to "2" then retry."

EMERGENCY LIGHTS:
There is a third procedure we could do with this setup, which would be to run the house electronics off of the the starting batteries in the event the house bank failed.  To do this I would change the 1/2/B switch to "B" and the normal ON/OFF to OFF to prevent combining the battery banks or I could leave it ON if I really wanted to combine the banks, though I feel combining is not a good thing in case one battery has shorted or is dragging down the others.

The main logic behind this setup is that it is dead simple.  There is an "emergency" switch (1/2/B) and the only time you touch it is in an emergency.  There is another ON/OFF switch that the crew can use and is easy and simple for a lubber to understand for daily operation.

Note that the alternator is wired directly to charge only the house bank and not the starting bank.  This makes sense because starting a small diesel requires less than 1amp of power.  Which will need the charging more, the starter bank or the house bank?  You want the charge source going directly to the house bank.  This also makes it impossible for the alternator to be disconnected while under load by turning a switch (this makes it impossible for your lubber friend to turn the switch and blow your diodes!)

To keep the starter bank topped off we use a Xantrex Duo which is not a simple diode.  The tech nerds get really fired up about battery combiners/diodes/etc so I'll not try to be overly technical and start a war, but I will say that it does this...  When the house bank is being charged, it will take some of that current and provide up to 15amps to the starter bank for charging.  If the house bank is not being charged, it separates the banks. If the starter bank is full, it will cease providing current.  If the Xantrex is not your style there is another device made by BlueSeas called an ACR (Automatic Charge Relay) that is functionally about the same but does it in a different way.  For more details on these simple/cheap/effective devices please see:
http://www.xantrex.com/power-products/power-accessories/auxiliary-battery-charger.aspx

and
http://www.bluesea.com/resources/1366/Automatic_Charging_Relay_%5BACR%5D_Explained

I know some of this can be confusing but if there are specific questions, please let me know.

Dick:  Nice write up - did I miss your info on how long your batteries are lasting for you?  and what are you doing for charging and controlling the charging?

I also recommend reading Nigel Calder book Boatowner's mechanical and electrical manual... he devotes 1/2 the book to electrical systems including charging systems and batteries.

Pygmalion ( a day sailer mostly) has a two bank system  - one bank is a single  type 25 for starting and the other bank is 2 type 27 deep cycle.  ie nothing special for flooded lead/acid batteries. 

I have the system designed with switches to isolate the house from the starter bank normally but I can combine the batteries if I need to start the engine with the house bank.

Each battery has its own charger output and I use a battery isolator to keep discharged batteries from draining down the charged batteries.

I also have a watering system for easy watering of the batteries without overfilling.  

Typically I get 6+ years out of the batteries.

I guess my point is that besides the batteries - the charging system needs consideration as not all chargers are good for all types of batteries.

Jay