Battery Bank Expansion

[Harry Brown]

I am looking for some information regarding battery banks on your Yachts.
It appears that at present (factory fit), I have two 140 AH leisure batteries both with separate isolation switches at the Nav station.
What I don?t understand is, why have 2 separate banks (apart from redundancy), why not have 1 bank with twice the capacity?
I would like to replace these 2 batteries with new ones and increase the capacity considerably. i.e. probably having 6 Trojans AGMs, or maybe LiFePO4 (but that?s another post!) .
So should I convert the existing wiring so that I will have 1 bank at say ~600 AH, or 2 banks at ~300 AH?
To me it seems better to have one 600 AH bank?
What am I missing here, why have 2 very small banks? What have you guys done in regards to battery bank expansion, if you have a similar 2 bank system?

Hope that all makes sense!
Thanks in advance.

[IslandAlchemy]

The reason you have 2 banks is to separate the starter battery from the house batteries.  There are 2 main reasons for this:-

1./ If you run the house batteries flat, you can still start the engine.
2./ Engine starter batteries are about cold-cranking load, so a car battery is what you need.  House batteries are about small loads over long periods, so leisure/deep cycle batteries are what you need.

My battery bank has been upgraded for long-distance cruising, and I have 6 x Trojan T105s in 3 pairs as my house bank, and an 096 car battery for starting.

[Symphony]

You may want to consider fitting a small engine start battery and turning both the 140AH batteries into one house bank. That is a pretty standard set up with a diode or a VSR splitting the charge from the alternator. If you go much above the current capacity you need to look at your charging capacity as older alternators may not have enough.

[Odysseus]

Put "warm battery" in forum search engine to see an interesting artical on linking the bank together. Something I found last year. And it did make a difference.

Odysseus
Bav 38

[Nigel]

Put "warm battery" in forum search engine to see an interesting artical on linking the bank together. Something I found last year. And it did make a difference.

I must see if I can remove the very limited search box at the top of the page, the one in the menu bar is much better.

See here for the "Warm battery" thread.

[tiger79]

If you go much above the current capacity you need to look at your charging capacity as older alternators may not have enough.

It doesn't particularly matter; it might take longer to recharge, but it won't be a problem.  Bigger battery banks accept charge more readily, so will make good use of whatever charging is available.

[Lyra]

It doesn't particularly matter; it might take longer to recharge, but it won't be a problem.

Although technically correct, I would not go for a system with severe undercharging capacity.
The  reason one would increase the house bank capacity is to get more autonomy, but if this is at the cost of charging more frequently or longer time then not much has been achieved (as this is not autonomy anymore).
Assume (totally hypothetical figures) I was able to run my boat loads for 24 hours, followed by a 3 hours alternator charge. I double the bank size to hold the loads for 48 hours, but now I need to charge for 6 hours.
I therefore think that the charging capacity should be proportional to the battery bank size.

[tiger79]

It doesn't particularly matter; it might take longer to recharge, but it won't be a problem.

Although technically correct, I would not go for a system with severe undercharging capacity.
The  reason one would increase the house bank capacity is to get more autonomy, but if this is at the cost of charging more frequently or longer time then not much has been achieved (as this is not autonomy anymore).
Assume (totally hypothetical figures) I was able to run my boat loads for 24 hours, followed by a 3 hours alternator charge. I double the bank size to hold the loads for 48 hours, but now I need to charge for 6 hours.
I therefore think that the charging capacity should be proportional to the battery bank size.

The flaw in your argument is the definition of "severe undercharging capacity".  What exactly do you mean by this?  My last boat had a 660Ah domestic bank, charged by a 90A alternator, and it all worked well for many years.  My Cruiser 37 has a 270Ah domestic bank, charged by a 115A alternator.  If I were to double my domestic bank, do you really think I'd need to fit a larger alternator?  My previous experience suggests not.

[Lyra]

Note the original answer which just recommended to make sure that the alternator is large enough, and this is why I did not specify a specific amperage.
Your alternators seem to be more than the standard, and may have been designed to provide enough current at lower rpm and high temp. The original poster did not provide any info regarding the current alternator, but for example the standard alternator on the smaller engines is is 60 amps, and I doubt if it can continously provide this current considering temperature, not to mention low rpm if you do it at near idle under anchor.
I agree that the issue of how fast the batteries should charge (or how many hours per day one must turn on the engine) is a personal matter, but I assume that you are much happier today with an alternator that is about half bank capacity compared to your older boat where the alternator was about 1/7 bank capacity, so even if you double your bank it will still be better than the old boat.
In my boat I also increased the battery bank to 300Ah and I felt that the 60A alternator is not enough - my solution to this situation was the addition of a solar panel.

[robgs]

My 2007 Vision 40 (same vintage) came from the factory with 2X140 AH house batteries paralleled as a single bank of 280 AH's and one smaller start battery. I would bet that yours is set up the same way. I have replaced the house Bosch 140's with 4 Trojan 105's paired in series for 12 V and then added two more for a total of 675 AH's at 12 V. My boat also came equipped with a 100 A alternator so you should check that as well. A further mod-I removed the battery isolator (inefficient) and installed a Balmar Duocharge between house and start banks. This operates both with the alternator and shore charger to separate the two banks. I live aboard with shore power on a lot so shut off the start battery switch to prevent overcharging the start battery.

[IslandAlchemy]

My bank.  3 pairs of Trojans + starter battery

[Moodymike]

How have you secured your batteries to the locker ?

[IslandAlchemy]

Battens glassed into the floor to stop them moving.

That's it really.

I I ever feel the need, I can screw the lid on.

[Impavidus]

My 2007 Vision 40 (same vintage) came from the factory with 2X140 AH house batteries paralleled as a single bank of 280 AH's and one smaller start battery. I would bet that yours is set up the same way. I have replaced the house Bosch 140's with 4 Trojan 105's paired in series for 12 V and then added two more for a total of 675 AH's at 12 V. My boat also came equipped with a 100 A alternator so you should check that as well. A further mod-I removed the battery isolator (inefficient) and installed a Balmar Duocharge between house and start banks. This operates both with the alternator and shore charger to separate the two banks. I live aboard with shore power on a lot so shut off the start battery switch to prevent overcharging the start battery.

Are you feeding from both ends of the bank Bob or just one end? Looks like both ends, there is a theory that both ends is best rather than one end and apparently it makes a big difference.

See here; http://www.smartgauge.co.uk/batt_con.html

BTW 2 x100 watt de-mountable semi flexible mono crystalline solar cells, charge regulator and wires with waterproof socket fitted this weekend. Getting 10 amps in bright sunlight!!!!!!!!!!  £210£240 with everything. Thanks for the tips 

Ant.

[MarkTheBike]

BTW 2 x100 watt de-mountable semi flexible mono crystalline solar cells, charge regulator and wires with waterproof socket fitted this weekend. Getting 10 amps in bright sunlight!!!!!!!!!!  £210£240 with everything. Thanks for the tips 

That sounds like a decent price, Impavidus - where from?

[IslandAlchemy]

I've got a semi-flexible 100w and a dual-bank regulator.  Got it off ebay.  Was about £150 for the lot.