New domestic battery options

wragges

wragges

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Nottingham, UK
www.minstercomputers.com
Hi,

Looking at renewing our standard "leisure" battieries this winter and wondered what the gang thought of this:-

Given a fixed amount of space (under the navigation bench) what is better 3 x 120Ah ganged in parallel, or one very large 340AH unit that will just fit the same space...taking in to account internal resistance and deep-cycle dischange abilities etc.
 
I don't really know about the electrical pros and cons but I do know that a 340 Ah battery will weigh in the region of 90kg which wil present an interesting proposition in terms of handling. I would go for the 3 x 110Ah batteries myself purely on those grounds.
 
If the three 120 Ah batteries have a life of say 5 years on average, one will possibly fail at year 4 and another at year 7, making the replacement an easier and cheaper proposition that having to replace all three as the 340ah in one huge lump - hernia aside that is!
 
3 little ones...

Go for the 3 little ones! A 340Ah battery is really too heavy to be safely handled on to or off a yacht.
 
As opposed to having one big one?

I would think that the likelihood of a battery failing is a function of the number of cells you have. So two 6v versus one big 12v puts you in the same position.

3 little ones gives you 3 times the chance of a failure, but possibly leaves you with power, if you detect it early enough, or the batteries aren't paralleled.
 
Not the best idea...

[ QUOTE ]
Go for 3, but dont just parrellal ....

[/ QUOTE ]The advantage of a high-capacity domestic bank is that, for a given usage, each battery has a lower depth of discharge than would be the case with fewer batteries. It's the depth of discharge which ultimately shortens battery life. So putting a number of batteries in parallel is actually the best way to provide for domestic power needs.
 
Re: Not the best idea...

Following advice here earlier this year I bought a pulser (desulphater) before returning to the boat. The one I bought cost £35. It has transformed the holding power of my domestic batteries. Before treatment, a day under the Greek sun would produce enough solar power to give a voltage of about 12.6 by sunset and 11.9 the following morning, fridge on 24 hours per day. After treatment these were 12.9 and 12.2 volts, respectively. Even last week, with only about 10 effective hours of sunshine per day (but slightly lower air temperatures) we still had 12.1 or 12.2 volts first thing in the morning.

So before ditching your present batteries I would strongly recommend a small investment on a pulser. It won't be wasted anyway, as all batteries begin to sulphate in time.
 
Re: Not the best idea...

Is now the time to consider true deep cycle batteries with thick plates - probably more expensive, but apparently much better at resisting the type of use they get on the domestic side of a boat.
 
Re: Not the best idea...

[ QUOTE ]
But you need some switching and isolation - no reason not to run them in parallel - just don’t simply connect them all together or you lose all flexibility.

[/ QUOTE ]If you're going to run them in parallel, you don't need isolation switches. Unnecessary complication. Just daisy-chain the batteries together, then take the positive from one end of the bank and the negative from the other end, thus minimising any potential voltage drops through connections.
 
I am sure a recent article in PBO said that batteries rigged in parallel allowed the duff one to drain the good one - Yes it was in the story of the delivery skipper who died after falling overboard off Portland
 
Probably more expensive...

[ QUOTE ]
Is now the time to consider true deep cycle batteries with thick plates - probably more expensive, but apparently much better at resisting the type of use they get on the domestic side of a boat.

[/ QUOTE ]You're right - it's probably more expensive. I reckon the most cost-effective solution is just to use a big domestic bank of relatively ordinary batteries. That way, each battery isn't suffering a huge discharge current, and the all-important depth of discharge can be kept within reasonable limits. Plus, there's the fact that a big bank will absorb charge more readily, thus minimising recharge time. I use a bank of 6 ordinary maintenance-free batteries (about 660Ah in total) - they're about 10 years old and still working well.
 
Re: Not the best idea...

[ QUOTE ]
If you're going to run them in parallel .... Just daisy-chain the batteries together, then take the positive from one end of the bank and the negative from the other end, thus minimising any potential voltage drops through connections.

[/ QUOTE ]

That sounds to me like series, not parallel - you would get 36 volts. (Or maybe you make daisy-chains in a funny way.)

Alan
 
Bad expression maybe...

OK, sorry to confuse you, maybe "daisy chain" was a bad expression. "Wire them all together in parallel" would be less prone to misinterpretation. I'd forgotten that "in parallel" might not be understood by all our gentle readers. For those who don't understand, it means joining all the positive terminals together to form a common positive, and joining all the negative terminals together to form a common negative.
 
Re: Not the best idea...

If you do as you suggest without isolation - you lose all flexability, for example keeping one battery in reserve fully charge. You also have a big problem in a fault situation as you will not be able to switch a defective battery out of the bank. You are correct that it is simpler - I guess it come down to pesonal choice at the end of the day.
 
Re: Not the best idea...

However, provided that the main domestic bank is not also the only way of starting the engine, I don't see that the flexibility offers any real benefits.

It is always better for the batteries to use the full capacity of the bank, rather than keeping one in reserve, because that results in less overall discharge on any one battery. Should one battery fail it is the work of only a minute or so to disconnect one of the terminals, so taking it out of the circuit.
 
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