battery banks

[Peter]

I’m in the process of fitting a new house battery bank, approx. 400 amp/hr. My question is it better to us the largest batteries size (amp/hr rating) possible to achive the final 400amp/hr i.e. keeping the number of batteries to a minimum, or to use a “lot” of smaller batteries, or is it horses for courses. For this question storage space is not a major issue. What are the pros and cons of either route? Any other comments on this issue appreciated.

Thanks for the answers

Peter

 

[Twister_Ken]

Not sparky enough to give you an electrical opinion, but two other points to consider. Smaller batteries weigh less, so it'll be easier to manhadle them aboard, take them ashore for winter, etc. And then there's the question of redundancy. If one big battery fails you're considerably stuffed. If a smaller one sparks out, it has less impact on your total amp hours.

 

[Mr Cassandra]

I have read the 12vBible for boats [ miner Brotherton ]and here says more smaller batteries. As if one cell dies, in a single bat instalation ,the battery is finished. if two or more you only have to replace a smaller cheeper one, you also have the benifit of more capacity throughout their entire life. cheers bobt

Bob T

 

[oldsaltoz]

Re: Miltiple battery banks

One thing you will have to consider is charging the bank of multiple batteries, because if any one of them develops a fault all the others will be overcharged. You can purchase “off the shelf” battery management systems for 2 or even 3 batteries, but I’m not so sure about multiple sets.
It will be an interesting thread to follow.

 

[HaraldS]

My short answer would be to go for fewer and bigger. But there are details:
(1) Choose a size that is very polpular, then you get better price and more choice of batteries, like Gel, Semi Traction etc.
(2) Do go in size over what you can handle without breaking your back.
(3) Fwer batteries fewer cables and losses and worries.
(4) Bank isn't bank: You can isolate groups of batteries or keep them all connected. Advantage of isolation is a potential backup and the possibility to replace them at differnt interval with different types. A single bank should have all the same batteries and the same age. Separated banks are more complicted to manage and may require separate sense wires and appropriate regulators and chargers to get the best performance.

 

[pvb]

Smaller...

The optimum solution would probably be for you to use a number of smaller batteries, say about 100Ah each. The main advantages are:-

* smaller batteries are much lighter and easier to handle
* it's often easier to find suitable locations for smaller batteries
* if one fails, you minimise the percentage drop in total battery bank capacity
* it's cheaper to replace if/when an individual battery fails.

For best results, you should wire them so that you connect the main negative lead to one end of the parallelled bank and the main positive lead to the other end. This ensures that any slight voltage drops in the inter-battery connections cancel out, letting each battery receive the same charging current.

If you can, get batteries with threaded stud connections, rather than traditional tapered posts - it's easier and neater to wire.

If you have space and funds, consider installing 500Ah or 600Ah, because the bigger the bank, the lesser the percentage discharge for a given use, and therefore the batteries will last longer. I use 6x110Ah sealed maintenance-free batteries, and have been very happy with the performance.

For best results, I guess you'll have a smart regulator on your alternator. And don't forget a "Mega Fuse" or similar next to the bank - a 400Ah bank contains a mighty amount of power if there's a short!!

 

[hugh_nightingale]

Just remember that if you connect all the batteries in parallel, when one "dies" they all die and your carefully contrived backup capacity will be momentarily lost until you can isolate the problem. Hence the use of so called "smart" chargers (and 1/2/both switches). I am not in favour of paralleled batteries.

Probably the best advice given here is chose a size you can EASILY lift, you have to get them down the companionway.

Hugh

 

[LadyInBed]

I go along with the 110AH solution.
A couple of alternatives on this :
1 split into two banks and use a 1/2/both switch.
2 leave as one bank, but put Isolator switches on each battery, so that you can isolate any individual battery.
3 Do 1 and 2.

 

[Guest]

A interesting solution I recently saw wast to split the bank in 2 and use each one on alternate days. This way the bank not used should always get up to 100% charge which extends life considerably. Particularly good for long term cruising or boats with fairly high permenant drain such as fridge etc which hold final charge voltage below 14.5. Got this tip from couple in Cardiff and they had 7year old bank still going strong and in daily use.

Roly, Voya Con Dios, Glasson, Lancaster

 

[pvb]

Don\'t split the bank...

Suggestions that you should split the bank in two are ill-advised. Perhaps the most crucial factor in determining battery life is the percentage depth of discharge. The larger the battery bank, the smaller is the percentage discharge for a given consumption. By splitting the bank in two, you double the percentage discharge!

In terms of recharging, getting back to 100% charge requires the use of a smart regulator, and this works just as well whatever the size of the battery bank. Indeed, larger banks can be recharged more rapidly, because they will accept more current and therefore replace the used Ah more rapidly.

 

[pvb]

Parallel connection is OK...

Connecting batteries in parallel is the only realistic way to create a large battery bank. A cell failure in one battery doesn’t ruin the whole bank. Cell failures are also very rare and are easily detected. Even if a cell failure isn’t detected immediately, it takes a long time before a bad cell adversely affects the other batteries. Remember too that cell failure is frequently linked to excessive discharge, so larger battery banks are less likely to suffer cell failure.

Parallel connection is well proven over many years. After all, we wouldn’t have had the early submarines without huge banks of paralled batteries!

I agree with you 100% on the weight issue - moving big heavy batteries around in confined spaces is a real back-breaker.

 

[Guest]

Re: Don\'t split the bank...

Very good point that I hadn't thought of - Thanks

Roly, Voya Con Dios, Glasson, Lancaster

 

[pvb]

There\'s another reason....

Thanks, Roly. There's actually another reason to have a big battery bank, but it's a bit technical. It involves something called the Peukert Equation. Basically, about 100 years ago, a scientist discovered that the effective size of a battery decreases as the rate of discharge increases. Batteries generally have a rated capacity in Ah (amp-hours). Usually, this is quoted at the "20 hour rate", which means that if you fully discharge it steadily over 20 hours, it will deliver the rated number of amp-hours. However, if you discharge it at twice the rate (ie over 10 hours), the effective capacity will drop to 84%. And if you discharge it over 5 hours, the effective capacity will drop to only 67%.

In leisure boat use, this effect can be important, especially when power-hungry equipment (refrigerators, etc) is in use. If the battery bank is small, the discharge rate can start to exceed the 20 hour rate, and the effective capacity of the battery is diminished. On the other hand, with a bigger battery bank, the Peukert effect is much less likely to happen, giving greater overall efficiency.