2v vs 6v Battery Bank - Pros / Cons

[neelie]

Looking at AGM L16s. Same manufacturer (Lifeline), wired to achieve the same total capacity at 12V (6V are 400AH, 2V are 1200AH)

i.e. 6 x 6V to get 1200AH or 6 x 2V in series to get 1200AH.

Pro's and Con's of each scheme?

What worries me is that there is no redundancy in the 2V scheme. I.e. in the event of a single cell failure of the 2V bank, I lose all electrics versus losing "only" 33% of capacity in the case of a single cell failure in the 6V bank. Why would one risk the 2V option?

Is charging and discharging the 6 x 2V cells wired in series easier to control and manage and possibly more efficient?

Is the large single cell 2V battery more "robust" and less sensitive to abuse?

[djmarchand]

The 2V batteries dramatically increase the number of interconnections and complexity. That is enough for me to pick 6V batteries.

David

[Aussie_Sequoia]

Coming from the domestic solar industry, many people pick 2 volt sets because if one fails, it is cheaper to replace a single 2 volt cell. I don't know about this particular brand/set up though

[Unicorn Dreams]

What size of 6V batteries are you using to get 400AH per battery.

2-6V 400AH batteries only equals 12V 400AH

Or did I read something wrong..

[CarlF]

Most 6v batteries are really three 2v batteries connected in series. In a flooded battery, each cap is a 2v cell.

If one of the 2v cells in a 6v battery fails, you have to replace the whole battery -- so there's an argument that you should always use 2v batteries since you never have to throw out a battery that still has some good cells.

With six volt cells, if you have a failure you can remove two batteries and have a smaller bank at full voltage. A real advantage.

But in the real world, you usually replace a whole bank.

The wiring is also not usually more complicated since you typically use big 2v batteries that equal the total amperage of the battery bank. This means that there are just five wires connecting the six batteries in series for a 12v bank. Your six 6v batteries will require more wires (11) since they are partly wired in parallel.

But the biq question is cost vs. number of cycles. Sometimes 2v batteries have thicker plates that will give you a much longer life (but usually at a higher initial price). I'd ask Lifeline about the number of cycles that can be expected out of these batteries.

Carl

[pressuredrop]

Originally Posted by Unicorn Dreams

What size of 6V batteries are you using to get 400AH per battery.

2-6V 400AH batteries only equals 12V 400AH

Or did I read something wrong..

2 x 6v = 400ah at 12v

6 x 6v = 1200ah at 12v....

[Unicorn Dreams]

And as I posted, where do you get a 400AH battery?

[twistedtree]

If I follow you correctly, you plan to use 6 L16s in either case. It's just a matter of whether you get the tradition 6V L16 (400Ah @6V), or the newly available 2V (1200AH @2V) variants. Either way you end up with 1200Ah @12V - it's just a matter of how they are cabled up.

Personally, I think the 2V L16s are a joke - basically a cheap attempt to take some business away from companies that make large 2V cells. These L16s are still 3 cell batteries, just wired internally in parallel instead of series. You still need to water 3 cells per battery, and you still end up with 3 parallel strings of cells, just with internal connections.

With a 12V system, I'd definitely stick with 3 parallel strings of traditional 6V L16s. As you say, if you lose one cell, you can bypass that string and keep on going with a 33% loss of capacity. If you use the 2V variant, bypassing a battery means dropping the system voltage from 12V to 10V which isn't going to fly, so a failed cell takes the whole system with it.

Where large 2V cells DO make sense is in larger, higher voltage systems. Big 2V cells (single cell, not multiple parallel cells faking it as a large cell) result in fewer cells to water as opposed to paralleling strings of smaller batteries, and that makes a big difference in maintenance. Also, in a 48V system you can bypass a 2V cell (even a 4V cell) and run at 46V (or 44V) without any problem. I had to do it this past year and all it takes is a few adjustments to charge voltages and you can keep on running while waiting for a replacement cell.

[twistedtree]

Originally Posted by djmarchand

The 2V batteries dramatically increase the number of interconnections and complexity. That is enough for me to pick 6V batteries.

David

It takes 7 jumper cables to wire up 6 6V batteries in 3 parallel strings of 12V each.

It takes 5 jumper cables to wire 6 2V batteries into a single 12V string.

So I think the 2V arrangement actually requires fewer jumper cables, but I'd still pick based on other considerations.

[roverhi]

How tall are these batteries??? The two volt cells and the 6 volt 300 plus amp hour batteries that I've seen are 1 1/2 to twice as tall as a typical golf cart battery. The footprint can also make a difference. For most people, the type of battery will have more to do with the available space.

Personally, I'm a little leary about swapping out individual batteries in a battery bank. If the cells are maintained properly, they will last about the same amount of time. Once they start to go, swapping in a cell or battery will pair a good one with the old and the capacity will be that of the old.

Because of space, I go with two banks of golf cart batteries in series for 12v/220 amps each and use each 220 amp bank separately with the ability to combine them if I get the urge and another 12v battery that is strictly for engine start. That way I can swap out a set of two batteries should something happen and not have to ditch all four batteries.

[twistedtree]

L16s are a standard size just like the T105 golf cart batteries, and you are correct that they stand taller - about 12"-14", maybe more. They are potentially harder to fit depending on the boat, but pack about 2x the Ah of a T105 with the same number of cells to water, so in some cases they can be a good choice. It all depends.

Single battery replacement would be more to address a defective battery rather than a normally aged cell. In my case I had a clearly defective battery after 2 years of service on batteries that are warranted for 10 years. But I had to wait for 6-8 weeks for a replacement, so I shunted around it, reset voltages, and kept running until the replacement arrived. Even if you do plan to replace all the batteries, bypassing a dead cell can be the difference between having power and not until you can get replacements.

[neelie]

twistedtree - Many Thanks, you answered my question. I had a closer look at the drawings of the 2V L16 and discovered you are correct - it is made up of 3 cells. The devil is always in the detail.

In which case, I can see no advantages in using the 2V variant for my application.

[denverd0n]

The basic lead/acid battery cell is always 2 volts. When you buy a 6, 12, or whatever voltage battery, the manufacturer has simply linked up the appropriate number of 2 volt cells, within the case, to make the voltage that you want.

[neelie]

Denver - thank you, but I suspect everyone already knows this is how most batteries are built and neither was it the basis of my question.

However, I did learn something useful from "twistedtree". And that was, that the particular 2V battery to which I was referring to is made up of 3 x 2 volt cells internally wired up in parallel, rather than consisting of a large single cell, as I had imagined it to be.