2 volt batteries vs 6 volt

Re: 2 volt batteries vs 6 volt

More or less, a 100 lb lead acid battery (or 50 kG) will store about the same amount of power whether it is 2/4/6/12 volt... For example

6 volt * 220 AH = 1,320 Watt*Hours of storage for "6 volt golf cart battery"
12 volt * 110 AH = 1,320 Watt*Hours of storage for a "12 volt GC sized battery"
4 volt * 330 AH = 1,320 WH for a 4 volt GC sized battery
2 volt * 660 AH = 1,320 WH for a 2 volt GC sized battery

Here is some real life similar sized batteries from Trojan at NAW&S (our host's store):



Trojan RE Series Deep Cycle Batteries for Solar & Renewable Energy Systems


Part Number
Volts
AH @ 20 HR Rate
AH @ 100 HR Rate
Length
Width
Height
Weight (lb)


T105-RE
6
225
250
10.375"
7.125"
11.75"
67


L16RE-A
6
325
360
11.575"
7.125"
17.7"
115


L16RE-B
6
375
410
11.575"
7.125"
17.7"
118


L16RE-2V
2
1110
1235
11.575"
7.125"
17.7"
119



Notice that weight (amount of lead per battery) is a big variable here... If, we instead take the V*AH and divide by the weight we see:

• 6v*225AH*1/67 lbs = 20 WH / lb
• 6v*325AH*1/115 lbs = 17 WH / lb
• 6v*375AH*1/118 lbs = 19 WH / lb
• 2v*1110AH*1/119 lbs = 18.7 WH / lb

All the batteries have about the same amount of energy storage per lb of battery weight.

The difference here is that we can put 2*6 volt * 375 AH in series 4,250 WH of storage (2 batteries * 118 lbs) or we can put 6*2 volt batteries at 1,110 AH for 13,320 WH of storage (6 batteries * 119 lbs each) in our system and still only have 6 cells to monitor (specific gravity, fill with distilled water).

I would still only have 120 lb (55 kG) cells to wrestle around, and I get ~3x the storage.

My other option would have been to get 6*6 volt cells (same weight) but then I would have 3 parallel strings of batteries (each string with 2 batteries in series). Now I have 18 cells to monitor, I have 3x as many parallel electrical connections to monitor. If I have any one cell fail (open or shorted), they will be masked by the other cells around it (and a shorted cell could take down a whole bank, or an open cell could damage a series string of batteries, etc.). Also, when you parallel battery banks, you should have a fuse/breaker per battery string (more wiring, more hardware, more costs) to prevent one shorted battery/wiring harness from being fed too much current from the rest of the strings.

Also, when paralleling batteries (or any voltage sources), it can be difficult to make sure that the current "shares" roughly equally between all the strings of batteries. A dirty connection, slightly corroded cable, one battery hotter (or colder) than the rest, and the sharing of current goes out the window.

None of the above are insurmountable issues... They just take a bit of extra care and money to address. My personal recommendation is to aim for one series string, and use 2-3 parallel strings maximum for a system (unless there are other issues--such as finding batteries with the larger AH capacity). Also, besides the usual tools of a hydrometer (specific gravity measurements) and a good quality volt meter (measure cell/battery voltages, voltage drops across wiring harnesses under load/charging), we now can get pretty reasonably priced DC Current Clamp Meters (like this $60 unit from a US store that does AC/DC and DMM--digital multi-meter--type functions). You can use a DC current clamp to just take a look at the current flow among the parallel battery strings once a week to once a month (under heavy load, and heavy charging) to make sure all is OK.

So, like most engineering problems--There is no one right or wrong answer. There is a big gray/grey area that we can usually help people understand and meet their needs. For example, there are lots of people with three or more parallel strings of batteries that are very happy with them--But they tend to be maticulus and monitor their batteries pretty closely (and some folks even move cells around once a year to keep everything "balanced" on average).

So, rather than trying to answer your question in the abstract--What are your needs and what is it you are looking for? For example, some folks in remote areas like two parallel strings of batteries with an A/B/Both switch between them. This allows them to switch out a failed string and still limp along until they can arrange to repair/replace the failing items.

Other folks have found that if they have a 48 volt battery bank and one cells fails--They can take the one cell out of service and still have everything work pretty normally (their batteries were 2 volt cells--so it was "easy" to bypass).

On the other hand, failure analysis tells us that having 2x as much of anything, pretty much doubles the chances that something will go wrong... Murphy's Law means there are no winners in his game.

-Bill

Re: 2 volt batteries vs 6 volt

Hi Tom,

I forgot to put in the Sears link to a $60 DC Current Clamp meter. In my previous post (it has been updated too).

-Bill

Re: 2 volt batteries vs 6 volt

Yea--Trojan sort of blew it on their two volt design. Those (appear to be) simply their standard 6 volt battery with the bus bars rearranged to put the three cells in parallel instead of series. Does not really meet the requirements for what I would suggest as a true "2 volt" cell.

However, in a large AH battery bank, it does at least allow you to have a large string with "replaceable" 2 volt cells. I have not read anybody here yet who has used these cells and offered a review. Do they have any issues with "balancing" cells SG and sharing of current--Kind of difficult to tell since the cells are paralleled internal to the battery.

I was trying to give the 50,000 foot view without adding a bunch of confusing caveats.

-Bill