6v vs 12v

[Chris1992]

Hi I'm new to this site I was wondering if anyone could help me I have a conundrum I have a choice of either 4×250ah 6v batteries wired up to make 12v at 500ah or 5×120ah 12v batteries at 600ah the 12v batteries are only £30 more for 100ah but overall what would be the better choice 

Thanks in advance to anyone that can help 

[Yarome]

Welcome to the forum!

That depends greatly on which 6v and 12v battery you are trying to compare. Make and model (size/group) would be necessary to know to offer any meaningful advice.

Just a glance at the ah capacities it would "appear" you are trying to compare a 6v "true" deep cycle battery to a hybrid starter/deep cycle 12v battery. If that is the case then you're really comparing apples and oranges (in a matter of speaking).

[Chris1992]

Thanks verry much for awnsering so quickly

if it would help the moddle numbers are lagm120 and agm-6210 would be grateful for any info I know there not the best batteries but with the warranty I think they will serve there purpose 

Thanks again 

[Yarome]

Gotcha. Leoch's, right? Both are AGM's, but the 6210 is the "true" deep cycle of the 2. The 120 is in fact what we would call a "hybrid" in the U.S. even though they may market it as a "deep cycle". I believe you folks call them a "recreational" battery.

What that boils down to is the 6210 being able to accept a higher charge rate, better discharge under heavier loads and more lifecycles. In other words... having 500ah's that will retain a higher ah capacity for much longer than having 600ah's up front, but will loose capacity at a much faster rate. 

The other factor to consider is levels of discharge. With a true deep cycle it's generally recommended to maintain a state of charge (SOC) no less than 50% of it's rated capacity to maintain optimal battery health. So 250ah's would be your "working" or "available" capacity between charge cycles. With the hybrids, dropping them to 50% SOC can dramatically impact their lifespan so "available" ah's... on paper... would be 300ah's, but to maintain optimal health you should not discharge them below 70%.. or 180ah's "available" between charge cycles.

Weight and size should also be considered. 5 120's is going to weigh just a bit more, that's to be expected with the additional ah capacity, but more importantly... will require much more floor space. Providing your battery comparment is tall enough the 6210's would require less.

Money-wise... £30 isn't much at all, but having to replace a worn battery bank a year or two sooner than you might with the 6v batteries could become even costlier.

IMHO, the 6210's are going to give you the better value per pound over the life of your battery bank.

Note to U.S. readers: "value per pound" is my version of "bang for your buck"

[Chris1992]

Ahh like a lesure battery

Thank you very much for taking time to explain it to me I did think as much but wasn't sure about it but it all makes sense I know it probably dosnt make much difference but I will not use it to deeply and will be charging it quite often i have a dc to dc charge controller to take power from the vans alternator wich will be on the move quite often waight and floor space which I considered already would not be a problem just one more question what would be the life expectancy of the 6 volt batteries, I know the 12v are approx 600 cycles but not sure about the 6v 

[Yarome]

Yes yes.. that's what I was trying to remember... "Leisure" battery. Thanks!

I would have to look up the specs on Leoch's site, but off the top of my head it shouldn't be unreasonable to expect, conservatively,  900 charge cycles @ 50% discharge. The lower the level of discharge the greater number of lifecycles you would have so it's quite difficult to place a year # on expected life span.

If you follow this link to a recent thread in this forum and scroll down to AL F's post on Dec. 25th he posted a graph illustrating the charge cycles based on level of discharge that can give you a great example as to what you might expect to see. If you notice, the 50% rate line is a bit higher than what you might see out of the 6210's, but if you follow the arc you'll get the idea.

In case you were wondering... lifecycles on the 12v's (600) are also based on a 50% discharge rate.   

[Yarome]

So... Leoch is showing things a bit differently and in series groups rather than in individual battery models. They are showing Total capacity percentage by life cycle based on percentage of DOD (depth of discharge).

The 6210 (or rather that series group) shows "useful life" (80% original capacity) @ 800 cycles @ 50% DOD. "Battery life" (60% original capacity) @ 1200 cycles @ 50% DOD.

30% DOD puts "useful life" @ 1800 cycles and "battery life" @ 2400 cycles.

The practical battery life, to me, is when the available capacity of your battery bank will no longer support your daily energy requirements between charge cycles. That figure may be higher than the 60% Leoch is portraying.

[Chris1992]

Well your realy helping me so its only fair I

really tryed hard to follow that  info and I'm grateful for all that writing I will re read it a few times but it's a little bit complicated so I'm guessing the original statement still stands that the 6v volt batteries would stand up better over all even with the 100ah difference 

[Yarome]

10 minutes ago, Chris1992 said:

...the original statement still stands that the 6v volt batteries would stand up better over all even with the 100ah difference 

E...Xactly! More ah capacity available over the life of your battery bank, and you'll save money by not having to replace the entire bank for a bit longer than you might have with the 12v batteries. Theoretically.. about a 30% increase in battery life if they are maintained properly.

[Kirk W]

3 hours ago, Chris1992 said:

Hi I'm new to this site

Welcome to the Escapee forums! I'll just say welcome as you seem to have your answer already. 

[Chris1992]

Thankyou for your welcome and a very very big thank you for the info sorry but  I've just thought of something else do you know the max current draw I can get out of the 6v battery bank as in surge not sustained ? So I know if I need a bigger bank thanks once again 

[Sehc]

Six volt batteries have the capacity for an unbelievable current surge. Very dangerous. The limiting factor is the battery cables. Always have fuses at the positive battery terminal. I have 300 amp fuse on 24 inch, 2/0 cables. My microwave draws 100 + amps while running. I have not noted the startup surge, but it is fused at 200amp.

[Chris1992]

Thanks for thats info So will 4 6v batteries wired for 12 v handle 300a This is where it gets complicated a bit I live in the UK we don't use awg here so was going to us 450 amp cable and fuse for 350 with a kill switch 

[Chris1992]

Sorry to correct the cable is 485 amp 70mm2 core cable I will be using 

[Yarome]

14 minutes ago, Sehc said:

Six volt batteries have the capacity for an unbelievable current surge.

X2. Agree with every point Sehc made.

He's referring to a 9.26mm cable.

I'm not familiar enough with the latest internals on a Loech 6v, but as an example, a lifeline agm 6v would be "capable" of a 600amp load, but that's certainly not at all adviseable. I feel, with some certainty, that a Loech is not going to be able to handle that much current without "dramatically" shortening it's useful life if it would even handle beyond 400amps at all. If you're talking about wiring for 450amp you're getting into an 11.7mm cable and very short distances. Manipulating that diameter of cable between battery connections is "quite" difficult.

Instead of wiring for "Max+", my advice would be to wire to the max specification of the device/devices that would be attached for minimal line loss and call it good. You may still be getting up into a 10.4mm cable, but If at all possible I would try to avoid the 11.7mil. Working with maybe 20-25cm lengths that stuff is a massive pain.

What exactly is it that you are concerned with pulling those kinds of loads?  Being aware of course that a 300amp load would use all "available" ah's in less than an hour... let alone calculate how long of a drive you would have to take to replenish that energy with an auto alternator?

[Yarome]

5 minutes ago, Chris1992 said:

Sorry to correct the cable is 485 amp 70mm2 core cable I will be using 

Ah. For us a 70mm2 would be the equivalent of a 2/0. An 11.7mm = 102mm2 and a 10.4mm = 85mm2.

Just to say... I would never consider pushing anything over 300amps through a 70mm2. That's "right" at the ampacity threshold for that size cable and temp's have to be accounted in there as well.

[Chris1992]

To keep it simple I'm using thick copper bar to connect the battery and yes I'm insulating the bars I would only be using about 250a at most but the uping in size is for length of cable and maybe the fuse is to big maybe a 270-300 would be better and I will only ever draw that load for a matter of 15-30 mins and not very often just nice to have it if I need it 

It's the old adage it's better to have a umbrella and not need it than need it and not have it 

[ALLOY]

When running heavy cable I'll use a tubing bender. Ether spring or mandrel type.

 

[Chris1992]

Or maybe 95mm2 core cable 

[Yarome]

1 hour ago, Chris1992 said:

...about 250a at most but the uping in size is for length of cable and maybe the fuse is to big maybe a 270-300 would be better...

Nothing wrong fusing for 300. I would... or to say... I DO! 

1 hour ago, J-T said:

When running heavy cable I'll use a tubing bender.

True that! There was a guy I did a few jobs with that had gorilla arms and "paws" bigger than my feet that could handle 4/0 like it was a spaghetti noodle, but for us mere mortals.. I'll take a tubing bender any day of the week. 

[Chris1992]

You have no idear how hard its been to find a forum that acutly helps rather than belittle you thats why ive had to look on American sites Ovously there is some stuff I know you won't be able to help with like a good  inverter make due to differences between 120 and 230 and 50hz and 60 50hz and ovously avaliblity but i love the help and the banter I can bend it by hand have done on my mates ww2 truck with f off big cables i find the trick Is with cables to have a high strand count and doing it walm not cold makes them more flexible 

so do I stay with 70mm2 and a 300amp fuse or use 95mm2 cost isn't really a problem here once you have it you have it im running about 3m cable max total that's positive and negative 

[Yarome]

1 hour ago, Chris1992 said:

so do I stay with 70mm2 and a 300amp fuse or use 95mm2 cost isn't really a problem here once you have it you have it im running about 3m cable max total that's positive and negative 

For inter battery connections, 70 is going to be "better than adequate", but 95 would of course, be "better". The battery to load run... that would depend on how far of a run you need to make. A "run" with DC being a complete circuit there and back... so a 3m distance is a 6m "run". Once you know your run distance then you'll want to size your wiring based on what you consider to be an acceptable amount of current loss vs. cost and ease of wiring. As the amps go up, so will your current loss.

So... a quick break down on a 6m run and calculated resistance at 20degrees C of 1.72e-8 (stranded copper), single conductor, 12.5v:

With 70mm2:

100amps 1.22% or .15v loss
300amps 3.65% or .46v loss

With 95mm2:

100amps 1.22% or .15v loss
300amps 2.86% or .35v loss

So with lower loads you won't see a lot of difference and a 1.22% isn't "substainal". If most of your loads are under 50amps then that number would drop down to a .6% loss which is completely acceptable, IMHO. If most of your loads are going to be more in the 200amps range then the loss percentages are starting to get up there.

As you see though, with heavier loads 95mm2 does make a difference. That's when you have to balance the amount of gain vs. cost and ease of wiring.

Either way you go... 300amp fuse will do ya.

Personally, I don't have a single thing in my rig that would pull 200amps. My inverter is "capable" of it so I fused for 300, however, my heaviest load would be my microwave at ~120amps that never runs longer than 3 minutes and my longest DCV run is 1.2m (.6m each direction).

I would run 70mm2. 

[Chris1992]

I thought so if i over size cable it wont have issues, most loads will be under 100a and again not for very long be for being toped up with charge and sorry didnt make it clear my apologies It's 3 m total 1.5m red 1.5m black

to sum up if I only use 300a for sort periods say 15 mins at this load i would be running the vehicle to help with load a small amount and not very often 70mm2 would be more than capable 

[Yarome]

2 minutes ago, Chris1992 said:

...It's 3 m total 1.5m red 1.5m black

...70mm2 would be more than capable 

Ah. My bad if I read that wrong. That does make a small difference, but really only reassures that, from everything you've posted so far, I "really" don't see any reason why you would want to run anything larger than 70. Efficiency is one thing, but for the VERY minimal efficiency increase for the VERY minimal amount of time it MIGHT make the smallest difference... it's an exercise in futility.

You'll never reach 0% loss.  

[Chris1992]

That's all good news I'm not concerned about losses to much as loss will be more noticed over time I was mostly worried about damage to the cable (thermal runaway is no joke)  thank you so much for all you have done to awnser me 

Ps if there was zero loss there would have to be zero resistance and if that was the case we would all have free never ending power