Is there an advantage if power drawn is from two strings instead of one?

Wbuffetjr1 · January 2020

I have a ~20 amp load that runs daily on my 48V off grid system. I currently have three strings of batteries and they have worked VERY well. So far no problems keeping them in line (knock on wood). Just thinking down the road a little.

Is it any "easier" on the batteries to have say a 20 or 30 amp load spread across three strings of batteries versus one string? I would much rather graduate to one string when I finally upgrade batteries.

Thanks for the input!

Estragon · January 2020

Assuming equal total capacity, it's generally better to run a single string vs three.

OTOH, if three strings is 3x total capacity vs the single string and the 20a load is significant relative to bank size, three may be better. For example if one string is 100ah (at C/20, would be less at C/4), and the load runs 4hrs/day, the single string would be near dead daily and not last long. Three strings (300ah total) would be much happier with a ~25-30% daily depth of discharge.

Marc Kurth · January 2020

Redundancy is a reason often cited by people wanting two or three strings using 6v batteries for a 48v system.

When possible, I prefer a single string of 2v batteries. Losing one cell simply requires adjusting setpoints to continue operating with a 4% reduction in battery bank capacity.

Marc

Wbuffetjr1 · January 2020

Thanks Estragon!

I am thinking in terms of same total capacity.

Someone told me that running 20 or 30 amps through three strings is easier on the batteries than running the same load through one string of the same capacity.

When I upgrade I would much prefer to have one string, but I want to keep life easy on them!

mike95490 · January 2020

Yes, it is easier on the batteries.

But - No, it's actually pretty tough to realize equal current in each string - see

http://www.smartgauge.co.uk/batt_con.html for more info

Wbuffetjr1 · January 2020

I 100% understand it is difficult to balance 3 strings and 3 strings are not recommended.

I would MUCH prefer to have just ONE string. I have always imagined upgrading to one single string. However, if it is SUBSTANTIALLY easier on the batteries, and would increase battery lifespan, to have two strings to "split" 20, 30 and 40 amp loads then I would do two strings.

Any thoughts on how much easier it is on the batteries to "split" a load like that across two strings versus one?

BB. · January 2020

In the end, it does matter "which batteries" are a looking at... Simply larger (heaveir, more lead/acid) cells, they, of course, have more ability to manage more current (charging and discharging).

Personally, I suggest 1-3 parallel strings, and 4 or more parallel strings become more expensive/difficult to build and maintain (more wiring, more connections, more cells to monitor electrolyte levels in, more fuses/breakers if you believe in a fuse/breaker per string when you get over ~2-3 parallel strings, etc.).

And, how big of battery (or cell) can you pickup/move/install/remove--200-300 lbs? >1,000 lbs, or 67 lbs (golf cart 6 volt @ ~200 AH).

While current is an issue with life (charging and discharging), there is also cycle depth, temperature, maintenance, "accidents", etc.) too...

Assuming weight and space is not a constraint (not an RV, plane, trailer, etc.)... It is usually better to simply limit yourself to one or two parallel strings (again, my suggestion is 3 max unless other issues prevail)... Simply going to larger AH cells (or batteries) is one way. Another is placing more cells in series (12/24/48 volts typical for our needs here). And as bank AH capacity needs go up, (my suggestion is 800 AH rough max for typical designs), go from 800 AH @ 12 volts to 400 AH @ 24 volts or 200 AH @ 48 volts (just examples).

Old WWII diesel electric submarines used something like 126 cells in series with 2 parallel strings of 2 volt cells maybe 25,000 AH per cel(?) (that weighed something like 1,650 lbs each):

https://fleetsubmarine.com/battery.html

Rather than trying to discuss this "in theory" --- What are your energy requirements? Volts, amps, Amp*Hours, Watt*Hours, etc.? What are your other requirements (space, weight, cost, etc.)?

A guy playing around with some submarine batteries:

https://www.youtube.com/watch?v=j25dh7w48Xs

The premise of the video is demonstrating "ball lighting"--However, from my limited understanding, this is not ball lighting, just lots of hot metal and some plasma (such as you get from welding). Still exciting.

-Bill

Wbuffetjr1 · January 2020

Thanks Bill!

I currently have a remote location three strings of golf cart batteries for a total of 48V and 690 AH. I bought them as my initial set to learn on and tear up. They are performing perfectly! They may actually last longer than I really want them to. I run a 1HP pump off a timer from them everyday. If we have a snow storm we can almost run the pump two full days before we hit the auto disconnect set conservatively at ~70% SOC. With one full sun day we can pretty much be back to 100% the following day. I would like to be able to run for more days before we auto disconnect. Ideally, I would go with a set of the Trojan industrial 4V batteries with an AH rating close to 1,200 AH. I will also add a few more panels to make sure my charge rate is still acceptable. Space and weight are no issue.

So, would you be concerned about running a 20 or 30 amp load (currently always less than 20, would just like to be capable of 30 if needed) thru a single string of 4V ~1200AH batteries? If two strings would be better I would go with a different battery to make two strings work out correctly to be close to the same AH rating.

Marc Kurth · January 2020

mike95490 said:

Yes, it is easier on the batteries..............

..............................................................

Mike,

Help me to understand your logic. Pulling a 30 amp load from three strings of 230 ah each, vs pulling 30 amps from a single string of 690 ah batteries equates to the same percentage of load vs. capacity. In essence, it is the same load per battery.

Not saying that you are wrong - just can't see it. If anything, I would think that realistically one of the three strings would end up carrying slightly more load and therefore become taxed more. I suspect that I am missing something.

Marc

BB. · January 2020

More or less, a leisurely discharge rate is C/20 hour:

690 AH / 20 Hour rate = 34.5 Amps (over 20 hours, in reality, discharge to 50% in 10 hours of continuous discharge)

Of course, with 3x parallel strings, each would be carrying ~11.x amps each...

A heavy discharge rate would be C/8 hours:

690 AH / 8 Hour rate = 86.25 amps for less than 4 hours (to 50% state of charge)

For "typical" deep cycle batteries that are full time off grid, 10% to 13% (to as high as 20%) Rate of charge (20 hour capacity) is usually the minimum.

For "industrial" type deep cycle batteries (forklift, traction, etc.), then you really should look at 13%+ rate of charge (20 hour capacity). The forklift type battery tends to be less efficient (higher self discharge) than other batteries. And many of those class of batteries have less space (volume) above the plates (and use more water)--So you would have to be very diligent checking electrolyte levels.

But, with only one string of 48 volt batteries, then you are looking at just 24 cells to check, vs the 72 cells you have to check on your 3 string of golf cart batteries.

Not knowing the present size of your array, a suggested "nice size" array for your existing battery bank would be:

690 AH * 58 volts charging * 1/0.77 panel+controller derartings * 0.10 rate of charge = 5,197 Watt array nominal

If your loads are not planning on changing, then your existing system sounds like it is a good size for your needs.

What is your present array size?

Of course, the downsides of larger battery bank... Large array, heavier wiring, more expensive battery bank, possibly a larger genset+charger, more risk of $$$ (damage to bank through miss-use, theft, etc.). For a 48 volt @ ~1,200 AH battery bank:

$1,305 per 4 volt battery (list price) * 12 batteries = $15,660 for battery bank
$150 * 72 6 volt @ 230 AH batteries = $10,800 48 volt battery bank @ 690 AH battery bank
$150 * 8 *(6 volt @ 200 AH) batteries in series * 6 parallel strings (for 1,200 AH) = @7,200

Thank you Paul C. for checking my math... It looked "good at the time". -Bill B.

So, price wise, they are roughly equivalent costs per AH @ 48 volts (depending on what your local pricing+taxes+shipping are).

Cheap Golf Cart batteries are still cheaper...

-Bill

mike95490 · January 2020

It's not the battery, it's the "capacity".

A 400ah battery is the same as an "ideal" 400ah bank. So if you use one string of 400ah batteries all in series, it would be the same as an "ideal" 400ah battery bank using 200ah batteries paralleled.

So you have to weigh the cost savings of cheap, parallel batteries vs the problems of

a) more cells to check and water (24 caps & cells vs 48)

b) more interconnects to check and maintain

c) problems with load sharing between strings

There is no advantage to using parallel to share the loading, only drawbacks, when comparing the same size battery bank

Marc Kurth · January 2020

mike95490 said:

It's not the battery, it's the "capacity".
A 400ah battery is the same as an "ideal" 400ah bank. So if you use one string of 400ah batteries all in series, it would be the same as an "ideal" 400ah battery bank using 200ah batteries paralleled.
So you have to weigh the cost savings of cheap, parallel batteries vs the problems of
a) more cells to check and water (24 caps & cells vs 48)
b) more interconnects to check and maintain
c) problems with load sharing between strings

There is no advantage to using parallel to share the loading, only drawbacks, when comparing the same size battery bank

Mike, was this in response to my post to you?

Marc

mike95490 · January 2020

Marc - no, that was a response to post #6 which hung in the buffer when my laptop crashed.

https://forum.solar-electric.com/discussion/comment/406579#Comment_406579

Wbuffetjr1 - there is one more thing, AGM batteries have lower internal resistance than flooded batteries, and therefor less loss and able to sustain higher surge amps when charging and discharging. However, that nice low internal resistance makes them very susceptible to unequal load sharing when paralleled.

paulcheung · January 2020

BB. said:

More or less, a leisurely discharge rate is C/20 hour:
690 AH / 20 Hour rate = 34.5 Amps (over 20 hours, in reality, discharge to 50% in 10 hours of continuous discharge)
Of course, with 3x parallel strings, each would be carrying ~11.x amps each...

A heavy discharge rate would be C/8 hours:
690 AH / 8 Hour rate = 86.25 amps for less than 4 hours (to 50% state of charge)
For "typical" deep cycle batteries that are full time off grid, 10% to 13% (to as high as 20%) Rate of charge (20 hour capacity) is usually the minimum.

For "industrial" type deep cycle batteries (forklift, traction, etc.), then you really should look at 13%+ rate of charge (20 hour capacity). The forklift type battery tends to be less efficient (higher self discharge) than other batteries. And many of those class of batteries have less space (volume) above the plates (and use more water)--So you would have to be very diligent checking electrolyte levels.

But, with only one string of 48 volt batteries, then you are looking at just 24 cells to check, vs the 72 cells you have to check on your 3 string of golf cart batteries.

Not knowing the present size of your array, a suggested "nice size" array for your existing battery bank would be:
690 AH * 58 volts charging * 1/0.77 panel+controller derartings * 0.10 rate of charge = 5,197 Watt array nominal
If your loads are not planning on changing, then your existing system sounds like it is a good size for your needs.

What is your present array size?

Of course, the downsides of larger battery bank... Large array, heavier wiring, more expensive battery bank, possibly a larger genset+charger, more risk of $$$ (damage to bank through miss-use, theft, etc.).

$1,305 per 4 volt battery (list price) * 12 batteries = $15,660 for battery bank
$150 * 72 6 volt @ 230 AH batteries = $10,800 48 volt battery bank @ 690 AH battery bank

So, price wise, they are roughly equivalent costs per AH @ 48 volts (depending on what your local pricing+taxes+shipping are).

-Bill

I am respectfully point out. There is an error on the golf cart batteries calculation . It is 24 6 volt @ 230 AH batteries make up the 48 volts @690 AH bank. At $150 each battery it is $3,600 for the bank compares to the very expensive 4 Volts battery bank.

mcgivor · January 2020

@Wbuffetjr1 Be very careful with theroritic one dimensional calculations, lead acid batteries need to be fully charged on a regular basis. The array needs to be sized correctly to recover the discharged capacity, preferably in one day after a 3 day sunless period, with a 1200Ah bank the charging current would need to be >120A without generator support, the exact figure would be dependent on location.

Cronic undercharging is the primary reason for premature lead acid battery failure, sulfation is often masked by the greater capacity, going unnoticed until the crystals harden and the damage becomes irreversible. Other chemistries such as lLiFePo4 are more forgiving with partial states of charge as well as being significantly more efficient during the recharge cycle, but do have disadvantages in some cases, primarily low temperatures and initial cost. The initial cost however is misleading as the cycle life expectancy has to be taken into consideration, overall the costs are very similar when everything is taken into account.

Wbuffetjr1 · January 2020

I currently only have nine 350 watt panels for a 3,150 watt array. However, we are at 10,000' in Colorado and I have seen a little over 4,000 watts on the Classic from this array. It has hit the amp limit on the classic 200. I plan on adding more panels on a separate classic in summer 2020. Chronic under charging is not a problem for me. This is our 3rd winter with the batteries and they are holding up perfectly. Like I said they will probably last longer than I want them to. When I do upgrade I want more capacity for the times we spend at the cabin. When we are there we still have to be careful not to draw the batteries down too far. Basically, I WANT bigger batteries and I will add whatever charging capacity is needed to support them LOL. I would prefer one string. 72 holes to fill gets old!

Estragon · January 2020

Personally, I'd probably prefer 2v to 4v at around that capacity. Maybe something like:

https://www.solar-electric.com/rolls-surrette-s2-l16-flooded-deep-cycle-battery.html

I don't have a good way of moving a ~300lb battery around. If I did, 4v might make sense in terms of fewer interconnect wires.

Trojan makes a 2v battery, but it looks to be a 6v L16 with 3 cells wired in parallel for 2v (instead of in series for 6v). Kinda defeats a big feature of 2v IMHO.

Photowhit · January 2020

Estragon said:
Trojan makes a 2v battery, but it looks to be a 6v L16 with 3 cells wired in parallel for 2v (instead of in series for 6v). Kinda defeats a big feature of 2v IMHO.

I think they make both 2v with 3 cells in parallel and a 2 volt single cell. I hunted this down several years back.

Yet another option, would be a forklift battery made with each cell having 'flags' so it could be moved and assembled in place reducing the weight of each component. I saw this recently and thought it was an interesting solution to dealing with the weight of forklift/traction battery.

BB. · January 2020

Thank you Paul C., I have corrected my bad math in the original...

Take care,
-Bill

Wbuffetjr1 · January 2020

I am not too worried about a 300lb battery if I only have to move it to install it and then it will sit there for a decade....or two.

I was really interested in the Trojan Industrials due to the ten year warranty. Maybe the warranty is irrelevant. I am sure if you mistreat the batteries they aren't going to cover it anyway.

Is there an advantage if power drawn is from two strings instead of one?

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