How many strings ?

nigtomdaw · September 2007

Hi, a friend of mine (yes real not made up) has just bought 60 x 100ah 12v gel batteries to use in his solar off grid Spanish home. He bought them I suspect because they were part of a commercial banks ups system 3 yrs old with a warranted *non transferable* 10 yr warranty and have only had a maintainance charge and cheap. I think he was under the impression that he could just parrallel them all up to make a giant battery bank. Alas I have burst his bubble ! To get the most capacity with as few as strings as possible and be able to use his bargain batteries I suggested he go for the largest base battery voltage available for his inverter which is 48v which isnt a bad starting point anyway. Thus 4 batteries with give a 1 x 48 volt 100 ah string.

My question is how many strings would you recommend or push to ?
The advantage disadvantage of wiring up a second battery bank with a automatic or manualchange over switch

I have no further details of the batteries and I am aware that he has now bought a SW3048E inverter charger. He has only 4 x 130 12v mitsubushi panels at the moment but intends to increase this to a minimum of 12 in the near future using an XW60 vapourware CC or more likely a readily available MX60.
An estimated power consumption of around 100 dc amp hours @ 48v every 24 hours is the likely estimated maximum.

I have a 3 string 24 v battery bank each string consisting of 12 x 2v 700ah cells. I believe the less strings the less problems and 3 was regarded as a safe max ! Can this guy push to 4 or 5 48 volt 100 ah strings or is he asking for trouble.

crewzer · September 2007

Re: How many strings ?

Nigtomdaw,

A single string of batteries appears to be the preferred solution. But, if your trying to work with a bunch of batteries that can’t be configured as a single string, the dependable limit for parallel strings seems to be between three (ref Rolls/Surrette) and five (can’t recall where I saw that).

Here are some comments I’ve made in the past on this issue:

Configuring batteries in parallel works well enough, but can also lead to problems if not done correctly. I believe the key factors to successful parallel deployment are: 1) same manufacturer, model, size and age batteries; 2) gaps between batteries to permit air circulation (otherwise the battery in the middle runs hotter); heavy gauge, high quality connecting cables between batteries; and 4) electrically diagonal connections to the primary (+) and (-) connections.

It’s my view that there’s nothing fundamentally wrong with wiring batteries in parallel. NAWS has apparently used this configuration in many deployed systems. However, when wiring batteries in parallel, there are many opportunities to create problems. Basic “rules” include:

1) Use uniformly short and large gauge battery interconnect cables (or buss bars) between the batteries in each string as well as when connecting the parallel strings. The idea is to minimize resistance – and therefore voltage drop – in the cables and between the batteries.
2) Periodically check the cleanliness and torque of the battery connections. Dirty and/or loose connections cause voltage drops.
3) Periodically measure the voltage drop across interconnect cables. Any cable found to have an excessive drop should be repaired or replaced. Considering the number of batteries in your bank, you should probably have a couple of space cables anyway. Again, the idea is to minimize resistance – and therefore voltage drop – in the cables and between the batteries.
4) Make the charge/inverter cable connections to electrically opposite corners of your bank. This configuration helps to deliver a uniform voltage to each battery string. See: http://www.solarray.com/Images/ImagesTGuides/drawings/24vsystem.gif for an example.
5) Leave an air gap between all of the batteries to provide for air circulation and to promote uniform battery temperature.

HTH.
Jim / crewzer

nigtomdaw · September 2007

Re: How many strings ?

Thankyou Jim ,

If I go for for five strings that will give him 500 ah @ 48v with an estimated maximum daily use off 100 amp hours that gives him a daily 20% discharge on a totally cloudy non solar day.

crewzer · September 2007

Re: How many strings ?

That should work pretty well in sunny Spain. In fact, your pal should be able to run for three days or so with little/no Sun. He'd use ~300 Ah or so, which, at 60% of the bank's capacity, is OK to do on occasion. The key to long life will be to fully recharge the battery bank at least 3-4 times a week.

Best of luck!
Jim / crewzer

nigtomdaw · November 2007

Re: How many strings ?

My friend now has the batteries in Spain I saw them yesterday and was able to take the following informtion from them

Brand or name is Marathon M12V100FT 13.5 volt-13.8volt
Made in USA by GNB

I suspect they are gel batteries and are the voltages bulk and float values,

crewzer · November 2007

Re: How many strings ?

Nigel,

The Exide / GNB Marathons are AGM batteries. Here's a link to their user manual: http://industrialenergy.exide.com/exidepdfs/Section%2092.30%202005-09.pdf

HTH,
Jim / crewzer

nigtomdaw · November 2007

Re: How many strings ?

Thanks for all your help, can these batteries be equalised the manual seems to indicate they can and also can a solar converters battery desulphator..... ( BDS2) be used safley with these batteries, thanks again Jim.

crewzer · November 2007

Re: How many strings ?

can these batteries be equalised the manual seems to indicate they can and also can a solar converters battery desulphator..... ( BDS2) be used safley with these batteries,

Nigel,

The Marathon AGM battery can be "equalized", but not in the traditional sense of the term. If the batteries’ performance appears to be suffering, you can apply an equalizing charge (</= 2.4 Vpc; </= 14.4 V for a 12 V battery) after a complete recharge cycle. See manual Section 10 and Para. 9.3 for more information.

Do not apply a traditional ~15.5 V equalizing voltage to these batteries. The overcharge will cause them to outgas and permanently lose capacity.

I’m not familiar with the BDS2… As long as the voltage doesn’t exceed 14.4 V (for a 12 V battery), then it shouldn't do any harm. Whether it will do any particular good is another matter. :roll:

HTH,
Jim / crewzer

westbranch · November 2007

Re: How many strings ?

re desulfation there is a lot of info on this site.

http://p198.ezboard.com/bleadacidbatterydesulfation

happy reading

Eric

niel · November 2007

Re: How many strings ?

for the record, so far nobody has reported good results let alone proof that desulphators work. if there be any of you out there that can confirm or deny that they work then by all means chime in. i personnally used an hf radio (100w) to try and desulphate some batteries of mine that had become sulphated. it did not even partially work and i feel it to be not only a waste of time and money, but a waste of precious power.

westbranch · November 2007

Re: How many strings ?

Evening Niel, et al. My circumstances are similar to NIGTOMDAW's.

So, I have one more live test to go in trying to determine exactly what/where my charging/ discharging problem is. If it is as I suspect ( a bad AGM cell, 1 of 6) then I have nothing to lose on my AGM but to try the desulfation technique. That is why I have looked into it... If it doesn't work, I am not out anything but time and some pocket change, and then I have to get a new batttery anyway

.
I see it as a last resort, FWIW.

Cheers
Eric

nigtomdaw · November 2007

Re: How many strings ?

I bought my batteries secondhand. 2v 700 ah tall Exide Wet Cells. I bought 42 of them and Im using 36 to give 2100 ah at 24v. I fitted the Solar Converters BDS2 for $90 and has been fitted to my bank for 2 years. My batteries rarely go below 90 % and at 90% dod it takes my 100 amp SW3024 set at 80 amps between 2 2.5 hours to get them back to 100 %. Ive had one cell failure very early on in there new life with me. I take a lot of effort to keep these cells happy as I havent the cash to afford to murder them as NAWS would say. At 90 dollars even if it helps a little Im OK with my prchase. I use it more as a preventitive rather than a cure . Although I will add that after 3 months use the white specs I had seen on top of the plates has gone. This maybe because Im charging correctly and keep them topped up.

I think the technology is sound, the wave length of the pulse seems important ,as for wasting energy it only activates when the batteries are under charge at 25.6 volt , I have mine at full power for the pulse because on my battery capacity . I believe some battery manufacturers use this technology in there chargers. So Im a believer with no evidence whatsover:p

Interesting feature on my batteries was that Varta had in installed a 2 foot long thin tube in each cell which recieved low pressure air to mix the electrolye to assist in cooling when under a fast cycle recharge from there chargers. Anyone heard of that before _?

How many strings ?

Comments

Categories