Max battery bank

Bretto · May 2018

Hi guys, I have a stand alone system. 36 x 260 w panels. 4 x 4 x 280 amp agm batteries linked to give 48 v. 16 batteries in total. I would like to add another set of 4 or 8 batteries for extra storage but I am told there are unbalance issues with more than 4 banks. Is this correct? Why? Simple answers please, not a electrician.
Thanks Bretto

BB. · May 2018

Here is an explanation of how to parallel your batteries with the least amount of issues:

http://www.smartgauge.co.uk/batt_con.html

Personally, I like to recommend only 1x or 2x parallel strings. With 3x being the comfortable maximum.

For flooded cell batteries--There are simply a lot of battery caps to check electrolyte levels in. And, over all, just more things to fail (4x24=96 cells in your present bank). And other issues (ideally, each string should have its own breaker or fuse to prevent short circuits from starting a fire (the other strings feeding current into the shorted string, etc.).

However, if you connect the strings in parallel properly (basically equal resistance for each string/wiring set so that each string has the same voltage drop/shared current flow). If you monitor your bank (get an DC Current Clamp DMM) so you can ensure that each string is sharing current properly... There are folks that do parallel lots of batteries together (large AH AGM batteries are harder to fine, in other countries, large AH batteries are hard to find, expensive to buy/ship, etc.).

AC/DC Current Clamp DMM meters are getting less and less expensive--So as part of your normal inspection, measring the current flow in each string (while under heavy charge or discharge) is much easier these days. I think you are in/around Melbourne Australia--But here is a US Amazon DC Current Clamp meter that is pretty inexpensive:

UNI-T UT203 Digital Handheld Clamp Multimeter Tester Meter DMM CE AC DC Volt Amp by UNI-T

Make sure that it is AC/DC Capable current clamp meter. AC Only clamp meters are very nice instruments and more common--But will not work for DC current measurements.

If you were to have a larger AH capacity battery bank--You might need a larger array (and more charge controllers)... Typically like to suggest 5% to 13%+ or so for solar array sizing (battery bank rate of charge). For example, 6x strings of batteries would look something like:

6 strings * 280 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 6,436 Watt array minimum
6 strings * 280 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 12,873 Watt array nominal
6 strings * 280 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 16,735 Watt array "typical cost effective" maximum

Your present array of (36*260W panels=) 9,360 Watts is already good sized, but would be nice to increase if you add more bank capacity--Are you looking for more storage? Would adding to the array be more cost effective (panels have a 20+ year life, battery banks tend to have a 5-7 year for typical AGM batteries).

-Bill

Estragon · May 2018

I assume the existing batteries are 12v, 4 in series for 48v string, and 4 strings for ~1100ah?

Ideally, this bank would be 24x 1100ah/2v batteries in a single string, or maybe 24x 550ah/4v in two strings. Any more than two gets harder to wire such that all strings have equal resistance, and the same charge/discharge. Also, with more than two, each string should be protected with a fuse or breaker. As more strings are added, the odds of all string resistances being the same go down. 4 is hard, 5 is harder, 6 is harder yet, and so on.

With AGMs, you can't check specific gravity, so the health and state of charge for individual batteries can only be estimated using voltage and/or individual load resting. Adding new batteries to an older bank can be problematic under the best of circumstances. Before doing so, you would want to be pretty sure the old batteries are quite healthy, as the new batteries will soon get as unhealthy as the old ones. Without being able to check SG, it's pretty hard to judge the health of the older batteries, and with 4 strings of AGMs, the odds of them all being really healthy aren't particularly high IMHO.

If you're wanting to add capacity, it follows the existing batteries may have had a hard life, and it may be best to do what you can to keep them going as best you can for now, and replace the bank when necessary. Adding to it is likely to be good money after bad.

You might want to check out:

http://www.smartgauge.co.uk/batt_con.html

It gives some suggestions on wiring etc which may be useful.

To help extend the life of the existing batteries, I would wire as suggested in the link above if not done already, and would check resting voltages of individual batteries regularly to identify any needing supplemental additional charging.

Estragon · May 2018

And Bill's suggestion of a DC clamp meter is also a good one!

Bretto · May 2018

Thanks Guys, my batteries get a easy life a 2 wire start gen set starts at 48.5 v but as the panels are big enough to supply power all day + charge batteries quickly the draw at night is making complete cycles daily from float to 70%. In a rural area, good sunshine but dusty panels are common. The house was built with an inbuilt cold room but only gets used for chilling down meat with generator backup a couple times a year because I settled on 280 amp batteries never thinking we would be running 5 big freezers a house fridge and a camp fridge when guest come around. Yes I should reduce freezer space! Yes I should get up on roof to clean panels more often! What causes this imbalance I am told about that stops me increasing batteries by 50% without having to sell mine and start again with lithium or sets of 2v high capacity batteries? My first thought to steer away from lots of 2v cells was because if one fails a hole bank should be replaced ( new and old drama) . Now I change 4 x12v not 12 x2v. But really even my setup at 2 years old if one 12v died all SHOULD be changed so I suppose I have shot myself in the foot Thanks for your input guys.
Look forward to your out of balance/ unstable battery answer.
Bretto

Bretto · May 2018

Thanks for the link B.B. to smart gauge. Did answer some Q
Bretto

mcgivor · May 2018

Having multiple strings of lower capacity cells would not only increase the maintenance, watering, checking interconnect terminals for resistance etcetera, the fact that there are so many potential points of failure make the 2v single string inherently more reliable. With the 12V arrangement one bad cell in one battery, or a high resistance terminal, could go unnoticed while creating ballance issues to the remaining strings. With 2V cells the potential for one cell going bad without being noticed is dramatically reduced. Since the effort to maintain 24 cells is only 25% than that of 96 cells, the frequency of maintenance checks could be increased and a well maintained system is a reliable system.

mike95490 · May 2018

Because of their low internal resistance, AGM batteries are the hardest thing to properly parallel and get the current equal.

Can you get 4V or 2V cells to wire the whole thing in series ?

Bretto · May 2018

Yes, I can source them ok but at a huge cost of course, on sell my 2 year old batteries and change up to larger AH 2v or lithium but I was hoping to add one more string of 4 x 12v but it’s looking to hard

Estragon · May 2018

At two years old the bank is likely 1/3 or more into its lifespan. If you're willing to accept that a new string added to the old ones will have a life expectancy of (eg) 4 years or less instead of six, and adding another parallel string increases the odds of prematurely failed string(s), add the string. Personally, I'd run the genny a bit more rather than add, but that's just a personal choice. Wired and monitored properly, adding can work too.

Do be sure to fuse/breaker each string though. As Bill mentioned above, an internal short in one string could put the combined SC current of the remaining strings through the short.

Bretto · May 2018

Thank you, yes the generator is the cheapest option it wouldn’t auto start Three times a week if you don’t use the hot plate and microwave together the volts stay up till sun up then we’re away. Just wanted to make use of the cold room in house 100% but that’s not to be. Thanks heaps for you come back, great site. Will be looking for a dc /ac amp clamp👍🏽

Max battery bank

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