Newbie / Batt bank question

Re: Newbie / Batt bank question

I am the guy that usually recommends ~3 parallel strings if possible. From my generic experience with computer designs, I find that large numbers of parallel connections with voltage sources become problematic with current sharing (even with all wire harnesses of equal length--you still can have a loose connection, a little corrosion, a dead cell, etc. that can take a string out of service--an open cell; or even slowly discharge the entire bank--i.e., a shorted cell). As well as the whole issue of having to check the water level of all of those parallel cells.

However, lots of people (including our host NAWS) have systems with more than 3 parallel strings and they are very happy.

If you have parallel strings--I would suggest two things. 1) get a DC Clamp Amp Meter (just a suggestion) so you can monitor heavy charge/discharge current between strings--If you find a high or low current reading, check out the string for opens/shorted cells/etc.).

Second, each battery string should have its own fuse/breaker rated for maximum current you expect from the bank (and fuse/breaker should match the maximum current supported by your interconnect wiring). A huge problem with large battery banks is they can support ~10,000 amps or more into a dead short. High Current Fuses/Breakers are not cheap and can be difficult to wire in--but are critical to reduce the chance of fire (shorted cells, dropped tool, etc.). A fuse per parallel string connection will reduce the maximum current to a "safe" value.

Wiring wise, here are some suggestions for balanced current sharing (equal resistance for each string current path).

It is recommended that batteries be installed in matched sets... The more mismatch (age, sulphation, differences in electrolyte fill specific gravity, manufacturing, etc.) can cause old vs new battery current sharing mismatches...

How much will that "kill new batteries" because they more deeply cycle than the older batteries--A shorted cell in one bank killing the whole bank (if missed), etc. all add "risk" to your setup.

So--you are left with the choice of simply using the existing bank until it "dies" (even if it means cycling deeper than you normally would)--Then re-engineering the new bank all at once (larger AH cells to reduce parallel string, adding fuse/breaker per parallel string, possibly different brand/model batteries, or even AGM vs Flooded Cell, etc.).

Another choice--you are happy with the batteries you have and would purchase them again for a new bank--So do a bit of rewiring on the existing bank (if needed), add over current protection per string, add the new batteries (hopefully same manufacturer/model), and monitor the overall bank performance with a good DMM and DC Clamp Meter--If you find a failed "old battery"--You may even replace one or two of those (or get some used batteries)--until it is clear that you will need to replace the entire "old string".

The problem is that batteries do not appear to fail consistently... With a large bank, you may have one early life failure (warranty return). Then the bank lasts 6+ years, and about 1/2 the batteries are end of life (usually capacity loss/failure to hold a charge). And the other 1/2 of the batteries may actually work fairly well for another 3+ years (give to somebody else for their smaller system, move to a back up system, etc.).

I am afraid there is no "right answer"... Personally, as I said, I like as few parallel strings as possibly--But many folks are limited by battery size/weight--
Simply cannot move a 500 lb large AH capacity cell around the battery room.

But, for you, wiring in a second set of new batteries into the existing bank and monitoring with voltage and clamp DC amp meter is the most cost effective solution for you.

Also, I would ask if you have enough charging current/energy for the larger bank... Typically we recommend a 5% to 13% of Bank's 20 Hour Amp*Hour capacity as available charging current... Installing a very large battery bank will many times take people below the 5% minimum rate of charge--then they have to run the genset more often or they end up "deficit charging"--basically operating the battery below the ideal 80-90%+ daily state of charge range.

-Bill

Re: Newbie / Batt bank question

dagelt wrote: »

Cariboocoot,
Not for sure... maybe I am communicating incorrectly. Each battery is 245 Ah @ 20 hrs and I have 4 in one string to make 48 volts. I have 3 of these so I think I have 735 Ah bank now. Is that correct?
I am trying to support 18000 watts a day.... my whole house with out discharging them past 75% 50.5 volts.

• 735 AH * 48 Volts = 35,280 Watt*Hours est. battery capacity
• 18,000 Watt*Hours / 35,280 WH = 51% discharged (49% state of charge)

Assuming I understand 18,000 watts a day to be 18,000 Watt*Hours per day. Assuming you charge during the day an use power at night...

If you use 1/2 your power during the day, and the other 1/2 at night--then (at least on a sunny day) you will only use ~25% of the battery capacity per night.

-Bill

Re: Newbie / Batt bank question

Negative/Positive leads do not have to be the same length--However, you want the same total string lengths to match between parallel battery strings. This keeps the resistance balanced between strings (otherwise, the low resistance string will take most of the load/charging currents and the strings will not properly share loads/charging current).

Smart Gauge Battery Wiring (sorry to be repetitive--link posted before)

However, unless the wiring balance is really bad--This is probably not going to cause a short term noticable problem with battery bank operation.

I have lost track, are these Flooded Cell batteries or AGM/Sealed type batteries?

If flooded cell, do you have a good Hydrometer and take readings to see how you bank (and individual cells) are performing?

In general, if you don't think your bank is performing well--Either you are not properly recharging the bank or you have some specific problem (failed cell, poor electrical connection somewhere, etc.). Or, you are simply using more current/power that you think you are (if you do not have a good load/charging monitoring system setup yet).

For example, it is not unusual at all for Solar Charge controllers to read 5-10% high for their output current (or even 30% off at times)--People never complain if their system appears to output more current than they planned (solar charge controllers still work well--they just are not designed for accurate current/energy logging--costs more money to do that). Accurate current monitoring (battery monitor, DC Clamp Meter, etc.) is useful if you are trying to maximize your power output of your system (variable solar power and variable loads day by day and even hour by hour causes lots of confusion).

-Bill

Re: Newbie / Batt bank question

Each cable does not need to be the same length or even in the same relative position.

Just the same over all length per string for same resistance path.

-Bill