Series rule of thumb

Re: Series rule of thumb

My personal recommendation is try to find cell AH ratings (even down to 2 volt single cells) that allow you to have one series string--ideally.

2-3 strings if you cannot.

You can certainly parallel 4 or more strings--but I personally would avoid it unless you have no other cost effective solution available.

And, you should, with parallel strings, have a fuse/breaker per string to prevent the other strings from feeding excessive current into a shorted string.

The downsides with parallel battery strings are (again, my list, others may have other issues or disagree with me):

• Sharing of current during charging/discharging should be monitored. A bad cell, poor wiring connection, etc. is not obvious unless you are using a DC current clamp meter to ensure that parallel strings are sharing the current flow (here like this inexpensive meter)
• An open cell can stop a parallel string from charging/discharging (see clamp meter to check)
• A shorted cell can discharge an entire bank
• Lots of cells to check electrolyte levels
• Lots of electrical connections/batteries to check and keep clean
• Lots of extra fuses/breakers/wiring with parallel strings
• More parts, more things to go wrong

When monitoring current sharing--It turns out for paralleled battery sources, that "very low resistance" paths are actually the weak point in the system. The lowest resistance path (shortest, heaviest, lowest resistance battery cells/string) carries the most current. From my experience of working with paralleled current conductors in computers/lab equipment, the lowest resistance path carries the most current. Since Power=I²R, those connections are the ones that overheat first and fail. That is why it is very important that the wire lengths be balanced between parallel paths/connections. (paralleling current sources--such as solar panels, do not have this issue of needing matched resistance wiring paths)

Some people will like the idea of parallel strings add redundancy to a system. If one string goes bad, it can be disconnected and the bank still functions until the bad cell/battery/connection can be fixed/replaced.

Also, large capacity cells/batteries are heavy. You can get around this, somewhat, by finding 6/4/2 volt cells (if you need to move around by hand).

And there are many people that have parallel strings of batteries and are very happy with their operation.

-Bill "my two cents" B.

PS:

vtmaps wrote: »

CallMeChaz, I agree with the advice given above. In order to diagnose and treat your massively parallel battery bank you should understand the nature of parallel battery problems. Bill has just an hour or so ago written a good explanation in another thread:
http://forum.solar-electric.com/showthread.php?14674 [this thread here. -Bill B.]

Not quite as useful as Bill's post is an article in the latest issue of Home Power magazine (#147 Feb-March 2012). The article describes a single string as "Most Desirable". It says "some designers prefer two strings for redundancy -- in case one battery or cell fails, there will still be one functional series string at the correct voltage to rely on". It also says "three parallel strings are considered marginally acceptable". More than three strings are "undesirable".

The Home Power article does not do a thorough job of explaining why multiple strings are so problematic. Let me add to what Bill has written: Parallel strings of identical batteries with perfect connections are themselves thermally unstable. If one string is a bit warmer than the others (inevitable) it will draw more charging current than the other strings which will make it warmer which will make it draw more current which will make it warmer which will make it draw more current which will _____ (fill in the blank and buy new batteries).
--vtMaps

vtMaps has a very good point. Matching charging on parallel strings that are not at the same temperature will over charge the "hot string" while undercharging the cold strings. In addition to the whole thermal run-a-way issues (hot string, voltage falls, accepts more current, string gets hotter, etc.).

Re: Series rule of thumb

The first limit is that of the sizing of your battery interconnect wiring... Your T type fuse is probably fine (in general) for your needs--You just need to review each physical string for its wire/cable ratings and your expected loads. So, at the very least, if you have three strings, you should have 3 T fuses (one per string). When cable leaves your common + bus, then each cable that leaves should have its own fuse/breaker for its protection (i.e., you have 2 awg connecting battery to bus, then some 6/8/12 awg wire leaving for various uses--each lead needs its own dedicated fuse/breaker).

The NEC table is very conservative. for example, 2 awg wire is rated around 95 to 130 amps. Type of insulation, conduit size/fill or free air, etc. all have an effect.

American Boating and Yacht Council allows much more current... 2 awg is 178-210 amps maximum.

The next limit (which should meet above or less) is the maximum continuous current that you would expect.

Obviously, for a single string battery, that current * 1.25 (NEC Design Factor) for the wiring/fuse minimum rating (i.e., 100 amps * 1.25, fuse and wiring should be rated for a minimum of 125 amps).

When you have parallel battery strings... How good will your current sharing be? For two to three or more parallel strings, I would plan on the worst case string being cable of carrying nearly 100% of the rated load current (don't count on each string carrying 1/3rd of the current in a three string battery bank). If you have lots of strings (like 4 or more), I would would plan on the worst case string still carrying at least 50% of the load (unbalanced batteries, good/bad connections, a failed cell somewhere, etc.).

Now, because most people (especially for low voltage banks like 12 volts), design for minimum voltage drop. The actual copper cable will probably carry 200+ amps when your load is (something like) 100 amps. So, you would have the option of fusing somewhere between 125 amps and 200 amps.

If you fuse 125 amps, you may have more nuisance trips. If you have 200 amp fusing, you may have more fire works (glowing cables, etc.).

Also, many fuses are available in slow or fast blow... If you have "surging loads" (typically pumps), you have a choice of fusing for the lower value with a time delay fuse/breaker. That can protect a motor's windings when stalled--If so, I would suggest that the time delay fuse be installed directly on the motor instead of at your inverter input fuse.

If you have a wiring/inverter fault, you probably want the fuse to clear/break/trip quickly.

Also adding up surging loads (pumps) with static loads (lights, electronics, etc.) is easier to use a larger fast blow fuse to support the wiring / inverter loads... Remember, branch circuit fusing is there to protect the wiring, not the devices (if an inverter pops a DC Input fuse--the max fuse rating for the inverter is designed to help keep the "flaming bits" inside the inverter--so too large of fuse on the inverter can be a safety problem--not just a "repair" problem.

For ground referenced systems (i.e., negative ground to earth/safety/metal chassis grounds), you only need one fuse/breaker in the "hot" (or positive in this example) lead.

For best current sharing--check your battery bank wiring--it should be something like this:

www.smartgauge.co.uk/batt_con.html

I suggest using a DC Current Clamp Meter (such as this inexpensive one) to insure that your battery strings are properly sharing current (on average--it will never be 1/3-1/3-1/3 equal sharing---As a guess, if the lowest and highest, worst case sharing, was within a factor of two of each other--such as 15 amps low vs 30 amps high--That would be my point of concern/investigation.

Note that lead acid batteries have very low internal resistance... On the order of a few feet of their interconnect cable (very roughly)... So, if one string had 2x the amount of cable (or poor connections, etc.), then that string would carry less current--both discharging and charging.

-Bill