More battery connection advise please

zozomike

Hello to my betters! I have some thought time, and one important thought is to redo the connections of my 10 each BB5024 batteries, because;
   A. I did a sucky job the first time around with salvage cable and homemade lugs, I trusted BB statement that the BMS balances at every cycle. Maybe...
   B Cables are a mix of 2/0 and 4/0 and have some length differences
   C. It has been suggested that I add to the battery, and these are on sale finally. So a couple more are in order. 
I will upgrade to all 4/0 new cable and get a real crimper. This will be a parallel  connection as the batts are 24 volt, and so is my
Flexpower VFXR3524A. I have existing 3 of the Victron Power In, but I struggled with 2/0 using those, Even though Victron says 
4/0 should work I would not look forward to it. Plus I have not found a way to use equal lengths without a lot of bending. I designed several ways with long bus bars, and that could work well I think. but cost a bunch. Another option with all 4/0 is below. I like it better but the confounded way they build this battery seems limiting for double tapping, (the lugs are down in a recess and both on one end) 
Advise me before I screw up!
(yes I do know the arc flash of this is lethal, this will be a slow and careful rework, with adult supervision) 

BB.

Regarding BMS units... Each brand/model seems to be different. Some actively balance (you may have to reach full battery SoC and battery bus voltage to balance), and others only "monitor" and raise an alarm. You have to read the specifications/manuals to see which your's does.

Regarding the various wiring schemes... Most of what you have are not what is recommended... And would be "fixed" by moving one battery bus/cable connection to the "opposite side" (kitty corner) to the other battery bus connection. You are trying to make sure that the "current path" is the same total length no matter which path is taken (through batteries ABCD or EFGH, etc.)--Same resistance/voltage drop for each parallel string of batteries.

smartgauge.co.uk/batt_con.html

There are two major reasons picking the "right" AWG wiring for your bank... One is maximum sustained current. You don't want the wiring to overheat and melt the insulation. Most battery bank wiring is in "free air" space and you have the advantage of cooling of the cables by circulating air and can "get away with" smaller AWG cabling that that inside conduit (restriction of free air circulation, other cables inside also self heating).

The other is voltage drop. This is just a "waste of energy". Long cables (longer than "needed") have more resistance and more wasted energy (p=i^2*r).

Also--You are rarely going to draw 100% of AC inverter rated capacity... Look at your loads. For example, if you have random electrical appliances (fridge, LED lighting, laptop, RV water pump, well pump without irrigation requirements)--You probably will only average 25% (pick a number) of the inverter's rated capacity (on average). And you may hit 100% (or even 200%) only in cases of starting a well pump, running a Skill Saw, etc.

Where you may have heavy/sustained loads--If you have a couple electric heaters, or an induction cook top... In these cases is it possible to run for at 30-60+ minutes at full rated power. But even then, most people seem to run (example) an induction cook top at 1/2 power when cooking for 1 or 2 people.

Also--If you have (5 parallel strings of 2x12 volt batteries for 24 VDC bus)--Each parallel string of batteries only (on average) carries 1/5th the total current... So running 2 or 4 AWG cable for the 3 kWatt main bus leads is only 0.6 kWatt is per battery string--So having 4 AWG for battery to battery interconnects are "overkill". I sill like to see battery parallel connections over-sized as current sharing between strings is not exact (maybe design for 1.0 kWatt of current per string in your case). But that is still much smaller than the 3 kWatt main bus leads to the inverter. Note for every 3 AWG "change" in wire size, is around a factor of 2 change in wiring cross section area (i.e., A 7 AWG cable has 2x more copper vs a 10 AWG cable).

And understand that some resistance is "good" too in parallel string wiring. A bit of resistance helps to better "balance" the current flow between strings. If one battery pair is supplying "extra current"--The string cable resistance means higher voltage drop, and naturally directs current flow to the string with less current and less voltage drop.

When charging, you generally want relatively low voltage drop from controller to battery bank so the controller "measures" accurate battery voltage (for optimum/quickest charging and stopping of charge). However, for most solar installations, the charging current is much less than the peak load current (i.e., you have 600 Watts of panels, not 3,000 Watts)--So the voltage drop is less during charging and you get your "better" voltage accuracy (v=i*r drop).

In the end, I would not "kill myself" ($$$ and pain of thick/stiff cables) trying for 4 AWG when 2 AWG is nominal recommendation (unless you have some installation were the batteries are 15 feet from the AC inverter). And for battery string wiring, 1/3rd to 1/5th the rated bus current, something like -4 AWG smaller cable is not "cheeping out"--Just good design based on actual needs.

Some things to think about... Look at Welding cable. Very fine stranded cable--Much more flexible. The downside is finding the "right" crimp connectors. Fine stranded cable is larger in diameter because of the "extra air space between fine strands" (connector still needs to crimp and remove airspace for good crimp--Getting "right crimp connectors" and tools is important).

https://www.solar-electric.com/search/?q=welding+cable

Note that "welding cable" cannot be used with most "binding screw" termination on devices... The fine strands tend to "squirm out" from under a simple screw termination used for solid or standard "house wiring" stranded cables (need "shoe" under screw to hold all fine strands for welding cable or add a short length of "standard" cable to binding screw terminal).

The other issue is to make sure that there is no way your battery parallel strings can short circuit (no sharp sheet metal to cut insulation, covered batteries in operation so no metal/tools drop on bank etc.).

In a large battery bank, using a fuse per string is the "engineering recommendation" to protect a single string from getting shorted... With many parallel "smaller" foot print batteries, a fuse/breaker per string (5 in your case?)--That is asking a lot (space, $$$, etc.).

-Bill

zozomike

Thank you Bill for the thorough education. In all my plans I was striving for equal length interconnects.  In the case of these 24 volt batteries, the posts are all on the same end, so it is difficult to see that plan 3 ( the colored one,) is diagonal. And as these are 24 volt and so are my inverters, I am not changing voltage, just adding amperage. I wonder if you mean 2/0 and 4/0 when you say 2 awg or 4 awg? But if I am understanding correctly I could interconnect with 1/0 or 2/0? ( not 0000) Then only use the 4/0 for the run to the inverter? ( which is outback spec) If I can use the smaller gauge cable for interconnects, then I can easily use my existing Victron Power In bus bars to tie the strings together, as shown in the attached. This makes doing the Smart Gauge plan easier as well. Since each battery can deliver a sustained 100 amps per Battle Born, would the string not deliver 400? in that case the ties between the Victron Power In Bus Bars should be 4/0 as well. And the string fuse should these be 300? I have tried to use the Smart Gauge balanced plan in the attached, but as the posts are on the same end it is difficult to see and draw. I think this plan effectively uses equal length interconnects?  Please advise. 

BB.

The battery wiring looks fine in terms of balancing...

I think I need the equipment list--I may have made some wrong assumptions.

1. What battery are you using? XXX AH @ YY Voltage?
2. What battery bus voltage (12/24/48/??)?
3. What AC inverter brand/model/link or x.xxx Watts @ yy DC battery bus voltage?
4. Charging current (inverter-charger, aux DC from vehicle, AC to DC charger, etc.)?

Are you looking at 50 AH @ 24 Volt Battle Born batteries?

https://1t1pye1e13di20waq11old70-wpengine.netdna-ssl.com/wp-content/uploads/2021/02/BB5024-Manual.pdf

The website has at least one set of mistakes... 14.4 volts charging a 24 volt battery... So their 100 Amp max current -- Not sure if correct...

From the above manual:

Multiple BB5024 batteries may be connected in parallel to increase the capacity and current of the system. When batteries are connected in parallel, the voltage of the system does not change, but the capacity and current limits are additive. For example, two BB5024 batteries connected in parallel (shown in Fig. 3) create a 24V 100Ah bank that can deliver 120A continuously and 200A for 30 seconds. 

So, each battery can support 100 Amps, but only for 30 seconds... Otherwise, each battery discharge rate of 60 Amps continuous (still pretty good for a 50 AH @ 24 volt battery...

While Li Ion batteries can surge lots of current and even continuous discharge current at C/1 hour discharge rate... What is it you need?

Generally--I would expect a battery bank to last at least 8 hours (discharge rate)--8 hours for one night (100% to 0% SoC), or 4 hours a night for two nights (and bad weather/no solar during day).

Your battery bank is 2x batteries in series and up to 6 strings in parallel... But are you using 24 volt batteries--Two in series is 48 VDC battery bus? Or will you be using 12 volt @ 100 AH (or other) batteries--Two in series for 24 VDC battery bus?

Before I make any more "suggestions"--Getting the equipment list right is the first step. And understanding your expectations the next step (i.e., want battery to last 1 hour or 8-20 hours under "Typical"/Average loads?

-Bill

zozomike

Correction, that 100 amps output per batt is 30 second surge current, continuous is 60 amps each, 

zozomike

I had corrected that surge last night, but never hit POST, 
Anyway:
Most of these answers are in my signature, 

1. What battery are you using? XXX AH @ YY Voltage? 

BB5024 Battleborn 24 volt 50 AH

2. What battery bus voltage (12/24/48/??)? 

24

3. What AC inverter brand/model/link or x.xxx Watts @ yy DC battery bus voltage?

Outback Flexpower Two, using VFXR24A inverters, (7000 w) 

4. Charging current (inverter-charger, aux DC from vehicle, AC to DC charger, etc.)?

VFXR3524A charge current 82 Adc 

I do not see any series connections in the diagram, it is all ++ or-- is that not parallel? 
I am planning for a 24 or more hour window, in the case of winter storms that will cut power dramatically for 2-3 days, 
with extreme conservation. I was thinking getting up to 600AH ( adding 2 BB5024 to my existing 10) would let the 
remainder of my equipment be satisfactory. That said I can still pretty easily trip by turning on the table saw while cooking and laundry are going on. 
I am not always present to make adjustments, fuel generator etc.  so having an autonomous system is a priority. 
BTW I was thinking 300 amp fuses at the strings, does that sound doable? 
Thanks again

BB.

Yes, you are correct, all batteries are in parallel for 24 VDC bank (no series connections--I "hate" that wiring scheme, I find it confusing to make sure it is drawn correctly).  (I need new glasses )

for 10x 50 AH * 24 volt batteries, assuming you drain them by 70-90% SoC (I like planning for 70% for Li Ion banks, but with BMS system you can drain to 90-100% (depending on battery/BMS specifics:

• 10 * 50 AH * 0.90 Discharge * 1/24 hours = 18.75 Amp draw
• 18.75 Amps * 24 volts * 0.85 AC inverter eff = 382.5 Watt average draw
• 382.5 Watts / 7,000 Watts of inverter = 0.055 = 5.5% of your inverter capacity

While you can get high current discharge:

• 10 * 60 Amps * 24 volts * 0.85 inverter eff = 12,240 Watt for a bit less than 1 hour

Lets assume your max continuous draw is 1/2 of the 7 kWatts of inverter:

• 3,500 Watt draw * 1/0.85 AC inverter eff * 1/24 volts = 124 Amp DC draw at 1/2 power
• 10 * 50 AH * 0.9 max discharge / 124 Amp draw = 3.6 hours of battery operation at 1/2 power

Looking at your "real" 24 hour loads (Watt*Hours, peak Watts, etc.) is important here... I would using 1x 3.5 kWatt inverter for this system (keep the other for spare?)... If you have loads that can drain your bank in less than 4 hours (running continuously)--I would be looking at a larger bank or running the genset for those loads.

If you have a home of "normal loads" (fridge, lighting, etc.) and one "killer load" such as a well pump--Maybe two inverter could make sense (only running well pump for a few minutes at time to refill pressure tank, etc.).

You have the fuse (or breaker) per group of 4x batteries (4x 60 amps=240 ADC continuous max).

And you have leads running from battery bus to your inverters and other DC loads. Each inverter should have its own Bus to Inverter DC connection and fusing to protect the DC wiring:

• 3,500 Watt inverter * 1/0.85 AC inverter eff * 1/21.0 VDC battery cutoff voltage = 196 Amps
• 196 amps * 1.25 NEC continuous load derating for branch circuit = 245 Amps "optional" breaker/fuse/wiring rated branch circuit

The 1.25 NEC derating is optional... Suggested if you are planning on running max power for more than a few minutes at time.

Per battery string, 240 to 300 Amps is OK... I would probably stick towards the 240-250 Amp range--And keep the wiring a bit smaller too (save on copper costs and thick copper wiring pain).

4/0 is already rated to 260 Amps with higher temp insulation... And I would guess your max current draw (continuous) would be 200 Amps or less in normal operation divided across 2-3 battery groups anyway.

The batteries have their own protection... The fuses/breakers here are for protection of your wiring.

And each inverter (and other DC loads) should have their own wiring ("star" wired vs "daisy chain") to major loads/branch circuits--Each sized AWG for loads and fuse/breaker protected.

You do have the option of fuses or breakers... Just a comment that breakers are not much more expensive that high current fuses and holders (plus any replacements). Plus breakers make for handy on/off switches for debugging/servicing/etc.

-Bill

zozomike

Thank you for your time Bill, In my original post I had tried to explain that I was adding two more BB5024 batteries, and that was the occasion for trying to find the best "balanced" interconnection scheme. The diagrams I have shown are using 12 batts for that reason. If the plan shown in the last reply is the best balanced  set up that's what I will go with. 
My inverters are like the attached, an integrated system, all breakers, shunts, connections  etc. are included in the DC wiring box. The fuses shown in my plan are just to protect the interconnect cable, etc.  (https://www.outbackpower.com/products/integrated-systems/flexpower-two-fxr

I have no DC loads, and as it notes in the signature, the well is on a separate system, I am conservative with power use, but soon to have more occupants. I also have no propane. Only the EU3000 IS. which I use only during maintenance or 3 day snowstorm events. I do have a dump load to water heating wired off one of the FM100s to relays. I can order two or even 4 more direct from Battle Born right now, as my bank can still be added to, (very shortly that time window will close) So that order will happen tonight,    (where I am at the moment is 12.5 hours ahead of Battleborn) 
As always I learn from the math you include, (not my skill set) 
Thanks again

BB.

That sounds good. That is why I give the math/numbers in equations so you (and others) can see what my assumptions were and modify without me having to give the "updated" magic numbers.

10-12 batteries in series is always a bit of an issue... Keep track of each battery's voltage (rest) and especially under heavy curent draw/charging. You want to catch any wiring issue (loose/hot/corrosion) before they can cause problems with your bank (unbalanced batteries).

You can also use a DC Current Clamp DMM to monitor charging/discharging current flow once in a while--Each battery or "pod of batteries" is (roughly) taking its share of current (checking wiring and battery health).

But being lithium Ion batteries with sophisticated BMS--Even an unbalance due to bad cable connection or other issue--Li Ion batteries are not going to be "harmed" like an unbalanced Lead Acid bank (too high, too low of battery State of Charge, storing at at less than 75% SoC, etc.)...

-Bill