Any advantage to common Bus for all I / O
Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
Is there any Advantage / Disadvantage to having a common Bus connection for all In / Out connections to a battery bank. If a Inverter is pulling from the Bus will the Input go to it first ??, then any excess to the batteries ??
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Not being facetious, but in my mind electrons follow the path of least resistance and do not differentiate what they are flowing to. ie if more than the pv output is needed by an inverter, the flow to it comes from all sources to satisfy the demand.
I don't think there is any question as to effectiveness of buss bars, just imagine the battery post with all the different connections made to a single point and think about my previous statement.
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In general, following the recommendations for connecting the batteries (balanced wiring) and creating a single "ideal" bus connection that has very low resistance to the battery bank--Then connecting all the loads and all the charging sources to the common bus is the best way to do it.
Note, there are some wiring diagrams that create two "ideal" connection points. You could place chargers on one and loads on another--But I am not sure there is any advantage to that (with an otherwise properly designed battery/bus bar system).
If your charge controller has remote voltage sense leads (MorningStar TS family, new version Rogue), then connecting those directly to the battery connection point (instead of the bus bar) could save a bit of voltage drop (my personal recommendation, try to not have more than 0.05 to 0.10 volt "rise" between charger and battery bank--Higher voltage "rises" will tend to slow down battery bank charging--reduce charge rate faster than "ideal").
A bus bar is just a different common connection point from the battery post. Usually they are only used when you have a lot of wires that have to go to the same place (parallel battery strings). Otherwise it's another connection point (possible something to go wrong) and expense. No real advantage in current flow otherwise. The batteries will not be deprived of charge by the current from the controller going to the inverter "first".
I was asked the question because a ME type posed it to me because he planed to use two charging sources and wondered if one had a advantage over the other. I usually do a diagonal battery bank connection or a paralleled bank and really didn't have a answer about one over the other. I'v used the Scan Gauge information on cabling.
Busbars are great!
Just remember that busbars are conductors. They have ampacity and tempurature ratings.
The panel-mounted busbars like the ones in Midnite E-panels and Outback power panels are typically rated at 180 amps (AC or DC). I have found overheated neutral wires in a common busbar. There were 4, 60 amp charge controllers all fed to one end of a 180 amp busbar of a grid-tied battery-bassed system. The insulation had been melting back and the copper #6 wires were discolored and very brittle.
Also, the "factory" built power panels are often pre assembled with a #2 wire feeding the positive busbar. This would limit the current to 130 amps.
I asked this question because of something I noticed when charging with a Honda EU 2000. This morning I was able to do a little test. I took 2 chargers one Iota 55 amp and a old 25 amp. The Iota 55 is wired to my battery bank. I hooked the 25 amp directly to the inverter input lugs. If being on a common bus makes no difference then I shouldn't notice any difference on the inverter draw or a change in the charger output or the Honda's ECO throttle. The bank is 10 GC-2, @ 12v.
I fired up the Honda and both chargers on ECO throttle. The Iota 55 was putting out a steady 55.8 amps and the 25 amp about 15 amps. Both being 3 stage, I thought they must be fighting each other and the 25 amp had cut back some. I kicked on the refrigerator as a Inverter load and the Honda started to increase the throttle in response to the load. The Battery / Buss voltage remained at a steady 13.3 volts and the Iota 55.8. The 25 Amp hooked to the Inverter was now putting out 25 amps.
So in essence the Inverter is taking a extra 10 amps from the charger and not pulling it from the buss or batteries because it's being fed off the 25 amp charger.
To test it farther I removed the Iota's 55.8 amps from the buss and left the 25 amp hooked to the inverter inputs. Every time the refrigerator would kick in the incoming amps would drop on the buss but the charger was still putting out 25 amps and the inverter was taking it and the voltage was steady.
My conclusion is it does make a difference and the inverter will take the power either from the buss or the charger. How much difference does it make ?? Is it more efficient the current is not being converted to battery storage and then to the inverter power ?? The Honda reacts to the draw thats for sure. In essence you have one inverter feeding another anyway. My only thoughts is can you cut conversion loss any ??
This would be a difference in the wiring resistance between the connection points where the two chargers were attached. This is particularly noticeable on 12 Volt systems where even a small amount of resistance makes a significant change in performance. And of course the higher the current involved the greater the effect.
Thats my thoughts also. I can kind of tell more over a period of time on fuel consumption and recharge times. I'll run a couple A/C's tonight and the mix the generator in. I usually charge at bed time and run them at night. If true, it would save me big time, anytime I can keep from running the 20 KW generator is $10.00 a hour saved, not to mention the batteries.
The biggest advantage to using a common buss is everything comes to and from a common piont of reference. Everything can be calculated from that point. Master battery cables to buss have a voltage drop of 'X". Volt drop from buss to inverter is "X".
Normally most power panels have the buss mounted almost directly to the inverter, That should put all charge controllers and inverters on the same voltage plane. But there are line losses from the controllers to that buss as well to take into account.
Is this what you are talking about? :
Attachment not found.
The inverter draws current from the DC buss. It does not "care" where it comes from. The resistance in the cable between the battery and inverter could be the reason the Iota 55 primarily charges the battery and the Iota 25 primarily powers the inverter. The greater the resistance the more pronounced the difference in the power sharing between the two chargers will be. It would make sense that if you are wanting the best efficiency for battery charging you should minimize the resistance between the chargers and the battery.
I suppose that if you want to convert generator power to DC to feed the inverter it would be more efficient to minimize the resistance between the charger and the inverter. Of course the nagging question is; why convert from ac to dc and then back to ac?
You might want to check out how the chargers would share the load if both chargers were connected at the same place. Using larger cable or shortening the runbetween the inverter and batteries should also have an impact.
This is the advantage to having "voltage sense" wires attached directly to the battery terminals. This allows charging equipment to get the actual battery voltage instead of a voltage reading on the current carrying conductors which can be seriously effected by resistance.
The voltage at the output terminals of an 80 amp CC at full power can be quite different than the voltage at the batteries. Fortunately, when the batteries are closer to full the current falls off and the voltage readings are more accurate anyway.
I've wondered what might happen if you have a CC with seperate voltage sense wires and a bad connection in either the current carrying conductors or the sense wires. It seems like things could get messy.
At least with computer power supplies, we had a few 1-10k Ohm resistors attached directly to the power supply output--So if the sense leads failed, the supply's output (at least) would still be in regulation. And attaching sense leads simply "overwhelmed" the high value resistors to measure the remote bus voltage.
What I have always wondered... It is not difficult for an MPPT Solar Charge controller to measure/estimate the output resistance of its wiring and compensate for that... Just change the output current by 10 or 20% and measure the voltage... I.e.:
R=V/I= change in voltage / change in current = (14.5 volts - 14.2 volts) / (60 amps - 48 amps) = 0.025 ohms
Or even let the installer program in the estimated harness resistance (can even use the controller's internal temperature sensor to adjust the copper's resistance due to temperature changes).
Now, the controller can figure out the voltage drop based on its estimate of V=I*Rwire and simply add that to the set point.
Oui! I can just hear boB and Ryan clamboring back into the lab...:roll:
That pdf. is the exact set up. I could run the AC into the Inverter, but it would require me to always be changing the setup. It's a SW 3000 w/ 150 amp charger and would way over power the Honda 13 Amp output. I also have a have a 24 volt inverter 4 GC- 2's that I use to overnight my Refrigerator that I will fill in with a charge or re-charge as a opportunity load when I run the 20 KW.
It seems to me there could be many other factors in play which would be a problem for this. If it was just a stand alone battery charger designed for use with a battery which was not in use at the time... (an EV charger?)
If it was a hybrid system (multiple charging sources and loads) it could work if it was fully networked. Outback and XW already do this with battery temp comp.