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Old 03-24-2020, 01:36 AM   #21
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Default Re: Airtime's electrical schematic for review please

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Originally Posted by 99sport View Post
Victron makes a 200Ah battery that is essentially the same size as the 100Ah battleborn (7/8" narrower and 3/8" taller) so you could have the same capacity in half the space (negating the need to place half the batteries on opposite sides of the van). The cost is $100 extra per amp hour (plus the cost of the external BMS) and you get a 1 year warranty instead of 10. On the plus side you get an external BMS that can talk to the rest of your system, individual cell monitoring and balancing, and bluetooth communication.
Interesting... I had looked at that earlier so I went back to look again. Actually the dimensions I see for Victron are 9.4H x 12.7L x 6W. That 12.6" is too long to fit in the undermount tray that Hein sells, for use in front of the wheel wells. The Battle Born GC2 batteries are 11 x 10.3 x 6.9 if you use them with the terminals on the side, and it is the 10.3" length that fits in the tray.
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Old 03-24-2020, 03:05 AM   #22
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Default Re: Airtime's electrical schematic for review please

I've done some analysis of a "Method 3" approach to cabling up my Battle Born batteries in undermount trays on left and right. For those of you who like to geek out, I'd really appreciate any feedback on my approach here, from a safety standpoint and anything else. While I've spent some time with small signal electronics, I haven't spent much time in the world of batteries, fuses, and big cables.

Method 3 is with individual cables from each battery to common bus bars. Here again is the link that was helpfully provided by Cheyenne:
http://www.smartgauge.co.uk/batt_con.html

To have balanced batteries, the cables need to be balanced. In my scenario, I experimented with matching resistance using larger guage, longer cables for the right bank, and smaller guage, shorter cables for the left bank. All while trying to meet ampacity, voltage drop, etc.

Jumping to the end... DC simulation shows good results with current imbalance of 0.4% without too much fine tuning, and total system voltage loss of less than 0.7% at 200A load and 1.5% at max 400A load. The cables seem smaller than I would have thought - 4 AWG for the left bank (each handles up to 100A continuous). But based on my understanding of the Ampacity specs and the Ancor wire specs, it seems to work. Comments?

Following are the details.
Method 3 simulation.pdf

Battle Born specs
- 100A continous
- 200A for 30 seconds
- 4X bank can supply 400A continuous

Inverter specs
- 2000W continuous
- 4000W surge
- 90% peak efficiency (assume peak is at full load)
- Calculates to 185A continuous, 370A peak @ 12V
- Kisae requires minimum 300A DC fuse or breaker, plus DC disconnect

Battery bank cabling and circuit protection
- 100A MRBF terminal fuse on each battery positive terminal
- Individual cables from each battery terminal to positive and negative bus bars
- 400A Class T fuse on output of positive bus bar feeding inverter

Wire sizing (would like feedback on these)
- Matched cable resistances between right and left banks for balanced battery current
- Each cable sized to have
- ABYC ampacity rating exceeding the fuse rating
- assuming 75C insulation spec (Ancor specifies 75C for wet, vs 105C for dry)
- and further derated by 0.857 for bundles of 4-6
- Overall system designed for
- 1% voltage loss from battery terminals at 200A load
- 2% loss at 400A maximum load (fuse rating)

Actual cable sizes
1) The left bank cables are selected to be minimum guage meeting the above constraints. This works out to 4 AWG and 3ft length. Resistance is 0.72 milliohms.
2) The right bank cables are sized to match the resistance of the left bank cables, using larger guage to compensate for longer length. Result is 1/0 AWG and 7ft length, 0.70 milliohms.

Simulation results at 200A load:
- Current imbalance between batteries less than 0.4%
- Voltage drop from battery terminals to load (assumed at bus bars) of 0.7%
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Old 03-24-2020, 03:30 AM   #23
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Default Re: Airtime's electrical schematic for review please

Thanks everyone for all the advice. I've made quite a few changes and I'm posting my updated schematic. I'll leave out the details on battery bank cable sizing, if you are interested in that please see the analysis I just sent in my prior post.

Here is the updated schematic:
Sprinter electrical schematic v3.pdf

Changes include:
1) Moved the positive lead to the BMV-712 shunt to one of the battery terminals, and added the temperature sensor
2) Eliminated the rooftop solar fuse and added a disconnect breaker/switch inside
3) Added 100A MRBF terminal fuses at each battery positive terminal
4) Rewired the battery bank for individual matched resistance cables feeding into positive and negative bus bars
5) Inverter fed by 400A Class T fuse followed by a remote battery switch--so my wife can have a button in the kitchen rather than a manual switch underneath the bench seat
6) Added a connection from House negative bus bar back to the ground post at the throttle pedal. Sized to match the DC-DC converter supplies, although I think it is more of a ground reference and won't actually carry much current? Depending on how balanced the supplies are on the charger.
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Old 03-25-2020, 04:51 PM   #24
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Default Re: Airtime's electrical schematic for review please

As a couple of notes, Airtime: your house bank chassis ground connection needs to be on the "load" side of the shunt, not the "battery" side, and the temp sensor is a two-wire connection with one wire being fused; it replaces the individual power lead that comes with the BMV-712. You may be showing that, but with the individual labeling it looked like you might be anticipating still using the individual power wire, so I just thought to clarify that a little.
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Old 03-25-2020, 09:23 PM   #25
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Default Re: Airtime's electrical schematic for review please

Quote:
Originally Posted by Justin Laureltec View Post
As a couple of notes, Airtime: your house bank chassis ground connection needs to be on the "load" side of the shunt, not the "battery" side.
Yes a classic mistake with the alt charge bypassing the shunt and the load going through it.

Quote:
Originally Posted by Justin Laureltec View Post
the temp sensor is a two-wire connection with one wire being fused; it replaces the individual power lead that comes with the BMV-712. You may be showing that, but with the individual labeling it looked like you might be anticipating still using the individual power wire, so I just thought to clarify that a little.
I missed your point here the installation manual shows it as he has drawn it.
Attached Files
File Type: pdf bmv700.pdf (3.08 MB, 8 views)
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Old 03-26-2020, 12:06 AM   #26
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Default Re: Airtime's electrical schematic for review please

100 amp battery fuse on each battery - that works from a technical viewpoint. Consider if these trip during a vacation or if your wife is using the system and the ability to swap them when it is raining.


The simulation results are interesting. I am wondering about the effect of contact resistance at connections vs copper conductor losses. It doesn't take much for a wire lug to bus bar connection to to be imperfect, especially at those currents. As an example, the lug of a 1/0 wire is going to have nearly 2x the surface area connection to the bus bar as the 4 awg. I have no data on the contact resistance of this connection point but if your design has a risk area this will be the place that needs to be verified on a bench or after the build.

Fortunately it is relatively easy to build this all up on a bench and verify the results before installation. Worst case you just buy a few new wires. A clamp meter will be your friend.

I have never attempted to down size the wire to match impedance but that makes sense. The traditional method used is to just make the wires all the same size and length and coil up the extra length. Not saying that is right or wrong, just common.

My approach has been to wire the batteries in series so that I don't have to match wire impedance, but it is hard to argue with the simulation results - at least from a bulk conduction perspective.
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Old 03-26-2020, 03:19 AM   #27
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Default Re: Airtime's electrical schematic for review please

Justin: Thanks, I need to remember that the shunt is really just part of the battery, and the load side of the shunt is the House Negative.

Kevin: Actually the alternator charging current has a return path on the alternator input from the Kisae DC-DC converter to the chassis ground. And then on the output side, battery charging return current is connected to the Load side of the shunt.

The connection between house ground and chassis ground is a separate question. I think current really shouldn't be flowing in that except in a fault condition--depending on the isolation in the internal design of the charger and inverter. I've got some questions into Kisae support. But either way, connecting it to the load side of the shunt--the House Negative--makes sense.

Harry: Yeah, good points on the fuse swap inconvenience. But that really shouldn't happen unless I have a cable short, and then I want it to trip rather than have a fire. Say if I hit something in the road while driving. Otherwise it would take a 400A load to reach 100A on one battery, less if one battery was very imbalanced.

I do think I'll lower the Class T main fuse inside the van to 300A, so it trips first if (someone) runs the microwave and the induction cooktop at the same time and then makes an espresso while waiting...

On the contact resistance, I was trying to find info on that with no luck.

As you said, nothing that can't be bench tested.
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Old 03-26-2020, 05:40 AM   #28
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Default Re: Airtime's electrical schematic for review please

Quote:
Originally Posted by Airtime View Post
Justin: Thanks, I need to remember that the shunt is really just part of the battery, and the load side of the shunt is the House Negative.

Kevin: Actually the alternator charging current has a return path on the alternator input from the Kisae DC-DC converter to the chassis ground. And then on the output side, battery charging return current is connected to the Load side of the shunt.

The connection between house ground and chassis ground is a separate question. I think current really shouldn't be flowing in that except in a fault condition--depending on the isolation in the internal design of the charger and inverter. I've got some questions into Kisae support. But either way, connecting it to the load side of the shunt--the House Negative--makes sense.
[.
I think you will find the input and output negative in the Kisae DC-DC converter are connected together, either way the added wire is a return path to the alternator so this way they are anyway.

It would be interesting what your BMV charge current would look like with a short across the shunt. Ohms law knows no difference between intended an fault current, it takes the path/s of lowest resistance.
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Old 03-26-2020, 06:06 PM   #29
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Default Re: Airtime's electrical schematic for review please

Not sure regarding your ampacity for 4 awg cable. You're using 4 awg for individual 100A max battery cables.

75 C, 4 awg cable is rated for 85A. It's close, but OK.

Looking at battery balance:

Cable resistance:
0/1 = 0.0983 m ohm/ft
4 = 0.2485 m ohm/ft

ratio = 0.2485/0.0983 = 2.528

That means if you're using 7 ft 0/1 cable, the 4 awg cable needs to be 7/2.528= 2.76 ft long. Your 4 awg cable is 3 ft in length. They are not balanced.
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Old 03-26-2020, 07:31 PM   #30
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Default Re: Airtime's electrical schematic for review please

Thanks for replies!

Quote:
Originally Posted by Kevin.Hutch View Post
I think you will find the input and output negative in the Kisae DC-DC converter are connected together, either way the added wire is a return path to the alternator so this way they are anyway.
I checked. Yes, all three negative returns (alternator, solar, battery) are connected in common. The Chassis ground is a safety ground for the case and is isolated from all positive and negative current-carrying lines.

Quote:
Originally Posted by calbiker View Post
Not sure regarding your ampacity for 4 awg cable. You're using 4 awg for individual 100A max battery cables.

75 C, 4 awg cable is rated for 85A. It's close, but OK.

Looking at battery balance:

Cable resistance:
0/1 = 0.0983 m ohm/ft
4 = 0.2485 m ohm/ft

ratio = 0.2485/0.0983 = 2.528

That means if you're using 7 ft 0/1 cable, the 4 awg cable needs to be 7/2.528= 2.76 ft long. Your 4 awg cable is 3 ft in length. They are not balanced.
I see somewhat different numbers in the Ampacity table I got from the Blue Sea website:
http://assets.bluesea.com/files/reso...ence/21731.pdf

75C (wet), 4 AWG (21 sq mm) is rated for 125A for single wire, multiply by 0.857 for bundles of 4 to 6, works out to 107A for wet, bundled 4 AWG cable.

Balance--yes not perfect with round numbers in feet, but close. I could tweak the 4 AWG cables as you suggest.

But since I'm down in this rabbit hole, I'm looking at another alternative that has larger copper and perfect balance, at the expense of extra connections. I'll post that.
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