24V Solar System for School Bus - Need advice! :)

Dre123

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May 15, 2021
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Hi,
I am currently sketching the solar system diagram for a School bus conversion into a motor-home.
It is my first time doing this, i have been reading a lot in the last couple months and learned a bunch, but still. If you guys could give me some input and advice it would be great.

I have conducted an energy audit and have come up with 4947wh/day
I am in Canada and i expect 4h of Solar Irradiance.
I would like to use the solar system year round if possible, but the bms have a low temp battery protection at 0 degree Celsius. I still need to think about how i can make this work when its -20C...
I would like to be as cost and energy effective as possible.

This is the setup:
- Solar Panels 5x 300W - Total of 1500W
- Midnite Solar Classic 250 MPPT
- Battery: 24V system 16 cells 3.2v 100ah 8S2P
- 2 BMS 24V 100A
- 24V to 12V 70A Victron Energy Orion DC DC Converter
- Giandel 4000w Inverter 24V to 120V

Thanks
Solar System Diagram1.0.png
 

time2roll

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For the amount of solar and inverter I think you could go 2x to 4x on the battery size. 200 amps to that inverter seems light. And discharging the batteries at 1C seems a bit heavy. That 35 amp fuse to the inverter will be quite limiting. Check the inverter installation manual again.
 

MisterSandals

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Looks REALLY well thought out.

Hopefully just the drawing but make sure wiring of parallel batteries is equal length.
Wondering if positive wire from one battery to the other battery positive, then to system creates out of balance scenario.
 

rmaddy

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The negative wire from the MPPT should not be going to the shunt, it should be going to the negative bus bar. Only the battery should be connected to that end of the shunt.

The batteries in parallel should be connected differently. Have a look at the Wiring Unlimited guide from Victron and read through chapter 3 for proper ways to connect batteries in parallel.

As a nit pick, the AC wires from the inverter to the breaker box and from the breaker box to the AC loads each have 3 wires, typically black (hot), white (neutral), and green (ground).
 

Dre123

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For the inverter, i wanted to go with the Giandel 3000W but its more expensive than the 4000W. The 3000W is 956$ and the 4000W is 637$.
The highest loads i plan on using are, small microwave(950W), blender(1000W), travel hair dryer (1000W) and 2 slice toaster (900W). Not all at the same time. Is going with the bigger inverter really a problem?

The loads on the 12V portion are led light bulbs, led strips, a couple usb chargers, 2 maxxair fans and 2 water pumps.
On the 24V portion i only have the fridge.

Is this right way to connect the 16x 3,2v 100ah batteries? Output goal is 24V 200ah controlled by 2 BMS.

Sans titre-1.png
 

time2roll

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The 4000 watt inverter could draw close to 200 amps putting you at 1C discharge rate. Ok so the max usage will be a bit lower with a typical 1000 watts max. (microwave at 950 watts cooking power is close to 1300 watts input) Maybe you only need a 2000? Your call. An oversize inverter is fine even if the idle draw is a bit more. Consider a 15 or 20 amp breaker on the inverter output to limit power vs opening the fuse. I think I would have a 120/150 amp fuse and wire to match.

The battery looks like 4800 watt hours and your solar is 6000 watt hours. You could start the day almost dead, use a lot of power and still get a full charge. Just seems like there is a lot of solar compared to the battery. If you are using that much power each day you could run very low on power with just one bad solar day. Nothing wrong with extra solar. Can always add more battery if needed.

I think I would skip the battery terminal bus at each end of the stack to keep it wired as two separate batteries.
 
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Dre123

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Thanks for all that helped. I think i will get back to the drawing board and take in consideration the discharge rate and add more batteries.
I will repost the version 2 when its ready. Budget is kind of a limiting factor, do you guys think 3500$Usd is a tight budget for my needs?
 

ed1380

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I'd look at circuit breaker size

orion is good for 70A so use an 80A breaker

4000W at 25v is 160A so use 180-200A breaker


like said above, the only thing that should be connected to the battery side of the shunt is the battery. everything else goes on the load side
 

HRTKD

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For the inverter, i wanted to go with the Giandel 3000W but its more expensive than the 4000W. The 3000W is 956$ and the 4000W is 637$.
The highest loads i plan on using are, small microwave(950W), blender(1000W), travel hair dryer (1000W) and 2 slice toaster (900W). Not all at the same time. Is going with the bigger inverter really a problem?

The loads on the 12V portion are led light bulbs, led strips, a couple usb chargers, 2 maxxair fans and 2 water pumps.
On the 24V portion i only have the fridge.

Is this right way to connect the 16x 3,2v 100ah batteries? Output goal is 24V 200ah controlled by 2 BMS.

View attachment 49228

You have interconnected your batteries before the BMS. You don't want to do that. That bus bar between the two batteries on both the positive and negative terminals isn't right. The problem there is that one battery could be at 100% and the the other at 90%. The 90% battery will continue charging, putting the other battery over 100%.
 

Dre123

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Ok, i have made a couple of changes to the diagram, i think now it's better but maybe still not enough battery capacity.
Let me know what you guys think.
2 questions;
Should i connect the negative bus bar to the chassis?
Also I would like to limit the charge to 93% of the total battery capacity and limit the discharge to 7%, How can i do that? Through the BMS or the MPPT?
Thanks
 

Attachments

  • Solar System Diagram 2.0.png
    Solar System Diagram 2.0.png
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ed1380

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some of your wire gauges are on the bigger end, but that comes down to distance and personal choice

drop the mc4 fuses down to 15-20A. short circuit they max out under 10A
 

MisterSandals

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Also I would like to limit the charge to 93% of the total battery capacity and limit the discharge to 7%, How can i do that? Through the BMS or the MPPT?
Thru your MPPT. The BMS is a last ditch safety disconnect.

How do you expect to achieve 7-93%? I suspect you will either use your he Victron connectivity and SoC or you have voltages in mind for your SCC.
 

HRTKD

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For limiting state of charge on the low end, if the Victron components can be set for that, then I would go that route. Otherwise, you'll have to do it in the BMS.
 

Sojourner1

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Ok, i have made a couple of changes to the diagram, i think now it's better but maybe still not enough battery capacity.

Also I would like to limit the charge to 93% of the total battery capacity and limit the discharge to 7%, How can i do that? Through the BMS or the MPPT?
Thanks
I would think about doubling the the battery bank, with the price of cells no reason to stress the pack or having to possibly think about is there enough solar recharge after a couple crappy days of weather. 100ah @24v might sound like a lot but for things you want run and if using it for long periods of time you'll eat through that power. To some 1,500w of solar seems like a lot also but if mounted flat, acouple crappy days combined with winter shorter days you'll be supplementing the charging with engine/ generator with hold over charges.

For wanting to use a lower SOC capacity just set your charging lower (28.0-28.4v) and low volt disconnect 24v or a little less on the inverter.

In 5 years of everyday use and at times drawing 180a for 3-5 minutes at 30% SOC (70% DOD) the inverter will show 12.2v, my lvd is set at 12.0v on the inverter.
 
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Dre123

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some of your wire gauges are on the bigger end, but that comes down to distance and personal choice

drop the mc4 fuses down to 15-20A. short circuit they max out under 10A
True, i will put a 20A instead.
I will have all the solar components all the way back of the bus.
The longest cables are 32' - 40', its the ones coming from the 12V Fuse box, 24V Fuse box and 120V Sub Panel.
 
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