solar school bus question

Hello forum, good day!

I am designing a 12 V battery bank off-grid mobile solar system for a school bus.

I am putting circuit breakers on each circuit and using the BLUE SEA DUALBUS PLUS, which you can see here:

I am running the battery wires to one side and the FLEXMAX 80 charge controller to the opposite end. Circuit breakers on both positive wires.

I am running an Inverter, Fridge, Stereo, and Lighting all off the Dual Bus Plus, which is rated at 150 amps.

The inverter will only be used in emergencies and the biggest load is the Fridge.

4 questions:

1. Is it common practice to run all this off the Dual Bus rather than put multiple circular connectors on battery terminal. Only running a + and - off the battery to the Dual Bus through a breaker seems easiest and safest because battery terminals are hard to access.

2. How would I interact a generator into this equation if available. Would I put MC4 connectors onto it and run it through the charge controller like the PV, or run a separate power strip for those times when its available. The solar is only 2 135 watt panels and will not get enough current on my batteries so Id like to supplement it with a generator, etc..

3. Is there a good online calculator for sizing wire? I know continuous usage requires 1.25 and its always best to oversize.

4. The starter battery on the bus has been acting up and is not linked to the solar system, but does not start the bus and there is indication there could be a short so we are running new starter wire, should I wait to install solar until this has been resolved aka.....could the + from the starter interact with the - from the bonded negative on the solar?



  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: solar school bus question

    1). With lots of connections, use a bus bar. Piling up rings on battery posts is asking for trouble. Be sure every branch circuit off the bar has a fuse or breaker appropriate for it.

    2). Generators DO NOT feed in to charge controllers. They are AC and power a battery charger, the output of which goes to the bus bar.

    3). There are many on-line charts for sizing wire according to its Ampacity. They do not always agree. There is no continuous rating down-sizing; the rating given should be for continuous usage. This is not always the case. Application has a bearing on Ampacity too, as much of it is based on how much heat can be dissipated. My favoured chart includes a basic V-drop calculator as well:

    4). Ideally the 'house' system is kept entirely separate from the 'auto' system (although they will inevitably share negative connection to the chassis). This way one can be used to charge the other if need be, and keeping them separate prevents one from draining the other (which is what you'd experience with your present undiagnosed problem).
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: solar school bus question

    Re the starter battery, get it tested with one of those 'load testers' that garages use. Do this after a good long drive so that you can also checkout the alternator and Voltage regulator. You should see it sending ~ 14.4 V to the battery right after discharge (starting) and then it will drop to ~13.2 V when the battery is charged.

    If it can also be charged up with a standard car charge and if it tests out OK, look for a parasitic draw (short) in that system. Always check the voltage ~3 hours or more after a charge. The volts should read ~13.2V, Less than this is indicating a failing battery.

    If it does not pass the load test replace it...

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