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Old 10-10-2021, 10:43 AM   #1
Bus Crazy
 
Join Date: Oct 2020
Location: Near Flagstaff AZ
Posts: 1,951
Year: 1974
Coachwork: Crown
Chassis: "Atomic"
Engine: DD 8V71
What battery bank voltage to design into a bus/motorhome system?

Hi all,

I wanted to share my thoughts (for what they're worth ) on designing an energy system for a bus or bus conversion. My perspective and approach is a little different than some, I know, and I don't in any way mean to argue that this is the best approach. And I certainly don't demean other system designs, because everyone has to build what works for them.

I like discrete components over multi-function components. For example, I prefer separate inverters and chargers...as opposed to combined inverter/charger units. After building off-grid systems in some pretty remote parts of the world, I came to appreciate redundancy and the ability to repair or find parts anywhere. If a system device does go down, I want that device to impact the fewest possible functions. And discrete devices are easier to find replacements for. Examples...in a pinch, you can find an inverter at any truck stop...or a battery charger at any auto parts store, or WalMart. And a replacement 12V water pump is also easier to find.

I strongly encourage running as many items off the native system voltage as possible. Generally, this is 12V DC. So often, I see people plan their system with a cheap 120V dorm fridge but they don't calculate the idle or running current draw of the inverter needed to run that fridge. I've done calculations for people in which the inverter used more energy than the fridge, which more than doubled their energy consumption.

I like multiple inverter systems, with one smaller inverter that you can "afford" to run all the time if you want...for any low draw appliances, like your TV or computer monitor (if those aren't 12V DC items). Then, for your induction cooktop or microwave or Nespresso machine, have a larger inverter that that you turn on/off as needed. Most inverters have a remote switching option, so this should be convenient. A large inverter can use a lot of energy just sitting there idle and on.

And...what prompted me to write this...I see lots of folks designing systems with 24V or higher battery bank voltages. In the off-grid village or house systems I worked with, we always went with the highest practical battery bank voltage possible. This was usually 56 volts. This made sense for a system which is designed to only provide battery power to inverters. We never had 56V appliances or the need to run 12V items. And the wattage requirements for power transfer between the battery bank and the inverters was usually much higher than that required in a bus, so the higher voltage allowed for more manageable cable and fuse sizing between the batteries and the inverters. But in a bus/motorhome, I've not had an issue with current carrying capacity for the short run between a 12V bank and the inverter/s.

A higher voltage battery bank does mean lower current between the battery bank and the devices which turn that battery bank power into usable power. If you happen to have a bunch of 24V DC appliances, then you're all set and a 24V battery bank is just dandy. But normally, we find either 12V DC appliances or 120V AC appliances. A 24V battery bank would allow smaller diameter cables between the batteries and the inverter, but that run is (or should be) short and once the inverter's spitting out 120VAC the battery bank voltage is irrelevant. And if you have a 24V battery bank you need a converter to change 24V DC to 12V DC. Normally, I don't find any compelling reason to go with anything other than a 12V system in a vehicle.

IF you want to go with a 24V battery bank, that's totally cool if it works for you. Just keep in mind these considerations:

- You'll most likely need a converter to provide 12V power from the 24V battery bank. If that converter fails, and you don't have some 12V cells in that battery bank from which you can steal power, then your 12V system goes down.

- It's not as easy to find 24V appliances as it is 12V appliances. Most modern cigarette receptacle USB chargers will work on 24V (check the tiny print) but beyond that, you won't find that emergency back-up coffee maker or fan or inverter at a truck stop or most stores. They'll generally all be 12V.

- If your house battery bank is not the same as the chassis (starting) battery, you can't jump start your rig if you drain the starting battery. You can use a 24V house battery bank to charge your 12V chassis battery, but it's not as easy as if you had two matching systems.

So...design the system that works for you. But please, if you're new to this, take all the pros/cons into account and have a plan for what you would do if something fails while you're driving through Pipestone, Minnesota.

As always, I welcome any comments and alternate thoughts or suggestions!

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Old 10-10-2021, 12:38 PM   #2
Bus Crazy
 
Join Date: May 2018
Location: topeka kansas
Posts: 1,769
Year: 1954
Coachwork: wayne
Chassis: old f500- new 2005 f-450
Engine: cummins 12 valve
Rated Cap: 20? five rows of 4?
Systems

My bus is my first excursion into 12v power/inverter/battery banks/charging systems. I have used nothing, but 12v items.

12v compressor refrigerator, the big ol magnum energy inverters, huge continuous duty 200 amp alternator, generator will be diesel to match engine fuel, I don’t know if I will have solar,

I think I am going to use a solenoid to connect house batteries when the engine runs. My driveline is mechanical. No angry pixies needed for operation.

Using solenoids for charging makes sense to me.

The items for inverters, might be computers, entertainment stuff, and power tools.

What are your thoughts on tools you can use with 12v?

William
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Old 10-10-2021, 01:01 PM   #3
Bus Geek
 
Join Date: Nov 2016
Posts: 2,775
The battery powered lines have 12V chargers
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Old 10-10-2021, 01:09 PM   #4
Bus Crazy
 
Join Date: Oct 2020
Location: Near Flagstaff AZ
Posts: 1,951
Year: 1974
Coachwork: Crown
Chassis: "Atomic"
Engine: DD 8V71
Quote:
Originally Posted by magnakansas View Post
What are your thoughts on tools you can use with 12v?

William
I'm a big fan of the Milwaukee M18 line of tools and, as John mentioned, they have a 12V charger for those. It's about $100, though, so I just plug in the regular 120V charger when it's convenient to do so.

On our "expedition" rig...the Freightliner ambulance conversion...I did install an exterior 120V receptacle for an electric chainsaw. I've used that to cut wood and remove downed trees from forest service roads and it works impressively well.
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Old 10-10-2021, 10:16 PM   #5
Bus Crazy
 
Join Date: May 2018
Location: topeka kansas
Posts: 1,769
Year: 1954
Coachwork: wayne
Chassis: old f500- new 2005 f-450
Engine: cummins 12 valve
Rated Cap: 20? five rows of 4?
thanks

I will probably put one 120v outlet for exterior use.

Nice idea I never thought of.

william
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Old 10-17-2021, 11:40 AM   #6
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Join Date: Jul 2019
Location: California, Bay Area
Posts: 896
Great post, well articulated perspective, and many points to consider.
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Old 10-17-2021, 01:25 PM   #7
Bus Crazy
 
Join Date: Jun 2016
Location: Orange County, CA
Posts: 1,358
Year: 1990
Coachwork: Crown, integral. (With 2kW of tiltable solar)
Chassis: Crown Supercoach II (rear engine)
Engine: Detroit 6V92TAC, DDEC 2, Jake brake, Allison HT740
Rated Cap: 37,400 lbs GVWR
I also share your views on RV/MH house electrical systems, and if anything have taken it to a further level! For example, I have essentially two completely separate solar systems running in parallel, with each row of four tiltable PV panels paralleled in its own combiner box which then sends power through its own 4AWG cables down to its own 50A circuit breaker before its own Morningstar TS-MPPT-60 charge controller. Each CC sends power through its own 80A CB to its own bank of four GC2 batteries wired in series and parallel; each bank of batteries uses a 250A Schottky diode to prevent one bank from back-feeding into the other, then both banks are combined at the main DC busbar that supplies the inverter(s) and DC loads. This way, if one PV array or one CC or one battery bank goes tits up, I'll still have 50% of original power available. Now that's redundancy!

My house system is 12V for the simple reason that the bus's own chassis system is 12V. I really don't want to faff about with Vanner equalizers and all that palava, like MCI owners need to do, and I can easily start the engine from the house batteries by just turning two Blue Sea switches that connect house Pos and Neg directly to the starter motor. Yes, there is some minor cost saving by using smaller cables if one were using a higher battery voltage, but in the context of a multi-thousands dollars bus conversion that's just noise. 4/0 welding cable is only $4 or $5 a foot, 4/0 lugs are cheap, and a perfectly functional FTZ circumferential crimper was less than $200 - big deal.

I not only plan on using as many 12V appliances as possible to reduce inverter idle loads, but I'll be using two small (4 or 5 cu.ft.) chest freezers instead of one upright fridge/freezer; one chest freezer will use an external thermostat to run as a fridge (a fridger?). Each chest freezer will have its own small inverter, and the big Magnum MS2000 inverter will only be switched on when needed. Again, I have redundancy: if one of the chest freezers or its inverter fails, I'll have a spare. I also have two SHURflo 2088 water pumps that can be used individually or together, and if either needs any attention it takes less than a minute without any tools to remove it for servicing. I'll also have multiple ways to heat water and multiple ways to deal with grey water, so I don't anticipate ever having things grind to a halt because of one single failure. I'm trying to keep single nodes of criticality to a minimum, similar to aircrafts' multiple-redundant systems.

I think the best engineers are pessimists. They simply assume that $hit will happen sooner or later, and they design accordingly. Would you want to fly in an aircraft designed or built by an idealistic optimist? That's how I look at my bus's design: when (not if) something fails, how easily can it be fixed?

John
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