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!