S-caper

Electrical System

The electrical system is really the heart and soul of a successful conversion; it can literally make or break the intended usage of the conversion.  Getting this right means a happy experience...getting it wrong means a frustrating one.  A bad (or inappropriate) electrical design can be fixed but it's less expensive (in the long run) and easier if you just get it right the first time.

Ok, I'm stepping up on the soapbox...

THE #1 thing with electrical systems is SAFETY!  This is NOT the area to pinch pennies in!  Do it right or don't do it at all; it's ok if you want to light up your life but you have no right to expose other folks that may be on board your bus to that risk.

You can come out now; I kicked the soapbox back under the table.

What you'll get here isn't (by far) the only way to do things; there are lots of opinions on this stuff.  What I will describe is a robust and safe system that will require very little user intervention.

Don't use house wiring; it's solid rather than stranded wire and is not designed to handle the movement and vibration of "goin' down the road".  Use the best wire you can afford; I suggest marine quality wire like Ancor and East Penn make.  Use crimp connectors on the wire ends; over the years in boats it's been found these hold up much better than solder connections due to the constant vibration and movement.  A good solder connection takes skill and they're impossible to check visually whereas crimp connections can be done by unskilled folks with certainty and can be checked visually.  I'd spend the money to get a good ratcheting double crimper; the second crimp supports the wire by the insulation rather than just relying on the crimped stranded connection.

I'm using marine quality switch panels, fuse blocks, distributions blocks and other components; most are by Blue Sea Systems.

Right from the get-go you're going to have to decide at what level you're jumping into the game and also plan for any future upgrades.  Some folks can get by with an extension cord out to the bus; others can barely manage with an 8KW diesel genset; make up your own mind as it's your quality of life you're messing with.  Just be aware that getting the same level of 'juice' in a bus as we've become accustomed to in a house is darn near impossible and even remotely approaching that is VERY expensive.  Now's the time to go back and read the Mother Earth News and get in touch with the back-to-nature side of yourself; to make this an affordable adventure (and why the heck would you consider a school bus or doing it yourself if it didn't need to be affordable) you need to have conservation and simplicity in mind.  Visit web sites dedicated to off-shore sailing or to rural off-grid living and you'll start to get a sense of what it's going to take.  It's not likely you're reading this if you've got the $700K to $1 million bucks for a high-end Marathon Coach or equivalent so that really means you've got to accept some limitations! J

I like a multi-energy source approach; in other words...I like to spread the load out so that I'm not totally reliant one one system for everything.  For instance, lighting can be powered by 12-volts, 110-volts, batteries, propane or kerosene (or lamp oil); if you have several choices it means you can conserve a particular power source when necessary.  If you happen to be in an area with cool nights and you're trying to maximize your battery resources you could choose to use propane lighting or kerosene (like a boat's cabin lamp) and save the batteries for things that have to have them.

It's a system and it's all about balance. 

In our bus electrical system we've got to balance our storage capacity (typically batteries) against our loads and charging capability.  It's like groceries...if we don't want to go to the store every day we need a bigger pantry and fridge; but if our only mode of getting groceries home is on a scooter or public bus we can't carry enough stuff to fill up the pantry...the situation that would let us avoid going to the store; so we go more often anyway to keep from having to carry too many groceries at once.  However, if we buy a car or pickup the scene drastically changes.  What's that got to do with electricity?  Well, if you have too much load for your battery capacity you have to recharge very often, so you put in bigger batteries, but if you don't have a big enough charger it ends up taking so long to charge that you charge more frequently anyway to avoid the really long charge cycles; the system isn't balanced.

One of the first things you've got to do is make a list of all the electrical items you want to run and figure out what your daily amp-hour requirement is.  Think of this just like gasoline; how many gallons per day do I need to go the distance I travel.  Most all electrical gizmos have a wattage rating on them; it not they should have an amperage rating.  The math here is really simple; voltage times amperage equals wattage.  So if you've got a 10-watt lamp running on 12-volts that's 10 divided by 12 or .83 amps.  Or, say you've got a 15-amp device on 110-volts; that's 1650 watts.  See, I told you it was simple.  Regardless of voltage watts is watts is watts; it's just a number representing the amount of power required to run the thing and it takes that much power whether it comes from 12 volts or 110 volts (the voltage and amperage change of course, but not the wattage).  So now that you know what you'd like to run you need to estimate how long each day you'll run the item.  Something that requires 3-amps that runs for 1 hour will consume 3 amp-hours of power; for half an hour it would be 1.5 amp-hours.  If you're going to run something off an inverter you need to figure its wattage then divide by 12-volts to get amperage on the 12-volt side and then factor in the efficiency of the inverter (usually given by the manufacturer) to find out how much 12-volt power (out of our batteries normally) that it will consume.  If we have an 800-watt coffee maker on our inverter that's about 7.3 amps on the 110 side but 66 amps on the 12-volt side; and if our inverter is only 90% efficient then we really need 74 amps.  If the coffee maker runs for 15 minutes then we'll have used about 19 amp-hours to power the coffee maker.

Now lets look at the battery side; what we need are some good quality high capacity deep cycle 'house' batteries.  These aren't going to be cheap so we'll want them to last as long as possible and that will take some paying attention on our part.  The first rule is that you don't want to discharge your batteries any more than 50% of their rated capacity (i.e., if you've got a 200 amp-hour battery only use 100 amp-hours of that), otherwise you'll damage the plates in the battery and shorten the battery life (number of 'cycles' you can get) a lot.  How do you know when you've hit 50%?  A battery monitor (like a Link 10, Link 20 or Tri-Metric) is the easiest way; it's really just a 'fuel' gauge only for batteries instead of a gas tank.  Without it you're just guessing unless you have a very sensitive voltmeter or take the specific gravity reading of the battery (believe me, the Link and Tri-metric monitors are easier!). 

The second point is that it's hard to recharge the deep cycle battery to more than 80% capacity without a good 3-stage (or 4-stage) charger and some time.  Because of the thick plates in a deep cycle battery they need a long, slow charge at the end to "top them off".  That means we'll often have available just 30 to 35% of rated battery amp-hours available for daily use.  Using a good 3 or 4-stage charger when 110vac power is available will allow you to recharge fully and equalize them (if they're wet lead-acid batteries).  A lot of the time you're going to recharge the batteries from the main engines alternator and as a rule -of-thumb your batteries shouldn't be any larger than 4-times the alternator's capacity; 3-times is better.  So at the outside with a 105-amp alternator we wouldn't want more than 420 amp-hours of battery capacity; and, 315 would be better (but we'll probably go to the most we can get anyway!).

Then there's this little thing called Peukert's Law; what it really says is the higher the percentage of load is on a battery the faster it will discharge in an accelerating manner.  Here's a example; a 100 amp-hour battery will sustain a 5-amp load for 20 hours (that's how they're rated).  If you put a 20 amp load on the battery it will not last for 5 hours; it probably won't make it past 2.5 hours; it's important to factor in your 'heavy' loads.  Let's go back and look at the coffee maker example where we're going to pull 19 amp-hours from the battery to run the device for 15 minutes.  If this is on a standard group 24 deep cycle battery of say, 85 amp-hours ,the battery is likely to need recharging after just one session if we follow our 50% rule and only started with the battery at 80%.  We only had 25 amp-hours available to start with and pulling 19 out in 15 minutes will actually take us below our recommended 50% level.  If this were a 245 amp-hour battery we'd have started with about 75 amp-hours (assuming 80% charge and 50% cut-off) and the 19 amp-hours may account for about 1/3 of available power.

The recommendation on inverters is that the battery capacity should be 20% (in amp-hours) of the inverter's rating (in watts).  So a 1000 watt inverter should be backed up by 200 amp-hours of battery capacity (1000x.20=200).  Otherwise Peukert's Law really takes a bite out of run time.  By the way, that 1000-watt inverter is pulling almost 90 amps from the battery when at full tilt.

Depending on your loads it may be more efficient and less expensive to run a generator.  In the example of the coffee maker above we'd likely use less fuel running a Honda EU1000 or EU2000 (or equivalent) for the time we needed to run the coffee maker than we will to recharge the battery to replace the energy we used through the inverter.  There are times when just being able to use the inverter would be really nice; especially in a park outside of "generator use" hours.  It again comes back to balance; it takes a large and expensive system to continuously feed heavy loads and given the cost of a small 1000 or 2000 watt genset that may be less expensive than the battery system/inverter/alternator/charger system that would handle a similar load.

In our bus I'm planning on a 'medium' battery system consisting of two 8D AGM batteries (200 amp-hours each) and a 2000-watt (Xantrex MS2000) inverter.  The only reason I'm going as high as 2000-watts on the inverter is to handle the microwave and to get the great charger that's built into the MS2000.  Microwaves are very efficient and rarely run for more than 5 to 7 minutes; if that.  My coffee maker is an espresso machine that needs to run for about 30-minutes; it isn't practical (at least to me) to install a battery bank that will handle this much load so I'll just plan on starting the genset.  We don't plan to be in too many parks at any rate and if things change and I do need the silent power of an inverter I can add more battery capacity later.

I'm still wrestling with the portability, fuel-efficiency and quietness of the Honda EU2000 versus having to go out and start it when needed.  A built-in genset (I have a Honda EV4010 in my current class-c motorhome) is certainly convenient with its plumbed-in fuel line and inside remote starting.  If I do go the built-in route it will probably be the Onan MicroLite running on propane; I have a diesel bus and don't want to carry gasoline and I can't afford a diesel generator (at least not based on those I've found so far).

It's been a long page...sorry.  It's probably the most complex thing you'll have to deal with in your bus conversion.