The 24-Volt Coach
Uncle Mervin insists he's going to rewire his coach from 24-Volts to 6-Volts...that's what his tractor uses. We're trying to persuade him otherwise, but he'll probably go ahead anyway. Aunt Mildred figures she'd rather have him messing with 6-Volts rather than 120, so she's not much help.

We've been asked many times to explain 12-Volt and 24-Volt differences. This presentation, however, is out of the AC wiring realm that we have been discussing, but is overdue, judging from the mail.

Why 24-Volts?

Electrical power is simply the product of Volts and Amps. That is 120 Watts of energy results from 10 Amps flowing at 12-Volts. The same 120 Watts only takes 5 Amps at 24-Volts.

Now, the more Amps that flow, the larger the wires must be to avoid loss of energy in the wires themselves. For instance, if you want to run 100 Amps through 20 feet of wire with only a 3% loss, it will take #2 AWG wire. If current is cut by one-half, then only #4 wire is required. A reduction in wire size is a reduction in weight and cost.

Ordinarily, the size of wire isn't of paramount concern; however, a manufacturer building motors can build motors for less, if smaller wire sizes can be used. If that manufacturer is building starter motors, and can get the same power out of a motor with smaller wire. Guess what voltage is used—24. In short, it's easier to get high power from a 24-Volt motor than a 12-Volt motor.

While the engine starter may be the bigger motor on a coach, there are other motors in a commercial people hauler, air conditioning, for instance. These will also be smaller and lighter for the same power rating.

While most people get rid of the bus air conditioning system, they still end up with a 24-Volt starter to contend with. What should be done? Is there some way to rewire everything for 12-Volts?

Learn to Love It!

Not too long ago, automotive gear was 6-Volts. Finally, the 12-Volt standard became universal, and since then, we've come to accept 1 2-Volts as the 'best system. Indeed, there ate so many things made to run on 12-Volts that even if we elect to operate a 24-Volt coach, some form of 1 2-Volts will be required.

Despite 12-Volt systems being more popular, there are some definite advantages to a 24-Volt system. The starter motor is smaller than it would be at 12-Volts, and for the same size alternator, about twice the power can be obtained. For instance, a 300 Amp, 12-Volt alternator is standard for Eagles, while most Prevost units have a 270 Amp, 24-Volt alternator...almost twice the power available. From such an alternator, over 7500 Watts can be delivered when running at 28-Volts, a typical charging voltage.

What can you do with 7.5 KW? Cook underway, for one. How about running a couple of large air conditioners while cruising the interstate? With a couple of Trace 4000 Watt sine wave inverters, there will be plenty of power to start and run a couple of 16 kBtu air conditioners. This will take some reasonable battery capacity to act as a filter between the alternator and battery, but it can be done. We'd suggest at least six 8D batteries, (or equivalent), as a minimum, with eight or ten batteries is better yet.

To make 24-Volt battery banks, at least two-12-Volt batteries are necessary. The batteries are wired in series to produce 24-Volts. Another pair of batteries wired in series may then be wired in parallel with the first pair to double total capacity.

But what about 12-Volts?

If you have an inverter, the need for 12-Volts shrinks. Why not just run AC appliances everywhere? This is not as bad as it may sound. Yes, running a small AC fan from batteries through a large inverter is not very efficient, however, in terms of total power wasted, it's not much to worry about unless you do extended dry camping and want to minimize energy drain from the batteries.

One disadvantage of running everything from the inverter is the impact of inverter failure. Even with two units, there will be occasions when both have failed. With this in mind; perhaps a minimal 12-Volt system is required after all. This can be obtained in a couple of different ways.

Shown in Figure. 1 is the basic DC circuits for a 24-Volt and 12-Volt system. There's a positive distribution point for both 24-Volt and 12-Volts. A distribution point is generally an insulated terminal post that is used to connect loads and feeds to circuit breakers.

Note that the alternator is wired directly to the house bank rather than the starter bank. This is done for the simple reason that the house bank will often be deeply discharged, and may accept all the current the alternator can deliver. It doesn't make sense to run this current through switches and solenoids.

Shown in the Figure is a 24-Volt to 12-Volt Converter. As mentioned, there are two ways of doing the conversion. First, is the "equalizer" device which allows tapping the 12-Volt mid-point in the 24-Volt bank. The equalizer works by keeping the bottom 12-Volt bank at the same (equal) voltage as the top bank. As a load draws on the bottom battery, the equalizer siphons enough current out of the top 12-Volt bank so that its voltage matches the voltage on the bottom half of the bank.

The equalizer circuit has the midpoint connection shown in a dotted line between the 12-Volt terminal on the converter to the 12-Volt tap on the house battery bank. With the equalizer circuit, you can intermittently draw more current from the bottom half than the equalizer can produce...the excess comes from the battery bank. As time permits, the equalizer will replenish the bottom bank.

There are e also DC-DC connectors that are designed to operate without a 12-Volt battery. That is, the input power comes only from the 24-Volt battery bank and is converted to 12-Volts electronically. The dotted line shown in Figure 1 is not used for these converters.

The DC-DC converter circuit is more foolproof than the equalizer circuit, however, intermittent overloads aren't near as welcome since there is no battery on the output of the converter...all loads must be powered by the converter within its power rating. If that happens to be 25 Amps, that's all you can get.

Whether you chose an equalizer or DC-DC converter, living with 24-Volts isn't so tough that a coach should be converted to 12-Volts. The 24-Volt system is more powerful, and with the large alternator already on the...engine, a major expense is avoided.

Charging the Starter Battery

Shown in Figure 1 is a device called the AutoSwitch. This is a latching solenoid that is operated by a microcomputer. Whenever the house bank is charging, the solenoid will connect the starter battery to the house bank so that both banks get charged.

Also shown is an emergency parallel switch. This can be used to permit starting from the house bank.

Yet another switch is shown in series with the starter motor. While not necessary, the switch will more than pay for itself if the starter solenoid ever fails to open. A starter running when the engine is also running is not a pleasant sound, and it usually ends in a bang as the starter motor comes apart. The same switch can also be used as a security switch if it is placed out of sight. Someone intent on stealing your coach isn't going to do a lot of wire tracing.

A Generator in the 24-Volt System

Is a generator even needed? With substantial generating capacity from the main engine alternator, and plenty of AC power from one or two inverters, when will a generator be needed? Probably only with extended dry camping. With a suitable size battery bank, overnight stops at truck plazas or rest areas no generator is necessary.

But before this issue can be finalized, decisions must be made about refrigeration. The type of refrigeration will have a profound impact on the rest of the electrical system. We'll start banging around some of the ideas.

The All Electric Coach?

Some people are opposed to carrying propane on-board. No matter how safely a propane appliance is installed, there is always danger if a leak which can kill silently, or in a big bang. Certainly insurance companies are interested in keeping propane out of motor coaches.

Electric cooking isn't something you'd want to do much of from a set of batteries. but is obviously feasible from a generator. A microwave, which can be successfully operated from a battery/inverter combination, can suffice for short heating jobs.

A generator is only operated for relatively short periods to run the electric cooktop.

As it turns out, the "rub"' isn't so much cooking as it is refrigeration. Refrigeration is an energy hog, and even the latest so-called energy efficient house models are not suited for general use from a battery bank.
There are some refrigeration alternatives that we'll explore in future articles that can make an all electric coach possible, maybe even without a generator!

Uncle Mervin's All Electric Coach

After reading about 12-Volt and 24-Volts, Uncle Mervin has started to convert his system to 6-Volts. He found tons of welding cable at a recent auction and figures he can always double it up for the high currents. Aunt Mildred is glad he's out of the kitchen. BC