The engine may drive the coach, but it's the charging system that's the power behind it all. All MCI coaches have 24-volt charging systems. There are two major reasons for the 24-volt system: First is that when compared to a 12-volt system, the same amount of power can be delivered to a device using wires that are half the size and weight; the second reason, related to the first, is that electric motors can be smaller and more powerful than their 12-volt counterparts. In other words, 24-volt systems offer higher performance. This is important in motor coaches, which use large amounts of wire and entail large air-conditioning loads.
In actuality, newer coaches have a combination of 12 and 24 volts on tap for whatever the situation demands. The newest MCI coaches could actually be classified as dual-voltage vehicles. They provide 24 volts for heavy loads and battery charging, along with the flexibility to use some of the newer 12-volt automotive electronics with a minimum of customization and voltage conversion schemes.
This "24-volt" classification is a legacy in the automotive and aircraft industry dating back over 100 years; 24 volts is the nominal voltage for a set of fully charged batteries at rest, which is why we call it a 24-volt system. Today's 24-volt charging systems actually produce about 27.5 volts when the engine is running; this is the correct voltage to keep 24-volt batteries properly charged.
All MCI coaches use 24-volt alternators. On coaches with two alternators, the outputs are hooked in common with each other. With the exception of the E4500 described below, all of the regulators, be they internal or external, should be set between 27.2 and 27.5 volts with the engine running on fast idle and under a light electrical load (exterior lights on and all HVAC turned off). However, there is one big exception with MCI® E4500s that use the large 50DN Delco with an external adjustable regulator and a smaller Delco 34SI alternator as an emergency backup unit. The smaller backup unit has an internal non-adjustable regulator set to 27.5 volts, so to avoid having the small alternator unit trying to carry excessive loads, we set the regulator on the larger stronger 50DN unit between 28.2 and 28.5 volts. That way, the smaller unit hardly ever needs to work very hard unless the main unit (or its drive belt) fails.
Where did the "battery equalizer" come from? In the days of the MC-5s and MC-9s, the entire electrical system was 24 volts. The two 12-volt batteries were in a series, but they did not have any 12-volt loads back then. No 12-volt video systems, no 12-volt lighting of any kind. Everything worked fine, as long as the batteries were well matched. But then customers started adding 12-volt accessories, and the trouble began. One battery might go dead while the other got over-charged, causing the bus to not start. Enter the battery equalizer. This little gadget keeps both batteries equally charged by dividing the charging voltage precisely in half, with one battery to each half. We now rely on them for all 12-volt loads and have effectively migrated to a dual-voltage electrical system. If you ever find that there is one dead battery and one overcharged battery on a newer coach, and system voltage is good, then suspect an equalizer problem.
To check the performance of an equalizer, check the system voltage as described above (engine running on fast idle), then measure voltage of each battery separately; they should be equal within ½ volt of each other, assuming that both batteries are in usable condition (no shorted or dry cells, not badly discharged, etc.).
Last but not least is the battery itself. They are being made with better, more durable materials then ever before. They last longer than ever before, but they are still lead acid batteries, and maintenance is just as important as ever. Automotive Batteries can last a long time if you take care of them, but don't ignore the battery box, or it's going to make your life a lot more complicated than it needs to be.
For more information, refer to Section 7 of the MCI Maintenance Manual for your coach or contact MCI technical support at 800-241-2947 with any questions you may have regarding your MCI charging systems.
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