1982/11/08 Caterpillar

SUPPLEMENT: 01/24/83

SERVICE MAGAZINE, NOVEMBER 8, 1982, PAGES 7 AND 8. In the article, "Do Not Center-Tap Batteries In 24-Volt Electrical System To Operate 12-Volt Accessories," Caterpillar's recommendation was to use a voltage converter to get 12 volts for operation of automotive-type accessories. The specific recommendation was to use the 9G6981 Voltage Converter Assembly.

The date of the Service Magazine article which announced the 9G6981 Voltage Converter Assembly is wrong. In the last paragraph of the first column on page 8, change "August 10, 1982" to "August 10, 1981".

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Most Caterpillar vehicles use 24-volt (nominal) electrical systems which have two 12-volt batteries connected in series. Some users want to operate 12-volt, automotive-type accessories such as a radio. To do this, users have center-tapped the batteries; they have made an additional connection between the two 12-volt batteries as shown in Illustration 1.

Illustration 1.

This connection will provide a nominal + 12 volts with respect to ground when the disconnect switch is closed. However, Caterpillar recommends against center-tapping the batteries for the reasons which follow.

1. Center-tapping the batteries will cause early battery failure. The 12-volt load will use current only from battery B, but the alternator will charge both batteries equally since they are connected in series. See Illustration 2.

Illustration 2.

As a result, battery A will be overcharged and battery B will be undercharged. The rate at which this overcharging and undercharging occurs depends on the size and the duration of the 12-volt load. For example, a solid-state FM receiver may use less than 100 milliamperes at 12 volts; it will take a few hundred hours for the effects to be noticed. But, a coffee pot which uses 8 amperes or a large business-band transceiver which uses 24 amperes in the transmit mode may cause a battery failure in one or two days.

Either battery can fail. For example, in hot weather, battery A may fail first because of excessive electrolyte loss due to overcharging. In very cold weather, battery B may freeze and crack first because the electrolyte freezes more quickly as the battery nears its discharged condition. As battery B becomes nearly discharged, starting problems also will result.

2. Center-tapping the batteries can cause other problems when the disconnect switch is opened. If left turned on, the 12-volt load may act as a ground path for battery A because the load is parallel with the open disconnect switch. This may cause lamps to light dimly as well as discharging battery A. This also may allow current to flow backwards through and damage the 12-volt load. See Illustration 3.

Illustration 3.

3. Short circuits and fires are possible. The only way the batteries can be center-tapped is to connect a wire to a battery terminal or cable. Installation of the wire between the cable clamp and the battery post, or under the cable clamp nut, increases the possibility of a bad connection.

The wire generally has no fuse. For this reason, a short to ground causes the length of wire between the battery and the short to burn almost immediately. Installation of the wire is never to Caterpillar's standards because it can not be put into a braid-covered harness. Some users have run wires through cab windows or near hot components such as turbochargers. Some times the wires are installed in places where they must be removed to make repairs on other components.

Caterpillar Recommends The Use Of A Voltage Converter

The 9G6981 Voltage Converter Assembly, announced in the August 10, 1981 Service Magazine, is a switching-type converter which has protection against external faults such as output short circuit, high input voltage, reverse polarity, etc. This converter provides 12 volts at up to 4 amperes continuously, or slightly more amperes if well ventilated. If more than 4 amperes of current is needed, two or more converters can be installed in parallel.