User manual

time not allow a light or some other load in the RV to drain the vehicle battery. It does this

by allowing current to flow from the “A” terminal to either the “B1” or “B2” terminals, but

not in reverse. This is accomplished, with some high current diodes. Current can flow

from the alternator to both batteries, but current cannot flow from the vehicle battery to

loads in the RV. The same is true in reverse, should you leave the vehicle headlights on,

the RV battery will not get drained.

There is another terminal on some isolators. This is required because many alternators

need power to start working. When the ignition switch is "on," 12 volts is present on this

terminal. Inside the isolator, a diode connected such that the A terminal will have power.

In the normal automotive configuration, the alternator output is directly connected to the

battery.

Note that the alternator voltage is higher than either battery voltage when the engine is

running. This is due to the natural voltage drop across the high current diode in the

isolator. Since the vehicle voltage regulator senses the battery voltage, the regulator calls

for slightly more than the normal voltage from the alternator to compensate.

The high current diodes will have some voltage drop when current is flowing though them.

If the batteries were identical and at an identical state of charge then under charging

conditions, the voltage at battery terminals “B1” and “B2” should be very similar. This

condition would rarely be the case in practice. A more likely situation is one where one

battery is at a much lower state of charge than the other. The voltage at the vehicle

battery is used to control the output of the alternator and if the battery is low, then the

voltage drop across the diode feeding the vehicle battery will be larger. The user then

may see a higher than normal voltage on the RV battery since there will be less voltage

drop across that diode due to the lower current. Again the actual readings may not seem

to make sense, but in fact be quite normal.

Battery Separator

The Separator, like the isolator allows the alternator to

"charge" the RV battery, but at the same time not allow a light

or some other load in the RV to drain the vehicle battery. It

will also allow the converter/charger in the RV to charge the

vehicle battery. It does this by monitoring the voltage on both

batteries, if either battery voltage is 13.2 volts or higher for

more than a few seconds, the separator, connects the

batteries together. Should the voltage fall below 13.2 volts for

more than a few seconds it will disconnect the batteries.

Unlike the isolator, the battery separator is a mechanical switch and there should be little

if any apparent voltage drop between the terminals when active.

In separator equipped RVs, the alternator is connected directly to the battery and the

alternator output stud will show voltage even if the vehicle is turned off.