Please Explain Battery Isolator Operation/Function

[John Suter]

Hi Eric,

Most battery "isolators" work in a similar manner,
regardless of coach model or battery combinations;
which generally total two banks, with each providing
12 volts of power for different applications (i.e.
engine starting and house/coach 12V appliances to
include the inverter(s) which then provide/convert to
110V). Sometimes we loosely refer to the sturdy buss
bar device as the "isolator," when we really mean the
entire system of electrical components, including the
insulated buss bar, sets of diodes, and relays.

A major purpose of the isolator is to permit the
engine alternator charger (via its voltage regulator)
to simultaneously charge both the engine battery bank
and the coach/house battery bank, and when not
charging, to separate/isolate one bank from the other
to prevent an engine electrical load from discharging
the house/coach battery bank, and to prevent a house
load from discharging the engine bank. Often by
default, the isolator system simultaneously "permits"
the house/coach bank to be charged, while it
"prevents" charging the engine bank, by the
inverter/charger system (regardless of whether its
110V source is shore power or generator power).

This "isolation" process can be
"defeated/overridden/controlled" by a dash switch
often labeled "auxiliary battery," which electrically
connects both banks to one another and permits
amperage to flow both "in to" (if charging) or "out
of" (if discharging), both banks as if they were one
single battery/bank. Some Birds have a 2-way switch
(isolated, not isolated) and some a 3-way switch
(isolated, not isolated, momentarily not isolated as
long as the button is held down). Typically, the "not
isolated" position is indicated by a small green LED
becoming lit next to the switch.

Whenever the engine alternator charger is
operational, owners will notice that regardless of
switch position, the LED will be lit because under
that condition, the system must electrically defeat
the isolation process to charge all banks
simultaneously, and under control of the engine
voltage regulator system.

However, simultaneous charging of all banks, by the
inverter/charger regulator system, can be a problem if
the banks contain different battery "types" -
Lead-Acid, Gel, and AGM, as each type has different
charging characteristics, and the inverter/charger
system can only be set to charge one battery "type" at
a time. (Lead-Acid and AGM battery types often can
use the same setting).

Although Lead-Acid batteries require maintenance of
their acid/water level to be at least above the plates
to avoid damage, they tend to "boil" out their
acid/water under prolonged charging by
inverter/charger systems. None-the-less, most battery
experts agree the best combination of cost and
operational efficiency is for the engine starting
battery bank to contain Lead-Acid batteries, previous
Forum discussions/opinions notwithstanding. However,
unless the isolator is overridden by the dash switch
at the risk of damage to the engine bank, most
inverter/charger systems default to charge only the
coach/house battery bank, thus leaving the engine bank
uncharged.

So what is the recommended method to charge a
coach/house bank of Gel or AGM batteries and an engine
bank of Lead Acid batteries?

Isolate the banks via the dash switch, which permits
the inverter/charger to only charge the house bank.
Add an Echo-Charger (second isolation device about
$150) between the house bank and the engine bank to
"sense" the appropriate engine bank charge-rate (i.e.
"take" only an appropriate charge from the house bank
to maintain the charge of the engine bank). Note the
isolation by the Echo-charger permits the
inverter/charger to only "see" the coach/house bank
and to keep the engine bank "invisible," so the
inverter/charger will not adjust its charging rate to
that of the engine bank.

As for the generator starting battery, BB has used a
variety of ways to charge it from a small alternator
on the generator engine, to connecting to the isolator
system, parallel-ling it with the engine or coach
banks, and I've seen an Echo-charger used to charge it
as well.

Hope this helps more than it confuses.

John Suter




--- Eric Johnson wrote:

> My coach has 4 - 6 volt AGM batteries that serve as
> starting and accessory DC supply. I have
> a gel battery mounted in the generator compartment
> that starts the generator. I have an
> "isolator" in the coach battery compartment with
> leads coming from the coach and generator
> batteries. I use a Xantrex 40 Truecharge, located in
> the center driver's side compartment, to
> keep the coach batteries charged. On the dash I have
> a rocker switch that reads "auxiliary
> battery". My understanding of the dash switch is you
> can use it momentarily to augment the
> coach batteries if they're too low to get the engine
> started.
>
> I've read a number of references to isolators but am
> clueless on how they function and would
> appreciate some education on the subject. I'm
> particularly curious if my Xantrex charger may
> be supplying the generator battery as well as the
> coach batteries. Xantrex says mixing battery
> types with the same charger is not a good thing.
>
> Eric Johnson
> 84FC35SB
> San Antonio Texas
>
>
>
>




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