Battery System Components
What do these things do and why do we need them?
A combiner is an electrical apparatus used to permit two battery (banks) to
be charged "intelligently." The combiner relies on the fact that the
voltage across a charging battery will rise as the battery charge
increases. When the charging voltage is great enough, the combiner
electrically connects both batteries in parallel to permit them both to
become charged. When the batteries are not being charged the combiner
electrically isolates the two banks. Using a combiner in place of the
isolator that is original equipment in the GMC permits the electrical
isolation (keeps the engine battery from becoming discharged) when you use
the coach 12V electrical AND allows both batteries to be charged when
charging voltage is present. This can simplify the electrical system if you
want to permit charging of the engine battery from the coach charger.

battery isolator
A diode based device that permits both battery banks to  be charged from the alternator (and allows the alternator to provide 12V  to the coach wiring) but blocks current flow from one battery to another.
This stops the two banks from discharging themselves in a short period of
time. The discharge happens because (generally) one battery bank is
different electrically from the other. This leads to the "stronger" of the
banks experiencing current flow out as the batteries try to reach
equilibrium electrically. As one banks discharges into the other it will
overshoot a little bit so that it becomes the weaker of the banks. Then the
second bank tries to charge the first - and overshoots a little bit. The
"seesaw" current flow eventually discharges both battery banks.

Battery bank - in the GMC one 12V battery or 2, 6V golf cart batteries in
series to create a 12V bank. The GMC has two banks - the engine (usually
one battery) and the coach bank (a 12V or 2, 6V). Some coaches have added a
third bank to extend the time between recharges.

Statpower - a company that manufactures a variety of battery related
equipment including chargers and inverters.

>statpower40, TrueCharge 40, TrueCharge 40+

A series of multistage battery charger/converters that convert 110 to 12V
(24V if you have that type of system - GMCs are generally 12V systems). The
multistage charger permits rapid charging of battery banks by employing a
high current phase until a specific battery voltage is reached. The current
is then reduced for another period of time until a different higher voltage
is reached. The process continues reducing current with the goal of
supplying the maximum charge current without boiling the battery
electrolyte. The chief advantage to the multistage chargers is that they
can restore a battery to a specific charge level much more rapidly than  an
automotive trickle or taper charger. In addition, these devices only supply
a float current when needed, so batteries aren't overcharged. Charge time
for a given battery bank differs based on the type of batteries used, but a
typical golf cart battery configuration will  recharge to the 75% point in
about 2.5 hours assuming a discharge to 25%. This represents the fastest
recharge possible and maximum useful battery capacity with reasonable
battery life. The Statpower products also function as a converter to
provide 12V from the 110V shorepower  (or genset). The charger can charge
batteries while supplying the coach 12V but it requires more time to charge
the batteries when they are operated in the eh 25-75% range.

A device for converting 12V DC from the batteries (or alternator) to 11V AC. There are many different types of inverter, each with different properties. The least expensive inverter supplies a near-square wave output which is passed through a simple filter to make it more sine wave-like. These are very inexpensive but relatively inefficient.The power filtered out to make the output more sine wave-like  is turnedinto heat and is lost. Some inverters lose 45% of converted energy this way(so if you are trying to power a 100W AC item at 110V you will actuallyexpend close to 200 watts of DC - half of which is turned into heat). Smallinverters of this type are useful for powering small items used occasionally (or if you don't care to conserve power).

The next most expensive inverter is a modified sine wave inverter. This
device is more complex than the simple inverter but is more efficient.
Where the square wave inverter loses much energy to heat losses, the
modified sine wave inverter is more efficient. It also costs a bit more. It
is better for running some type of motors and electronics. most Statpower
inverters are modified sine wave inverters.

The most expensive inverter is a sine wave inverter. It produces AC that
looks just like the power company. It is generally the most efficient of
the inverters. Electronic equipment and motors operate like normal when
using these inverters.

You do not need an inverter unless you dry camp AND want to not run your
genset often.

Wire size or gauge - wire must be of a size sufficient to carry the current
that will flow through the wire. Wire has a resistance. Resistance means
that the wire will lower the voltage from the source to the item using the
electricity. The large the gauge the lower to voltage drop (loss) from
source to the item. This means that you lose less battery capacity while
getting brighter lights, faster turning motors, etc. Some items like motors
can fail when the voltage drops too low because there isn't enough torque
to turn the motor but there is enough heat generated to destroy some parts.
Henry Davis

Diode Isolator Replacement    (This page created in 1999)

Battery Combiner

The Battery Combiner   replaces the Diode Isolator provided with the GMC.

If you use a smart charger for your house battery, the Combiner will supply this same smart charge to the Engine battery with out any rewiring.  You can use a cheaper charger and replace your failure prone diode isolator at the same time.

How it works
The alternator is connected to the engine battery and the shorepower charger is connected to the house battery.  When the alternator has charged the engine battery up to 13.2 volts, the COMBINER  connects the two battery banks together to charge the house battery.  The same thing happens when the house charger has charged the house battery up to 13.2 volts the COMBINER connects the two battery banks and the engine battery is charged using the smart charger.  When the chargers are off or the battery voltage drops to 12.8 volts (off the charger) the COMBINER releases so that both banks will not be discharged. .

The features are:

        1  No one half volt drop in the charging voltages-- allows full charge, read this link.
        2  Automatically senses the charger, shore power  or  alternator
        3  Disconnects when the chargers stop
        4  Only draws .1 A when the chargers are in operation
        5  Maintains both battery banks charge.

I did some research before jumping into this abyss.  Here are my references: for the one shown) (Used on Monacos' called MAINTAINERS) (pathmaker)

Here is a drawing of how to wire the combiner by Gil Hibbs

Here is a drawing of how to use a combiner to charge the Onan Battery (9/3/05) Rick .


Some have said that diodes are dependable, as they are solid-state - no moving parts to fail, like a relay has. I agree. However, I have seen failed diodes, and I have seen failed isolators (which are diodes). Dependability is not my issue - it is the voltage drop across the diode, which prevents the  battery from getting fully charged.

Duane said the alternator has a sensor lead that bypasses the isolator so the  alternator voltage regulator will sense actual battery voltage. OK, that  works fine - for that alternator. If you need a replacement alternator, you will need to make some revision to it to make the voltage regulator see actual battery voltage. SurePower, who is the largest maker of RV isolators, has instructions for each brand of alternator, telling you what you have to modify to make the alternator work with their isolator. Check out the Service Bulletins on their web pages at Some of these are simple modifications, some are complicated, but they are a must - ie. -you have to alter something else so THEIR product will work.

Our company builds custom sleepers for big trucks, and we used to use isolators for the house batteries. We did not like to make these alterations to brand new trucks because 1. each time we got a different brand or model of alternator we had to get the instructions and learn how to modify the alternators, and 2. invariably, if that truck had an alternator failure later on we got blamed for the problem, as we had altered something. We now use combiners exclusively (we call them separators) which are made by - you guessed it - Sure Power. They do a great job - the only failures I recall were caused by the solid-state circuit board, not by the relay. I am not
saying that relays will not fail eventually - this has just not happened yet. However, these are well built relays, made for continuos duty, and I expect them to last a long time.

I have not even mentioned the problem of other charging devices which have to work through an isolator. What if you use a converter/charger, like a Statpower TrueCharge? You have to run a charge line to each battery bank so the charger won't have to work across the isolator. Statpower uses internal diodes to separate the three circuits, but their voltage regulation circuit
is designed to overcome the voltage drop that is present.

On our sleepers we use a single-output converter/charger (Progressive Dynamics 3-stage) for less than half the cost of the Statpower, and connect the output to the house battery only - the combiner takes care of the engine battery.

Now, what about the charge circuit on the generator? - we'll have to see what we have to tweak on that if we want to charge the engine or house batteries also. And my GMC has solar panels - so I would have to change the voltage regulator on that if I plan to charge more than one set of batteries with an isolator.

The point I am trying to make is that we have to tweak every charging device we have, or put up with not fully charging the batteries, if we use an isolator. Not because they are not dependable (they do a great job of isolating) but because of an inherent design feature that causes a voltage
drop during the charge cycle.

With a 2-way separator (combiner) between two batteries, it does not matter what is used to charge any battery, as long as it has a regulated output. If the voltage rises above 13.2-13.4 volts, the combiner will close, and both batteries will be charged. If there is a third battery in the equation, just add another combiner.

If your use of the motorhome depends mostly on the main alternator for charging, and other charge devices (converter/charger or generator) are only used a day or two at a time, there should be no problems with slightly undercharging the batteries, and an isolator will do the job. In this case, a heavy-duty relay which is triggered off the alternator circuit would also work fine. However, if you spend long periods in the motorhome without running the engine, I would recommend you use a combiner, or adjust the other charging devices to work around the isolator voltage drop.

Another point - all these automatic devices have been developed because we just can't depend on remembering to flip a switch to shut of the AUX battery relay. And I would surely be one who would forget to do this!

These are my opinions, and I will admit that they are based for a great part on battery systems in heavy-duty trucks, which see a much different usage than most of our motorhomes. But I have been through too many problems with isolators to just let it go.Erv Troyer

SOLENOID IS AN ELECTO-MECHANICAL DEVICE  This was my strongest concern.  There are industry applications that still use these devices over solid-state devices( breakers over fuses, GFI protectors,) The applications  are usually high voltage, high current, and high overload situations. The boating folks use switches and relays for reliability since it is real hard to get a jump in the middle of the ocean when you suffer  component failures.  I have had to replace every diode isolator I have ever owned ( three different rigs) and my GMC has had three isolators installed that I can identify.  So I am going to risk trying this approach.

THE SOLENOID DRAWS POWER .   The solenoid ONLY draws power when the chargers are in operation. The 0.1 amp is about 0.1% of the alternator output.  There is one combiner that uses a bi-stable solenoid that requires no power in either position (ref #2).   This power drain is not a problem and some of the units are made to allow 600 amp overloads..

COMBINERS COST MORE.  Yep, about two times the cost of an isolator.  But I was going to buy an isolator that was two times larger hoping it would last.  So would have cost about the same.

DIODE DROP IS NOT IMPORTANT---   A Duane points out, the GMC is normally wired so that the alternator compensates for the voltage drop of the diode to the engine battery.  This does not adjust for the voltage of the coach battery which should be the same unless you are running your 1000 watt inverter (or refer, etc) and dropping more voltage across the coach-diode in the isolator.


1) The combiner will allow the smart house charger to also charge the engine battery in the smart fashion without adding any new wires.  My engine battery now gets bulk, soak and float charging on shore power.  You have to wire around the diode isolator to use the smart multi-bank house charger. ( see the wiring diagram on the web page)

2) The failure mode of a relay is normally open unless you ARC the contacts closed.  This open circuit  will leave your batteries isolated and connected to their own chargers.  The diode isolators failure mode is shorted diodes which will connect the batteries together and allow them to discharge together. This is what left me stranded.
3) There is no heat generated in the combiner so little power is wasted and no heatsink is required.
 There are internal overload sensors which protect the combiner.  With the diode isolators the heat generated and high transient loads might be causing the failures.

4)  My dash battery voltmeter now tells me the voltage of the House battery, since they are connected together during driving.

5)  I get all of these features without any rewiring.  Smart alternators and multi-bank house chargers require rewiring adding current sensors, etc. The really good smart chargers use temperature compensation and do a better job of charging.  I just did not want to go that far. Gene

High amp solenoid relays were not that reliable for many years unless they were very expensive.  I would have to think that reliability and cost are probably among the major reasons that these battery combiners have not been more popular in the RV industry.  A diode isolator has no mechanical parts and should last forever if operated below its rated capabilities.  A battery combiner (of the relay type at least) has mechanical parts which are probably more likely to fail.  They are also often rated for lower currents than the isolators.  We have a high amp alternator which can put out more amps than the continuous rating of the battery combiner mentioned here.  A cheap isolator, on the other hand, is rated for the high amps and has worked for several years with no problems.

Another thought is the idea of 12V batteries in parallel.  That seems to be basically what the battery combiner does.  It is only when a charge is being applied, but if one battery dies suddenly, the other would probably be affected as well.  The isolator seems to keep the two banks more separated, so maybe it would not be such a problem.  Anyone have any thoughts on this?

Current draw might be another consideration.  If solenoid relays are used, there would be some current draw.  The very small (30-40 amp rated though) automotive type relays we have used draw at least 1 mA.  Assuming that a solenoid like the one used in the GMC battery booster switch is used in these battery combiners, I would think that the current draw would be quite a bit higher (.1 A or so).  The stats listed below indicate a current draw of only .1 mA though.  Do they use something other than a solenoid relay to connect the batteries, or should it have been .1 A.  Even .1 A would be better than the voltage drop the isolator causes though, so this idea is probably not very valid.

Overall, IMO cost is probably the biggest factor.  Looking at all the high dollar stuff in the West Marine catalog and stores, I think that average boaters might spend even more on their boats than average RVers spend on their RVs (yes even GMCers :).  This may help explain the difference in popularity of the more expensive battery combiner when RVers and boaters are compared.  For an example, just take a look at which catalog has the $400 Statpower converter/charger.  West Marine carries it, and ,IIRC, even has a version with their name on it.  I assume that they have carried this type of converter/charger for many years, while the RV industry seems to be switching over only recently.  Most of the RV catalogs still only offer old style convertors or possibly cheap new style ones.  I don't expect a "Camping World" version of any $400 convertor/chargers any time soon :). Each market carries what they think they can sell.  This is why we have to look around at many different stores/markets if we want to find the best products for our GMCs.  We always look through all our racing, industrial supply, marine, RV, etc catalogs when we are looking for upgrade parts for our GMC so that we know all the options available.  Zak
You are generally right about the isolator. However, there are conditions under which the 0.8 volt drop is important. For one, the isolator dissipates heat proportional to the current flow. At full alternator current flow the isolator diodes dissipate at most 80 watts or about 5 amps. For most MH uses you are right that the loss is a "don't care." But, for those folks who rely on the alternator's full  capacity, the lowered loss through a relay is preferable.

Most automotive type regulators do not provide the multistage high current charge profile of smart chargers. Conversely, many marine applications include the smart regulator so that the engine can be used to recharge the batteries in significantly less time.

My view on the GMC isolator is that the diode circuit is preferred unless you replace the regulator with a smart multi-stage regulator - in which case you can really use all of the available current.Henry
 The Diode Voltage Drop (approx.  0.8 v) is of no concern in charging batteries because the Voltage Sense Line to the Alternator is connected to the output of the Isolator & is therefore compensated  for in the  Alternator's output voltage.  It would be crime, in my mind,  to revert back to an unreliable Electro Mechanical device in lieu of the solid state Isolator.  Like going back to vacuum tube TVs...low reliability & high cost.

Thanks for clarification of other User Applications that may be concerned with the power dissipation (loss) within the Isolator.  The GMC OEM Alternator wire & Isolator does get hot (power loss) when the Alternator is even at 1/2 current capability.  However, normally the heat reduces in a very short time as the batteries/loads decreases.Duane

GMC Battery Charging 101
by Emery Stora

You have two separate battery systems in your GMC.

First, you have a battery that is just for starting and running the engine, running lights, dash lights, gauges, distributor ignition, etc.

Second, you have what we often refer to as you house battery.  This is either a single 12 volt. battery or two 6 volt golf cart batteries in series, or sometimes (but not recommended), two 12 volt batteries in parallel.

The engine battery is designed for heavy loads for short times and it is charged back up quickly by the alternator.

The house battery is designed for less heavy loads for longer times and is often called a "deep discharge" battery.  It is designed to be taken down low and then recharged whereas a battery designed for starting an engine will fail after relatively few deep discharges.

The engine alternator will charge both sets of batteries.  It does this by use of an "electronic switch" called a "battery isolator" (some GMCers now use a "combiner" which does the same function but operates a bit differently).
The battery isolator has three terminals (some have 4 and charge 3 sets of batteries but the GMC normally has only two sets of batteries to isolate).  The alternator goes to the center lead and then the charging current of the alternator is split by the isolator so that some goes to each set of batteries.  The isolator contains diodes (think of them as electric check valves) which allow the current to flow from the alternator to each set of batteries but the current cannot flow back through the diodes so that a load on one battery bank cannot draw current from the other battery bank.

In the event that you have a dead battery (either the engine starting battery or the house battery) you can "boost" the dead battery from the other set by pushing your "boost" switch on the dash.  This supplies power to a coil in the boost solenoid that jumpers the two heavy wires going to the solenoid.  One wire leads to the engine battery and one leads to the house battery.

The house battery is charged when you are plugged in or are running the Onan by the battery charger.  The original GMC charger, affectionately called the "buzz" box because of the noise it makes, is connected to the house battery by the wire going from the battery to the 12 volt "house" fuse panel.  It will not charge the engine battery because of the action of the isolator.  So it will only recharge the house battery bank.

Now, an exception.  If the buzz box is plugged in and you were to hold down the boost switch then the buzz box would also charge the engine battery.  Some of us carry a small jumper wire that can be used the same as holding the boost switch down. The boost solenoid, which is located behind the front hood panel on the passenger side, has four terminals: two large ones and two small ones.  One of each of the large ones goes to cables that lead to the two battery banks. One of the small terminals has a short wire that goes to ground.  The other small terminal has a wire that leads from the boost switch.  If you take a short wire with alligator clips at each end and clip one end to the terminal that the boost switch goes to and clip the other end to a "hot" lead, such as the positive battery post or to the large cable that goes to the side of the solenoid from the house battery, you can actuate the solenoid just as if you had pushed the boost switch.  You can leave this jumper in place to charge the engine battery from the buzz box.  Just don't leave it there all the time.

In case you should have a failed alternator while on a trip, or a bad engine battery, you can use this jumper to actuate the solenoid and then start your Onan and allow the house battery to provide power to your engine.  You would leave the jumper in place until you get home or to someplace where you can get your alternator repaired.  Instead of doing this, once you have the engine running you could also disconnect the two outside wires from the isolator and connect them together.  This would keep your engine battery charged from the Onan and the buzz box.  Just don't try to start the engine with only this wire connected as you would draw too much current by your starter. Use the boost switch to start.

If you have purchased a "smart battery charger" such as the StatPower 40+, there are separate leads to charge sets of batteries.  The 40+ can charge 3 sets of batteries.  The charger has internal circuits so that the batteries stay isolated.  It would be necessary to run an additional heavy wire (6 gauge will work) from the charger to the engine battery.  This would allow the charger to keep both sets of batteries charged up when plugged into 120 volt or when running the Onan but the different sets of batteries will not interfere with each other and will not discharge each other.

Emery Stora