GMC Western States

 Tech Center Number 30 - August 21, 2000

By Chuck Aulgur


I have had several requests to provide some info about batteries as there seems to be a lot of confusion or misunderstanding as to which batteries are the best for use in our Classic GMCs.

The material presented here is based on my personal experience and the personal experience of other club members. It is our viewpoint and does not represent authorized data pertaining to the GMC Motorhome. It is the responsibility of the readers to make their own judgment as to the validity of this material in relation to any repairs and/or modifications to their own vehicles.

As a starter, there is no one battery that fits all applications. We all use our motorhomes in different ways, and the battery that is best for Mary may not be the best battery for John. There are basically three different methods for constructing batteries that I will be discussing, and in each construction method there are "starting" batteries and "deep-cycle" batteries. Starting batteries are designed with the capability of supplying very high amperes for a short duration. They can be destroyed in a short period of time if they are repeatedly deeply discharged. Starting a large engine like we have in our GMCs can require up to 400 amperes for a few seconds. They are constructed with a large number of thin lead plates and separators, making it possible to create the short burst of high energy. Deep cycle batteries are designed with the capability of supplying low amperes for a long duration as when we are living in our motorhomes and are not connected to shore power. They can be repeatedly deeply discharged without damage. They are constructed with much thicker lead plates and therefore do not have as much area exposed to the battery acid, which limits their current generating capability.

Flooded Batteries
Flooded batteries are the oldest type of design, and they are the type that came in new GMC Motorhomes. They depend on the chemical reaction of lead and sulfuric acid to create electrical energy. Flooded (or wet cell) batteries rely on a reservoir of liquid sulfuric acid to act as a pathway between positive and negative plates. These plates produce hydrogen and oxygen gasses when being charged that are vented into the atmosphere around the batteries. As a result, wet cell batteries require periodic inspection and topping off cells with distilled water. They must be well ventilated to prevent a buildup of hydrogen gas as accumulations above 4 % are highly flammable.

Flooded batteries have the advantage of lowest initial cost, and they contain the least weight per amp-hour of capacity. They can also accept higher recharging voltages and are less vulnerable to overcharging. The main disadvantage is the required periodic maintenance of adding water which tends to be a problem in our GMC Motorhomes as the battery storage areas are in locations that are not easily accessible. They also have the highest rate of self-discharge (up to 7% per month) and require off-season charging, which if not done properly, can severely shorten the battery life. Another disadvantage with wet cell batteries is the tendency to leak acid around the battery storage area which can cause corrosion problems.

Gel Batteries
Gel batteries were developed to get away from the periodic maintenance problem of adding water, which is why they are called "maintenance-free" batteries. Gel batteries have a totally sealed case and therefore do not vent gasses overboard in normal operation. The oxygen produced by the battery's positive plates recombines with the hydrogen produced by the battery's negative plates forming water that is recycled back into the electrolytic gel. The battery case has built-in pressure relief valves for each cell that holds the gasses under pressure causing natural recombination to take place. The chemical composition of the gel material varies with different manufacturers, but it is generally composed of phosphoric acid, pure water and fumed silica. This combination creates a thixotropic gel that offers a cycle-life up to three times that of flooded batteries.

Gel batteries are much lower temperature tolerant, and they are less sensitive to shock and vibration than flooded batteries. Also, their self-discharge rate is about 3% per month which is approximately one-half that of a flooded battery. Gel batteries weigh more per amp-hour capability than wet cell batteries, and they require a more accurate charging regulation for proper battery life. If they are severely overcharged, they can vent water overboard through their relief valves, and the water cannot be replaced.

Absorbent Glass Mat (AGM) Batteries
AGM batteries were designed for the dual-purpose of starting and for deep-cycle functions. They have a dense absorbent glass mat separator compressed tightly between the battery's positive and negative plates. These uniquely designed separators allow the oxygen produced on the positive plates to migrate to the negative plates and recombine with the hydrogen gas. The dense glass mats embed themselves into the surface of the plates providing greater plate support which allows the AGM batteries to absorb more shock and vibration than conventional batteries. The dense packing also results in lower internal resistance allowing AGM batteries greater starting power and greater charge acceptance than other types of batteries. The AGM batteries have a long cycle-life if properly charged, but similar to the gel batteries, the water cannot be replaced if overcharged. They too are heavier than the wet cell batteries. The main disadvantage with the AGM batteries is their higher initial cost, and their design is relatively new technology. Their primary usage is in the marine applications, however they can also be used in motorhomes.

Battery Terminology
When you go to buy a new battery it can sometimes be confusing when looking at all the data that is listed. The most critical value is the amp-hour (Ah) rating which defines the capacity of the battery. Amp-hours capacity is measured by determining the total energy that it can deliver in a 20-hour period, at a constant rate of discharge, before the battery voltage drops to 10.5 volts. As an example, a 200 Ah battery will run a 10 amp load for 20 hours at which point the battery is dead. Another data point you may see listed is the "reserve minutes" which is a measure of how many minutes the battery can supply a 25 amp load before the voltage drops to 10.5 volts. Some batteries will also list the "cold cranking amps" (CCA) which is a measure of how many amps a battery can deliver at zero degrees F without having the voltage drop below 7.2 volts. When buying deep-cycle batteries, usually labeled RV and/or Marine, you may see the MCA (Marine Cranking Amps) value listed. This value is determined the same as the CCA value, except the test is performed at 32 degrees F. The CCA and MCA data has very little meaning when selecting a deep-cycle house battery.

Sizing Your Battery
To determine the amp-hour capacity you need for your motorhome, you will need to make an estimate of how long a period you want to stay in one place without any shore power or generator. You can then estimate what 12 vdc appliances you plan to use during this period and how long you may want to operate each appliance, most of which will list how many amps or watts they use on their nameplate. If the usage is listed in watts, divide the value by 12 and that will determine how many amps it draws. Multiply the amps of each appliance times the number of hours you plan to operate them, and total up all the individual amp-hour values. Double the total value, and you will know how much battery capacity you need. The reason for doubling the total amp-hour is because it is not feasible to draw a battery down to 10.5 volts as they do in testing the batteries for their amp-hour capacity. Also a battery will normally lose around 20 % of its capacity as the battery ages if they are not properly maintained and charged. A fully charged battery with no surface charge and no load will produce approximately 12.7 volts, and it will be about 70% depleted when the voltage drops to 12.0 volts. A lot of the modern day electronics we have installed in our motorhomes will not operate properly much below 12.0 volts.

People that use their motorhomes mostly for travel and usually park in campground where there is electrical power can get by with one Group 27 deep-cycle battery for their house power. If you occasionally camp out for a long weekend without shore power, you will most likely need two Group 27 deep-cycle batteries. A lot of people have gone to two 6-volt golf cart batteries connected in series because of their large amp-hour capacity and much longer cycle life. There are a few people like myself, who dry camp for long periods, and have four 6-volt golf cart batteries. Two golf cart batteries in front, along with the starting battery, and two golf cart batteries in the rear, next to the generator, that takes up about all the battery storage available in a GMC.

Whatever size and type of battery you decide you need for your GMC 12-volt house supply, it should be the same type of construction as your starting battery. Therefore if you decide to use gel batteries for your house power, you should also have a gel battery for your engine starting battery. If you use wet cell house batteries, you should also use a wet cell starting battery. Gel and wet cell batteries have slightly different charging characteristics, and if they are intermixed on a common charging system, it can result in one of them not being properly charged. Also if you have more than one house battery, they should all be the same brand and age. You should never install a new house battery in parallel with an old house battery as the old battery can draw down the new battery even if there is no load on the system. If you have more than one 12-volt (or two serially connected 6-volt) house battery, they should be interconnected with switches so you only use off of one 12-volt system at a time. The simplest switch to use is a rotary "Perko" battery switch that you can purchase at most RV supply stores. If the two 12-volt systems are connected together in parallel, they can feed off of each other and gradually run down both batteries.

Batteries need to be properly charged in order to maintain their full capacity. This subject has been thoroughly discussed in various issues of GMC Motorhome News (Cinnabar). If you want more information about batteries and charging you can find additional information listed in a current "West Marine" catalogue, or you can review their material at

By Frank Condos


Over the last several years there has been much discussion and implementation of methods available to improve the GMC coach braking system. Discussions range from the stock system being considered adequate if properly adjusted to the whole system needing replacement such as adding large full disks to both rear axles. Several of the better known approaches are discussed and analyzed in this article.

If you are not happy with your current brakes, remember that you are stopping a vehicle well over 11,000 lbs, not your family Honda. The first recommendation would be to make sure the rear shoes are free and properly adjusted. The self-adjusting mechanism of the rear wheels generally will not keep them adjusted since most of us do not apply the brakes hard enough when backing.

The original brakes were not inadequate for the time and met the Federal Standard of CFR Title 49 Vol. 5 part 571 Std.105 for vehicles of this weight class. Since 1978 and most currently, a number of modifications and after market parts are available to further refine the braking system. The purpose of this paper is to do a comparative analysis of several systems using generally accepted engineering methodology as outlined in the referenced books. We will calculate braking forces for the stock system, larger front calipers, larger rear wheel cylinders and three different rear disk conversions.

Braking Fundamentals
The braking force is the force which is applied where the rubber meets the road. It can be expressed as the force applied at the disk or drum multiplied by the coefficient of friction of the brake material times the disk/drum diameter divided by the tire diameter.

The force applied at the disk is the brake line pressure as generated in the master cylinder by your foot and the vacuum booster times the caliper piston area.

The force applied at the rear drums is also generated from the brake line pressure times the wheel cylinder area but is modified by the self energizing or duo-servo design of drum brakes. It has been determined from tests to be 3 to 3.5 times the force generated by the wheel cylinder for cold brakes, depending on the friction material but rapidly decreases as the drums heat (more on brake fade later).

With these basic understandings of how the braking forces are generated, we can compare the stock brake system with several of the current modifications. First we need to look at the vehicle weight distribution between front and rear wheels both while moving at a constant speed (static) and the weight transfer that results from rapid braking. Understanding the weight distribution is important since an ideal braking system would apply just enough force at each wheel to be just below the sliding point on dry pavement. Hence the force ratio, front to rear (F/R), should be the same as the vehicle weight ratio under braking. The effects of weight transfer can be calculated using an estimated center of gravity (cg) location and an assumed braking (deceleration) rate. I used stopping in 200 feet from 60 mph. The center of gravity was estimated approximately a foot above the floor and in between the wheels proportional to the weight ratio.

A further complication is the effect of force transfer between the front and rear wheels on the rear bogie. The force transfer can be calculated for constant braking knowing the rear suspension geometry and the assumed deceleration. I will discuss the effect of this force transfer in a later Tech Center. Figure 1 shows both the static and braking weights for each wheel set. The static weights are based on the average of over twenty 26-foot coaches measured at the Mt Hood rally. The braking weights assume constant braking, stopping from 60 mph in 200 feet (better than minimum Federal requirements).

 Table 1. Braking forces comparisons (in lbs)
   Rear  Front  Total  %F/R
 Stock  3600  3360  6960  48/52
 80 mm front with stock rear  3600  5300  8900  60/40
 80 mm front 1-1/8 wheel cylinder both rear  5200  5300  10500  50/50
 80 mm front TSM 11" both rear disc  6700  5300  12000  44/56
 TSM 10" disc intermed rear  5150  5300  10450  51/49
 12.5" both rear disc (Harrison)  12230  5300  17530  30/70

Calculated front and rear braking forces at the road for various brake systems are shown in Table 1 along with the total forces and the front to rear ratio (F/R).

The results in Table 1 indicate that the stock configuration is well balanced according to the weight distribution albeit a bit anemic.

Converting only the front calipers shifts more braking to the front. This configuration can cause overheating of the front disc and increase the temperature surrounding the bearings. This is a consequence I have personally observed.

Using the large front calipers with larger wheel cylinders in the rear as described by Jim Anstett, provides a good balance although drum related brake fading could still occur.

The effects of disc conversions in the rear are shown in the last three configurations. Full rear discs shift the balance to more braking in the rear. The configuration with the large 12.5" discs clearly shifts much of the braking to the rear. Under dry pavement conditions this high percentage of rear braking may not be a problem. However with wet or icy conditions an emergency stop can cause rear lock up and loss of control with excessive rear braking (the reason ABS is standard on most pickup trucks).

The purpose of this discussion was to show through engineering analysis the relative differences between some modification options. The master cylinder pressure and coefficient of friction for the braking material is the same in all cases. The examples show that it is possible to increase braking force from 28 to 250%; actual results can vary.

I am making no recommendations. What you do to your coach is your business, not GMC Western States.
Formula used can be found in "Brake Handbook", Fred Puhn, Hpbooks and "Race Car Engineering & Mechanics", Paul Van Valkenburgh published by author.
In a later article I will discuss ways to change the master cylinder pressure and the effects of different brake pad material.

 This material is based on my personal experience and the personal experience of other club members. It is our viewpoint and does not represent authorized data pertaining to the GMC Motorhome. It is the responsibility of the readers to make their own judgment as to the validity of this material in relation to any repairs and/or modifications to their own vehicles.





 Please send your comments and ideas for the Tech Center to:
Chuck Aulgur, Technical Vice President,
9805 Ogram Dr., La Mesa, CA 91941
Phone: (619) 465-9875



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