The Trinity: Properly Balanced ERV Charging Systems

Christian P. Koop   Christian P. Koop

One of the worst situations an emergency response vehicle (ERV) driver/operator can find himself in, when called on to respond to an emergency, is when an engine won’t start because the batteries are low or dead.

Pushing the starter button and hearing the tell-tale click-click-click sound of a starter solenoid that won’t engage because the batteries are low is never a good thing! This should not happen with ERVs, but it does and more often than you would think. For ERVs to be reliable, the charging system – alternators, batteries, and onboard shoreline charger – must be system-matched. Think about it as the trinity, a phrase coined to express how important it is to understand how these three critical components need to be properly matched so they can operate as a complete and balanced system in a cohesive manner.

This is something that should be accomplished when ERVs are being manufactured; however, there are many rigs that leave the factory with alternators, batteries, and shoreline chargers that don’t make for a happy marriage and are destined to be unreliable until a solution is found. Some of these units could be the result of poorly written technical specifications while others could be from wiring that is the wrong gauge or from a poorly designed charging system circuit that has built-in high-resistance connections that create excessive voltage drops. Many reading this can probably relate to and think about rigs that have had problems in this critical area. This article will delve into the basic types and operation of alternators, batteries, and vehicle-mounted shoreline chargers to help explain the importance of how these components must be matched to become a balanced system where the components can support each other.


Alternators are the main power source of the electrical system when a rig’s engine is running. Not only do they keep the batteries charged when the rig is not connected to shore power, they also provide the current to keep the electrical accessories operating properly when the engine is running. To do this, alternators must be sized properly to produce enough current or amperage to maintain all the connected loads used during an emergency. I refer to this as the total connected load, and it will vary from rig to rig depending on the specified 12-VDC electrical accessories. When ERVs are specified, a load analysis must be done based on the electrical accessories that will be used to determine what size alternator is needed. Although switching from incandescent lighting to LED lighting has reduced the lighting load considerably, more and more 12-volt accessories are being added. This, in turn, requires higher-amperage alternators to keep up with power demands. If memory serves me, I believe 500-amp alternators are now available. Back when I started working on apparatus, the biggest alternators in use were between 100 and 160 amps. Matching the size of the alternator to the loads that will be imposed on it is critical for proper accessory operation and longer alternator and battery life. Think trinity.

Keep in mind that although alternators do keep batteries charged, they are not battery chargers. They were never designed to charge dead batteries, and when you have six very low or dead batteries in a pack and jump start before recharging, you can overheat the alternator and shorten its useful life. In a nutshell, alternators generate alternating current (AC) when magnetism produced by a rotating direct current (DC) field coil is induced into a stationary stator and rectified to DC by a set of positive and negative diodes also referred to as bridge rectifiers. About how the field coil circuit is energized, there are two basic types of alternators in use today, brush type and brushless. I personally prefer brushless because brushes are wear items and subject to failure if not replaced in due time. Some are easily replaceable while others require alternator disassembly. Most shops today replace the entire alternator when the brushes fail. The field coil produces the magnetism, which is varied by the voltage regulator to increase or decrease current output via the stator windings and diode bridge rectifiers. Many years ago, automobile alternators started producing temperature-compensated voltage regulation, which is important for precise output and longer battery life. Today, there are heavy-duty brushless alternators, such as ones produced by Niehoff, that are very advanced and are not only temperature-compensated but also have remote sense, which takes into account voltage drops and ensures the proper voltage is charging the batteries.

Stator windings are commonly configured in two ways: a delta winding and a Y winding, the former being used more in heavy-duty higher amperage units, whereas the latter is more commonly found in lower-amperage lighter duty units found in some automobiles. Keep in mind when testing alternators that they should be at operating temperature and they should be able to produce at least 80 percent of the current output on the tag or specifications listing. I also think it important to use a ripple meter or oscilloscope to ensure the diodes are not leaking or bleeding too much AC voltage into the DC output circuit. Excess AC voltage being bled into the 12-volt system can cause radio interference or noise and can also affect other sensitive electronics on the rig. Alternators should be load-tested any time there are battery issues and at least once per year as required by National Fire Protection Association (NFPA) 1911, Standard for the Inspection, Maintenance, Testing, and Retirement of In-Service Fire Apparatus.


Battery types and their use in ERVs have evolved steadily over the past 30 years. I can remember when two lead acid (LA) 8D size batteries were common for fire apparatus. Today a bank of six group 31 batteries is the norm on most large rigs. Although there are basically three types of batteries to choose from depending on the application, the role the batteries or bank plays in the system is still the same. They provide power for starting, stabilize charging system voltage, provide a level of spike protection for the electrical system, and operate the electrical accessories when the engine is off.

Batteries produce electricity because of a chemical reaction. A common 12-volt LA battery has six cells that produce 2.1 volts per cell for a total of 12.6 volts when fully charged. They are easily recognizable as they have removable caps for each cell so they can be refilled. Each cell is composed of alternating lead (negative) and lead dioxide (positive) plates that are filled with an electrolyte solution that is 64 percent water and 36 percent sulfuric acid. The chemical reaction that occurs between the plates when immersed in electrolyte produces electricity. LA batteries produce a highly explosive hydrogen gas during charging, so take care for proper ventilation so gases do not accumulate. How much current (amperage) a battery can produce and for how long are directly related to the size of the plates and the depth of the material used.

How batteries are rated is very important to know, especially for anyone involved in specifying new ERVs. Reserve capacity (RC) refers to how many minutes a battery can produce 25 amps and not drop below 10.5 volts. The 10.5 volts is also very close to the minimum voltage most engine and transmission electronic control units need to function. Cold-cranking amps (CCA) is the amount of current or amps a battery can produce at 0°F for 30 seconds without dipping below 7.2 volts. Cranking amps (CA) is the amount of current or amps a battery can produce at 32°F without dipping below 7.2 volts. Here is another important point to consider: When a battery reaches 12.2 volts open circuit voltage (no loads) across its terminals, it is only 50 percent charged or 50 percent discharged. You can look at it as the glass half full or half empty. From 12.6 volts (fully charged) to 12.2 volts may not seem like much, but it does mean a lot to how hard the alternator or shoreline charger must work to bring that bank back up to 12.6 volts.

Most batteries used for ERVs today fall into three basic categories. The common LA battery mentioned above is a flooded LA battery and is also known as a starting lighting ignition (SLI) or cranking battery. It is designed to produce a lot of current instantaneously for cranking the engine. A subcategory under the LA battery is the GEL type, which uses a gelling agent in the electrolyte to keep it from sloshing around. Next, and very popular with ERVs, is the maintenance-free or low-maintenance cranking battery. What sets this battery apart from the standard LA battery is that the cells are sealed, and the plates are constructed of calcium, cadmium, or strontium instead of antimony used in the LA battery.

Because of these different plate materials, the batteries do not produce much gassing during charging, allowing the sealing of the cells and hence their being maintenance-free because they do not need to be refilled. The third type is the absorbed glass mat (AGM) cranking battery. Glass mat refers to the fiberglass material used to seal in the electrolyte within the cell as opposed to the LA battery where the electrolyte fills the entire void inside the battery’s rectangular housing between the plates and can slosh around and leak out. AGM batteries can be manufactured in any shape and can be mounted in any position because the electrolyte won’t leak out. All three types are also available as deep-cycle batteries. Deep-cycle batteries can discharge current for longer time periods and for many more cycles than other batteries because the plate material is thicker. They do not provide as much instant amperage as an SLI; however, they can be discharged to as low as 20 percent of charge while still producing rated current output for many more cycles than conventional SLI batteries, hence the name “deep cycle.” To compare, conventional SLI batteries may only be deep cycled 10 times before they kick the bucket, while deep-cycle batteries may be able to last for 300 cycles.

As I have written in the past, I compare batteries to a checking account: If you withdraw more from your account than you are depositing, you will go broke. The same holds true for batteries – taking out more current than you are putting back in will discharge the batteries and eventually damage them and shorten their lives. This is referred to as heavy cycling, and it happens when battery voltage dips or is allowed to go below 12.6 volts. Common LA batteries don’t hold up well to this abuse – not only does battery life suffer but also alternator life. Here is what happens: When LA batteries are discharged, soft lead forms on the plates. While the battery is being charged, the lead sulfate mixes into the electrolyte and crystallizes and forms a barrier on the plates. Over time, as the cycle is repeated, this barrier thickens and reduces the amperage capacity of the battery, shortening its useful life. This is commonly referred to as battery sulfation and is the number one cause of battery failure, also known as battery murder. Over time, this can be very costly to the organization and cost many thousands of dollars.

Shoreline Charging

Rig-mounted shoreline chargers are a must for the modern ERV. This is mainly because of the parasitic drains imposed on the batteries from the many onboard mounted computers and accessories today’s rigs have. These include engine and transmission computers that need electrical power to maintain keep-alive circuits and a host of other parasitic loads such as laptops, radios, thermal imaging camera chargers, flashlight chargers, and others. In the early days of onboard chargers, users could get away with small battery chargers in the 10-amp category because parasitic loads were nil to very minor, and the battery banks were smaller. This has not been the case for quite a few years because of the greater amount of parasitic loads and the larger battery banks. Today, it is quite common to use shoreline chargers that can produce 80 or 90 amps to keep up with all the parasitic loads and also recharge battery banks that have been discharged. I have seen many battery banks drawn down because someone simply failed to plug in the shoreline to the auto eject. Couple this error with a battery switch and some accessories left on, and you have a recipe for low or dead batteries that need an effective charger with automatic regulation that can produce enough amps to get the job done.

On top of that, the charger’s voltage output must be matched to the types of batteries with which the rig is equipped. LA (SLI) batteries require static 14.1 volts at room temperature while maintenance-free or low-maintenance requires 14.7 volts, and AGM needs 14.7 to 15.0 volts. Gel batteries can be easily damaged by excess heat during recharging, generally have a very narrow voltage band, and should not be allowed to exceed 10 amps during charging. There has been much advancement in technology when it comes to onboard chargers. Some double as inverters that are also used to power 110-AC voltage equipment, while others are designed just for keeping batteries fully charged and ready for response. Kussmaul, which recently celebrated 50 years, has been manufacturing various electrical accessories for ERVs, such as auto ejects for the shoreline, and also offers shoreline chargers with parasitic load compensation.

It is critical, given today’s complex rigs with huge electrical systems, that the alternator, shoreline charger, and batteries are system-matched to provide reliable and long life. This is not only important for good response times; it will provide a considerable cost savings to the organization and can also reduce liability in today’s highly litigious environment.

CHRISTIAN P. KOOP retired as the fleet manager for the Miami-Dade (FL) Fire Department after 35 years with Miami-Dade County and four years in the military. He has been involved in the repair and maintenance of autos, military track and wheeled vehicles, heavy equipment, and emergency response vehicles for the past 40 years. He is a member of the Fire Apparatus & Emergency Equipment Editorial Advisory Board. He has an associate degree from Central Texas College and a bachelor’s degree in public administration from Barry University and has taken course work in basic and digital electronics. He is an ASE-certified master auto/medium/heavy truck technician and master EVT apparatus and ambulance technician. He is a member of the board of directors of EVTCC and FAEVT and a technical committee member for NFPA 1071, Standard for the Emergency Vehicle Technician Professional Qualifications.

No posts to display