Alternators & Batteries

Alternators have long surpassed generators as a method of charging batteries and providing power to the various devices on-board modern vehicles. There are several reasons for this change over. They are lighter, smaller, more reliable, and most importantly much more powerful. Chrysler is recognized as the first domestic car manufacturer to switch over to alternators on standard production vehicles. These early units were just a little more powerful (40 amp rating) than the generators they replaced. Nowadays it is not uncommon for cars and pickup trucks to sport 140 to 160 amp alternators, and in a couple of cases 225 amp units. All of this extra power wasn't meant for us amateurs. Instead they're intended to provide the necessary power to defrost our windows, heat our seats and mirrors, and all the rest of the accessories we've become accustomed to.

There are a lot of web sites which explain the inter-workings of alternators, so I won't go into detail here. In simple terms, they consist of a rotating, multi-pole field which is nothing more than a rotating electromagnet. This rotating magnet spins inside an inter-woven tri-filer wound stator coil producing a three phase AC output. This output is full-wave rectified producing a near ripple-free flow of current. By adjusting the amount of field current, the output can be maintained (regulated) at a constant level, nominally 14 VDC (13.6 to 14.4), up to the rated amperage of the unit.

To clarify some fine points; some alternator regulators pulse the stator current similar to a switching power supply, while the others (usually older models) use an analog method like a linear power supply. Similar to a switched bench supply, the frequency and/or the pulse width may be varied (independently or together). The RFI from these models sounds like the old cartoon rat-a-tat-tat used for machine gun sounds. Although it may sweep across the bandpass, that is not always the case. If you're plagued with this particular noise, the alternator is a good place to start looking.

In the last couple of years, both OEM and aftermarket rebuilders have started to use two independent tri-filer wound stator coils, and 12 diodes instead of just 6. This allows the alternator to be smaller, lighter and more powerful. They also have less ripple, and are less prone to producing whine.

There is at least one aftermarket rebuilder using three separate, tri-filer windings, and an 18 diode array! Looking similar to the example at right, these special-made units are capable of producing as much as 500 amps! About the only way to produce more, is to use multiple alternators which greatly exacerbates the wiring and regulation problems. If you really need something this big, you'd be better off with an on-board AC generator.

In the old days, most all cars were equipped with ammeters connected between the battery and generator. It was easy to see if the generator was delivering enough current. With the advent of alternators, the ammeter lost its effectiveness. Electronically controlled alternators always put out just the right amount of current and except for brief periods an ammeter would always show "0". In other words, not charging or discharging. A more meaningful indication is a DC voltmeter. But even a voltmeter is a little superfluous so most cars don't even have these, relying instead on the proverbial idiot light. Since we're not idiots, we need to add a voltmeter (if you don't already have one) to be safe and not stranded with a dead battery.

If the voltage doesn't stay above 13VDC or more while you're transmitting, you might not have enough reserve power for your rig. The left photo shows my voltmeter while I was key-down with 500 watts out and the engine at 2,500 rpm. The right photo shows the same key down condition except the headlights and air conditioning are on and the engine is idling. It wouldn't take long at this power level to drain the battery. Fortunately, most miniature radios shut down when the voltage drops below 11.6. Incidentally, at highway speeds the voltage maintains 14 even with the lights and air on. This is possible because my car is equipped with a 130 peak amperage alternator. Yours may not have one this large so a voltmeter becomes especially important if you run high power.

Finding out the size of your alternator can be problematic. Sometimes you get lucky and the amperage is written on the outside of the unit. This is true of most Denso, Akia, Nippon, and Bosch units. American made units are a mixed bag. Your dealer's parts department is a good source in case yours is not marked.

Because we typically don't run all of our accessories at the same time, there is usually enough capacity left over to power even a moderately powerful amateur transceiver. In some cases, a lot more. Enough, in fact, to operate a 500 watt mobile amplifier if we're careful with our electrical accessory use. Determining if you have enough capacity isn't all that difficult if you use some basic logic. If your car has a rear window defroster, you have about a 30 amp reserve when it is not in use. This is enough for any of the late model 100 watt transceivers like the FT100, FT857, IC706, and even the 200 watt TS480.

If you use an amplifier, you'll need even more reserve. At 500 PEP watts out (1,000 watts input), the peak amperage draw is about 100 and the average around 40 to 60 amps including the transceiver. This requires a reserve of at least 70 amps. If your car has heated seats and mirrors in addition to the rear window defroster, you might have enough. In any of these cases, a good voltmeter is a valuable asset as long as you pay attention to it.

Just for the record, most of those dash-mounted voltmeters GM is so fond of, aren't very accurate. If you are attempting to rely one one, I suggest you check its accuracy with a know-good DVM. The one shown in the above photos was purchased from DigiKey. It's a Martel two-wire unit (read that as easy to install), and the part number is 227-1042-ND. It's about $45.

You don't always know what you're getting when you buy a new vehicle. Vehicle manufacturers know the alternator will only be delivering its full output for short durations, so they cut every corner they can. It is not uncommon for a 100 amp alternator to have a continuous duty cycle of less than 60 amps. Under the extra load imposed by high power mobile applications, the alternator and/or the interconnect wiring may over heat. This is especially true of low content (minimal accessories) vehicles.

If you're contemplating purchasing a new vehicle, consider purchasing a heavy duty electrical system if one is available. The big three all offer heavy duty electrical systems on mid to large size vehicles (and some compact ones), and universally on trucks. The up front premium is small, usually under $100, for which you get a bigger, heavier duty alternator and a larger battery. As an alternative, order a trailer towing package even if you don't plan to pull a trailer. You typically get a solidly mounted hitch from which you can mount an antenna if you're not a hole driller; a transmission and/or oil cooler; and a bigger alternator and battery for one relatively low price.

Another good Internet site for really BIG amperage demands is Alternator Parts. They both offer a variety of different types and manufacturers. Some of their offerings will supply up to 250 amps (100% duty cycle), and no one makes a more powerful standard-sized alternator.

One word of caution. Talking on the radio while in heavy traffic should be discouraged due in part to the distraction it causes when your full attention should be on safe driving. Further, air conditioning, cooling fans, headlights, and slow engine speeds all add up to low alternator reserves. When in doubt, error on the side of safety and hang up the mic.

Whine can also be caused by a large AC component impressed upon the DC wiring. This occurs when one or more of the rectifier diodes are open (or leaky). This is a rare occurrence nowadays, and is typically signalled by the Check Engine light.

If you're not sure if you have alternator whine, visit my Noise page for more information.

There is a lot of confusion about which type of auxiliary battery to use in a mobile application, and some even question if one is needed at all. If you run a nominal 100 to 200 watt mobile transceiver, you probably don't need one. However, if you use an amplifier it is usually best to use one to maximize peak power capability which helps reduce the IMD products associated with poor voltage regulation.

There two important points about batteries which need to be mentioned up front. First, the term Deep Cycle is a misnomer, as all lead acid batteries are considered discharged when their voltage reaches 10.5 volts under load. They are actually designed to have a better Reserve Capacity (RC) than an SLI (Starting, Lighting, and Ignition) as I explain below.

Secondly, batteries designed for marine applications are not the battery of choice for any mobile installation, the fact they have screw terminals notwithstanding. A marine battery is designed to maintain at least an 80% charge after sitting uncharged for 12 months or more. They are a form of SLI battery, but typically have less starting amps and less reserve power than a true SLI.

There are two types of mobile operation, and each one needs a different approach with respect to battery type, and whether or not an isolator is required. The first being operation while underway, and the other more correctly called portable operation. This is where you're stationary and using only batteries for power. Since the demands on the batteries are different, they require different types of batteries if we're going to get the maximum serviceability and bang for your buck. We'll take them in order.

There are some important things we need to know about batteries before we get into the different uses. In our case, we're talking about lead acid batteries which come in a variety of sizes, current ratings, configuration of posts (+ and - connections), and most importantly their construction. The latter is a major consideration if the auxiliary battery is trunk mounted (more of this later).

We're also narrowing our selection to two basic types of batteries; SLI and RC. In general terms, SLI batteries are designed to provide a large amount of current over a short period of time. Where as RC batteries are designed to provide a low amount of current over an extended period of time. Under the right conditions, both types can be charged at rather high rates.

Both types come is a large number of sizes from 2 ah (ampere hours) to over 400 ah. No matter the type, a lead acid battery is considered discharged at a nominal 10.5 volts (while under load). The specific length of time each type will supply a specific amperage until it reaches a nominal discharge level, varies over a wide range. It is always best to refer to the published specs from the manufacturer.

Fully charged, the nominal resting voltage of a lead-acid battery is 12.2 to 13.1 depending on construction. By construction I'm referring to the plate construction.

Standard car batteries are referred to as "flooded" in that the sulfuric acid electrolyte is in liquid form, and the plates are usually flat. This is true of every OEM battery I am aware of (gasoline or diesel). The only exception are hybrid vehicles.

In an AGM (Absorbent Glass Mat), like an Optima Red Top shown at left, the electrolyte is a gel-like. As a result, they may be mounted in any position, even up side down. Their plates are spirally wound, which makes them almost impervious to vibration. What's more, they don't out gas under normal conditions so they can be mounted inside a vehicle. They will, however, out gas if you abuse them by over-charging. Speaking of which...

In any application, there are limits to the rate of charge, temperature of operation, and the applied voltage especially if the battery is maintained in a float condition (constant charge). These vary with battery construction, type, and manufacturer so it is always best to consult with the manufacturer when in doubt. The right photo is an Exide Orbital, and is similar in construction to the aforementioned Optima batteries.

Any auxiliary battery should be installed in a battery box and properly restrained. The rule of thumb for battery restraints is 6Gs lateral and 4Gs vertical. The last thing you want is a 60 pound battery flinging acid all over the insides of your vehicle! If you use a standard car battery, the box must be vented to the outside.

A word of caution is in order. A typical lead acid battery has an internal resistance at full charge of about .003 ohms although some types are slightly higher, and some slightly lower. Under direct short conditions, the maximum current can exceed 3,000 amps! Doing so can cause some batteries to self destruct which will hurl sulfuric acid far and wide, so handle them accordingly! It's best to keep their plastic post caps on until you actually make the connections. When removing them, or installing them, the negative lead should be removed first, and installed last. Why? Think about it!

For those of us who run high power, an auxiliary battery is almost a necessity unless we wish to use welding cables for our connections. For those who do not understand this logic should read my articles on Amplifiers and on Wiring. It's basically related to I²R losses and minimizing IMD products. The fact remains, it is cheaper to use an auxiliary battery than to use welding cable which costs $5 or more per foot.

In this application, as long as the batteries are both lead acid (AGM or otherwise), the batteries may be connected in parallel. In fact, using an isolator on such a set up defeats our purpose which is to keep the voltage as stiff as possible. Quite obviously we need to isolate them with fuses in case any of the wiring gets shorted. As I point out in my fuse article, don't assume circuit breakers are the answer to replace fuses. Circuit breakers cannot handle instantaneous currents much above 2,500 amps, and some can't handle 1,000 amps! Under direct short conditions the contacts in a circuit breaker will weld together with obvious catastrophic results!

The preferred remote-battery type is an AGM. As pointed out above, this is the safest approach with the respect to out gassing and mounting considerations. The battery should be designed for starting like an Optima RedTop, not a marine battery like an Optima BlueTop, or an RC battery like an Optima YellowTop. Again, this application is high amperage for short durations.

Not all battery isolators are created equal. Simple diode isolators have a number of drawbacks, not the least of which is their forward voltage drop. They can also play havoc with the charging circuitry in some alternators. This leads some folks to use isolation relays, and they too have their drawbacks. There is one brand of isolator which combines the attributes of both, and that is the one made by Hellroaring.

The Hellroaring uses diodes and FET switches in its design. It is also remotely controllable. This fact allows all of the batteries (two or three) to be combined if necessary. It is competitively priced, and available directly from the factory. If you have to use an isolator, this is the one to use.

Perfect Switch makes both isolators and switches for high current applications, and both employ FET technology. One of their units is shown at right. One very good advantage is there are no contacts which could fuse together in a dead-short scenario. In fact, drawing current over their rating will cause them to disconnect and require a reset. This is about as fail-safe as you can get.

There are a couple of important items to keep in mind when using any isolation technique. First, some automobile engine control strategies (Honda, Toyota, et. al.) use the alternator current and voltage levels as data inputs to the engine control computer. Suddenly connecting a second battery can cause fault codes to be written to the OBD II memory, which will turn on the Check Engine light.

Secondly, most alternator circuits incorporate a fuse. If you suddenly connect a fully discharged RC battery to the charging circuit, it is possible to draw enough current to blow this fuse. Obviously you should keep a spare fuse on hand, and you should also follow Hellroaring's recommendation about delaying the FET closure until the current and voltage stabilize.

In order to maintain a solid 13.8 volts to operate the radio, a lot of amateurs opt to use a Battery Booster. Known by at least a dozen names, they act like a switching power supply, and although the battery voltage drops, the output remains steady. The one shown at left is a W4RRY unit, and the one below right is the MFJ unit.

These units include a low voltage cutoff, but some don't which can lead to problems. As stated above, any nominal 12 volt lead acid battery is considered discharged when the voltage drops to 10.5 volts under load. Discharging them lower than this will drastically reduce the life of the battery. Without low voltage protection, the end result can be a ruined battery.

Odds & Ends

The use of a power relay to disconnect an RC battery from the SLI battery and charging circuit (rather than use an isolator) isn't always a good idea, and here is why. Assuming we discharged the RC to 10.5 volts, and then start our engine to recharge it, when you close the relay the difference in their voltages can cause some alternators to misbehave and put out too much current and/or voltage. I know of at least two cases where this situation caused the alternator to fail, and most likely caused damaged to the battery as well. The use of an isolator minimizes this problem.

Isolation relays have another potential problem, and that is their instantaneous current carrying capacity. As I point out in my Wiring article (and above), circuit breakers also have this problem. Assuming there is a dead short, relay and circuit breaker contacts can weld together. Unless there is proper circuit fusing, catastrophic failures can result.

This can be a problem with diode isolators too, but for a different reason. Since almost all isolators are encapsulated, large amperages like those caused by dead shorts can ionize the semiconductor material allowing the current to continue flowing. Again, this is why proper fusing is a necessity.

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