Last Modified: December 16, 2011
Contents: Caveats; Basics; Measuring Capacity; Alternator Ratings; Alternator Whine; Auxiliary Batteries; Battery Isolators; Battery Boosters;
Please read this section carefully. If in doubt about your vehicle's electrical configuration, refer to the Owner's Manual and/or ask your dealer's service manager for assistance.
Modern vehicles must meet strict, and ever increasing pollution standards. To meet those standards, the engine control computer (known by a lot of different names) often interconnects with the alternator, through devices referred to as Electrical Load Detectors (ELD). As amperage loads vary, the computer can adjust the various engine controls (injection duration, timing, etc.) to better control emissions. In some cases, the CPU can shut off the alternator when required, increase or decrease the output voltage, and preload the alternator when the load suddenly changes—when the air conditioning comes on for example. This fact all but eliminates the replacement of the OEM alternator with a high-amperage unit.
Other digital systems monitor the condition of the battery (BMS). They do this in a variety of ways, including shutting down the alternator, and measuring the voltage drop under the imposed (at the time) load. The engine CPU then adjusts the alternator's voltage output accordingly. This assures that the battery is in optimal condition for its age, load, etc. In some cases (2002> BMW vehicles for example), the engine CPU needs to be reprogrammed if the battery is replaced.
It should also be noted, that flooded SLI batteries have a different charging strategy than AGM batteries do, if long life is to be expected. Here too, some engine CPUs have to be reprogrammed. This also limits how, if any, second batteries are connected to systems so designed.
Not all late-model vehicles use lead acid batteries. A few have switched over to Li'ion (Lithium Ion) or NiMH (Nickel Metal Hydride). Due to their unique charging requirements, is may not be possible, or advisable, to change the battery type, size, or even the make! Again, read your Owner's Manual and/or contact your local service manager for assistance.
Based on the aforementioned, it is always better to think ahead whenever possible, and buy the heartiest of electrical systems. One way to do this, is to order a trailer towing package even if you have no intention of using it. It also brings up a multiple (house) battery issue which is covered below.
For more information on Electrical Load Detectors and Battery Monitoring Systems, please read the Caveat section in the Wiring article.
There is one very important point to remember when reading this article; It is the alternator, not the battery, which provides the requisite electrical power to operate the various, on-board devices. All the battery does, besides providing starting power, is to act like a very big capacitor. Oh, now, don't go asking me how big of a capacitor, as it is a difficult to answer question without knowing a whole lot more than what is in evidence. However, consider it quite a few Farads!
Alternators have long surpassed generators as a method of charging automotive batteries, and providing power to the various on-board electrical devices in 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, and their stand-alone regulators were less than stellar performers! 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. In addition, more and more vehicles are being equipped with electrically-driven water pumps, power steering, and even power brake assist. This puts extra demands on both alternator and battery systems.
In simple terms, alternators consist of a rotating, claw-pole field which is nothing more than a rotating electromagnet. Both the north and south poles of the field are energized by a single winding. This rotating magnet (rotor) 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.
In a load matched condition, the output power of a claw-pole alternator is proportional to the number of field ampere turns squared. With this in mind, both OEM and aftermarket rebuilders have started to use two, independent tri-filer wound stator coils, and 12 diodes instead of just 6. This nearly doubles the field ampere turns, which allows the alternator to be smaller, lighter and more powerful. A few high-end suppliers offer alternators with three, tri-filer windings, and an 18 diode array! An example is shown above right as supplied by Alternator Parts. These 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. There are better ways!
If your power needs are really big, you'd be better off with an on-board AC generator. Better yet, an Auragen system as shown at right. The AuraGen® is a new class of axially symmetric induction machines. The machine's design is really quite intriguing. It generates 400 volts AC, which is fed to an inverter, which supplies both battery charging current (nominal 12 vdc), as well as 120/240 volt AC. It can also operate backwards, supplying 120/240 voltage from a battery bank. Even if you don't own a huge motor home, the data on Auragen's web site is a good read.
Some alternator regulators pulse the rotor 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's regulator is a good place to start looking.
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. The reason is, electronically controlled alternators always put out just the right amount of current. So, 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 remains ≈14 even with the lights and air on. This is possible because my vehicle is equipped with a 130 amp 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, IC7000, 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 in-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 known-accurate DVM. The voltmeter shown in the above photos is a Martel two-wire unit (read that as easy to install). Its current draw is under 2 mils, so may be left connected even for extended periods of time. The part number is QM-100V. It is about $45. Martel also sells similarly-sized hour meters, for those who want to keep track of their equipment on time.
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 accessorized) 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 500 amps, and no one makes a more powerful standard-sized alternator. They even sell a stand-alone, fan-cooled, rectifier package allowing a standard OEM alternator to safely handle more amperage.
Lastly, 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.
If you think you have alternator whine, it should sound like this. Whine can be caused by a large spike in the AC component as shown in the oscillograph at left; a very rare occurrence nowadays, and one which typically turns on the MIL (Maintenance Indicator Light). This said, almost without exception, alternator whine is caused by a ground loop (another reason to properly wire your installation). In most cases the whine is only apparent on the transmitted signal which is another indicator of a ground loop problem. If this is the case, chances are the cause is the use of a mag mounted antenna, or a poor RF ground return (open coax shield connection).
Recently, several manufacturers have started selling power cable filters, sometimes referred to as brute-force filters. They're advertised to cure alternator whine. The truth is, they only mask the real problem. If you have to resort to such devices, it is a sure sign you didn't wire your installation correctly, or you're using a mag mounted antenna. Just one more concrete reason to through-hole mount VHF antennas.
One popular myth for curing alternator whine is to tightly twist the radio's power cable with an electric drill. Supposedly, this increases the capacitance, thus shunting the whine to ground. Let's explore this theory. A 10 foot power cord has about 150 pF of capacitance. The average whine ranges between .5 to 2.5 kHz. Thus, the effective shunt reactance is well over 200,000 Ω. The effective impedance of the power cable is a few tenths of an ohm, thus the affect would be nearly impossible to measure, even if we increased the reactance by a factor of 100,000! Thus twisting up the power cable is junk science at its best!
There is a lot of confusion about which type of auxiliary battery to use in a mobile installation, 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, unless you're operating without the engine running—portable operation in other words. If you are operating as a portable, then the highlighted article is for you.
For those of us who run high power, an auxiliary battery is almost a necessity unless you wish to use $5 per foot welding cables for the inter-connections. For those who do not understand this logic, read my articles on Amplifiers, and on Wiring. It is basically related to I2R losses and minimizing IMD products. The preferred auxiliary-battery type is an AGM (Absorbent Glass Mat), and there are several reasons why they are.
All lead acid batteries generate hydrogen gas during normal operation, and becomes excessive during overcharge conditions. Hydrogen gas is very explosive, and even a minor spark can ignite it. Any resulting explosion isn't a pretty sight, as the electrolyte is sulfuric acid! Since the gas is vented to the outside, flooded batteries shouldn't be used in enclosed areas like the trunk of a vehicle. AGM batteries outgas too, but the hydrogen gas is absorbed by the glass mat. Here too, overcharging can cause more gas to form than the mat can absorb. Thus manufacturer's maximum charge rates shouldn't be exceeded.
Thus the safest approach with the respect to out gassing is an AGM type. What's more, unlike a flooded battery, an AGM can be mounted in any position, even upside down! This said, any auxiliary battery should be installed in a battery box, and properly restrained. Doing so, also prevents accidental shorting of the exposed connections. 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!
Auxiliary batteries used in high-power applications should be SLI (Starting, Lights, Ignition) types, like the Optima RedTop® shown at left, or the Exide Orbital® shown at right. Remember, this application (amplifier peak current support) is high amperage for short durations, and not long-term reserve capacity like you would need in a portable application. Since both batteries are lead-acid (AGM or otherwise), the batteries may be connected in parallel. In fact, using an isolator on such a set up defeats the purpose which is to keep the voltage as stiff as possible. Quite obviously we need to isolate them with fuses in case the wiring gets shorted. As I point out in my protection (fuse) article, don't assume circuit breakers are the answer to fuses. Circuit breakers cannot handle instantaneous currents much above 2,500 amps, and some cannot handle 1,000 amps! Under direct short conditions the contacts in a circuit breaker will weld together with obvious catastrophic results!
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, and a decent quality AGM, almost 4,000 amps! Doing so can cause some batteries to self destruct, hurling sulfuric acid far and wide, so handle them accordingly! It is 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!
Here's another important point to remember. Batteries designed for marine applications are not the battery of choice for any mobile installation, the fact they have screw terminals notwithstanding. A battery rated for marine use 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. Incidentally, the terms Marine and Deep Cycle seem to be synonymous terms at least in the amateur community. They're not, even though some battery manufacturers would have you believe otherwise.
There are few more important points about batteries which need to be mentioned. 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. This includes every, lead-acid type, no matter the plate construction, the type of electrolyte (liquid or gel), the name on the outside, what service it is intended for. Discharging any lead-acid battery lower than this, drastically reduces its charge-cycle service life. Further, so-called Deep Cycle batteries are often (not always) designed to have a longer (in minutes) Reserve Capacity (RC) than an SLI (Starting, Lighting, and Ignition). By no means, does this convey, indicate, or concur that the battery can be discharged lower than 10.5 volts under load!
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, and the electrolyte type (liquid or gel). 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 most OEM batteries, (gasoline or diesel). The only exception are hybrid vehicles.
The only time batteries need to be isolated in a mobile application (other than RVs) is in portable operation. Their use in high power applications should be discouraged for several reasons, not the least of which is SOC (State Of Charge) considerations. Simple diode isolators will always have forward voltage drop of about 1 volt, and they can also play havoc with the charging circuitry in some vehicles, leading to illuminating the MIL (Maintenance Indicator Light). This leads some folks to use isolation relays, and they too have their drawbacks. But 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 just have to use an isolator to stay warm, and fuzzy, this is the one to use.
Perfect Switch® also makes isolators and switches for high current applications, and both employ FET technology. One of their units is shown at left. 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. Proper fusing is still required, however.
Lastly, most alternator circuits incorporate a fuse. If you suddenly connect a fully discharged auxiliary 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.
There are a couple of important items to keep in mind when using any isolation technique. First, most automobile manufacturers use some form of BMS (Battery Monitoring System). Battery and/or alternator current and/or voltage levels are used as data inputs to the engine control computer (see the Wiring article for more information). Suddenly connecting a second battery can cause fault codes to be written to the OBD II memory, which will turn on the Maintenance Indicator Light (MIL). It can also cause the main battery fuse (≈120 amp fuse) to blow. In any case, you should carry a spare. Incidentally, the battery fuse in most modern vehicles are OEM proprietary, and usually contains part of the ELD (Electrical Load Detection) circuitry (Hall device). Read that as expensive!
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. By the way, the November, 2008 issue of QST has a product review on these units including one made by TGE.
These units include a low voltage cutoff, but some don't which can lead to problems. As stated several times 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 typical end result can be a ruined battery.
The real question is whether a battery booster is needed, and there is no clear-cut answer. Certainly for portable operation, they have a definite use. However, in most mobile-in-motion operation, where the wiring is correctly sized, and the alternator is of adequate amperage, their use is all but moot. Personally, I view them as just one more thing to fail.