11.2 Brushless/Brush Alternators, Internal Regulators & Charging Circuit Testing
Key Takeaways
- Brushless heavy-duty alternators utilize stationary field coils and stationary stators with rotating iron pole pieces, completely eliminating slip rings and brushes to endure harsh mining and forestry dust environments.
- Stator windings are arranged in either a Wye (Star) configuration for high voltage and charging output at low idle speeds, or a Delta configuration for maximum current output at high engine speeds.
- A standard three-phase bridge uses six diodes to achieve full-wave rectification; an eight-diode bridge incorporates two extra diodes on a Wye neutral tap to capture third-harmonic AC energy, boosting output by 10% to 15%.
- The remote battery voltage sensing wire ensures proper battery charging by measuring actual voltage at the battery posts, automatically compensating for voltage drop across long output cables.
- Excessive AC ripple voltage (>0.3V to 0.5V AC) indicates a leaking or blown rectifier diode, which discharges the battery bank at rest and induces severe electrical noise into machine CAN-bus networks.
11.2 Brushless/Brush Alternators, Internal Regulators & Charging Circuit Testing
Heavy mobile machines depend entirely on the charging system to replenish battery energy consumed during cranking and to supply continuous electrical power to high-draw loads including electro-hydraulic proportional valves, electronic control modules (ECMs), cab climate blowers, communication radios, and high-intensity LED working lights. In modern off-highway equipment, alternators must deliver 100A to 350A+ across extreme ambient temperature swings (-40°C to +50°C) while enduring continuous vibration, abrasive rock dust, and moisture. A certified Red Seal technician must master electromagnetic induction theory, rectification electronics, voltage regulator logic, and specialized charging system diagnostic procedures.
Alternator Electromagnetic Induction Principles
An alternator converts mechanical rotational energy from the diesel engine crankshaft into alternating current (AC) electrical energy through electromagnetic induction, and then internally rectifies it into direct current (DC).
ELECTROMAGNETIC INDUCTION CYCLE
Crankshaft Pulley ──[Serpentine / Poly-V Belt]──► Alternator Rotor
│
▼
[Rotor Field Coil] ◄── Regulated DC Current ◄── [Voltage Regulator]
• Generates rotating magnetic flux (N and S poles)
│
▼
[Stationary 3-Phase Stator Windings (Phase A, B, C)]
• Faraday's Law: Rotating flux cuts stationary conductors
• Induces 3-phase alternating current (AC Sine Waves)
│
▼
[Full-Wave Rectifier Bridge (6 or 8 Diodes)]
• Converts 3-Phase AC into overlapping DC Pulses
│
▼
[Alternator B+ Output Stud] ──► Machine Loads & Battery Bank
Faraday's Law and Alternator Advantages
According to Faraday's Law of Electromagnetic Induction, an electromotive force (EMF, or voltage) is induced in a conductor whenever there is relative motion between the conductor and a magnetic field: $E = -N \frac{d\Phi}{dt}$, where $E$ is induced voltage, $N$ is the number of turns in the coil, and $\frac{d\Phi}{dt}$ is the rate of change of magnetic flux.
In legacy DC generators, the high-current output windings were located on the rotating armature, requiring massive carbon brushes and segmented commutators to collect heavy electrical currents. This design suffered rapid brush wear, arcing, and poor low-speed output. In contrast, an alternator reverses this arrangement:
- The magnetic field is rotated on the internal rotor, requiring only a small field excitation current (typically 3A to 7A).
- The heavy output windings are stationary on the external stator core, allowing high current (100A to 350A) to be transferred directly to heat sinks and output studs without passing through sliding contacts or brushes.
3-Phase Alternating Current Generation
The alternator rotor features intermeshing claw-pole fingers (Lundell rotor) enclosing an internal field coil. When energized, one claw-pole ring becomes North (N) magnetic poles, while the opposing ring becomes South (S) magnetic poles (typically 12 to 16 poles total). As the rotor spins, alternating North and South magnetic fields sweep across the stationary stator windings.
The stator core consists of laminated iron stampings supporting three separate, independent phase windings (Phases A, B, and C) physically spaced 120 electrical degrees apart. As the rotor turns, three sinusoidal AC voltage waves are generated, each peaking 120 degrees out of phase with the others, producing continuous, smooth 3-phase electrical power.
Stator Configurations: Wye (Star) vs. Delta
Manufacturers configure the three stator windings in one of two arrangements depending on the machine's intended duty cycle:
WYE (STAR) CONFIGURATION DELTA CONFIGURATION
Phase A Phase A
│ / \
│ / \
[Coil A] / \
│ [Coil A] [Coil C]
▼ / \
Neutral ── ( * ) ── [Coil C] ── Phase C / \
▲ / \
│ Phase B ──[Coil B]── Phase C
[Coil B]
│
Phase B
-
Wye (Star) Configuration:
- Connections: One end of each of the three phase coils is connected together at a common center point known as the Neutral Junction (or neutral tap). The opposing ends form the three phase outputs.
- Voltage & Current Relationships: Line voltage is higher than individual phase coil voltage by a factor of the square root of three: $V_{line} = \sqrt{3} \times V_{phase} \approx 1.732 \times V_{phase}$. Line current equals phase current: $I_{line} = I_{phase}$.
- Performance Profile: Because phase coils are wired in series pairs across any two phase legs, Wye stators produce high voltage at low rotor speeds. This enables early "cut-in" charging at low engine idle RPM. Wye stators are ideal for vocational equipment, urban service trucks, and machines that spend extensive hours idling.
-
Delta Configuration:
- Connections: The three phase coils are connected end-to-end in a closed triangular loop. Phase outputs are tapped from the three corners of the triangle.
- Voltage & Current Relationships: Line voltage equals phase coil voltage: $V_{line} = V_{phase}$. Line current is higher than phase current by a factor of the square root of three: $I_{line} = \sqrt{3} \times I_{phase} \approx 1.732 \times I_{phase}$.
- Performance Profile: Delta stators have parallel current paths through the coils, allowing them to deliver massive amperage outputs at medium to high engine speeds. However, they require higher cut-in speeds to begin charging. Delta stators are standard on high-load production machines such as large wheel loaders, excavators, and haul trucks operated at sustained governed RPM.
| Operational Characteristic | Wye (Star) Stator | Delta Stator |
|---|---|---|
| Coil Arrangement | Connected to a central neutral point | Connected in a closed triangle |
| Voltage Formula | $V_{line} = 1.732 \times V_{phase}$ | $V_{line} = V_{phase}$ |
| Current Formula | $I_{line} = I_{phase}$ | $I_{line} = 1.732 \times I_{phase}$ |
| Cut-In RPM (Low Speed) | Low cut-in speed; excellent output at engine idle | Higher cut-in speed; low output at idle |
| Maximum Output (High Speed) | Moderate maximum current capacity | Exceptionally high maximum current capacity |
| Neutral Tap Availability | Yes (permits 8-diode rectification) | No (closed loop, no neutral point) |
Full-Wave Rectification & Diode Bridges
Because heavy equipment battery banks and electronic systems operate on direct current (DC), the 3-phase AC generated in the stator must be rectified into DC before leaving the alternator housing.
3-PHASE 6-DIODE FULL-WAVE BRIDGE RECTIFIER
Positive Heat Sink (B+ Output Stud)
│ │ │
┌┴┐ ┌┴┐ ┌┴┐
▲ │ D1 ▲ │ D3 ▲ │ D5 (Positive Diodes)
└┬┘ └┬┘ └┬┘
│ │ │
Stator Phase A ─────────────────┼────────┘ │
Stator Phase B ─────────────────┼─────────────────┘
Stator Phase C ─────────────────┼────────────────────────┐
│ │ │
┌┴┐ ┌┴┐ ┌┴┐
▲ │ D2 ▲ │ D4 ▲ │ D6 (Negative Diodes)
└┬┘ └┬┘ └┬┘
│ │ │
┴ ┴ ┴
Negative Heat Sink (Chassis / Housing Ground)
1. The 6-Diode Full-Wave Bridge
Rectification is achieved using semiconductor diodes, which act as one-way electrical check valves, allowing current to flow only when forward-biased (anode more positive than cathode):
- Three Positive Diodes: Anodes connected to Phase A, B, and C; cathodes connected together on an insulated aluminum heat sink linked to the B+ output terminal.
- Three Negative Diodes: Cathodes connected to Phase A, B, and C; anodes connected together on a grounded heat sink pressed into the alternator housing.
- Operation: As the 3-phase sine waves oscillate, whichever phase is most positive directs current through its positive diode to B+, while whichever phase is most negative routes returning current through its negative diode from ground. This produces six overlapping DC pulses per electrical cycle, creating a smooth DC voltage profile with minimal ripple.
2. The 8-Diode Bridge (Wye Stator Neutral Tap)
In standard Wye-wound alternators, an unrectified third-harmonic AC voltage wave develops at the central neutral junction. While normally wasted as heat, heavy-duty alternators capture this energy by installing two additional diodes connected between the stator neutral tap and the B+ and ground heat sinks.
At high rotor RPM, the neutral voltage oscillates above B+ and below ground. The two neutral diodes conduct this third-harmonic energy into the DC bus, boosting total alternator amperage output by 10% to 15% at rated speed without increasing copper stator wire gauge.
Brush-Type vs. Brushless Heavy-Duty Alternators
CONVENTIONAL BRUSH-TYPE BRUSHLESS HEAVY-DUTY
[Rotating Rotor with Field Coil] [Stationary Field Coil on Housing]
• Field coil spins on shaft • Field coil fixed; does not rotate
• Requires carbon brushes & slip rings • No brushes, no slip rings, no friction
• Vulnerable to dust, grit, oil wear • Impervious to silica, coal, abrasive dust
│ │
▼ ▼
[Stationary 3-Phase Stator] [Rotating Iron Pole Pieces on Shaft]
• Stator output windings fixed • Channeled magnetic flux sweeps stator
│ │
▼ ▼
[Rectifier Bridge & B+ Output] [Stationary 3-Phase Stator & Diodes]
1. Brush-Type Alternators
In standard commercial alternators, the rotor field coil rotates with the rotor shaft. Excitation current from the voltage regulator is fed into the spinning rotor via two stationary carbon brushes spring-loaded against smooth copper or bronze slip rings on the shaft:
- Advantages: Compact size, lightweight, low initial manufacturing cost.
- Failure Modes in Heavy Equipment: Operating in rock quarries, surface mines, or forestry sites introduces airborne silica dust, coal fines, moisture, and road salt. Abrasive particulates act as grinding compound on the spinning slip rings, grooving the copper and wearing carbon brushes down within 1,000 to 2,000 operating hours. Furthermore, extreme machine vibration causes brush bounce, resulting in electrical arcing, radio frequency interference, and intermittent field loss.
2. Brushless Alternators (e.g., Delco Remy 33SI / 35SI / 36SI, Niehoff, Prestolite)
To eliminate premature charging failures in hostile industrial environments, manufacturers engineered the brushless heavy-duty alternator:
- Stationary Field Coil: The field coil is mounted to a stationary iron spool securely bolted to the alternator rear end frame. It does not rotate!
- Stationary Stator: The three-phase stator windings remain stationary around the perimeter.
- Rotating Segmented Pole Pieces: The rotor shaft carries segmented, interlocking forged iron poles separated by non-magnetic rings. As the rotor turns, these iron poles channel magnetic flux from the stationary field coil across a precision air gap and sweep it across the stationary stator windings.
- Zero Friction Electrical Contacts: Because both the field coil and the stator windings are completely stationary, there are no slip rings, no carbon brushes, and no sliding electrical contacts of any kind.
- Industrial Durability: Brushless alternators operate reliably for 10,000 to 15,000+ hours in extreme mining dust and aggregate environments. The sole mechanical wear components are the heavy-duty sealed ball bearings. They can withstand direct pressure washing and aggressive vibration that would instantly destroy brush-type units.
| Design Feature | Conventional Brush-Type Alternator | Heavy-Duty Brushless Alternator |
|---|---|---|
| Field Coil Location | Rotating on the rotor shaft | Stationary, bolted to rear housing |
| Electrical Transfer | Sliding carbon brushes on copper slip rings | Magnetic induction across air gap (no contact) |
| Environmental Sealing | Open or vented; susceptible to dust/oil fouling | Fully sealed or dust-protected stationary coils |
| Brush Replacement | Periodic service required (1,000–3,000 hrs) | Never required (zero brushes) |
| Bearing Life / MTBF | Moderate MTBF (limited by brush wear) | Exceptionally high MTBF (limited only by bearings) |
| Primary Applications | On-highway highway tractors, light commercial | Off-highway mining trucks, excavators, loaders |
Electronic Solid-State Voltage Regulators
An alternator cannot regulate its own voltage output. Because induced voltage is proportional to rotor speed and magnetic field strength, an unregulated alternator spinning at high engine RPM would output 40V to 80V+, instantly frying electronic control modules, blowing bulbs, and boiling battery electrolyte.
ELECTRONIC VOLTAGE REGULATOR ARCHITECTURE
System Voltage Sense Line (B+ / Remote Sense Wire)
│
▼
[Zener Diode Reference Circuit]
• Establishes precision reference threshold (e.g., 28.0V)
│
▼
[Differential Comparator Amplifier]
• Compares sensed voltage against reference
│
┌─────────────────────┴─────────────────────┐
▼ (Voltage < Target) ▼ (Voltage ≥ Target)
[Turn Power Transistor ON] [Turn Power Transistor OFF]
• Field current flows through coil • Field current interrupted
• Rotor magnetic field expands • Rotor magnetic field collapses
• Alternator output increases • Alternator output decreases
│ │
└─────────────────────┬─────────────────────┘
▼
[Field Decay / Flyback Diode]
• Snubs reverse inductive voltage spikes
when field transistor turns OFF
Regulation Operating Principle
The solid-state regulator continuously switches field current ON and OFF using pulse-width modulation (PWM) at frequencies of several hundred to several thousand Hertz:
- When system voltage drops below the regulator setpoint, the power transistor (MOSFET or bipolar Darlington) turns ON, allowing field current to flow and expanding rotor magnetic flux to raise alternator output.
- When system voltage reaches the setpoint, the transistor turns OFF, interrupting field current. The magnetic field collapses and output voltage falls.
- By rapidly modulating the duty cycle (percentage of ON time vs. OFF time), system voltage is held stable within fractions of a volt regardless of engine RPM or electrical load changes.
- Field Decay Diode (Flyback Diode): When field current is abruptly interrupted by the power transistor, the collapsing magnetic field induces a massive reverse inductive voltage spike (Lenz's Law) exceeding 100V. The field decay diode is wired in parallel across the field coil to safely circulate and dissipate this inductive surge, protecting the switching transistor from dielectric breakdown.
A-Circuit vs. B-Circuit Regulation
- A-Circuit (Ground-Side Switching): One side of the rotor field coil is connected to positive power (B+ or auxiliary trio). The voltage regulator is located in the ground side of the field circuit, switching the ground path to regulate current. A-circuit regulation is the predominant standard on modern equipment.
- B-Circuit (Power-Side Switching): One side of the rotor field coil is permanently grounded to the alternator housing. The voltage regulator is located on the power side, switching positive voltage to the field coil.
Temperature Compensation Logic
Battery electrochemical efficiency changes dramatically with ambient temperature. In cold temperatures, electrolyte viscosity increases and internal chemical reactions slow down, raising the battery's internal resistance. A cold battery requires a higher voltage to accept charging current.
- Winter Setting: Electronic regulators incorporate a negative temperature coefficient (NTC) thermistor. At sub-zero temperatures (-20°C to -40°C), the regulator increases regulated voltage to 14.4V to 14.8V (12V system) or 28.8V to 29.6V (24V system) to force charge into sluggish battery cells.
- Summer Setting: In hot operating environments (+40°C), internal battery resistance drops. If charging voltage remained high, the battery would overcharge, causing rapid electrolysis of water into explosive hydrogen/oxygen gas, boiling off electrolyte and warping lead plates. The regulator lowers charging voltage to 13.8V to 14.2V (12V) or 27.6V to 28.2V (24V).
Remote Battery Voltage Sensing Wire (S-Terminal)
In large heavy equipment, the alternator B+ output cable may run 4 to 8 meters through chassis harnesses to reach the main battery bank. Under a full load of 200A, even a well-sized 2/0 cable will experience a legitimate voltage drop of 0.3V to 0.6V across its length.
- Without Remote Sensing: If the regulator sensed voltage only at the alternator B+ output stud, it would maintain 28.0V at the alternator. After cable drop, only 27.4V would reach the battery posts, leaving the battery chronically undercharged.
- With Remote Sensing: A dedicated sensing wire connects directly from the battery positive post back to the regulator Sense (S) terminal. The regulator ignores voltage at the alternator stud and regulates based strictly on true battery post voltage. If it senses 28.0V at the battery while the cable drops 0.5V, the alternator will automatically output 28.5V at its B+ stud, compensating perfectly for line loss.
Load Dump Protection
A catastrophic electrical event known as a load dump occurs if the battery disconnect master switch is accidentally opened, or a battery cable breaks while the alternator is operating at full output (e.g., 200A). The battery acts as a massive electrical capacitor. When suddenly disconnected, the energy trapped in the rotor and stator inductances produces a transient voltage spike of 80V to 120V+ lasting up to 400 milliseconds. Modern heavy-duty alternators utilize Zener-avalanche rectifier diodes that break down safely at 35V to 40V, clamping the spike and dissipating the energy to ground before it can destroy chassis ECMs.
Alternator & Charging Circuit Diagnostic Procedures
A structured diagnostic sequence prevents misdiagnosis of good alternators caused by simple external wiring faults.
CHARGING SYSTEM DIAGNOSTIC WORKFLOW
[Complaint: Batteries Discharging / No Charge]
│
▼
[Step 1: Regulated Voltage Check]
• Engine at 1,500 RPM, baseline no-load
• 12V: 13.8V–14.6V | 24V: 27.6V–29.2V
│
┌───────────────────────────┴───────────────────────────┐
▼ (<13.2V / <26.4V) ▼ (>15.2V / >30.5V)
[Undercharging Detected] [Overcharging Detected]
Proceed to Step 2 • Defective voltage regulator
│ • Open remote sense wire
▼ • Corroded regulator ground
[Step 2: Full-Load Amperage Test (Carbon Pile)]
• Apply carbon pile to battery bank to pull voltage down to 12.6V / 25.2V
• Measure alternator B+ output with inductive amp clamp at governed RPM
• Must produce ≥ 90% of rated nameplate capacity
│
├───► Output < 90%? ──► Internal stator / diode failure or belt slippage
▼
[Step 3: AC Ripple Voltage Test]
• DMM set to AC Millivolts across battery posts under load
• Maximum allowable AC ripple: < 0.3V to 0.5V AC
│
├───► AC Ripple > 0.5V AC? ──► Blown rectifier diode (open or shorted)
▼
[Step 4: Charging Circuit Voltage Drop Testing]
• Positive Side: Alternator B+ stud to Battery (+) post (Max 0.5V under load)
• Ground Side: Alternator metal housing to Battery (-) post (Max 0.2V–0.3V)
1. Regulated Voltage Output Check
- Check drive belt condition and automatic belt tensioner alignment. Belt slip under load is a leading cause of low charging output.
- Start the engine and operate at governed working speed (approximately 1,500 to 1,800 RPM).
- Measure voltage across the battery posts with no auxiliary loads active:
- 12V System Specification: 13.8V to 14.6V (at 20°C).
- 24V System Specification: 27.6V to 29.2V (at 20°C).
- Turn on all machine work lights, cab HVAC blowers, and auxiliary loads. Voltage should not drop more than 0.5V below baseline and must recover within seconds.
2. Full-Load Amperage Output Test (Carbon Pile)
An alternator may produce 28V under no load, but collapse completely when asked to deliver current due to an open stator phase or burnt diode:
- Connect a heavy-duty carbon pile load tester across the battery bank.
- Clamp an inductive DC amp clamp around the alternator main B+ output cable.
- Operate the engine at governed RPM.
- Gradually load the carbon pile, watching system voltage drop to 12.6V (12V system) or 25.2V (24V system).
- Read the inductive amp clamp: The alternator must deliver at least 90% of its rated nameplate amperage output (e.g., a 200A alternator must deliver at least 180A).
3. AC Ripple Voltage Testing (Diode Integrity)
A digital multimeter set to AC volts can detect defective rectifier diodes without disassembling the alternator:
- Connect the DMM leads across the battery posts with the engine running at 1,500 RPM under a moderate electrical load (lights ON).
- Select AC Millivolts (mV AC) or AC Volts (V AC) on the meter.
- Specification: Maximum allowable AC ripple is < 0.3V to 0.5V AC (on a healthy system with good diodes, ripple is typically under 0.1V AC).
┌─────────────────────────────────────────────────────────────────────────────┐
│ DIODE FAILURE MODE DIAGNOSTIC MATRIX │
├───────────────────┬────────────────────────────┬────────────────────────────┤
│ Diode Condition │ Primary Diagnostic Symptom │ Clinical Consequence │
├───────────────────┼────────────────────────────┼────────────────────────────┤
│ One Open Diode │ • Alternator output drops │ • Battery undercharges │
│ │ by exactly 1/3 (~33%). │ under heavy loads. │
│ │ • AC ripple rises to │ • Mild whine from stator. │
│ │ 0.3V–0.6V AC. │ │
├───────────────────┼────────────────────────────┼────────────────────────────┤
│ One Shorted Diode │ • High AC ripple (>1.0V AC)│ • Severe parasitic battery │
│ │ • Alternator case runs hot │ drain at rest. │
│ │ even with engine OFF. │ • Rapid dead batteries. │
│ │ • High-pitched audible hum │ • CAN-bus communication │
│ │ and burning stator odor. │ faults and sensor noise. │
└───────────────────┴────────────────────────────┴────────────────────────────┘
- Open Diode: The alternator loses one phase leg. Output drops by approximately one-third, but the alternator will not drain the battery when parked.
- Shorted Diode (Catastrophic): A shorted diode permits AC current to cross directly into the DC system, creating massive AC ripple voltage (>1.0V AC). More critically, when the engine is shut down, the shorted diode provides a continuous forward-biased leakage path from the battery positive post directly through the stator coil to ground. This generates a 3A to 15A parasitic draw that completely discharges a heavy-duty battery bank overnight and can overheat the alternator stator to the point of catching fire.
4. Charging Circuit Voltage Drop Testing
High resistance in charging cables forces the alternator to work harder, generating excessive heat while starving the battery:
- Positive Output Cable Drop: Connect Red lead to alternator B+ stud; Black lead to positive (+) battery post. Operate engine at 1,500 RPM under a 50A load: Maximum allowable drop is 0.5V.
- Negative Ground Circuit Drop: Connect Red lead to alternator metal housing; Black lead to negative (-) battery post. Operate under 50A load: Maximum allowable drop is 0.2V to 0.3V.
Worked Diagnostic Scenario: CAN-Bus Communication Errors & Dead Batteries
Clinical Complaint
A 30-tonne wheel loader experiences intermittent CAN-bus communication error codes across transmission, engine, and hydraulic controllers whenever the engine is running above idle. Additionally, the machine suffers dead batteries if left parked for more than 18 hours. The operator reported a high-pitched electrical whining sound from the engine compartment.
Step-by-Step Diagnostic Procedure
- Parasitic Drain Static Check: With engine stopped and all keys OFF, install an in-line ammeter between the disconnected negative battery terminal and cable. Meter reveals an abnormal 6.8A continuous parasitic draw.
- Component Isolation: Disconnect the heavy B+ cable from the rear of the alternator. The parasitic drain immediately drops from 6.8A down to 0.035A (35 mA), which is normal for ECM memory keep-alive circuits.
- Deduction: An internal path to ground exists inside the alternator, strongly pointing to a shorted positive rectifier diode.
- Running AC Ripple Test: Reconnect the B+ cable. Start the engine and run at 1,500 RPM. Connect DMM set to AC Volts across battery terminals. The meter displays 1.85V AC of ripple (exceeding the 0.3V specification by over 500%).
- Oscilloscope / Ripple Analysis: The massive 1.85V AC ripple is superimposing high-frequency electrical AC waves directly onto the machine's 24V DC power rails. This unconditioned noise bleeds through controller power supplies and corrupts the differential voltage signals on the CAN-H and CAN-L twisted pairs, setting intermittent J1939 communication fault codes.
- Resolution: Replace the alternator with a heavy-duty brushless unit. Verify AC ripple drops to 0.08V AC, charging voltage stabilizes at 28.2V, parasitic draw at rest is 35 mA, and all CAN-bus communication faults are permanently cleared.
Why are brushless alternators widely specified on heavy equipment operating in surface mining, oil sands, and forestry applications instead of conventional brush-type alternators?
A technician tests a 24-volt heavy excavator charging system that experiences dead batteries after sitting parked over a weekend. With the engine running at 1,500 RPM, the technician measures 1.25V AC across the battery terminals with a DMM set to AC millivolts. What does this test finding indicate?
Why do electronic solid-state voltage regulators incorporate negative temperature compensation into their charging profiles on heavy mobile equipment?