# Current-limited Alternator Battery Charger Control

A common problem exists when driving automotive alternators via motors. In the process of attempting to regulate the charging voltage of a low battery, the alternator shaft torque usually exceeds the motor capability so that is likely to stall and /or overheat. The solution is to regulate (limit) the charging current because the shaft torque is a function of output current. This system is a control circuit for a motor-driven automotive alternator. In this scheme, the alternator has no voltage regulator so it continues to charge at a constant current until the voltage reaches the upper voltage limit (full charge) – this feature allows it to easily adapt to 24 or 48VDC systems.

The charging system runs only on demand thus eliminating idle power consumption. The alternator field current regulator compares an adjustable set point with the DC feedback voltage developed across a shunt resistor. The field current driver also runs on demand thus reducing idle power consumption. A soft start feature ramps the current up gradually when the system starts thus enabling the motor to reach base speed before maximum load occurs. This is an enhancement of two previously published circuits: External Battery Charger Control and 6V, 24V, 48V External Battery Charger Control.

Schematic

System diagram

Why doesn’t the stalling problem exist in automotive applications?

Well, it does occur – we all know how an alternator behaves when the drive belt needs tensioning – the belt screams like a banshee. Since the engine cannot be stalled, the alternator stalls instead via belt slippage. The typical current overload lasts for only a few seconds until the maximum battery voltage is reached, and then drops back to a maintenance level – you have all seen this on automobile ammeters.

While the automotive alternator is often used for wind power, it is substantially modified and controlled differently. The wind power people are smart and well understand this problem.

How the current limit functions?

A low value shunt resistor is added in series with the negative side of the battery. When the voltage drop across this resistor exceeds a specific set point (R21), voltage comparator (U4A) turns off the field driver transistor (Q2). At this point, the field current starts to decay as it continues to flow through the free-wheeling diode (D5) that is connected across the field winding. As the field flux decays, the alternator output voltage is reduced along with the charging current. When the charging current drops below the set point, U4A turns Q2 on again thus causing the field current to increase and so on. This is also known as a bang-bang type of regulator where either full voltage or no voltage is applied to the field winding – it is ON-OFF rather than linear. This works well because the field inductance is so high that it takes milliseconds for the current to charge or discharge.

U4B discharges the soft start capacitor, and U4C turns off Q2 when K1 is dropped. R20 provides just a little positive feedback to improve the speed of switching – otherwise, it could oscillate. R18 and C4 make up a 100uS time constant noise filter.

While the voltage regulator portion has been tested, the current regulator has not. It works OK in my cranial simulator, but there could be surprises. Fortunately, the field current control is simple and forgiving or I would not be so bold. One caution is the possibility of burning out the field winding in 24 or 48V applications – if problems are experienced while getting this working, adding a power resistor in series with the field winding may prevent field winding burnout.

Voltage Regulator

For voltage regulator function, refer to the previous posts.

Alternator modification may be required

If an old style alternator is used that requires an external regulator, modification is unnecessary unless one side of the field winding is grounded rather than tied to the positive output terminal. If the regulator is internal, the alternator must be disassembled, the regulator removed and the field connections brought out. At the same time, one end of the field winding may need to be wired to the positive output terminal rather than ground because this circuit is a low side controller.

Low side field current control

Low side field control is the most practical in this case. High side field control is more difficult at 24 & 48V outputs. As a result, one side of the field winding must be tied to the BAT+ terminal

Direct drive or belt drive?

Assuming that the alternator has full output at an engine idle speed of 600RPM and the alternator belt drive ratio is 1:2, the alternator shaft runs at about 1200RPM. So for 12V applications, direct drive is feasible using the common 1500 or 1750RPM 4pole induction motor. For 24V applications, a 3000 or 3450RPM 2pole motor is a feasible choice. For 48V applications, a 1:2 or 1:4 belt drive ratio is required. Overspeed is generally acceptable provide that it does not exceed about 8000RPM.

There is also a simple stator modification that can double the output voltage in some alternators. Check out this very informative wind power site: http://www.poiesisresearch.com/Power.php

Calculating the charging current and shunt resistor value

Alternators have a relatively high current rating (e.g 100A). At rated output, the efficiency is low – perhaps 50% or so. I believe that this current rating is intermittent rather than continuous due to the extreme power loss in the alternator at this low efficiency. For continuous operation, I recommend derating this current to 70%. So a 100A alternator may run at 70A continuously at about 70% efficiency.

Shaft input power = Eout * Io /% efficiency = 14.6 * 70 /0.7 = 1460W or about 2HP.
2HP is on the large side for a single phase motor, so consider using something smaller – note that the alternator efficiency will be even higher running with a 1HP motor – perhaps 80%. Size your drive motor accordingly.

Output current (Io)
This is simply a power calculation. Assuming that the limiting factor is your AC induction motor, simply calculate the output watts: P = HP * 746 W /HP * efficiency
Next, divide this power by the full-charge battery voltage: Io = P /14.6V (or 29 or 58V)

Resistor calculation
R = 100mV /Io (e.g if Io = 40A, Rshunt = 100mV /40A = 0.025Ω)
Power of Rshunt = I² * R (e.g. P = 40² * 0.025 = 40W)
Derate by 50% or so. This calls for an 80W resistor.

Kelvin connection
For proper operation, the common and shunt connections must tie directly to both sides of the shunt resistor.

For the future

Dual-mode alternator voltage /current regulator – actually simpler than this control