Electrical – Onboard charging system for hybrid electric tractor


I’m in the process of converting a Cub Cadet SLT1554 from gas to a hybrid-electric tractor using a Motenergy ME0909 PMDC Motor. See description/specifications below:

Motenergy ME9090 PMDC Motor
12-48V, 4 hp cont, 12.8 hp pk
4 Hp Brush-Type PM DC. 4000RPM at 48VDC

The Motenergy ME0909 is a Brush-Type, Permanent Magnet DC motor with very high efficiency. Capable of 4.8 KW continuous and 15 KW for 30 seconds. For voltages from 12 to 48 VDC input and 100 amps continuous (300 amps for 30 seconds). Designed for battery operated equipment. It makes a great replacement for the original Etek motor because it has the same bolt pattern and is actually lighter in weight.


Power: 4 cont – 12.8 pk hp
Voltage: 12-48 Volt rated
Speed: 2150-4850 rpm
Size: 6” OD, (w/o shaft)
Shaft: 7/8” x 1-3/4”, 3/16” key
Weight: 24 lbs.

Voltage Constant: 0.0107
Revolutions per volt: 93.45 RPM
Torque Constant: 0.102 Nm/Amp
Generated Current: 9.8A per Nm

If possible, I would like to operate the electric motor at 24 volts powered by two 12-volt deep cycle batteries in series.

At the same time in order to guarantee sustained power, I would like to have an onboard generator powered by an 8hp I/C horizontal shaft engine driving a 12 volt, 253 amp alternator recharging the batteries. I’m under the impression that I would need at least 2,984 watts of electricity to create 4hp, and 3,730 watts (5 hp) would be even better.

QUESTION: Is it possible to create some type of configuration so that a 12v alternator can charge two 12-volt batteries in series. Also, I’m concerned that the ME0909 ad speaks of only 100 amps for continuous service.

Would the battery not act as a “cushion” between the alternator and the electric motor?

Any assistance in this project will be greatly appreciated.

Best Answer

If you want to charge a 24V battery from a 12V alternator, you will need a huge boost converter, which will be spectacularly expensive. It would be better to just buy an alternator designed for 24V systems, e.g. from a truck.

Yes, a battery can act as a short-term power source, which means that you can get a bit more power in the short term than the alternator can supply. But you still need to pay attention to the thermal limits on the motor: 100A continuous or 300A for 30s (and probably off or very low load for a few minutes after a high-power burst, so that it can cool down again).

For example (totally making up numbers here) if you calculate that your average load will be 60A, you could use an 80A alternator. It would supply the base load, and then when you want a 300A burst, you can pull that from the battery. While the electric motor is cooling after its burst (e.g. drawing only 20A), the alternator will charge the battery.

With a motor of this design, voltage = speed and current = torque; normally you use a PWM speed controller which acts in conjunction with the motor's internal inductance to behave like a buck converter. If you run it from 24V not 48V then you can put in only 24*100 = 2400W continuous (electrical) power; it will run half as fast on 24V (about 2400RPM) as it does on 48V (about 4800RPM) while providing the same torque (a function of the 100A limit). If you want to get the full 4.8kW (electrical) / 4hp (mechanical) power, you need to run it at 48V and 4800RPM.

Note that the motor is rated as 4.8kW electrical and 4hp (3kW) mechanical. That means its efficiency under full load is about 63%, which is terrible. If you want 3kW of mechanical power, you need to supply 4.8kW of electrical power.

Because your choice of supply voltage affects motor speed, it will affect your choice of gearbox ratios.