Power Sources 2, or, Why ATR5 is powered by two combustion engines
Small garden tractors, such as ATR5 - b0 , are not designed to support heavy electrical loads.В Their engines contain integrated alternators to power headlights and maintain the starter SLA, with an output around 40 watts.В This might be sufficient for logic electronics, but actuation and lighting add at least 50 to 100 average watts.В Even optimally efficient CFL or LED headlights will add 35 watts, and the physical actuators (especially steering) will add another 30 to 60W average.В [For instance, pneumatic actuation at 90psi/.06CFM would require a 300W compressor at a 10% duty cycle, or 30W average.]
ATR5’s existing 14hp (10.4kW) combustion engine could be retrofitted with an automotive alternator ($100 – $120 with necessary parts) to deliver power in the kilowatt range at the cost of a couple horsepower. Though it is tempting to avoid the complication of a second independent energy source in this way, an external alternator would require extensive chassis modification (positioning, belt tensioning, ground clearance) and an investment that must be customized and dedicated to each new chassis. Moreover, ATR5 would experience power fluctuations in response to suddenly varying drive loads [before the engine throttle could be modulated to compensate]. A modular, decoupled energy source is the general solution.
As discussed previously [see Power Sources ], there are two feasible options to store and deliver energies in this range: SLA batteries and combustion engines . If average energy requirements are around 100W, a deep cycle SLA ($130 + $40 inverter) can provide the necessary power for 10 hours at 4.4 cents an hour (assuming Peukert constant of 1.4, $0.15/kWh utility rate, 100% efficient charger, 80% charge-to-discharge efficiency, 100% efficient inverter), followed by a charge cycle of at least an hour.В [The efficiency will decrease if the power consumption is concentrated in short bursts. See also DieHard 27582 SLA unverified capacity function .]В If consumption is closer to 1000W, and assuming a minimum 3-hour consecutive runtime (75% runtime duty cycle, assuming 1-hour simultaneous charge), the initial cost would increase to $450 (three $130 SLAs + $60 kW inverter) and the ongoing cost to $0.71 per hour. [Running at 1000W from a single SLA would increase energy costs to $1.10 an hour.]
...