Solar Charging Trailer for an Autonomous Pasture Mower

November 2021

By Eric Buchanan, Renewable Energy Scientist

A final autonomous mowing test was completed in a pasture at the WCROC this October. The project has been a collaboration between the WCROC, the Department of Computer Science and Engineering (CSE), the Department of Bioproducts and Biosystems Engineering (BBE), and The Toro Company (Toro). The mower, named “Cowbot” by Toro, performed well taking down thistles and other weeds while leaving the recently grazed grass uncut. Over the next few months data will be analyzed to determine how the much smaller Cowbot compares to traditional tractor mowing of pastures from an overall energy usage standpoint. From a weed control standpoint, the Cowbot has already shown itself to be equivalent to or even better than a tractor pulled mower and cows don’t seem to care if a driver is present or not.

An autonomous, electric powered, pasture mower needs two things to successfully operate in a remote pasture: a means of transport to the pasture and a means of recharging its batteries at the pasture. Both needs were satisfied by adding solar panels and a battery storage system to an enclosed cargo trailer. The trailer is a tandem axle, V-nose model 14 feet long by 7 feet wide.

Several factors must be considered when designing an off-grid solar/battery system. Chief among these are the power and energy requirements of the electrical load and how long the storage system must supply that load without sunlight. In this case, the load is the Cowbot’s battery pack which contains 29 kWh of energy when fully charged. For perspective, that is about half the battery size of the all-electric Chevy Bolt. Ideally, the trailer and Cowbot batteries would be recharged daily since the Cowbot can operate about 3 or 4 hours before recharging.

The next decision is to determine the degree of discharge (DoD) that will be allowed on both battery packs (Cowbot and trailer). There is an inverse relationship between DoD and battery life meaning that allowing batteries to discharge more will reduce the number of charge/discharge cycles the battery can provide. Moreover, allowing the mower battery to discharge too much could lead to the mower being stranded in a pasture if it finishes mowing far from the charging trailer. A DoD of 35% was selected for the Cowbot as a reasonable compromise between mowing capacity, battery life, and reserve capacity leading to a daily recharging load for the trailer battery pack of 18.9 kWh.

The average solar insolation in central Minnesota during the summer months means a solar array of about 3.1 kW is required to meet the daily load. Ten panels will provide a 3.25 kW array satisfying the load requirement, but only four solar panels can fit on the roof of the trailer without extending over the sides. Brackets were designed and attached to the trailer roof to hold four fixed panels high enough above the roof to accommodate two rows of three panels each underneath the fixed panels. The lower two rows of panels are mounted to fully extending, heavy-duty, drawer slides allowing each row to slide out – one to the left side of the trailer and one to the right side – for access to sunlight. Two pins retain each moveable panel row in the retracted position for travel or in the extended position for battery charging.

Degree of Discharge is also an important factor in designing the trailer battery system in addition to battery type, voltage, and discharge rate. When this system was designed lithium ion batteries were relatively expensive so deep-cycle lead acid batteries were selected. Eight 6 volt batteries are connected in series to provide a 48 volt battery pack with a combined 18 kWh of storage capacity. Reserve capacity and battery life are of lesser concern for the trailer batteries than for the Cowbot battery, but even with a DoD of 0%, the battery pack will be a little short of the desired capacity. When designing a stationary battery system good practice is to round up all load estimates and battery quantity calculations to err on the safe side, but with a mobile battery system there are other considerations. Namely, each battery weighs 115 lbs resulting in a total battery pack weight of 920 lbs and the pack must be placed in the front of the trailer to allow room for the mower. This means the battery pack weight is mostly added to the tongue weight of the trailer. While a larger battery pack would better meet the load requirements, trailer towing considerations led to the slightly undersized battery pack.

Balance of system components include a PV combiner panel, charge controller, power inverter, and AC load center. Solar panel output wires are combined and fused in a PV combiner box which then feeds directly into a charge controller. The charge controller manages the battery charging process. The DC output from the battery pack is wired to a power inverter where electricity is inverted into AC current and fed to the load center to be distributed to any AC loads. In this case the AC loads are standard 120V outlets and a 240V, Level 2 electric vehicle charger made by Clipper Creek.

The solar charging trailer worked very well for providing power for the Cowbot at remote pasture sites and could also be used as a portable power station for other purposes such as emergency power during utility power outages.

Funding for this project was provided by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR).  The Trust Fund is a permanent fund constitutionally established by the citizens of Minnesota to assist in the protection, conservation, preservation, and enhancement of the state’s air, water, land, fish, wildlife, and other natural resources. Currently, 40% of net Minnesota State Lottery proceeds are dedicated to growing the Trust Fund and ensuring future benefits for Minnesota’s environment and natural resources.