I have one battery-powered electric mower in my collection. Run time per battery charge is inadequate for an extended run. Or should I say batteries recharge since this one was designed with two small 12-volt batteries to develop 24 volts allowing use of a 24-volt electric motor, which higher-voltage motors are generally more efficient than lower-voltage motors.
Conventional horizontally rotating mower blades have lousy cutting efficiency per watt-hour. Higher capacity batteries and even higher voltage battery packs like 36V and 48V, allowing use of even more efficient motors, are possible design paths to improve that. Even the little, low-production special purpose "deep cycle" batteries found in most electric mowers are poor buys compared to much larger "deep-cycle" batteries widely sold and used to power marine electric drives. But those larger batteries are quite heavy, which makes their use in push mowers a "mixed bag" of costs and benefits. Vehicle designers have known for over 100 years that taller tires consume less energy when rolling over uneven terrain. That applies to mowers as well, especially those which are heavy.
The range of DC motor efficiency values is wide, running from over 90% down to about 70%. Permanent magnet motors, are typically more efficient than motors which depend on input electrical power to create one side of the magnetic pull-pairs which spin motors. While permanent magnet motors were once much more expensive than brush-type DC motors, their expensive magnets have become cheaper while cost of copper, used in motor windings, increased. So PM DC motors cost/benefit ratio has become more competitive. Lots of clever people have been working on developing better-performing DC-motor electronic controls. A few years ago, we might just called those devices "speed controls," but with clever feedback sensing designs, rotor positioning is being interpreted by back-electro-motive-force WITHOUT even requiring a separate position-sensing feedback circuit! That's REALLY clever design work. So some of the new DC motors can maintain motor rpm despite unpredictable driven loads, much as "cruise controls" do in road vehicles.
Others may find what I'm about to say unlikely. But I expect one of the biggest breakthroughs enabling extended run time with battery-powered mowers will come from more energy-efficient blade design. I won't expand on that right now as I'm one of those involved in that design work.
Fueling gasoline and diesel powered mowers occurs in smaller, less economically painful steps. Replacing a push mower's battery set for $75 or more has seemed like a "deal breaker" to many electric mower owners. Electric recharging cost is pennies. That's not the cost that bites. Replacing expensive batteries bites. If you want to prevent batteries from turning to junk when not used during extended non-growing seasons (in the north where grass doesn't grow every month), you should invest in and use a battery-desulfating pulse-type charge maintainer. I'm just guessing, but I'd be surprised if more than 1 in 50 electric mower owners do that. So their batteries fail from sulfation and extended internal-loss discharge during their non-use periods. Reading forum discussions by battery-powered mower owners clearly indicates that most of them experience lousy battery life, most commonly related to failure to maintain those batteries during the "off season."
As usual with me, I've written more than most of you wanted to hear. If your attention span can't be sustained much longer than reading bumper sticker-length messages, it would be best to ignore my posts.
John
