Frankenstein Retirement
Published 3/3/2018

Frankenstein Retirement
Frankenstein has been a great lawn mower. He started out as a regular plug-in electric lawn mower. A couple years later, I added a large 12 V marine battery and an inverter. This made him cordless and worked well but was very heavy. A few years later I upgraded to larger wiring on the 12 V circuit which bought a longer run-time.

Frankenstein further evolved by upgrading to a 120V DC battery pack and feeding the DC motor directly. Finally with the availability of inexpensive lithium cells I fashioned a large 14 volt, 0.5 kWh battery pack out of 18650 cells, added an inverter and powered the mower from its 120 VAC output. This was by far the lightest lawn mower yet. Even our smallest child could mow the lawn too.

Timeline of Events: 
  • 2004 Piston shot out of our original Craftsman gas lawn mower. 
  • June 2004 Bought a 21” Black and Decker plug-in electric lawn mower. 
  • June 2004 Added a 12V Group 27 marine battery and 750 watt inverter. 
  • 2006 Welded broken handle (after lifting an overly heavy mower) 
  • 2009 Upgraded to 120VDC 24V 4Ah gel cell pack (Frankenstein is alive) 
  • 2012? More welding of the handle and swapping out cells as individual ones die Replaced defective switch lever with regular light switch.
  • 2014 Replaced burnt out light switch with breaker switch. 
  • 2015 Upgraded to 14.8V lithium pack.
  • October 2017 Retire Frankenstein. 
The down side to this pack was it was more technical to charge it up and turn on the mower. 
You had to put it on the 12V battery charger and when it was 100% charged the voltage was too high and the 12V inverter would trip. 
You had to clamp on the 100 Amp 12V battery tester to load it down and then turn on the inverter. 

Before disengaging the battery tester load you needed to flip the main switch to the lawn mower. Then you could mow the lawn. If you turned off the mower too early before the battery pack was sufficiently discharged, the pack would still be over-voltage and it would trip out the inverter and you had to repeat the process all over again. 
After 9 years, perhaps it was time to retire Frankenstein and go with a commercially available cordless electric lawn mower that was easy to operate and ran on the same standardized batteries as all our other yard tools. 
The new 40V Black&Decker cordless lawn mower is not as powerful as old Frankenstein, nor does it run as long on a charge. But with an abundant supply of standardized commercial battery packs, it isn’t that big of a deal to swap out 3 battery packs in order to mow all our lawns.
Battery Voltage Ah rating Theoretical capacity Actual 12V 27 Group marine 12V 100 Ah 1200 watt-hours 800 Wh

120V lead-acid battery 120V 4 Ah 480 watt-hours 350 Wh 14V lithium pack 14.8V (3.7V x4) 3.2Ah x12 strings 568 watt-hours 555 Wh 40V lithium pack 40V 4 Ah 160 watt-hours 156 Wh 40V lithium pack 40V 2 Ah 80 watt-hours 78 Wh As you can see the 40V lithium batteries are tiny in comparison to the batteries I was using before. They were also much lighter. It takes x2 of the 4Ah battery packs and x1-2 of the available 2Ah ones to mow the lawn. Fortunately they charge fast.

I will continue to use the old lithium battery pack and inverter as a portable 120VAC backup power source. 
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