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Note 2 - Summary of Significant Accounting Policies (Tables)
3 Months Ended
Mar. 31, 2021
Notes Tables  
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block]
  

March 31,

 

Excluded potentially dilutive securities (1):

 

2021

  

2020

 
         
Options to purchase common stock  1,040,522   1,709,106 
Unvested restricted stock units  4,771,537   2,397,299 
Financing warrants to purchase common stock  6,372   805,747 

Total potential dilutive securities

  5,818,431   4,912,152 
Schedules of Concentration of Risk, by Risk Factor [Table Text Block]
  

Revenue

 
  

Three Months Ended March 31,

 
  

2021

  

2020

 
         

Clarios (successor of Johnson Controls Battery Group, Inc.)

  %  100%