Ending today, Jelly Comb via Amazon offers the Jelly Comb Voltmeter USB Wall Charger for $11.99. Coupon code "23O2KIS3" drops it to $5.88. Plus, Prime members bag free shipping. (Non-members can spend $35 to bag free shipping; otherwise, shipping costs $6.11.) That's $24 off list and the lowest price we could find. It features a voltage display and over-heat, over-current, and over-charge protection.