It is not a defect, rather it is a natural process that occurs with age. Listed bglv2. - Light cleaning may be required.
Vintage Blitz Battery Charger A6 Model
Vintage Gambles Battery Charger 5 amps 6-12v antique garage
Sears 10/50 Amp 12V Battery Charger Engine Start Vintage Automotive Charger 608
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Vintage AUTO-LITE TRICKLE BATTERY CHARGER VINTAGE TYPE A-L-6-12-51 1940 OR 1950?
Vintage Battery Doctor 12v 1.25 Amp Automatic Battery Charger BFA-012-120 20070