15F20 is an older design of an electrical battery , consisting of 15 carbon-zinc or alkaline manganese cells in series connection with a nominal voltage of 22.5 V . This type of battery is specified in the International Electrotechnical Commission (IEC) standard IEC-60086 .  The battery is cuboid with the outer dimensions 50 mm × 25 mm × 15 mm, the two electrical contacts are opposite on the two narrow sides. The capacity depends on the cell type and manufacturer - it is usually in the range of 100 to 150 mAh .
A secondary application were photographic flash units with Vacublitz - flash bulbs , which instead of a high-current battery low voltage at this higher voltage and a storage capacitor could be ignited safer (so-called condenser ignition).  
- International Electrotechnical Commission (IEC): IEC 60086: Primary batteries, 2014, IEC
- https://photobutmore.de/exakta/zubehoer/blitzgeraete/ Horst Neuhaus: Description of old photo accessories (private website), accessed on September 5. 2018
- 22.5 V anode battery / Pertrix No. 72nd Lippisches Camera Museum, accessed on October 22, 2014 .