The Arlo Battery Charger - 2.1 mm is designed to charge 12 VDC sealed lead acid batteries, such as those used on the Arlo Robot system. This device will charge either 1 or 2 SLA batteries at a time, and it mates with the charging jack that comes standard on the Arlo Power Distribution Board (#28996).
This charger fits the 2.1 mm inner diameter power jack in the Arlo Power Distribution Board (#28996) kits made beginning June 2015. (Prior to that, a 2.5 mm power jack was used, to fit the original Arlo Battery Charger - 2.5 mm)
If you have the new Arlo Battery Charger - 2.1 mm Plug
and need to use it with the earlier Arlo Power Distribution Board - the one with the 2.5 mm barrel jack you need an adapter. Adafruit has what you need here - https://www.adafruit.com/product/2897
They say, ""Hrmph! All I have is a power supply with a 2.1mm plug and my doodly-whatsit has a 2.5mm jack. I keep trying to force the plug to fit but that 0.4mm difference just won't work....what am I going to do!? ENTER The Adapter. This 2.1mm to 2.5mm adapter is the best way to adapt a 2.1mm plug so that it will fit into a 2.5mm jack."
With limited description on the web - I ordered another elsewhere that wasn't what I needed. This one is just right and inexpensive too. It cost way more to have it shipped - so order some other goodies too!