The YUASA Battery Analyzer as sold by YUASA at most dealers.
Cost? YUASA MSRP is $11; usually less at dealers.
This all-in-one device has many uses in that it indicates if the open circuit voltage of the battery is above or below 12.5V, it can be used to connect any battery maintainer with an SAE connector and it can be used to power accessories such as a Gerbings heated jacket liner.
The connector lugs are connected directly to the battery and the inline fuse protects against short circuits. Since my YUASA Battery Analyzer is connected directly to my accessory fuse block, I cut the fuse holder out of the wiring.