geedubya it is a two pin quick disconnect. You use it to plug in a trickle charger, a usb charger, voltmeter, etc.
On the positive battery post you can see where one leg of the disconnect is attached. The neg. side not so much.
Yup - that is what is commonly called an SAE connector (
Society of Automotive Engineers) and they're usually sold with a short wire pigtail to enable them to be wired into a vehicle. The advantages of this style of connector is that they are inexpensive, robust and reliable plus they can carry a good deal of current -
BUT - one of the terminals is fully exposed (
if you leave off the little cap that is normally supplied with them) and so you need to be cautious about how you wire it into a vehicle.
They are usually supplied with battery maintainers (
aka trickle chargers - which are not
quite the same thing -
read on) as noted by WeekendRider above.
Here is a passage from the Wikipedia article on this style of connector:
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SAE connector[edit]
An SAE connector
The SAE connector is a hermaphrodite two-conductor DC connector commonly used for solar and automotive applications (also motorcycles). It is so named for the Society of Automotive Engineers who created its specifications.
This connector is typically used for applying a maintenance charge to a vehicle battery. The polarity of the connector, when installed in a vehicle and attached to a battery, is always such that no short circuit will occur if the exposed terminal were to touch the vehicle chassis. In most vehicles, this means that the exposed terminal connects to the negative terminal of the battery. Conversely, the positive terminal on a battery charger is exposed, to mate with the concealed one on the vehicle side. (On vehicles with a positive-ground frame, such as vintage British motorcycles, this is reversed.)
Although there is a risk of short-circuiting a battery charger, the risk is minimal and often mitigated by the circuitry of the battery charger itself. On the other hand, the short circuit current of the lead-acid batteries installed in vehicles is sufficiently great, that a short circuit could result in a fire or explosion. The priority is therefore given to avoiding short circuits of the vehicle battery, rather than of the charger.
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Thus, on the "bike" end of the SAE connector wiring - you should arrange to have the
exposed metal pin of the SAE connector - wired to the
NEGATIVE battery terminal so that if that pin touches the frame of the bike, you will not short out the battery.
BTW - if you do not presently have a
battery maintainer on your bike - I recommend one. It will keep your battery topped up so that the bike is always ready to go and if you buy a "smart" unit (and not just a "dumb" trickle charger), it will help keep your battery healthy much longer.
I have one of these on all of my bikes and the Miata and I simply leave a little connector pigtail sticking out of a discrete location so that I can hook up whenever I pull into the
Disaster Central Workshop. The Deltran maintainer is a good one - for less than $30 CDN at Walmart and it even comes with a convenient SAE connector pigtail ready to install on your vehicle battery - but they're widely available in lots of places.
Cheers,
Pete