Battery tender over gpu plug

[digits]

I know you love your battery tenders, so I figure I'd ask about the folllowing:

A 310, with 2 x 12V batteries in series would need some kind of battery tender during winter. Accessing the batteries is a bit cumbersome, but the plane does have a GPU plug (2 prongs).

Can the battery be charged via this plug, or will that cause issues? You could theoretically start the engine with the batteries off and the GPU hooked up, so that makes me wonder if that trickle charger might just be powering the whole airplane all the time if used in this manner?

[Colonel]

Look at the wiring diagram (or post it here, I will look)

This probably won't do what you want. Ideally, an external power plug
disconnects the battery (with a relay) to avoid the dead battery
dragging the voltage (provided externally) down during the start. You
need to look at the POH. Master on or off with external power?

A really crappily designed external plug will just be parallel to the battery,
like a pair of jumper cables, with perhaps some reverse polarity protection.
That would work for you, but remember that if you're energizing any relays,
that current draw is really going to confuse the charger software if it's looking
for phantom draw.

[Liquid_Charlie]

winter ops - simple battery warmer and tanis heaters/strap heaters (car warmers back in the day but there is some question of insurance now) and engine covers, yes a few bucks but you want to fly your twin in the winter and be a little self sufficient - you will surpass the expense if you want to have that heated hanger everywhere you go. Of all the years operating twin cessnas and buck 80-85 this is what we did and it worked.

[digits]

Okay, so not much luck on the wiring diagrams. They exist, but are cut up in different parts in the service manual, so not too clear on how to combine them.

There is some relay/solenoid magic going on. So I'll try to find an extension wire to screw on to the battery terminals, and then have it connected to the same spot as the blockheater to plug in a proper battery tender.

On a side note, lots of GPU plugs online have a "3 prong cessna style" connector. With 3 female connectors on it, 2 larger one, 1 smaller one. Yet quite a few airplanes only have 2 male end to fit the 2 larger holes, yet a missing (?) pin lining up with the samller female connector. What's up with that?