battery charger to get through Piper power port?

I bought a cheap SLA trickle charger at a ham fest awhile ago thinking
I might use it with a cable I made for my Aztec. On a warm (but not
warm enough) day last week I tried start the plane without heating the
engines. I failed. I thought it would be a good time to try the
charger.

I knew my bigger charger wouldn't work because it requires a battery
with *some* power to trigger it to start charging. That fails with the
Piper power port because the charger has to supply enough current to
trip the relay before connecting to the battery. It's a standoff.

Well, this trickle charger apparently works the same way. It wants to
sense a charge before it puts out.

Anyone know of a small charger that is safe for extended use and will
produce a charge without first sensing a battery? I'm about ready to
take my inverter and voltmeter to the auto parts store for testing.

Thank you.

--kyler