Posted for "Colyn Case on earthlink" <colyncase@earthlink.net>:

I don't know enough to answer your question on HOW to do it but it seems like
a bad idea if there's a possibility that when you plug it in, the two
batteries will be in different states of charge/sulphation etc. The tender
will not know what to do. Maybe best to get two tenders?
 
[We recently fought that battle on Ted Noel's IVP with two Odysssey batteries... one 17AH, one 55AH.  I took the single Battery Tender and sent its output through a SPDT relay that has a 120V coil.  I used a common lamp timer to trigger the relay coil in a 4-hour-on/4-hour-off pattern.  When the relay is de-energized the hot side from the Battery Tender goes to battery #1, and when it's energized the output goes to battery #2.  The Battery Tender, relay, timer, extension cord and battery connection cable (with a 3-conductor connector... one ground, two hots) are all housed in a nice plastic toolcase that looks rather like a briefcase... the whole thing sits inside the baggage compartment and gets plugged in when the airplane is sitting in the hangar.  So far it appears to be working as intended.    <marv>    ]