> The engine was off, so the Yandina should be off and not combining. That is w=
> hy there is a problem.

OK, I have a Yandina and I have UBB12220 22 A-hr batteries.  Here is
how it works:

The Yandina has a ground and it has two leads, one for each battery
(positive) that are about 24" long and cannot be shortened or the
warranty will be voided.  The Yandina uses the wire size and length
to limit the power to the device.

The Yandina will "combine" if EITHER of the two + battery leads
is 13.0 V or above.  If BOTH are below 13.0, it will disconnect.

The UBB12220 batteries like to take a full charge and sit at
13.0 V or just above for a while.  A typical wet cell starting
battery will sit at 12.8 or 12.7 V when fully charged and then
pretty quickly slump to 12.6 or so.  That is why 12.6 is usually
cited at a fully charged wet cell battery.

OK, everything is charged and you turn off the engine (alternator)
and---the Yandina light is "on" because the UBB1220s are at or
above 13.0 V.  Then the batteries start to drop, the starting
battery sucking from the UBBs until they drop below 13.0 when
the Yandina disconnects and the green light goes off.

Well, then the UBBs rest and slowly go up in voltage--until they
are back at 13.0 V--and the Yandina combines and the green
light goes back on.  That cycle can happed for quite a while--
several/many times.  If you have a load on the UBBs--I have a
Vitrifrigo--when it kicks on, the UBBs won't ever recover to
13.0 V and the cycle will be broken.

I have had this setup for several years and am very familiar with
it--and the 13.0 above/below Yandina cycle.  This is how it should
work, given where the UBBs like to sit in voltage.  Homework:
factor in battery temperature on this cycle....

BTW--I have a battery switch to combine the two batteries--the setup
from a PO.  With the Yandina in addition, I almost never use it to
charge.  If I am driving 2-3+ hours, the Yandina (with its limited
wire size) will fully charge the UBBs.  It makes life simple.

I only use the battery switch to switch everything to the UBBs when
camping--to isolate the starting battery from everything.

Here is some info I collected about UBBs and charging, solar and
alternator:
http://jones.colorado.edu/VW/ChargingAuxBatteries.txt
I now have 80 watt solar panels.  That is more than enough
for the Vitrifrigo, the Propex and my lights.  Note what
50 watts cost from GoWesty in 2010!!

Richard