I measured the voltages again and I have to make a small correction:
The batteries have 13.4V out of the circuit (no load) and 13.8V in the unit,
when operated from AC power.
When the 300W load is connected, the battery has still 13.8V (AC power) and
the low battery LED comes on.
Thanks.

"Johnboy" wrote in message
...
I would check the batteries real carefully. I have found even new SLA
batteries are bad from the get go. I would guess old stock. They must
hold about 12.9 to 13.0 under rated amp load. If they drop to your
measured 11.75, the battery has bad cells. Fully charged brand new
ones are 13.0 after a 24 hour rest and will not drop volts under the
rated load.

I checked the battery
voltage and I found 26V (2X13V) with 300W load (or no load) and AC
power,
and 23.5V (2X11.75V) when operated on battery with cut AC and 300W
load,
which seems OK. Is a recalibration required after replacing the
batteries?
Thanks in advance.
Istvan
icsaki at hotmail dot com