Urgent advice Defender TD5 not starting

[CraigE]

Hi all,
After some urgent advice. Been away for 2 weeks and had the Td5 on trickle charge. Got home went to start and the engine turns over once and then stops and seems to activate the immobiliser, central locking etc.
Get a bit of a click after and amp squeals. Seems to be enough battery power initially, then dead. Activate immobiliser and then does it all again.Is it likely the main start batter has dropped a cell or something more sinister not allowing the motor to turn over? Have checked the inertia switch. Was running fine before I went away. Coolant and oil ok, cant find any issues there.

[weeds]

Battery dropped a cell??

[strangy]

Electrolyte level if it's a wet cell?

[CraigE]

Ok its running now but strange. Its a sealed AC Delco battery, was showing full charge. Maybe the dual battery system not sure as I know the aux battery is buggered. Will be monitoring.

[CraigE]

Looking like my Rhino RAv2 alarm / immobiliser may be the issue, centre button not locking or unlocking.

[DiscoMick]

If the main battery was fully charged the isolator would have connected the two batteries. If the auxiliary battery was failed it could have sucked the charge out of the start battery, but failed to hold that charge, so just kept sucking more charge from the start battery, which would have kept falling