Hi Griff,

The first thing you need to do is to work out how much drain the "body
electrics" are putting on the battery when the car is parked up.

A simple home multimeter should be able to measure this low current.
Set the meter to amps/current Disconnect on side of the battery, and
connect the meter between this lead and the battery terminal you just
removed it from. (just don't try and start the car with the meter in
line, or you'll melt it!). Sorry, but you'll have to reset the code on
the radio after this. :-)

Once you have the standby current figure for the car, you can go about
trying to find something to provide this current.

For example, if your meter says 1amp, and you have a 70AH (amp hour)
battery, this means in a perfect world your battery should be able to
last 70 hours. Although I wouldn't like to try and start the car by
the 69th hour.

If the car take 2 amps, the battery will only last 35hours... Etc.

Most cheap solar panels I have seen that plug into the cigar lighter
provide a pitiful current (in milliamps), which most modern car alarms
would easily exceed, leaving you with a net drain on the battery.

Although having said that, there's nothing to stop you adding several
panels together in parallel... Just remember, they'll have to provide
all the current the car's going eat during 24 hours using the 8 hours
of daylight in the winter time, so they need to provide 3 times the
parked up drain current.

I suspect your 840 might start looking like one of those solar racing
cars they race in Australia!

Simplest solution... Buy yourself a jump pack, which is basically a
small 12 battery in handy carry case... You charge it up from the
mains at home, and then put it in the boot. When you come back to a
dead car, clip it onto the dead battery, start the car, disconnect the
jump pack and remember the put it back on charge when you get home.

Something like this

http://www.maplin.co.uk/Module.aspx?ModuleNo=49038&doy=22m6&C=SEO&U=strat15

Dodgy.

Don't feel bad -- I have a 7 series, and after 5 days of sitting; the battery is
dead.  I'd be happy for 10 days -- I disconnected the alarm / comfort locking
and didn't gain much.  

Personally - those two things don't bug me; at this point the radio code is
second nature to me; and I think car alarms are mostly ignored by people; and a
real car theif, or a smash & grabber doens't care about an alarm the later just
grabs whatever and is gone; the former just disables the alarm quickly.

They could help; only one way to find out.

I use a battery-tender to maintain the battery when I put the bmw in the garage.
It plugs into the cig lighter in the dash; I have it hanging from the ceiling;
so I just open the door; grab it; plug it in; close the door -- not that hard,
really.

The solar panel would help for "Not at home" storage, ala: airports, or hotel
stays; but probably wouldn't keep it topped up indefinately.

I decided the radio code/car alarm issue was secondary to being "strandanded" (I
often leave teh car where I can't use a charger for days) -- so I installed one
of these, and was VERY pleased how nicely they work.  Really amazing.

http://www.prioritystart.com/ps_pro.html

That said -- I think i'll be installing a solar panel (or two) on the back deck.

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No sh.t. Solar panels require SUNLIGHT. They're not powered by RAIN
or DAMP so they're not much use in the UK.

In countries where it doesn't rain every 5 minutes the things
are marginally usable but not real popular.

If parasitic load is killing two batteries in 8 days then
that has to be the all time record parasitic load and does
not sound remotely correct. A load that big can be measured.

Yeah I used to live there.

Both batteries?  You mean one at a time right?
Neither of mine like bign left for a long time but often when I'm at
the dealers I hear of people in 5-series whose batteries don't even
last overnight - BMW solution: "leave the car unlocked overnight sir"

Those work fine if they have enough output and if you park your car
outside.  You will want one with at least 1 sq ft of area, or two
smaller ones. I've used these for a variety of vehicles that aren't used
much, tractors to race cars. I love 'em.
  VW ships out every new car with a nice solar charger on the dash and
connected so the cars arrive with a hot battery. Most dealers just pile
these in the parts dept and sell them cheap. Ask your local dealer.
   I noticed Harbor Freight has a large solar charger now that puts out
something like 5 amps for under $50.  That is big enough to charge a
dead battery in just a few minutes if you get stranded and are parked
outdoors.

Get one that plugs into the cigar lighter.
The jumper battery is also a good idea. They make some small ones that
connect (and also charge) through the cigar lighter. Often all your
battery needs is that little extra nudge to get your going, and you
don't have to open the hood. If your car has a power outlet in the
trunk, you can leave the jumper charging all the time.

Contact BMW - in writing - and ask how long your model can be left safely
without the battery going flat. I'd expect at least 14 days with a good
battery in a good state of charge.

Unless not parked near an outlet I would try to use a auto charger
like batteryminder of batterytender. They keep the battery full, and
you don't have to bother with switching it off when full.
They have quick connect terminals or you can use the cig lighter.
Works great for my motorcycle. Haven't needed a new battery in years.