I think solar trickle chargers are available. Connect it up to the battery or cigarette lighter, put the solar panel in one of the windows and its all happy days.
I had an email from BMW that said I needed to run the engine for āabout 20 mins a weekā to keep the battery in good condition.
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If the battery voltage drops below 10.6 volts it causes permanent damage and cannot be properly recharged
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I would say a little more than that due to loss from start current. A car battery becomes fully charged after about 4 hrs at 14.4volts. A charge consists of a constant current to 14.4 V followed by constant voltage (absorption) charge for 4 hours after which it can be considered fully charged. AFAIK used to maintain 8400 Amp/hr 250 V FLA batteryšŖ
You may well be right. I am just saying what the official advice from BMW UK was a few months ago.
Do a trial: measure the voltage at start of the week, then measure at end of week then run engine for 20 mins and measure again.
Iām really not in the least interested in doing that! Sorry.
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They may be correct if it has a large alternator some have 200 Amp alternators which will recharge quickly but 80 or 90 Amp look more standard.
I keep an eye on how the Auto Stop/Start operates to get an indication of battery condition.
The condition of the battery is monitored by the engine management system, if certain conditions arenāt met, the Auto Stop/Start does not operate. I went for a drive this afternoon (first time for 7 days) and noted that it didnāt work until Iād driven 8 miles.
I checked the battery voltage when I got home, it was 12.45v. 
Agreed, a clever trickle charger can make a battery last a loooong time.
I would recommend Optimate / Accumate
Note that any external socket must, by law, be installed by an electrician who is āPart Pā qualified.
I use one, sort of, itās actually rated at 18W and uses a small chopper stabiliser circuit to prevent overcharging.
Itās very effective.
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Itās not an external socket. Itās an extension lead with the socket end screwed to the wallā¦
Iām not sure that is exempt.
If itās permanently fitted to the porch, then depending on whether the porch is considered an integral part of the building or an external attached structure (N.B. I canāt remember the exact wording), I think it may well fall under the relevant regulations. Public access to the porch may be relevant as well, unless the porch is normally securely locked.
Itās an integral part of the building, but itās only live when I plug it in and I only plug it in when I want to use it. Anyway I think itās unlikely anyone in authority is going to argue with me over it!
Best
David
That bitās definitely not relevant in law!
But itās part of why itās an extension lead and not a permanent installation.
Still irrelevant in the Electrical Regulations! 
Let it go!
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Yes irrelevant, because the IET Wiring Regulations donāt apply to temporary extension leads.