Cigarette lighter adaptor

[lakeside]

Can these adaptors for a cigarette lighter port affect the cars electronics.
I have leisure battery in the rear of the Berlingo for interior lighting; it also has a cigarette lighter port. When I am camping I use a cigarette lighter extension cable from the leisure battery to the bed area for use recharging my phone or tablet, sometimes a small kettle, recharging one device at a time. On these trips I avoid using the lighter ports going to the main car battery other than when travelling with the phone Satnav.
On a bit of a whim I bought an Energiser twin adaptor as I thought it might be occasionally useful to charge more than one device at a time.
I also have single adaptor with USB ports which I have not needed to use. To be happy I might not use these adaptors but it would be useful to know of opinions if they are any use; some folks on YouTube do consider they can cause problems.
I hike long trips, stealth camp in out of the way places and do not want to be stuck in the middle of nowhere with the car not starting.

[oilyrag]

If you use and adaptor with a built in voltmeter that monitors the car battery voltage and also provides a readout for the amps being supplied to the device via the USB it will give you a good indication of whether you might be compromising the starting capacity of the main battery. These devices are low cost, straight from China, and can be bought in different configurations/port types. 
One possible issue if trying to use an existing cigatte lighter port is that previous use can corrode the main terminal and make getting a decent connection difficult.
https://www.ebay.co.uk/itm/403326691090?...SwESVhpcfq
Additionally you can fit a panel voltmeter or one that plugs into the cigarette lighter to monitor the main battery voltage. https://www.ebay.co.uk/itm/185068015661?...Sw4Rxgpnak
If you carry jump-leads a leisure battery is fine for starting a car.
   

[cancunia]

Depending on which model Berlingo or Partner you have, most or probably all of the 12v outlets will be switched with the ignition so having something plugged into them should not drain your main battery. The Modutop on the M59 has permanent live 12v sockets, maybe the B9 does too but am not sure.
Your leisure battery should be connected via a split charger, so draining it won't affect the main battery. To answer your original question, I've never heard of any device plugged into a 12v outlet causing problems, but maybe there are others that have. I suppose it's possible to overload a 12v socket but that should blow a fuse.
Being a bit paranoid, I fitted a lower amperage fuse in line with the extra 12v outlets that I added.