Evening all, I was playing with my new E55 this morning and thought I would try and use the feature for testing battery before startup and alternator health once started, unfortunately battery reading 11.6v and car turned over but failed to start. I've taken main battery out (mercedes/varta) out and charged up to 12.6v I will leave over night without charging, it does seem to be losing charge already (down to 12v in 1hr) so I'm assuming it's the main battery. My question to all is I've got a yuasa 100ah 900a wet battery that's in my Audi RS6 C5 ( selling after chrimbo to pay for new ride) which is only 3 yrs old, can I use that battery short term to check if the battery is the cause and until I can get the correct battery from mercedes. The only other cause I can think of is that the boot catch was stuck up, so maybe car thought boot was open (sedan) and the lights have stayed on and drained battery. From what I can see the battery manufacturer date is 2014 from what I can see, but I'm not 100% on that. Luckily I have the day off tomorrow but need to sort ASAP, so would really appreciate anybody's help, thanks.