For resting voltage you want to take the reading as many hours after parking up the car as you can and allow for perhaps a 0.2v or 0.3v drop with the car's computers (obviously more if you have other stuff running live too). As someone pointed out in another thread this doesn't fully tell you battery condition, a load test would be needed for that, but if done reasonably with a known good battery it'll give a good idea from accumulated experience of previous readings and conditions. If you own an Audi and/or Beemer, particularly if one or both are black (and not just from diesel smoke) then I think it's the law that you have to buy a CTek charger maintainer and they're fine just that I personally think they're overrated and over-valued as well of course as over-priced for normal average car owner use. I know the £15-£20 ones (Aldi(?) and) Lidl (I always confuse the two stores) are good as so many I've seen have used them over the years and the one my neighbour bought has been fine for a good number of years regular use. I use a Ring - other makes, models, brands and labelled, and stores are available, possibly for a few pennies less. - Ring Smartcharge 4 (4-amps, I very much favour going as low as reasonable with charging amps, takes longer but more of a Hieneken effect. -https://shop.ringautomotive.com/rsc904-4a-smart-battery-charger-maintainer.html Do read the car's 'Owner's Manual' and charger maintainer's instructions for what and how to use a battery charger properly on that car. On the Beemers I've seen you'll be lucky if you could get near the battery (or batteries) but they usually have two lugs you connect to, be sure the clips grab the lugs well for a good connection as some hold better than others . Bosch batteries I was told, when I fitted one to my wife's 2015 Fabia, are Varta. If you kept a battery going for 13 years you may well already know but if you've not already seen/suffered (I'm reasonably sure I've seen your name in other thread(s)) my usual verbiage on charging/chargers/batteries and you want or need any I can cut 'n' paste any here for you, just ask. HTH.