I agree that the CTEK stated voltages, if true, are overly cautious so I just use it as a guide rather than gospel.
After 5-6 weeks outside, I reckon mine was probably dipping below 12 volts based on past experience and the relative charge that was left in battery on the brand new Suzuki in the garage. Either way, it started and apart from disgracing itself at MOT time, it’s been great to be back in it.
12.4 V is not a fully charged battery. Assuming the battery is reasonably healthy it indicates 70% charge at best. It may well start the car but that’s not the point. I would fully charge it to a minimum of 12.6 volts measured after an minimum of 3 hours rest then get it load tested.
May as well reuse my old thread !
I’ve been using my MX-5 for a weekly run to the supermarket, which is about 5 miles round trip, and I thought would be enough to keep the battery fettled, or at least break even on the fairly decent level of charge it was at before lockdown.
Today, I put it on my trusty Ctek MXS 5 and was surprised that after 4 hours, the battery still hasn’t reached the 80% mark according to the charger status. Must make a note to charge it more regularly or take a really long way round to the shops.
I thought to bring myself up to date on modern theory about charging car batteries by having a deeper look at how the batteries work (while looking for some curves on state of charge for a given rest voltage), and found the wiki article was very informative and worth a read, as was the main article on Lead Acid batteries if a bit long-winded.
But what really caught my eye as being novel was a reference showing that the lead-acid battery depends on Einstein’s relativistic effects to produce most of the volts.
I must also make a note to change it from motorbike mode (used for my Sealey pressure washer) and car mode !
It shouldn’t have made a massive difference as the Mx-5 is relatively low capacity, but I think it slowed down too early. Another 90 minutes and all was well in the world.