First: battery size. It appears battery sizes are BCI (Battery Council International) standard numbers, though there's no logic to them. Mine had an "075" (Halfords HCB075). The numbers identify the layout and type of terminals, and the overall dimensions. According to several web sites, my V12 should take a much larger battery (an 096), but it's a very early Facelift car, and my guess is Jaguar were clearing the parts bin of the old pre 1988 "HE" battery trays. Whatever, an 096 wouldn't fit, much as I'd prefer it. According to Halfords web site, later Facelifts get an 019 size, which is getting really big.
Second: battery capacity. On cars with dynamos and those dreadful trembler regulators, at low engine speeds there's not enough juice, so the battery powers the ignition and lights whenever you're stuck at the lights or in traffic, hence the need to supply lots of Amps for an Hour or two, hence the Ah ratings. On cars with modern electrics and alternators, the old Ampere Hour ratings are largely irrelevant - if the engine's running, the alternator will provide enough. What matters is starting the beast. The V12 takes 750-1000 A to turn it over, and the battery needs to provide at least 7.2V at that current to power the ignition system. (FWIW, that implies it takes something around 5.4-7.2 kW, or over 7 horsepower just to turn the beast over!). The rating that matters here is Cold Cranking Amps (CCA), which is the max current a battery can deliver when cold, whilst still providing 7.2 V. So really a V12 wants a CCA 750, but the biggest "075" battery I can find claims to be CCA 620, so in theory it may have a battle in cold weather. I use 10W40 semi-synthetic oil, which should help, and haven't had a problem, but I tend not to drive when it's -18C! (For comparison, the comparable 096 battery claims CCA of 660. Halfords sell a Bosch 019 battery rated at 830A, which is much more like it should be.)
There's another useful rating, the modern equivalent of Ampere Hours, the Reserve Capacity, which is the number of minutes the battery can support a 25 A drain, and is roughly how long you have to reach the garage when the alternator drive belt snaps! My old Halford battery would have given me nearly 2 hours, which is nice!
Third, make. The old battery was, as mentioned, a Halfords "Calcium" HCB075. In fact the case says it was made by YUASA, a Japanese company that tool over virtually all the UK lead-acid battery makers and I believe invented the Calcium maintenance-free batteries. (The older batteries alloyed the lead with Antimony to add mechanical strength, but it compromised battery life.) It was actually 10 years old, and even though I don't use the car much, it's not surprising the battery was dead! A replacement from Halfords was about £100, which seems steep, though a few test reports on the web rate them highly.
I eventually settled on a Ducelier battery from a web supplier, for £47-50 including delivery. A very good price, CCA of 620, sealed maintenance-free with silver/calcium technology, and a known name. At least that's what it claims; in fact, now I've checked (after buying
), the known name was Ducellier S.A. (two l's) and their trademark lapsed in the 80's; I can't find out anything about Ducelier (one l) apart from the fact they seem to have supplied a number of companies with virtually identical web sites. It does have a 3 year guarantee, whereas the Halfords one had 4 years and Bosch "Silver" batteries have 5 years. I suspect the Ducelier won't last all that long, but at half the price, it might still be a good buy.Overall, next time the battery dies, I may see if I can get a battery box for a later Facelift XJS and splash out on the Bosch 019 battery from Halfords.
