I'd suggest a decent smart charger, one that has a "desulphating" cycle, used every 3 months and allowed to run through it's sequence of cycles - CTEK is the obvious choice but cheaper smart chargers from Aldi/Lidl now seem to have those sort of mode cycles.

Unfortunately not an option for me, as I live in a flat, unless there's a type of unit that can be left by itself (i.e. it has its own battery pack and thus not connected to the mains) for a week and then taken inside to recharge and then put back under the shut bonnet.

I probably should've mentioned about my home in the original post. Apologies for the oversight.

Edited by Engineer Andy on 10/11/2020 at 14:23

My Caterham does at best 1500 miles a year, this years I have done just under 1000 miles. The last battery was on the car from spring 2009 to spring 2017, 8 years. The battery before that was on from spring 2005 to spring 2009, only 4 years. Both were quality batteries, an Fiamm and then an Exide (the Exide was a warranty replacement for the Fiamm which failed just days short of its 4 years warranty).

As far as I am concerned the difference was the alternator. From 1988 to 2010 I used Lucas alternators which were not the best and most reliable but the wiring loom on the car etc was terminated for these and when they failed (3 year life on average) at least they were cheap. But they only put out a max of 13.8 volts at all revs which is below the 14,2 to 14.4 of more modern alternators. So in 2010 when the alternator failed yet again I decided to make the change to a Japanese Denso alternator which are smaller, lighter and from the experiences of fellow kit car owners far more reliable. Bought a kit to suit my engine installation and the alternator did about 5 miles before screaming, took it back. They gave me a new one and that lasted 4 miles. Told me it was my fault, I told them I would be contacting Barclaycard, they refunded me there and then.

The next day I had the mobile tyre fitter form up the street for me 4 new tyres and was chatting to him about the alternators. He asked If I remembered the Charade GTTi he used to own and said he still had the alternator off it, minutes later it was in my hands FOC. Made a bracket and sourced a belt the correct length and all seemed well. The voltmeter said 14.4 volts when cold and about 14.1 to 14.2 when hot and over 10 years alter it still does. I donated some money to his kids toy fund.

Some time later I was chatting to a bloke at a race meeting about this and he told me a story about these fake Denso Alternators that had flooded the UK kit car market place. Made of cheese in China and to the trained eye obviously fakes due to the outdated manufacturers name and label style. Looks like I had 2.

During the winter months I take the battery off the car and keep it in the house (downstairs bathroom) and charge it every month or so using my faithful Gunsons smart charger from the 1980's. Leave it on auto at a low amperage for the best part of a day does the trick.

As a bit of insurance I bought another Denso alternator off a Daithatsu as a spare, still in the cupboard.

Since your current usage pattern seems fairly infrequent, you could perhaps disconnect a terminal between uses. This'll reduce any parasitic drain and wouldn't be incompatible with a solar charger if directly connected.

MUCH more speculatively, I've wondered if there would be any benefit in putting some capacitance between the battery terrminals. This could perhaps provide some of the initial current to the starter, allowing starting on a slightly tired battery.

Complete battery replacement is practical on a kickstart motorcycle but I THINK this'll require prohibitively pricy supercapacitors for a car, though I've seen Youtubery to the contrary.

www.youtube.com/watch?v=z3x_kYq3mHM

I've also, unfortunately, seen it alleged that the higher instantaneous charging load causes early stator failure

There were quite a lot of lighting capacitors on the dump last week. Probably don't have enough capacity to be useful, but if they are still there I should snag some and check them out.

Thanks for the reminder.

Lighting ballast capacitors. Look quite big but they seem to top out at around 50 microfarads, about the same as a start cap on an electric motor. This probably isn't big enough to be useful, even if I use a lot of them.

Dynamos and regulators. I remember peeling a bit of card off a fag packet and slipping it under the spring of a voltage regulator to try and extend the life of a tired, old battery. Think all it did was cook the battery.

and a USB charger for the fag lighter which has a voltage (and current display).

Sounds useful.

How/where does it sense the current?

I'd assume its measuring current through itself (to whatever is plugged into it) rather than alternator output, which would be more interesting but difficult to do.