Ah, poverty, makes us very cunning.
I remember keeping big boxer batteries going as an Adult Apprentice in Mt Isa in the '80's.
My blue RS at the NT border, with a mate on his RS.

I had traded my R90S in on brand new RS, supposedly the last of the RS's, just before being offered my Adult Apprenticeship, hence the poverty.
The heat and vibration in western Queensland was hard on batteries, lasting about one and a half years on average. But I didn't always have the readies for a new one when needed.
But as a conciliation, the full resources of a large mining company helped.
To keep the dieing battery going I would do the following.
I measured the specific gravity of every cell with a hydrometer, usually finding one down on SG, I always assumed that this was due to rubbish from the plates shorting the bottom of the plates.
I would then empty the cells out, flushing them with fresh water, until all the rubbish was out.
I would then refill each cell with the same SG acid as before, this was necessary or the chemistry of the plates in the cells wouldn't match the acid SG.
I could get another six months, minimum, out of a battery easy using this method.
When I was a tradesman, I bought a Mareg BMW battery, at what seemed to be vast expense, with its two year warranty, I went for 7 years with this battery (or its replacements) as they would die before the warranty expired.
BMW didn't really care as it wasn't their warranty that was paying out.
They didn't always die in a convenient place though.
Dead Dekar battery behind the Queensland sunshine coast. last year, some things don't really change do they.
