On Sun, 02 May 2021 15:38:28 +0100, Dave Plowman (News) wrote:

In article ,
newshound wrote:
For most people, car engines don't run for much of the time. They do
need to top up the starter drain reliably, it's not really a float
charge regime.

Quite. A car is unlikely to run continuously for more than a few hours.
Given the driver will want a comfort break of some sort, or indeed fuel.

A float charger could be running for weeks on end.

How about months on end? As I mentioned in a previous post, car
batteries are no substitute for the SLAs specified for UPSes. I learnt
this the hard way a few years back when the first set (luckily only a 60
quid investment on a set of four 36AH NOS SLIs bought from my local car
parts/breaker supplier) failed after just 6 or 7 months of being kept at
a float charge voltage of 13.8v.

Rather stupidly, I chanced yet another 60 quid's worth with pretty much
the same result before the lesson was finally learnt. :-(

I had Ass U Med that the much gentler operating conditions in UPS
service would extend the service life of SLI class batteries. Oh, just
how wrong can one be? Well, in this case, VERY wrong!

To use an anthropomorphic analogy, SLI lead acid batteries could best be
described as having a masochistic character. They positively thrive under
the sadomasochistic regime of automotive use where they're not only
subjected to a few hundred amps of abuse from the starter motor load but
also suffer the various acceleration forces throughout a typical journey
which they crave in order to keep the electrolyte nicely stirred up to
keep electrolyte stratification at bay, all of which is absent in UPS
service.

It turns out that my UPS was killing them with an excess of 'kindness'.
Who'd have guessed? Well, more to the point, who'd have guessed that car
batteries had been SO optimised for SLI service as to make them so
incredibly unsuited to the gentler routine of UPS service? Well, I don't
have to guess any more - I now _KNOW_ (and it has only cost me a mere 120
quid for that knowledge).

I did briefly toy with the idea of using yet a third set of SLIs but on
a reduced to 13.5v float charge setting but, considering the other need
to avoid stratification of the electrolyte, decided against running this
experiment.

I've since come to realise that LFP batteries are a far better fit for
this task than the SLA battery technology in current universal use with
UPSes. However, I'm still considering the best way to make such a long
term investment in something that'll outlast not only the UPS itself but
also my remaining lifetime.

--
Johnny B Good