On Fri, 23 Apr 2021 19:00:40 +0100, Fredxx wrote:

On 23/04/2021 13:44, Dave Plowman (News) wrote:
In article ,
Fredxx wrote:
At 50mA that would be less than the self discharge rate of a lead acid
battery.

Not so. A lead acid in good condition has a very low self discharge
rate.

More than a 1,000 hours?

How about more than 8760 hours?

A flea market purchase of a 2nd hand 12AH SLA I'd purchased for a fiver
(after checking the voltage was at least above the 12v mark on a borrowed
DMM) some five or six years ago. It was only when I retested the open
circuit voltage with my own DMMs on my return from the flea market that I
saw it was actually just below the 12v mark. I guess the cheap DMM I'd
borrowed either wasn't blessed with a low battery indicator or else I'd
simply not spotted whatever low battery warning symbol it may have
possessed.

Even now, it still in good condition (12.87v rest voltage reading about
a week or two after charging it up to 13.8v with my bench supply.

At that time, I did not have an SLA capable charger (or variable bench
supply) to safely charge it so used a pair of 1.2Wpk solar panels hung
out of the office window to take advantage of the summer sunshine. I
checked the voltage daily until it just topped the 13.8v mark about ten
to 14 days later. Checking the resting voltage the next and subsequent
days showed a 12.85v reading which over the months, until the next summer
solar charging season came around, dropped to 12.75v.

It did get used as a test voltage source for brief periods, often just
to check it could still produce amps with very little sag using a 50W
halogen headlamp capsule bulb (I wasn't planning on using it as a starter
battery).

However, after two such summer charging seasons one winter's evening, I
used it to jump start a 1.6l automatic whose battery had let me down
outside of our local chippy. It was my own fault, I knew the battery was
on its last legs but had still waited, engine off, with the parking
lights on for SWMBI to return from the chippy just 10 or 15 minutes
later, in spite of the street lighting allowing me to switch the lights
off to avoid just such a situation. Being an automatic, bump starting
just wasn't an option

I was rather surprised that a 2nd hand 12AH SLA purchased 2 1/2 years
earlier and only ever charged up twice with a couple of 1.2Wpk solar
panels could crank a 1.6l petrol engine into life so easily as it did
some six months after it had last been charged up.

I learnt a valuable lesson that evening, namely to reduce the float
charging voltage from the 2.133v per cell typically set by default with
UPSes down to the less abusive 2.1v value if you want your very expensive
"consumable" to last more than just a lousy two or three years (assuming
few to no brief mains outages in that time and that the automated weekly
battery test feature is also disabled).



--
Johnny B Good