I would like to learn something about proper care of sealed lead acid (SLA) batteries. Is there any good authoritative science on how SLA batteries should be cared for? I am searching... I suppose battery makers/distributors would be one of the best sources of information about that?
Specifically... I wonder why instructions that come with SLA batteries so strongly emphasize charging the battery immediately after use, without specifying how much use. Are they concerned that the battery might eventually self discharge too deeply? Or does it degrade an SLA battery to rest it in any state but fully charged?
** If an SLA battery is left in a discharged condition (ie less than 2 volts per cell ) for a long enough, ie weeks or months, it will corrode internally and be ruined. Same goes for most lead-acid car batteries too.
All lead-acid batteries degrade with time. The degradation rate goes up with increasing temperature and/or decreasing charge state.
In other words: Keep them cool and fully charged.
Note that proper charge voltage is dependent on temperature, so if you keep the battery in the freezer, your charger must be able to adjust the voltage accordingly.
Is the degradation versus charge state rate linear? I guess not.
Does the act of charging an SLA battery also degrade it? I have always thought that charging a battery degrades it, but maybe that is a misinterpretation of the maximum number of charge cycles.
Anyway... Keeping it fully charged is impossible here since it has to be used. But of course degradation takes place over time, so I guess that is why they emphasize recharging the thing immediately after using it. And the deeper the discharge, the more important that is.
Thanks.
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> Note that proper charge voltage is dependent on temperature, so
> if you keep the battery in the freezer, your charger must be
> able to adjust the voltage accordingly.
Very long trickle charge will kill one. That sulfates the plates, and is difficult to recover from. Also leaving a small consumer on for the winter period is a great killer. Often someting like a radio, a voltmeter, a computer chip, etc. Best to remove one cable from the battery, to make sure.
Don't discharge the battery. Don't overcharge the battery.
That kinda impedes on your actual use of the battery, so, a more useful compromise would be:
Don't discharge the battery any deeper than you can help it, and charge as soon as possible afterwards.
While charging, don't exceed the maximum charge rate for your battery.
Use a float charger, and ensure the maximum float voltage is not higher than the maximum spec. This varies with temperature, so may be an issue if the environment changes widely.
As an example, we used 100A/hr or so SLAs in an application where they were deep discharged (over a week or so) as a matter of course. Perhaps left for a couple of days in that state, then charged afterwards.
We'd get end of life after about two years of constant abuse like this
- about 80% or less service capacity left before it was no longer acceptable in our application.
Which is not bad, considering two years is considered the service life of SLAs in more critical applications like UPSs and such, in an environment SLAs must love.
Not quite, at deep discharge states, the plates under go irreversible chemical reactions that prevent an ideal charge from taking place afterwards.
There's more that can go wrong too. Which is why I said "don't discharge the battery" not entirely tongue in cheek. :-)
As a "point" of reference, i have an old UPS by APC and replaced the batery only once so far in the appx 15 years of "use". Most of the time it just sits there - maybe on trickle charge, who knows. But on rare occasion when the power goes out and i am online, it keeps all running for a careful save and shutdown.
Just because you *can* do that, doesn't mean you *should* do that.
The recommendation is two years, to ensure the battery doesn't magically fail soon afterwards. In fact, in those conditions, they're probably almost guaranteed to last double or more of that.
Problem is, "almost" doesn't cut it in a mission critical application, thus the two year replacement recommendation.
This is why some UPSs have an automatic scheduled "test", where they shut down the server but leave it powered on, go to battery backup, ensure it lasts pre-determined amount of time and power up again. Otherwise you have no way of knowing a battery has failed.
I should qualify my quoted constraints assume normal domestic SLAs. Higher grade UPSs use higher grade batteries that can last many years longer before the recommended replacement.
And it is at least worth the occasional visual inspection of the batteries for obvious signs of swelling and distortion of the case.
UK replacement is annual test and replace on fail or 5 years old whichever happens sooner. Mine live in a fairly cool environment and would still hold a decent charge when retired. I have seen some 8 year old ex UPS cells that I took to the recycle centre in a bucket because I was unsure if the plastic cases would hold out. They were very obviously mangled shapes with big bulges but still not leaking.
Exactly. No point in having an emergency back up that doesn't actually work when you need it. Same applies to emergency lighting.
Even so it makes sense to check them annually. You don't really want a failed SLA spewing corrosive electrolyte over your UPS.
Unfortunately, that's not an effective enough test. Most fail at end of life looking like new. So a visual test alone is not enough.
Some of the large commercial scale units perform an internal battery test with a limited load, with the marketing blurb claiming it's good enough to detect a dud battery even while the unit is live and running, never having to go offline for the test.
We were so poor, we had to used other's disposed of batteries as our new ones.
We went so far as trying to recover open circuit SLA cells. We dumped
10-15+ volts into the things (single 2v cells!), till they took a charge, then they somehow magically worked. For a while anyway.
That's on good quality (albeit old) cells. I've never really had any luck reviving cheapies.
True. I've seen batteries go short, overheat, dry out and become basically useless. A real chore to get the buggers out because they had warped so much.
No-one noticed because the power had never gone out in that time (many years) till someone thought it would be a good idea to actually look at it...
I've chatted with and watched quite a few large (tens to hundreds of kVA) UPS maintenance techs.
the standard procedure seems to be a visual check, retighten all connectors, measure voltage across all batteries, then again with the UPS in test mode where there is a real load across the batteries.
They can usually spot a bad battery before it would cause problems.
larger UPSes have redundant strings of batteries so even with a dud you can still operate.
The telephone industry has been using vented lead acid batteries for decades; and has very precuise instructions for charging and then maintaining the 'maintenance' or float voltage. Also checking for discrepancies between individual cells.
Mostly telecomm. batteries are 'floated' so that they are sitting there neither charging or discharging ready for instant use if/when the power fails.
So while most auto and similar batteries will charge at anything up to a voltage of 2.33 volts per cell (that's 14 volts for a so called, six cell, 12 volt auto battery) and be completely discharged at just below 2.00 volts per cell ( 11.9 volts for auto) the telecommunications batteries are carefully floated at ether 2.15 volts or 2.17 volts per cell, dependent on which administration e.g. ATT or UK Telecomms. and whether the battery cells are Lead-Calcium or Lead Antimony construction.
As said; you can ruin a battery by overcharging it (have seen them boiled dry) and or leaving them in a discharged state for long periods of time. Heat is also a factor, under the hood of a motor vehicle in hot climate being one of the worst environments. In some parts of the Middle East auto batteries sometimes last only a couple of years; whereas in a cool climate with light use our Nissan pickup battery is already 8 years old and may not have to be replaced yet, even next winter!
UPS batteries are also floated; but when they are needed they are probably discharged aggressively for relatively short periods of time to operate inverters etc.
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