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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi,
Somewhere I read that there is an optimal charging current for AGM SLA batteries, which is around 0.25*C and that charging with lower than this current isn't good for the battery. Do you think this is true?
Another question goes to the "topping charge", which basically means the charger lets the battery charge to the final voltage (around 14.7V) but then it holds this voltage until the current falls below a predefined small level and then switches to the float charge (13.7V). Do you think this is good for the health and life of the battery?
Finally, what do you think is the best way to treat the batteries (electrically) to ensure the their longest life?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I've often charged SLA batteries by hooking them overnight to a bench power supply set to 2.4V/cell (14.4V for a 12v battery) and a current limit of something like C/8. It seemed to work OK: I've had several years of life from batteries treated like this.
In a standby application, I think they can be float charged at 13.8V constantly and last many years. The higher charging voltages are for cyclic applications.
Lead acid batteries hate deep discharge, so for a long life don't discharge them deeply and don't leave them in a discharged state any longer than necessary.
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
I don’t know about a lower limit for charging current, but perhaps anything above 0.25 of C they regard as fast charging, so regular fast charging would likely shorten the life of a Batt, and a don’t think you can get all the energy back in at the fast rate with out reducing it to a slow rate at the end anyhow. So it a figure they probably use to limit the max safe continuous current. A typical rate is 0.1 of C for 14 hours isn’t it?
Yes most good chargers use a two stage method to equalise all the cells, either for a set time, or voltage as you noted.
I’ve heard if you are cycling them, the less you are discharging them by each time is better. . As well as making sure they get a good extended charge every so often. Though deep discharging seems to be a technique to try to revive a battery with reduced capacity. If you’re storing them don’t forget about them, but charge them occasionally.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Each battery chemistry is quite different, warnings for one type usually do not transfer to another type without considerable modification, "do not eat" and "do not put in fire" are about the only ones valid for all chemistries.
Lead acid, whether sealed or not, doesn't really have the concept of fast charge.
Unsealed batteries where overcharge causes gassing, which you can fix by adding more water, don't have an end point, you can charge them with whatever current for how long you like.
With sealed batteries, you *must* avoid gassing, all of the restrictions are about getting a reasonable charge time while avoiding gassing. Their electrodes have been carefully sized so that oxygen-evolving plate reaches full charge before the other, and the oxygen released diffuses through the cell to the other electrode, where it recombines back to water. Obviously this is a very slow process, and cannot cope with fast evolution of oxygen, 0.01C is the absolute max current that can be supported in this mode. To charge an SLA with a constant current of this value will be safe, but very slow.
SLAs will have a final current this low if you limit the charging voltage to 2.3v per cell. This allows a much faster charge for a discharged battery. If you are running the battery cyclically, and you limit the charging time, you can use a higher terminal cell voltage of 2.4v (or even 2.5v if careful) with minimal impact on the life of the cell.
++ about deep discharges. Plan the battery size so that in normal use you use only 50% of the rated capacity. It will be cheaper to buy 2C's worth of cells and replace them every 500 cycles, than 1C's worth and replace them after a few 10s.
Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
I just use a Numax 12v 1 amp automatic charger with my lead acid battery's and leave it going over night. Usually by morning the light on the charger has gone green and it is fully charged.
Not sure on the method of charging it uses but I don't really care as long as it charges it.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Here are some Internet references about AGM SLA charging, that may be more trustworthy and/or detailed than anecdotes and summaries on a hobby forum. Note that minor differences between charging for standby power, vs. shallow or deep cycling service, may depend on the battery structure (intended application) as well as the actual application.
I like the comprehensive tutorials at Battery University:
Don't overlook OEM data from the battery manufacturers. This example took a while to find, because of merger and acquisition trail (Gates - Hawker - Enersys). It appears to be about an AGM-type product line.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Somebody asked about pulse charging earlier. It seems that smooth DC is best, for the same reasons as it is best for driving Peltiers: the average current does the work of charging, the RMS just heats things up.
battery university wrote ...
Ripple voltage imposed on the voltage of large stationary batteries also causes a problem. The voltage peak constitutes an overcharge, causing hydrogen evolution, while the valleys induce a brief discharge that creates a starved state that results in electrolyte depletion. Manufacturers typically limit the ripple to five percent, or 5A for a 100Ah battery.
Much has been said about pulse charging of lead acid batteries. There are apparent advantages in reducing sulfation; however, manufacturers and service technicians are divided on the benefits, and the results are inconclusive. If sulfation could be measured with accuracy and the pulses applied as a corrective service, then the remedy could be beneficial. Assumptions without knowing the underlying results can be harmful.
As always in the electronics industry, "best" is quite different to "good enough", and many chargers use unfiltered pulsating DC straight from a rectifier.
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SLA charging
