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Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
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This depends on the manufacturing technique used for the SLA. There are Gel-Cell, Starved-Electrolyte and AGM (Absorbed Glass Mat) types which all have different requirements in terms of their optimum charging technique.
It seems the AGM batteries are the best for our application, since the fibreglass-mat seperators not only give the cell very high peak current capabilities, but also offer good resistance to mechanical shock (the mats stop the plates from touching and shorting when bent under impacts), as well as not having any limitations on "charge inrush current", unlike other technologies.
Apparently Gel and Starved-Electrolyte batteries dont like having big gushes of current poured into them when they are nearly totally flat and are then connected to a big beefy power supply that can supply 10 amps+ of current.
The current inrush does something nasty to the electrolyte or the plates (I forget which), so a lot of "Gel-Cell" chargers have what they call a CC/CV charging technique, where the charger starts off operating in a constant-current (CC) mode, where the maximum current that the battery is allowed to absorb is limited, until the charge state builds, then the charger switches over to a CV (Constant Voltage) mode to finish it off.
Absorbed-Glass-Mat batteries supposedly have no such limitations due to the increased surface-area of their electrolyte-soaked mats (the same surface area that gives them a higher peak output current than gel-cells) allowing them to soak up current as fast as your charger can feed it to them. So it is acceptable to run an AGM battery on a simple high-power constant-voltage (regulated) charger with no current limiting circuitry.
The actual charging voltage (when in CV mode) can vary in steps as well with some chargers. Initially, 14.4volts can be used to speed up the bulk of the current getting into the cell, then some back it off to 14.0-14.2v after a bit as the battery approaches full, and finally, if the battery is going to be left on "float charge" (where its basically 90%+ full, but you want to keep it "topped up") then the float-voltage should not be higher than 13.8v.
As I have mentioned before, one of the most important things an SLA charger needs to have is a smooth regulated output, something that 90% of them do not have (because its expensive to do at high currents). An output filtering capacitor big enough to smooth a 10amp supply can cost $50 all by itself, and thats without regulators and so on. If your battery charger isnt "rectified, filtered and regulated" then I wouldnt think about using it to charge SLA batteries.
Theres an awful lot of hype, marketing and wank in the battery charger market with "intelligent pulse controlled double overhead burp charging zap algorithims that revive tired batteries" and stuff like that used to sell chargers to the uninformed public, but your best bet is to check out the battery manufacturers spec sheets on what they advise and ignore the wank from the charger merchants. The battery manufacturers want their batteries to have better life and capacity than the competitions, so you can bet if there was some advantage to be had by charging their batteries in some "special" way, they would be telling everyone about it.
have a look at
http://www.rcbatteryclinic.com/burp.html
or google for "burp charging" for examples of the sort of bull that is out there for the unwary.
I hope all that covered your question glen.. short answer. if its a gel-cell, charge it slow, if its an AGM you can fast-charge it. in both cases, use a quality regulated charger if you want maximum life from them.
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Sat Jul 31, 2004 12:52 pm |
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