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1-7 of 7 messages
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Charging SLA Batteries
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by KI4CUW on September 24, 2008
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More of a portable question, haven't frequented these forums but hope it's about the right place to post.. Seemed most appropriate to me anyway...
I've read several different sources and wanted to summerize my understanding of charging SLA's to make sure I've got it all right, being a poor man and wanting the best life and quality.
I passed up buying a 1.5A 12V 2stage charger at wal-mart thinking I'd too quickly charge my PS-1270 7.0AH battery I got at work yesterday for free (They have to be pulled from service after 5 years but the thing is in like-new condition according to a smart battery tester).
Now my understanding is the battery will draw as much current as it needs from the charger, depending on how much it needs. "Bulk charge at 14.4-14.7v until current drops to 70mA." Looking now, the datasheet states limit inital current to 2.1A.. But let's say I do have a charger that's too quick, say 6A. What can be connected between the two to limit it? My basic electronics skills are just that -- basic. R=E/I, 14.4/2, so 7.3Ohms of resistance in series with the battery, but that resistor needs to be rated for P=IE, 2*14.5, 29W? ... Not very plausible, do I have other options?
I suppose just float charging it at 13.5-13.8 would be save with a cheap regulated supply, but one source said best battery condition is kept with over-charging being part of it's charging routine.
Thanks for putting up with a newbie. :)
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RE: Charging SLA Batteries
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by K0BG on September 24, 2008
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One major problem is, there's way too much misinformation on the net about charging SLAs, AGM, etc. If you want to bone up, go here first: http://www.batterycouncil.org/
Exide and Optima also have good information on their web sites.
Alan, KØBG
www.k0bg.com
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RE: Charging SLA Batteries
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by AA4PB on September 24, 2008
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A lot depends on how quickly you want to charge the battery and how far you let it discharge. According to the PowerSonic specs, to obtain the fastest safe charge you need to limit both the voltage and the current during the bulk charge process. The current must not exceed 2.1A and the voltage must not exceed 14.4-14.7V. When the bulk charge drops to <70mA then you need to limit the voltage to 13.5-13.8V where it can be left indefinitely on float charge.
To accomplish this requires a rather sophisticated charging circuit. The easiest way is to use one of the specialized ICs designed for charging SLA batteries (or purchase a 3-state charger with the appropriate bulk charge rate).
Now if you don't let the battery discharge too far (below 10V) and you don't mind taking longer to charge you can use a resistor to limit the current and a regulated 13.8VDC power supply. The regulated supply limits the voltage to the maximum float charge value and the resistor limits the maximum current to 2.1A when the battery voltage is down to 10V.
The maximum voltage drop across the resistor is 13.8 - 10 or 3.8V. R=E/I so 3.8/2.1 = 1.8 Ohms (use the closest larger size that you can get). P=I*E so 2.1 x 3.8 = 8 watts (use a 10W resistor or a parallel combination of smaller wattage resistors).
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RE: Charging SLA Batteries
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by K5LXP on September 25, 2008
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There is a saying that most batteries don't die, they're murdered. A properly maintained lead acid battery is good for at least 5 years and many hundreds of cycles, an improperly maintained one can be junk in just a few cycles. To coin a battery engineer I met, "you can't uncook a cooked egg". Once you've vented some electrolyte, incurred gas bubbles on the plates or reversed a cell, the game is over.
For about a buck from Mouser you can get a dedicated regulator that is made specifically for recharging and floating gel cells.
Manufacturer Part #: PB137ACV
Mouser Part #: 511-PB137ACV
Manufacturer: STMicroelectronics
Description: Voltage Regulators 13.7V 1.5A Positive
Powersonic has good documentation on their website.
Mark K5LXP
Albuquerque, NM
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RE: Charging SLA Batteries
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by K0BG on September 25, 2008
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Don't you know Mark, I almost said... until the battery expert arrives! You did!
I guess the only question I should ask is, why is there so much misinformation on the web about batteries and charging, alongside the really correct data?
Alan, KØBG
www.k0bg.com
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RE: Charging SLA Batteries
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by KG6WOU on September 26, 2008
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How much power?
I have been testing charging options using an amp meter, a 13.8 regulated 3 amp power supply AND a small 600 ma/200 ma regulated gell-cell charger.
Now, I'm going to note that I'm talking about 12 amp hours and above and charging rates under 3 amps....
Once you top a battery to float at 13.8 amps, I change to the gell-cell specific charger and the amount of 'extra' power above and beyond the 13.8 is almost negligable.
IOW, I don't think the three-stage system yields much more storage, but I have no idea of potential impact on the battery chemistry.
There are simple things:
1. Don't overcharge either rate or voltage
2. Don't over-discharge [under 10.5 volts = end of battery life]
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RE: Charging SLA Batteries
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by AD5TD on October 5, 2008
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I have been using a Walmart "Smartcharger" (less than $20) to keep my 255 ah SLA battery charged for more than 3 years now. It stays hooked up 24/7. The charger puts out enough power to actually run a transceiver in receive mode. I have had no problems what so ever.
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