Can I use any wall wart to charge a 12v SLA battery?

[gellfex]

I'm looking to charge a 12v 8ah SLA battery on the cheap. Can I just use any 12v dc "adapter" as long as it's amps is less than 10% of the battery, like a 500ma converter? Dos it make a difference whether it's a switching or copper wound transformer? I don't care how long it takes, it's for a fishfinder on my kayak to charge it for the next weekend. What about those HF "float chargers"? Will they eventually get an 8ah charged?

I've got a milk crate full of these adapters of every voltage saved up over the years, they've got to be good for something!

[Ohio Mike]

Your typical wall wart is a power supply not a charger so it doesn't have any regulation. It won't know when to stop charging, and will cook a battery.

[loply]

Your typical wall wart is a power supply not a charger so it doesn't have any regulation. It won't know when to stop charging, and will cook a battery.

So long as the wall wart only puts out around 10% of the batterys capacity (ie 2ah battery = 200ma) then the battery will be fine. It will take 12 hours+ to charge though.

The bigger problem is that the rating on the wall wart may not mean it gives a constant current of whatever the rating is, it's possible that's simply it's rated maximum and you may be able to draw more and overheat the thing. You'd need to check & monitor it with a couple of multimeters to make sure what you're dealing with.

[tmarks11]

If your wall wort puts out exactly 12V, it will not charge your battery above 24% of full capacity.

A battery charger puts out 13.5-15V (depending upon charging current, ranging from C/40 to C/10 where C= amp-hour rating of battery). The open circuit voltage on a fully charged "12V" lead acid battery is 12.6V. The open circuit voltage of a battery at 50% of capacity is 12.3V.

wrt overcharging your battery: it is very unlikely that the wall wort will overcharge the battery, even with a small 8 A-hr battery like you are talking about (assuming you are getting >12V out of the wall wort to begin with). Personally, I wouldn't leave the wall wort connected more then 16 hrs without checking open circuit voltage of the battery. This is assuming you are talking about a typical 500 mA wall wort.

[CCWKen]

Put in or wire it to your battery backup for your PC. That will charge it. Most of those use 12v 7-8Ah batteries.

[gellfex]

Put in or wire it to your battery backup for your PC. That will charge it. Most of those use 12v 7-8Ah batteries.

Actually, I pulled the battery from the FIOS box on the wall behind my desk when I bounced back to Comcast last month. I suppose I could just pop it in and plug in the box to charge. Funny how they don't want these boxes returned, they say they think of them as a permanent utility installation. I think they figure it'll lower the friction to returning to the fold.

Thanks to the rest of you for interesting comments. Sounds like the worst it could do is fail as long as I use a low amp unit. Like I said I've got a crate of these things, I might have one in 14-16v.

To digress even further, I've been mining the box looking for a 9v 1A dc to power my brother USB labeler. It drains the internal 6 AAA's like crazy even with little use. But what's bizarre about the whole wall wart universe is the utter lack of standardization. You can find the barrel plugs in any size for any voltage and in any polarity. The labeler has the outer barrel + contrary to most. They should all be shot. Makes the Micro USB phone standard seem like a miracle. Thanks EU.

[J Tiers]

You CAN indeed, but it is best to use a unit of more than 12V, and put a resistor in series. I am rejuvenating some dead and sulfated SLA AGM batteries now with a supply of about 20V peak, with a 15 ohm resistor in series.

They gradually come up, until they reach around 12V, and then they start legitimately charging. Works pretty well, and the series resistor can be chosen to not overload the wall wart even if shorted out totally in the battery.

I'd put something in series in terms of resistance, just to control currents. Wall warts have a habit of opening up if overloaded, and never working again.

[darryl]

Most of the little wall warts have enough internal resistance that your choice becomes one of voltage. With an open circuit voltage of 14-15 volts, you could hook it up direct and never have a problem. What I would do is first find one with 14 volts or more on the output, then hook it up to a battery through a 1 ohm resistor. Measure the voltage across the resistor, and that number will be the charging current. Check the battery voltage from time to time, and when it reaches about 13 volts or so, read the voltage across the resistor again. If it's more than about half a volt (corresponding to half an amp, or 500 ma) then it's probably too much for a continuous charge unattended. If it's under 200 ma, you may never get a full charge into the battery, but that will be better than having it self-discharge.

A full charge into a lead acid battery normally takes 14 volts or more, but you would not want to maintain that voltage as a semi-permanent charge. I've read that 13.2 volts is the maximum level for a permanent charge, and 13.6 volts is the voltage you bring it to on a one-shot basis to top it up. You might do this once every couple of weeks or so.

An option might be to couple a wall wart with a solar panel charge controller. As long as you feed it more than about 16 volts it will automatically keep the battery charged appropriately- provided the controller is half decent. I've seen these for about $20, and as low as $7 on sale. No need for the solar panel, but you could add one if you wanted to.

[Black_Moons]

No.

Buy a proper float charger or expect to deal with such potential hazards as: Sulfuric acid mist, Hydrogen, Fire, Explosion, Corrosion to any nearby electronics/metal structures, etc.

And make sure the float charger does not exceed 13.8v at 20c. (Shouldn't be any lower then 13.6v when the battery reaches full charge)

[J Tiers]

Ooch, ah.... better you don't mess with that dangerous electricity stuff... that can kill you, yuh know.......... call a professional.

Well we have had the safety nazi input, can we get back to reality?

This IS a tiny battery, SLA, almost certainly AGM.

No.

Buy a proper float charger or expect to deal with such potential hazards as: Sulfuric acid mist, Hydrogen, Fire, Explosion, Corrosion to any nearby electronics/metal structures, etc.

And make sure the float charger does not exceed 13.8v at 20c. (Shouldn't be any lower then 13.6v when the battery reaches full charge)

[Mike Nash]

Just a little personal experience. I have a Radio Shack Wall Wart Battery Charger rated 1 amp. It will (and has) charged regular automobile batteries to 15+ volts not once but twice. That's two different batteries. If I had left that on for more than overnight it would have killed the batteries over time.

I have some Harbor Freight battery chargers rated higher amps, but they properly maintain a battery rather than cooking it out. I won't leave them on long term either though because they ARE Harbor Freight.

[J Tiers]

The general rule for a FLOODED CELL lead acid is that you can charge it pretty much indefinitely at a C/30 or less rate. So if it is 100 AH, that would be less than 3A. Better if a bit lower, but generally OK there.

AGM are not as tough, and so they need to be treated better. Still, an 8AH battery should tolerate a charge rate of a few tens of milliamps pretty much forever.

If you pay some attention to the charging, you can use nearly any source that won't produce too much charge current, which one may define for general safe usage with an unknown battery as about C/10 for flooded lead acid, and C/20 for AGM. If you know better about the specific battery, then you can go with the spec.

[PStechPaul]

I think it is a valid suggestion for the OP to use a charger made for the purpose. Here is one rated 1250 mA that is less than $5 including shipping (from China, of course). I have used one that is very similar (probably an identical "Chinese copy"):

http://www.ebay.com/itm/12V-1300mA-Sealed-Lead-Acid-Rechargeable-Battery-Charger-For-Car-Motor-Truck-hg-/281521705804?pt=LH_DefaultDomain_0&hash=item418c00534c

It may be a trivial project for someone who knows what they are doing, to select the right wall-wart and add current limiting and monitor the current and voltage. But for someone who needs to ask about such details, there may be significant risk.

[Black_Moons]

I think it is a valid suggestion for the OP to use a charger made for the purpose. Here is one rated 1250 mA that is less than $5 including shipping (from China, of course). I have used one that is very similar (probably an identical "Chinese copy"):

http://www.ebay.com/itm/12V-1300mA-Sealed-Lead-Acid-Rechargeable-Battery-Charger-For-Car-Motor-Truck-hg-/281521705804?pt=LH_DefaultDomain_0&hash=item418c00534c

It may be a trivial project for someone who knows what they are doing, to select the right wall-wart and add current limiting and monitor the current and voltage. But for someone who needs to ask about such details, there may be significant risk.

Exactly. If you have to ask, Just spend the $10 to get it ready made by someone who already did their homework. (Or at least, copied someone who did)

[CarlByrns]

Ooch, ah.... better you don't mess with that dangerous electricity stuff... that can kill you, yuh know.......... call a professional.

Well we have had the safety nazi input, can we get back to reality?

This IS a tiny battery, SLA, almost certainly AGM.

I have an older Coleman electric lantern that uses a small 12 volt SLA battery. It burst from overcharging. No mushroom cloud, but the gel made quite a mess.