SLA charging question...... ?

[Nicksan]

I am using a 12v output at 500 ma wall wart to charge a 12v 12ah SLA battery, does that sound like a good charger for the job?

Does anyone have any calculations so I can figure out how long the charging might take using different chargers?


Thanks.

 

[Hero999]

Please do not make duplicate posts or threads, they do not increase your chance of a helpful response but only reduce it, as people get confused and can't always see what others have suggested in the other thread(s).

If you have made this post or thread in error then please do not be offended. We get lots of people making duplicate threads under the misapprehension that it will increase their chance of a helpful response when in practice the reverse is true.

A 12V wallwart will do the job but it will take awhile.

Also if it's regulated and output never reaches 13.8V the the battery will never fully charge.

 

[Nicksan]

umm...... I only posted this once, in one sub-forum. If it got posted in another sub forum It wasnt by me.
Ive been posting on forums for almost 10 years.
The charger says it charges at 12v but its more like 11.5, should I switch to another charger that outputs slightly more that 12v?

 

[Hero999]

There were two posts made, both by you but this forum does that every now and then, it isn't always the poster's fault. I reported the other thread to the moderator and it has been deleted.

Yes, you need a minimum of 13.8V to fully charge a lead acid battery.

The best way is to use a LM317 regulator.

You could get 15V 1A mains adaptor then add an LM317 regulator on the output set to 13.8V.

 

[Nigel Goodwin]

Yep, deleted the duplicate.

Moderator.

 

[Nicksan]

Oh, Could I get away with simply using an automotive battery charger instead of rigging up a LM317? Im not sure how to hook that up, I guess it cant be that hard, is it kinda like using a LM7805? but with a meter or somthing to see the output voltage?

This battery is a SLA but not a auto battery, if that matters.

 

[Hero999]

Yes, an automotive battery charger would be fine.

 

[Nicksan]

Cool, thanks.

 

[Roff]

Yes, an automotive battery charger would be fine.

My car battery charger puts out 10A when the voltage is low. Might that not be a bit much for a 12AH battery?

 

[Nicksan]

I think this 12v, 500ma wall wart might be too much or defective, It was getting hot so I kept checking it and after a few hours the casing started to warp and almost come apart.... eek. Im not sure why that would be, it says "Floating Charger (To Recharge Sealed Lead Acid and Ni-Cd batteries)"

The battery did charge but I cant imagine much since it was on for only 4 hours or so total.

 

[ericgibbs]

Yes, an automotive battery charger would be fine.

hi Nick,
A car battery charger is NOT suitable for charging sealed lead acid SLA batteries.

The charge voltage for an SLA must be limited to 13.8V for float charging and 14V for cyclic charging.
Car battery chargers can have end point voltages over 15V, which would ruin your battery or worse, make it vent!.

There are a number of circuits already on the forum for SLA chargers.

An overnight charge rate of 1.5A at 13.8/14V will recharge a discharged [10.8V] , 12Ah battery, in good condition.

The charge current input is not a 100% conversion to Ahr, allow only about 60% conversion efficiency.
That means 12hrs at 1amp will not fully recharge a discharged 12Ahr battery, it will only be about 60% recharged.

 

[Hero999]

Thanks for that information, I didn't know that, I incorrectly thought that all lead acid batteries had maximum voltages of 14V/

 

[ericgibbs]

Thanks for that information, I didn't know that, I incorrectly thought that all lead acid batteries had maximum voltages of 14V/

hi hero,

The 'old' style car batteries could 'gas off' when over charged, usually when the charger voltage went upto 15 to 16Volts.

Problem with the sealed lead acid, its really a 'gel' and being sealed, if they 'gas' too much the internal pressure can blow their safety vents.

 

[Nicksan]

Thanks for the info.
No prob Hero, I didnt try the auto battery.
I will have to look for some of charging circuits here because this wall wart got really hot and if I left it plugged in any longer it would have probably melted.

I found these plans....
**broken link removed**

I am a bit confused though, why would the input voltage be 18V? Why not design the circuit so that it starts from the main?

 

[ericgibbs]

Thanks for the info.
No prob Hero, I didnt try the auto battery.
I will have to look for some of charging circuits here because this wall wart got really hot and if I left it plugged in any longer it would have probably melted.

I found these plans....
**broken link removed**

I am a bit confused though, why would the input voltage be 18V? Why not design the circuit so that it starts from the main?

hi Nick,
Sorry if its getting a little confusing.
If I was selecting a transformer for that 1.2A charger circuit, I would choose a 18Vac.
The LM317 regulator requires at least about a 3V difference between the input voltage and the output voltage.
As you require about 14V to charge the battery this means the LM317 needs about 17Vdc input.

The problem is with a lot of cheaper Wart psu's, is when they have to supply a small current their output voltage rises and when its a large current their output voltage falls.
This is often referred to as 'regulation'

Why not design the circuit so that it starts from the main?

If you mean direct connection to the mains, it would not be isolated and could be a electrical shock hazard.

 

[Nicksan]

Oh I see. What I meant about starting from the main is why make a circuit, then use a wall wart. Why not make the circuit using a transformer and whatever else is in the wall wart so you have a compete package. Maybe because its easier?

 

[audioguru]

The 18VAC transformer makes a peak voltage of 25V at its rated current, not only 18V. A 15VAC transformer should be used, it would give an input voltage to the LM317 of about 18VDC.

 

[ericgibbs]

hi Nick,
Rethinking it thru, as its for a SLA, I would agree with 'agu' a 15V tansformer would be a better choice,
it will reduce the heating of the LM317.
A 25VA rated transformer should be ok, for the initial 1.2A charge rate.

 

[mcs51mc]

UC 3906 : Better than the rest

Just finished my SLA charger using a UC3906 from TI.
Attached datasheet, app note and my design for a 6V 4Ah battery

Sorry but can't upload the datasheet
You'll have to search the TI site
While you're seaerching don't forget to order your free samples.
Thanks again TI for mine

Like I read somewhere: "This IC based charger is a few orders of magnitude better than voltage or current based ones."

So please reconsider...

 

[Nicksan]

Well, I would like to go ahead with the 1.2 amp circuit in the link but I just found a 12v 1.2 amp wall wart in my closet. Would this be ok for the 12v 12ah SLA I have? The other wall wart I was using was only 500ma and got pretty hot.
I may still go ahead and make that one but I need something temporarily until I can find time.