For a upcoming robotic station (universal onboard PIC Programming) I wanted to have 1 12V 7AH Sealed Lead Acid Batteries to provide a lot of power. The only problem is that I need a charger for it. I do not know what to search for on TEH google. I am looking for an automatic shut off battery charger. SLA's usually want to get charged up to 14V, then drop down immedately to 12 volts when they are in use. Does anybody want to help meh out?

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You can charge that battery with a fixed voltage between 13.5 and 13.8 and you can leave it connected to the battery forever and the battery won't overcharge.

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Oh ok cool! All I need now is a little circuit that will tell me when it is full! That helped a lot.

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This Charger was designed for a 1.2 A/H SLA battery, but with the formulas shown you can make it work for your 7A/H Battery. It Works very good.

For 7 A/H the ouput should be about 14.5 volts and the current should be about 700 ma.

You might want to use an LM350 in place of the LM317.
Its the Same Pinout, Just a higher current device.

http://www3.telus.net/chemelec/Proje...er/Charger.jpg

Gary
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thank you! does it have automatic shut off? It doesnt matter if it doesnt.

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If you like I can post a schematic of one I have designed and built that is set for the float charge that I mentioned(13.5 to 13.8V), plus has an indicator that senses the current and when the battery is charged it turns on a led. It has the following other features: A resetable fuse on the output if you happen to connect the charger to the battery with the wrong polarity, and if the power goes off during charging the battery will not be discharged by the charger. It is a little more complicated by works real well.

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It Doesn't shut off but it reduces to a very slow trickle charge.
Its perfectly OK for a battery in "Cyclic" useage.

If your battery is in "Stand-by" useage, Reduce the voltage to 13.6 Volts.

I have used this charger on Many batterys without any problems.

I Show a Center Tapped Transformer and 2 Diodes, But a Regular transformer and a diode bridge is perfectly OK.
Both create Full Wave Rectification.

Its somewhat difficult to Test for a Full Charge on SLA Batterys.

Easiest way is use both an Ampmeter and a Voltmeter.

At 14.5 volts and Near Zero Current flow, you have a full charge

OR In Standby, at 13.6 volts and near zero current flow.

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Really all I need is to get the positive input above the cutout of the regulator. Kelp could you post that schemmy? I will try both.

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Krumlink,

Here is the schematic for a PowerSonic 12v SLA battery charger that I drew up while repairing one. D4 is on during fast charge, and goes out when the charging current drops below about 100mA.

Ken

Attached Images

PowerSonicCharger.JPG (29.6 KB, 71 views)

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With the 18 volts Rectified, it will put 25 volts into the regulator. (A 15 Volt Transformer Might be Better)
Or I can see that 7812 running a bit hot.

Also, Not Sure What the Inital Charge current will be on this circuit?

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A dead LM7812K was the reason I was repairing it...maybe overheat?

Its been a while since I worked on it, but I think that was the measured voltage on the secondary. The initial charging current is determined by the difference between the charger's fast-charge voltage and the battery's terminal voltage, and the inherent over-current limiting of the LM7812K. So, I suppose the maximum would be something above 1A. The settings for the Fast and Float charge pots were what I could pull from battery manufacturing company specs...nothing from Power Sonics. But, these could be anything that works for you.

Ken

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I Believe the LM7812K is 1.5 Amps Max.

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Thanks for both schemmys, I will try them out.

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One more Q:

Could I charge it with around 1 amp? Say a range between 0.75 and 1.25 Amps? That is where most of my wall warts range is around.

Last question Promise

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I'm Sure you will have More Questions!
But No Problem.

A wall wart is NOT a good idea.

A typical 12 volt one, rated at 0.75 amps will actually try to charge the battery to a Much Higher Voltage.

Example: Some of the cheaper 12 volt adapters will put out as much as 20 Volts at lower Current levels.
Definately Overcharging the battery.

It is only rated at the 12 volts When Drawing 0.75 amps.

PS: I updated the Charger article and clesned up that old schematic.
http://www3.telus.net/chemelec/Proje...er/Charger.htm

Take care...Gary

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