Battery tender "recycle"

[rodster_67]

I lost 2 battery tenders in a recent thunderstorm. Pretty sure the logic got zapped as I can still get an output bypassing it.

My question is, can I still us the center tap transformer to make a 'dumb' tender and a few off the shelf parts? If so, suggestions?

There is a circuit that creates 13.65 VDC (normal output of a working unit in HOLD mode) which the logic somehow used to drive the SCR's. I tried applying it to gate of the SCRs and get a nice output but when connected to a battery, the output drifts high (15VDC+)

I checked a good unit and when the battery is charged and in HOLD mode the voltage is 13.65VDC and drawing about 100ma. Not sure if the unit limits the current or if that's all a fully charged battery will draw at that voltage??

Will probably replace them with new units but before I toss the old ones figured maybe I can salvage them. Maybe even a little bench 12v supply even though they only supply 2A.

Thanks for any suggestions.....

 

[ForceFed70]

If the transformer is still good, then yes - you could hook it up to a simple bridge rectifier and make a "dumb" charger out of it.

Don't think it's worth it - but you could do it.

 

[rodster_67]

I think the bridge rectifier would output 15VDC+ which seems a bit high for a battery tender otherwise I would consider it.

 

[justsam]

I would hang on to them.
Looks like a full wave rectifier with center tapped transformer.
Add a three terminal regulator if needed. They would make a good power source for undercounter LEDs.
Hard to find a linear supply today since most are now switching supplies. If low RFI is important these would work.

 

[rodster_67]

I agree, they should be useful for something. I wonder if I can find a fixed 12vdc three terminal regulator that would handle 2A?

 

[Cruzingoose]

As a motorcycle repair shop, (and electronics tech) I'm done with these charger/maintainers. The regulator circuit is in failure mode from the time you plug it in. I've given up on the number of pi$$ed of customers and melted batteries caused by this POS. IF you keep it, gut it, put a cheap 5 amp diode in it and use it for a simple UNREGULATED charger. Modified, hey make nice trickle chargers.

 

[Cruzingoose]

I agree, they should be useful for something. I wonder if I can find a fixed 12vdc three terminal regulator that would handle 2A?

You can use a LM317 adjustable or a 7815 with a diode in series with the output to bring the voltage down to 14 volts. You can't use the 12 volt (7812) version as the voltage is not high enough to charge the battery.

 

[dogdog]

UA723 with external transistors is a better regulator for anything under 40VDC. but why not just get that chip reprogrammed from factory?

 

[HOTFR8]

Let a good Auto Electrician have a look at them as they usually are easy to fix and if not toss them. I had one I had sold once come back as it had got wet with a Cat doing well you know on it. Out of warranty but was cheaper to fix than replace.

If you have to replace them pay a little more and go for something a little better.
I have all mine set up with surge protectors now.

 

[rodster_67]

You can use a LM317 adjustable or a 7815 with a diode in series with the output to bring the voltage down to 14 volts. You can't use the 12 volt (7812) version as the voltage is not high enough to charge the battery.

Yea duh, 12v is not enough, it was late when I posted that.

The LM317 looks interesting and I think I'll give it a try. I guess I need a bridge rectifier before the regulator? Any suggestions for an off the shelf piece?

Thanks for the suggestions!

UA723 with external transistors is a better regulator for anything under 40VDC. but why not just get that chip reprogrammed from factory?

Checked with the factory, no support for repair. Typical in our throw away society. (Which isn't always bad, I made $90 on a compressor my neighbor tossed out but only needed minor repairs!)

 

[gayler]

Let a good Auto Electrician have a look at them as they usually are easy to fix and if not toss them. I had one I had sold once come back as it had got wet with a Cat doing well you know on it. Out of warranty but was cheaper to fix than replace.

If you have to replace them pay a little more and go for something a little better.
I have all mine set up with surge protectors now.

Did the cat learn it's lesson?

 

[HOTFR8]

Did the cat learn it's lesson?

I do not know. The owner did and made sure the tender was protected.

 

[Cruzingoose]

UA723 with external transistors is a better regulator for anything under 40VDC. but why not just get that chip reprogrammed from factory?

The 723 is the most unreliable regulator out there. When it fails it always shorts providing the pass transistors with full and excessive base current blowing them and time placing full unregulated DC on the output cooking anything connected. If you want to see the reliability of the 723 in action, look at the circuit of an ASTRON or PYLE power supply. When they fail they will put 28-40 volts on the output connections.

What makes the 723 popular is the high input voltage at which it runs. It uses tiny filter capacitors (READ CHEAP). So 40 volts in, the series pass transistors (acting like resistors) reduce the voltage to 13.8 or so and get good and hot dooing it.



The 78xx series regulator only requires a few volts higher than its output, is simple to connect, needs NO external components, self regulating in case of overload AND failure mode results in NO OUTPUT. It is easy enough so that even the noobiest noob can wire it up. Only three wires, Ground, Input power and Output power. It is wired up like the capital letter T.

The left top is unregulated power
The bottom is ground
The right top is output.

 

[Cruzingoose]

Yea duh, 12v is not enough, it was late when I posted that.

The LM317 looks interesting and I think I'll give it a try. I guess I need a bridge rectifier before the regulator? Any suggestions for an off the shelf piece?

Thanks for the suggestions!

If you have a "Ratshack" or any electronics house available, or mail order like Jameco, (shipping will cost more the the parts), a 50 volt bridge with a rating over 3 amps will do fine. You could salvage the rectifier from the old board also. In this application a filter capacitor is not needed or wanted. you can PM me and I can email you diagram and construction details.

 

[rodster_67]

PM sent.

I've had more ideas about this project.

 

[dslabuda]

The LM317 is rated for 1.5amp. Wouldn't a LM350 work better being it's rated for 3amp?

Sent from my SAMSUNG-SM-N900A using Tapatalk 4

 

[rodster_67]

Yes it would if it's the same function just a higher current rating. Thanks for the info!

 

[cbennett]

I had his happen a few years ago, battery tender replaced them. I got lucky?