efficient SLA battery charger

[tab a]

I'm going to build a charger--or chargers--for various 6, 12, and 24v SLA's I have, in order to prevent the crummy chargers that came with these tools from destroying the batteries.

I have been using a 7805, 7812, 7815, or 317 for this, but reading past threads leads me to believe this is not terribly efficient. Someone recommended a switching regulator, and I looked at the 2576. The adjustable version shows an efficiency of 77% but perhaps that's due to using 12v in 5v out parameters?

Is a switching regulator the way to go, and are there better options than the 2576 I should be looking at?

And, would it make more sense to build a separate charger for each battery voltage, or just one charger with a switch and different plugs for each? I.e., is it theoretically better to have an input voltage close to each output voltage?

Thanks!

 

[k7elp60]

I'm going to build a charger--or chargers--for various 6, 12, and 24v SLA's I have, in order to prevent the crummy chargers that came with these tools from destroying the batteries.

I have been using a 7805, 7812, 7815, or 317 for this, but reading past threads leads me to believe this is not terribly efficient. Someone recommended a switching regulator, and I looked at the 2576. The adjustable version shows an efficiency of 77% but perhaps that's due to using 12v in 5v out parameters?

Is a switching regulator the way to go, and are there better options than the 2576 I should be looking at?

And, would it make more sense to build a separate charger for each battery voltage, or just one charger with a switch and different plugs for each? I.e., is it theoretically better to have an input voltage close to each output voltage?

Thanks!

What kind of charging current do you want? and what method of charging do you want(float type or cyclic)?
One thing to consider with switching regulators is that they generally have a minimum load current. This may be a problem as SLA batteries get near full charge the charge current decreases.

 

[Diver300]

The LM2576 is a good start.

The advantage is that you can get a good efficiency with a range of output voltages. For a first regulator I suggest you stick to a buck regulator. That is one that always drops the voltage. That means that you need a bigger voltage than your maximum output voltage, so you need a supply of about 30 - 40 V.

That shouldn't be a problem as you won't be loosing efficiency badly when supplying the lower voltages. Just use a 24V transformer and full wave rectify.

The LM2576 comes in different voltages. It is simplest to first look at the LM2576-ADJ. It has a feedback input that limitss the regulator as the feedback input exceeds 1.23 V. You provide two resistors to divide whatever output voltage you want to 1.23 V. When you have done that, the regulator will limit as your output voltage gets to what you want.

The LM2576-3.3, -5, -12 are versions where National have put the two resistors inside the package for popular voltages. You are best to use the -ADJ version as the feedback current is much lower in that version, making your choice of resistors easier.

You can simply switch between resistors to pick your output voltage. You have two resistors, one between ground and feedback, and one between feedback and the output. It is best to switch the one between ground and feedback, because if your switch doesn't connect correctly, the output voltage will be too low and you won't charge the battery.

If you switch the other resistor, and your switch doesn't connect, you fry your battery.

 

[Diver300]

One thing to consider with switching regulators is that they generally have a minimum load current. This may be a problem as SLA batteries get near full charge the charge current decreases.

The LM2576 is fine at no load.

 

[blueroomelectronics]

Buying a decent charger would be cheaper and better than most DIY solutions.

 

[Boncuk]

A free sample of an UC3906 (TI) is the best solution.

 

[carmusic]

there a lot of better chip than lm2576 that has low efficiency, check out national.com and their desing switchers

 

[tab a]

Buying a decent charger would be cheaper and better than most DIY solutions.

I don't think I've ever seen charger that will do either 6, 12, or 24v. Do you have a source for a 6/12v charger that's cheap and good?

It was the 2576-adj I'd been thinking about using. Thanks for the tips on it.

I'd not been to the national.com design site. Thanks for the heads up.

< A free sample of an UC3906 (TI) is the best solution.>

Interesting. I have 3 different 24v chargers from electric mowers. The only one that works correctly has a 3906 chip.

 

[Endolith]

A free sample of an UC3906 (TI) is the best solution.

From everything I've read, it seems like this is the only solution. Are there any alternatives? Nothing else seems tailored for lead-acid batteries, but it's a little expensive for mass production compared to a simple solution like the constant-current --> constant-voltage L200 circuit that's always floating around. Are the extra benefits of the UC3906 really worth the extra cost? Is there a way to build the same thing with discrete parts?