# SLA battery charger

Joined May 27, 2013
5
Hello everyone.. I have been researching and buying electronic tools and supplies for about 9 months now.. and this will be my first real project

What I have are a lot of sla batteries used in my rack-mounted UPS devices. I need a nice method of charging them before they go bad. I also have new sla batteries which I want to use in portable led lattern projects

http://www.talkingelectronics.com/projects/BatteryCharger-12vSLA/BatteryCharger-12vSLA.html

Can someone take a look and see if this one is worth it? I have all the components but was rereading it tonight and saw the following which stopped me in my shoes (If this isn't a good design, have a suggestion for a better design/source?
The circuit is actually a half-wave rectifier. It only charges the battery on every half cycle. The plug pack doesn't like this as it leaves residual flux in the core of the transformer and causes it to overheat. But that's the only drawback with the circuit.

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#### #12

Joined Nov 30, 2010
18,076
Mixed emotions. If it tends to overheat the wall wart, remove one of the 1.8 ohm resistors to cut the current in half.

There are simpler ways to do this. I just used an LM317L chip to make a float voltage according to the label on the battery and watched it charge the battery until the charge current got down to .008 amps. It stays there until I need my lawn mower. Seems to work. and it's doing as much as your pictured charger does. It arrives at a float voltage.

You can add an LED circuit that glows bright when there is a lot of current and doesn't when the battery is full.

Which parts are important to you?

#### MrChips

Joined Oct 2, 2009
19,159
Same here. This is what I use for a battery charger:

#### wayneh

Joined Sep 9, 2010
16,102
I had to chuckle that the "only" problem is that the circuit overheats. That's like saying the only problem with your house is the missing roof.

Like #12 and Mr. Chips, I use a LM317 in a constant voltage configuration. Works great. Or I use a $5 trickle charger such as this one. (It's often on sale for half off.) Both work fine unless you need to charge a big, deeply discharged battery, which I don't think applies to you. In that case you need more current than these solutions can supply. The simple LM317 circuit doesn't limit current so can cause heating of the transformer or internal limitation by the LM317 itself. That float charger probably protects itself from over-current. #### #12 Joined Nov 30, 2010 18,076 Disagree. All 317 chips limit current at some level between .1 amp and about 2.2 amps (if I remember correctly). I intentionally used a 317L to limit current to 100ma because my lawn mower has a week to charge, so current isn't very important. And...MrChips showed how one resistor can be added to make any current limit you want, as long as it's lower than the design limit for the chip. #### Metalmann Joined Dec 8, 2012 692 I had to chuckle that the "only" problem is that the circuit overheats. That's like saying the only problem with your house is the missing roof. Like #12 and Mr. Chips, I use a LM317 in a constant voltage configuration. Works great. Or I use a$5 trickle charger such as this one. (It's often on sale for half off.)

Both work fine unless you need to charge a big, deeply discharged battery, which I don't think applies to you. In that case you need more current than these solutions can supply. The simple LM317 circuit doesn't limit current so can cause heating of the transformer or internal limitation by the LM317 itself. That float charger probably protects itself from over-current.
I also have that float charger, but I'm wondering if it can also be used for a 6 volt SLA? It is 6V @ 4.5aH.

#### wayneh

Joined Sep 9, 2010
16,102
Disagree. All 317 chips limit current at some level between .1 amp and about 2.2 amps (if I remember correctly).
I don't disagree but I should have been more clear. I merely meant that, with a simple LM317 voltage control circuit and no attention to current control whatsoever, you will be relying on the LM317 for current control. That can be fine as you noted, and the fact that the LM317 can protect itself has no doubt saved the bacon of scores of DIY battery chargers. But it could also lead to a problem if the LM317 is calling for enough current to damage the transformer, for instance 0.5A from a 0.1A transformer. As the datasheet shows in MrChips' post, the solution is simple enough.

#12

#### wayneh

Joined Sep 9, 2010
16,102
I also have that float charger, but I'm wondering if it can also be used for a 6 volt SLA? It is 6V @ 4.5aH.
Nope, sorry, it's specific to the voltages relevant to a 12V battery.

#### #12

Joined Nov 30, 2010
18,076
Agree. There are several parts that would need to be redesigned to make that a 6 volt charger.

#### Johann

Joined Nov 27, 2006
190
Agree. There are several parts that would need to be redesigned to make that a 6 volt charger.
Surely it can be modified to charge a 6V battery by changing the value of R2?

Joined May 27, 2013
5
Thanks everyone for your replies.. I have a bunch of lm317's from a planned bench powersupply (ended finding one cheap at mcmelectronics)

Ideally I would like to add a switch and perhaps a red and green LED(one on when powered and the other when charging)

Perhaps I will dissect the harbor freight option and go from there.

#### #12

Joined Nov 30, 2010
18,076
Here's an old drawing of how to attach indicators to some other circuit. Adapt as necessary.

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Joined May 27, 2013
5
Went to harbor freight.. that trickle charger would seem to only be half on my circuit(I may need to also charge used batteries for my led lantern projects down the road)

Thanks for the circuits #12.. I think I am headed in the right direction now

Joined May 27, 2013
5
With the TI circuit example, do the r1,r2, & r3 control the output to 12v? would this be determined by the input voltage less the 1.5v drop?

#### #12

Joined Nov 30, 2010
18,076
Use the formula Johann posted in #10. It only works if you have enough voltage to cover the requested output plus the drop across the regulator. Also, that's "Rs" in MrChips drawing. You really need the esteemed "datasheet". It will tell you ALL about it.

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