# Charging SLA battery with wall warts?

#### majhi

Joined Jul 2, 2014
55
Hello all. I have a couple questions regarding how to charge a 12V SLA battery.

1) I have read elsewhere that using a 12V wall wart can damage the battery. Why is that?
2) In order to more closely approach the cyclic charge of 14.5-14.9V of a battery, can I use a 14V laptop/TV/monitor wall transformer? I can't really afford a proper SLA charger.

#### ericgibbs

Joined Jan 29, 2010
11,159
hi majhi,
The wall wart 12Vdc is most likely 12V when the current load is equal to the current specified on the transformer, as the load falls the output voltage will rise.
The same applies to a Laptop psu, some can be as high as 19Vdc when lightly loaded.
E

#### majhi

Joined Jul 2, 2014
55
hi majhi,
The wall wart 12Vdc is most likely 12V when the current load is equal to the current specified on the transformer, as the load falls the output voltage will rise.
The same applies to a Laptop psu, some can be as high as 19Vdc when lightly loaded.
E
Thanks for the quick response! So if I'm understanding correctly, a 14V transformer would be less safe than a 12V one because of the voltage rise?
On that note, on the battery site I'm looking at, the chargers they have are only 12V as well. So what would be the problem with using my existing 12V 2A wall wart?

#### ericgibbs

Joined Jan 29, 2010
11,159
hi,
OK, plug in your wall wart, do not have a load/battery connected and then measure the voltage output from the wall wart, using Voltmeter, post what you measure.
E

#### majhi

Joined Jul 2, 2014
55
hi,
OK, plug in your wall wart, do not have a load/battery connected and then measure the voltage output from the wall wart, using Voltmeter, post what you measure.
E
12.2V

#### ericgibbs

Joined Jan 29, 2010
11,159
hi,
In that case it will never fully charge a 12V SLA battery.!
You need approx 13.8V or 14.2V in order to fully charge a 12V SLA. depending upon the cycling required.

E

#### Just Another Sparky

Joined Dec 8, 2019
62
Old school battery chargers worked by applying a relatively constant 13.3-13.6 volts to the battery at all times (line voltage and load current dependent). You knew the battery was charged when the ammeter dropped to zero. Slower than the new chargers, which apply more voltage at the onset and all sorts of other jazz, but it's still a valid technique.

In fact I've switched back to using those old transformer types exclusively again. I don't trust the new computerized ones anymore: I caught two of them applying over 15 volts once I started wondering why I was constantly replacing batteries all the time. I could actually hear the acid bubbling quite vigorously inside the cells as it electrolyzed. Not to mention the old ones just seem to last forever; there's nothing inside them except a center-tapped transformer, a fuse and two diodes.

K.I.S.S. I guess.

#### dl324

Joined Mar 30, 2015
11,926
1) I have read elsewhere that using a 12V wall wart can damage the battery. Why is that?
It depends. I use a jump starter that has a SLA battery and charge it with the supplied 12VDC adapter (which probably puts out closer to 18V). The device has it's own charging circuitry, so hopefully, it charges the battery properly.

If the adapter has low current capability, it may float charge the battery, but that could also damage the battery. Lead acid need to be charged "hard" initially to minimize sulfation. If you only float charge the battery, sulfation will build up and kill the battery.
2) In order to more closely approach the cyclic charge of 14.5-14.9V of a battery, can I use a 14V laptop/TV/monitor wall transformer? I can't really afford a proper SLA charger.
Sounds like a case of being penny wise and pound foolish.

Can you afford to replace batteries that die prematurely because they weren't charged properly?

#### Audioguru again

Joined Oct 21, 2019
2,090
Some countries have poor voltage regulation of their electricity. The 12.2V measured today might become 10V or 15V tomorrow,
That is one reason that a battery charger is used instead of a wall wart.

#### marcf

Joined Dec 29, 2014
260
Hello:
You may want to consider a 'taper' charging scheme for your application. It has its disadvantages, but is very simple.
I have seen this used before, and the "R" used was a light bulb, such as a automotive headlamp. When the battery is charging the light is on, when the battery is charged, the light is off.

This document covers a lot of information on the charging of SLA batteries.

#### majhi

Joined Jul 2, 2014
55
Thanks for the input, everyone! You've been very helpful, I greatly appreciate it.

#### MrChips

Joined Oct 2, 2009
22,113
This is what happens when a commercial battery charger fails to turn off after the battery is fully charged.

Use an LM317 regulator with 12V-15V wall adapter to create a simple battery charger.
Check to make sure that the adapter outputs more than 15VDC.

#### crutschow

Joined Mar 14, 2008
25,686
Use an LM317 regulator
RS=0.2Ω will give a theoretical short-circuit current of 6.25A, which is well above the LM317's intrinsic short current of about 1.4A.

Below is the LTspice simulation:

I reduced the value of RS in the second simulation to give a faster charge.