Item: 25W amplifier/speaker with SLA batteries.
I've done a little bit of modding in there. I doubt the warranty is valid any more, lol.
In case you don't want to read the link about my modding, basically, I just added a wire to the power switch of the amp, sending 12V from the battery to a LM2577 DC/DC boost converter set at 12.5V, which powers my mixer. So far, no explosions or short circuits since July 2015.
So here's my deal. Mains voltage is 240V (supposedly, but it's always lower). There's a transformer inside which is outputting almost exactly 12V, and this 12VAC is rectified and fed directly to the SLA battery. There doesn't seem to be any sort of regulating circuit that I can see, at all.
The SLA battery is two 6V 4.5Ah in series. To the best of my (admittedly poor) knowledge, lead acid batteries are supposed to be pretty robust. But that's not the point.
I want to save a tiny bit of weight in the device, and increase its efficiency a little, by taking out the 240V-12V transformer, and replacing it with a 12V SMPS, of which I have several lying around (old external HDD power supplies).
Would you advise against this? I'll have to say I'm not totally up to speed on battery management so please do educate me on the pros and cons here. I am also aware that SMPS can induce interference in the audio circuits. Is there a way around that?
Later on, I'll also look into replacing the battery with something lighter if it's possible, but that's a future concern. After all, the major amount of the weight is the speaker itself.
TL;DR: I have a transformer directly charging a 12V SLA battery for an amplifier/speaker. I want to replace the transformer with a SMPS.
Best Answer
To fully charge a 12 V Lead-Acid battery you need more than 12 V, you actually need about 13.6 V. So you will not manage with the 12V power brick (SMPS) alone.
You could increase that 12 V a little by adding a boost converter set to 13.6 V. Do add a series resistor though to limit the charge current when the battery has a low voltage.
Also check that the power ratings are OK, that you do not load the power brick more than it can handle.
I would not worry too much about the SMPS interfering with the audio as most SMPSes will switch at a frequency outside the audio range. But if needed you could add decoupling capacitors on the supply lines and / or a ferrite core around the wires. But I doubt that is needed here.
Regarding charge management, that's the resistor I mentioned. If you make sure the highest voltage you charge from is 13.6 V via a resistor, the battery will be fine. Lead Acid batteries are far more robust and less picky than Lithium based batteries for example.