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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.

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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.

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  • \$\begingroup\$ How do I calculate the right resistance value? \$\endgroup\$ – Boloar Mar 18 '16 at 9:23
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    \$\begingroup\$ By using Ohm's law ! Example: lowest battery voltage is 11 V then you'd get 13.6 V - 11 V = 2.6 V across the resistor when charging starts. Let's charge with a current of C/10, C is the capacity of the battery (it will be printed on that battery). For example C = 7 Ah, then we charge with Icharge = 7 / 10 = 0.7 A. Now use Ohm's law: R = V / I = 2.6 / 0.7 = 3.7 Ohms. Note that it will dissipate 2.6 V x 0.7 A = 1.8 W so a power resistor is needed ! 3.7 ohm is not a standard value but 3.9 ohm is. So use a 3.9 ohm, 2 Watt resistor. \$\endgroup\$ – Bimpelrekkie Mar 18 '16 at 9:30
  • \$\begingroup\$ Lol, I needlessly oversimplified myself. I meant, and should have said, what voltage/current values specifically do I need to use for calculating the resistor. Which you were kind enough to work out for me, so that saves time hahah. I do know my Ohm's law but I always end up using the wrong values. Thank you ^_^ \$\endgroup\$ – Boloar Mar 18 '16 at 9:50
  • \$\begingroup\$ I have a few old SMPS lying around, so say I use a 12V 1A hard drive power supply - will I really need the resistor? Since it won't be able to supply more than 1A \$\endgroup\$ – Boloar Mar 18 '16 at 9:59
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    \$\begingroup\$ Yes, I would still use a resistor. The supply is rated for 1 A meaning, it will work fine if you draw 1 A max. It is possible that it can deliver 1.5 A and will deliver it but that it will get very hot for example. Or fail after a short time. You can not / should never rely on a supply to be designed well enough to handle such an overload condition properly. So simply do not overload it, use the resistor and avoid problems :-) \$\endgroup\$ – Bimpelrekkie Mar 18 '16 at 10:33
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Lead acid battery has a floating voltage of 13.8V. With such voltage the battery will have 100% capacity and no risk to explode. Fast chargers do charge with a little more voltage (approx 15V) and then they switch to floating phase 13.8V when the battery is full. Therefore I suggestyou to serach a SMPS with output voltage 13.8V and continous overcurrent protection - if the battery is empty then the SMPS at begining will not reach 13.8V due to overload (overcurrent), so it will be chrging with constant currrent and then it will switch automaticly to floating mode when the battery will be almost full and maintain this voltage.

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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.

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