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July 15, 2011 2:20 PM   Subscribe

I have a portable rechargeable amplifier that was custom built for me when I was in Peru, it no longer makes sounds when unplugged, although an LED lights up. When plugged in to the wall, it emits sounds.

I opened it up today and noticed the 12v 4.5ah battery was bulging, and I am pretty certain that is the culprit.

But, I suppose further details could help clarify the problem. I let a friend borrow it, and when it came back it would no longer hold a charge. Seeing as Peru uses 220v, I have a step up/step down convertor to allow me to use it in the states [I suppose here, I need to step-up the power].

Anyway, I've noticed that now the amp works when plugged directly into the wall without any transformer, that's probably not a good thing, but it can do it anyway.

I looked inside the amp and no capacitors look blown and the wiring looks fine, I think it's the battery, but maybe there could be another issue. And why would the LED still light up if the battery is bulging? Does it still hold a minute charge?
posted by cloeburner to Technology (9 answers total)
I just read that bulge can be caused by overcharging. When my friend was borrowing it I recall him having it continuously plugged in, could that have played a role in the problem?
posted by cloeburner at 2:30 PM on July 15, 2011

If your charging circuit isn't smart enough to stop, then, yes, that could have caused your problem.

Swapping out batteries should be relatively easy: Two blade connectors and whatever physically holds the battery in place. I'd bet money that if you replaced the battery you'd be good to go.
posted by straw at 2:33 PM on July 15, 2011

It takes very little power to light an LED. It sounds like your friend plugged it in here without the step up transformer and cooked the battery.
posted by cosmicbandito at 2:35 PM on July 15, 2011

scratch that, misread your question. Yes, battery got overcharged. Replace battery, good to go.
posted by cosmicbandito at 2:36 PM on July 15, 2011

If it's a sealed-lead-acid battery (gel cell, AGM, etc) then yes, overcharging or too-fast charging could kill it and make it bulge; it doesn't sound unreasonable that it'd still have enough capacity to light an LED. Replace the battery (they have a finite lifespan anyway) and resume portable rocking-out.
posted by hattifattener at 2:39 PM on July 15, 2011

It does seem like the battery can be replaced and you're back to normal.

I would like to suggest some simple modifications: A better charging mechanism. What you want is a smarter charger, as indicated by straw; Then you want to add a 'switched mode' power supply that runs off any wall power tension and frequency in the world.

I would guess that there is a charge management circuit board and then an external power supply brick of some sort. If that is the case, then you could probably swap out the board to get better protection of the battery - and then you can swap out the external supply brick for one that can take 110 to 240V. These would be off-the-shelf parts, selected for compatibility with the battery chemistry.
posted by krilli at 2:43 PM on July 15, 2011

The battery is sealed acid, thanks for the tips everybody. I will attempt to track down a battery tomorrow.

Also, my convertor wall wart has a switch to select between 115v and 220v, the only problem is I've messed with that switch now and I cannot remember which setting it should be at. Should I have the switch at 220 for the appliance or 115 for the wall voltage? It's a JWin, if that's any help.
posted by cloeburner at 2:53 PM on July 15, 2011

Oh, and I would love to try my hand at converting the entire amp to accept 110v, any idea what would that entail? Swapping out the transformer?
posted by cloeburner at 2:53 PM on July 15, 2011

I don't know for sure what's in there ... but :)

Probably! Probably you want to rip out the transformer and anything between the transformer and the battery - i.e. the charging circuit. Then I'm not sure what's best.

Either add one of these smart charger bricks: Smart chargers for lead-acid batteries at Batteryspace.com. One from the 12V category. Looks like either the 0.8A or 1.0A charging rate types fit to your 4.5Ah battery, according to the 4th column. I guess I'd take one of the 1.0A ones :) This one is going to take 110 to 240V, turn it into a nice battery-charging level, deliver this kindly to your battery, and stop when it's done. That's it.

Or you add one of these charge controllers: Solar / DC Charge Controller (120W, 10A Rate ) for 12V SLA or LFP Battery. Then because this is just charge controller and not a charger, you need a separate generic power supply, i.e. one that is not dedicated to batteries. You could use this cheap supply. The supply supplies a steady 15V, and the charge controller downregulates and feeds what is appropriate for the battery at its particular charge level.

The charge-controller-plus-supply route is somewhat more expensive but protects against a wider range of hazards, like overcharge and overdischarge, short circuits and polarity reversal.

Fun! :)
posted by krilli at 3:46 PM on July 15, 2011

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