Converting computer speakers to portable, battery powered iPod speakers.

[Pir8 Pablo]

I have a set of stereo computer speakers that run off a 9v power supply, I'd like to replace this power supply with a bank of 8 1.2v cells (9.6v) that I could recharge with the original (9v 94mA) power supply. First of all, would I need to regulate the voltage down to 9v, or will 9.6v be fine on a simple amplifier circuit? Secondly, would I need any circuitry to allow the 9v supply to charge 9.6v worth of cells? Would some kind of auto cutoff circuitry be necessary to prevent overcharging of the cells, or would common sense suffice by not leaving it charging for long periods suffice?

Sorry if I seem to have asked lot of questions. It would be splendid if you could help me with all of them but please don't feel obliged to answer them all and be discouraged from posting. Any help is appreciated.

Thanks,

Pablo.

 

[giftiger_wunsch]

First of all, would I need to regulate the voltage down to 9v, or will 9.6v be fine on a simple amplifier circuit?

Should be fine. Though you may need to regulate the supply to avoid fluctuations in volume; I'm sure another user will know more about this than I do and will know if that will cause a significant effect or not.

Secondly, would I need any circuitry to allow the 9v supply to charge 9.6v worth of cells?

You need a higher voltage supply than the battery to be able to charge the cells. If the supply is 9V and the battery is 9.6V, then the voltage is higher at the battery so you can't possibly transport charge to the battery (i.e. have a positive current).

It depends on the type of battery you're using; I'm not an expert on charging batteries but if they're NiMH or NiCd you could simply trickle charge them; but if you want it to be done quickly or efficiently and to optimise the battery life, you should use an IC designed to charge that type of battery.


I'm using a very similar battery (8 x 1.2V NiMH cells, 2.8AH capacity per cell) to power a microprocessor development kit I needed a portable power supply for, though personally I opted to just use a commercial charger and remove them to charge them. My charger can charge four of these batteries in under 2 hours.

 

[audioguru]

I have never seen a power supply for computer speakers that is only 9V at 94mA which is a power of only 0.85W. Then the amplifiers heat with a total of 0.425W and each speaker (stereo) gets only 0.213W each max. Almost as low as headphones.

My computer speakers have a 12V/1000mA power supply so the speakers can be heard.

 

[giftiger_wunsch]

True, that is surprisingly low, I never noticed that. Perhaps he missed a 0 and it's actually 940mA? My speakers have a 9V / 500mA power supply and are perfectly loud though. I'd get a lot of complaints from neighbours if I played my music at max volume, anyway.

 

[Pir8 Pablo]

You are correct in saying that I read it wrong off the supply, that was the INPUT, Output is 9v at 1A, Many apoliogies. I found a power supply with variable output which goes to 12v at 300mA, would this suffice?

Also, the battery I have is 9.6v 2200mAh Ni-MH. Its connector is one I've never encountered before - like the one marked 'L' or 'H' here. Anyone know what this connector is and where I could buy a female version of it?

Thanks again,

Pablo.

 

[Pir8 Pablo]

Actually, 94mA was the input, sorry, it's actually 1A, so no worrys there.

I've decided I shalln't use that power supply to charge it, but the adjustable one I have that goes to 12v. My last qualm is about the connector on the battery, I have never seen one similar before. It's like type 'L' or 'H' here. At the very end of it, it's 6mm wide. Anyone know what this is?

Thanks,

Pablo.

 

[audioguru]

A 9.6V ni-MH battery is 12V when fully charged. The charger needs maybe 3V additional so a 15V supply must feed the charger.
The 300mA max allowed current from the variable 12V supply is barely enough to charge the 2200mAh battery overnight and does not have enough current for amplified speakers.

 

[giftiger_wunsch]

A 9.6V ni-MH battery is 12V when fully charged

How does that make sense? Surely a 9.6V NiMH battery is 9.6V when fully charged...


Edit: I just checked my 9.6V NiMH battery (it's pretty new and hasn't been used yet, so it should be fully-charged); it reads 10.4V. I have often checked "1.2V NiMH cells" and found they're at 1.3V, so the 800mV difference in a battery comprised of 8 1.2V NiMH cells makes sense to me.

 

[audioguru]

A 9.6V Ni-MH battery has eight 1.2V cells. Each cell averages 1.2V when discharging.
The voltage reaches 1.5V per cell when charging.

I charged some cells yesterday and now they measure 1.38V each so eight would measure 11.04V.

Here is a charging curve for a Ni-MH cell:

 

[giftiger_wunsch]

Ah I see, thanks for the clarification AG.