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A battery of batteries... good idea or bad?


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Hello!

I'm working on another portable stereo project and am bouncing a few ideas around about the power supply.

The stereo I've been using for the past year or so is powered by twelve NiMH D cells (which power two 20 watt amps.) This is is fine, but it makes it big, heavy, and is a pain to recharge. Due to the particular design, there's a certain amount of dissasembly required to get the batteries out to recharge them. On top of that, my charger only holds four batteries at a time, so a full recharge takes three shifts. All in all, it's not so bad, because on a full charge, the stereo lasts about two weeks.

So this summer it's time to build a newer, smaller, lighter, sleeker stereo.

My question is this:

Would it be feasible to use a battery of twelve NiMH AA cells that never needs to be removed from the stereo? My intent would be to build my own charger (researching that now) that could charge the ~14 V pack when plugged into a jack on the stereo's casing. Theoretically speaking, is it okay to recharge the battery pack as a whole? Could this fry or otherwise damage the AA cells by not charging them individually?

I would be using similar amps and speakers, so obviously the stereo wouldn't last as long, but it would make up for it because it would be so much easier to recharge, I could just plug it in at night and it would be ready for the next day.

My other thought was perhaps a lithium ion battery from the battery story, but I think I would still need to build the charger, which is new territory for me.

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What's the impedance of the speakers?

They need to be 4R for an output of 20W, if they're 8R, the theoretical maximum output with 14.4V will be 12.96W which will be 10W maximum in reality.

Are they real D cells or are they AA or C cells with extra padding - many cheap D cells are.

What's the capacity?

They should be at least 8500mAh, if they're 4500mAh they're really C cells and if they're 2500mAh thery're AA cells.

The problem with charging NiMH batteries in series is it's difficult to sense when they're charged. If you've researched NiMH battery chargers you'll probably know that they sense full charge when there's a fall in voltage. The trouble is, if they're connected in series this will happen when the first battery is charged and the drop will be smaller, relative to the battery voltage so will be more difficult to detect.

There should also be a timer to terminate the charge if the peak voltage detection fails and a temperature sensor to stop the charge if the cells get too hot, to prevent a fire.

Li-ion cells are slightly easier to charge but if you get it wrong there's a risk of the batteries catching fire. Fortunately all Li-ion cells have to come with a protection circuit built-in to minimise the risk of this happening. Charge at half the Ah rating until the voltage reaches 4.1V per cell, monitor the current and stop the charge when the current drops below C/20. If the battery voltage is under 3V the current should be reduced to C/10.

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Maxim have battery charger ICs for charging Ni-MH cells in series.
Each cell is 1.4V to 1.5V when fully charged so the charger must be able to produce 12 x 1.5V= 18V. The battery voltage will average 1.25V per cell when discharging which is 15V total.
You must detect and disconnect the battery when its voltage drops to 12V.

With a 15V supply a single-ended amplifier (TDA2003) produces only 2.8W into 8 ohms or 5W into 4 ohms.
With a 15V supply a bridged amplifier (TDA7240A) produces 10.2W into 8 ohms or 16W into 4 ohms.

How can you get 20W per channel? By turning up the volume control too high so that the output is horrible-sounding square-waves.

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