Originally Posted by ex0du5
Is there any cheap and efficient way of making a rechargeable CMOY. My problem is, running off of 9V can be especially costly (at 5$ per battery, cdn). I do have many NiMH 1.5V battery, but I'm not sure how many of these in series I would need to properly run my amp. I'm using the OPA2107 opamp with 32ohm phones. Also, my source is an iPod.
Are there any good writeups out there for this kind of solution, or does anyone have any suggestions?
There is a pretty easy trickle charger using a LM317, a resistor, and two diodes, that was done in the PINT and some homemade chargers. You should even be able to get parts at radio shack. Tomb's post in this thread
has a really nice and easy layout.
Most people have theirs set up for charging a pair of 9V NiMH batteries. I made one to charge AAA cells, I needed more battery life than a 9V has. Using different capacity battery means changing the resistor value.
From Tangent's PINT writeup
|This resistor sets the current used by the NiMH battery charging circuit.
The formula for computing the proper resistor value is R=1.25÷I, where I is the desired charging current. I recommend that you use the standard trickle charge rate: 0.1 times your battery's mAh rating. (The notation for that is "0.1C".) Since the default resistor value gives a charging current of about 17 mA, it's suitable for NiMHs of 170 mAh or higher.
Charging current is a trade-off between charge time and battery cycle life. You can choose to charge faster than 0.1C, but your battery will become useless after fewer charges than if you didn't abuse it so. If you charge much greater than 0.1C, you must use some sort of intelligent charger, which the PINT does not offer. Contrariwise, you can charge slower than 0.1C to get more cycle life from your battery, but I wouldn't go lower than 0.05C. Below that, the battery's self-discharge rate becomes too significant: the battery won't charge as fast as it "should", and it may actually fail to charge.
Since my AAA cells were 700mAh, the resistor value needed to be different than the one used for a 9V. I calculated the value I'd need like this: R = 1.25 ÷ ( Battery mAh rating ÷ 10 ). I then picked a resistor as close as I could.
If you take 8 of your batteries (I'm guessing they are probably AA sized since it is the most common) and connect then in series, you'll have about the equivalent of a 9V battery and it will last a lot longer. The only thing is, they take up a ton of room.