Do you have an M-Audio Microtrack 24/96 that keeps eating batteries like mine? The last time I took the battery out, it was swollen to the point of exploding, so I think there's something wrong with the design of the charging circuit. Or, possibly mine has a problem. At any rate, I came up with a way to run the Microtrack on an external battery without any internal battery. The trick is to replace the battery with a low-profile super capacitor (1 farad):
The capacitor is mounted in the same location as the battery
with a piece of double-sided tape. Make sure to keep the leads close to the body
as there is barely enough room. Also, keep in mind that those super-capacitors
come in different heights and you want a flat one. You could also parallel
smaller values which tend to be thinner. I'm not sure of the minimum needed
capacitance, but 0.22 farad didn't work and 1 farad did.
(Update: I switched to four tiny 220uF capacitors and they work fine with an external USB power source. Maybe the internal resistance of the lower value super caps is too high.)
The capacitor will charge up, just like a battery, and the Microtrack will come to life within about a minute of being plugged into a USB port. To power the Microtrack in the field, build a battery pack and regulator that will supply 5 volts to the Microtrack via the USB cable. Such devices are commercially available and typically run on three or four AA cells. The Microtrack draws about 300 mA and needs a low impedance voltage source to start up. Although I didn't try it, it is conceivable that a simple battery pack of three AA or C cells will work, without any regulator.
(Update: I'm using a Tekkeon MP1550 with perfect results. Just replace the battery with a large electrolytic ( a total of 880 uF in my case) and plug in the Tekkeon to the USB port. I'm sure other portable USB supplies will work, too.)
The external battery is actually a relief. I can run the recorder on ordinary AA cells in a pinch, using the MP1550. The Microtrack draws about 200mA idling and about 300 mA when recording so the four AA cells will last for many hours.
Here's another great approach:
Notice the recorder is magically charging itself. On the back is a Duracell 1800mAh rechargeable Li-ion battery pack. It's held on by Velcro, so it's easy to switch out or it can be charged in place. The capacity is high enough to run the Microtrack for five or six hours and it would be easy to carry a couple of these packs. My recorder has the battery replace by a capacitor as described above but if you battery isn't completely shot you can probably just leave it in. This blob on the back is totally acceptable to me, and I can completely kiss battery problems goodbye. I pull one of the USB connectors out when not using the recorder. Another nice feature is the remaining charge indicator on the Duracell pack.
(I had to do a hard reset (power and delete buttons for a few seconds) at one point in my experimentation, probably due to all my fooling around with low voltages, so make sure to try that if you have any trouble.)
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