Sony says that the MZ-RH1 can record for 6 hours of WAV tracks, or 9 hours Hi-SP ATRAC
under ideal conditions (backlight off, for example).
http://www.minidisc.org/manuals/sony/Sony_MZ-RH1_user_manual.pdfsee page 16
I think you're safe with an estimate of 5 hours WAV or 8 hours Hi-SP when using a reasonably new, fully charged lithium "gumstick" battery.
Sony equips the MZ-RH1 with the LIP-4WM lithium ion battery. Other models are sold with a lower capacity NiMH battery. Figure an hour less record time if you use NiMH. You can use the LIP-4WM in these models to gain that extra hour of record time. Cost of the LIP-4WM is about $20 via eBay.
The least expensive Sony HI-MD recorder that has a mic input is the MZ-NH700. It's a discontinued model that is available on eBay for about $100 used, $150 new. The MZ-NH700 uses a single AA battery. Use a rechargeable NiMH or a one-use Lithium battery for longest record time.
All Hi-MD machines have a USB port. You can power the recorder for longer periods using an external USB battery pack. The Hi-MD machine will run on the USB battery only,
without an internal battery at all.
When planning your recording session, remember that a Hi-MD disc has a capacity of 94 minutes of WAV files, or 7 hours and 55 minutes of Hi-SP ATRAC files. You'll need to stop and change discs at that point. It only takes a few seconds to pop in a freshly charged internal battery at the same time.
Hi-LP, the ATRAC format with maximum compression, can squeeze 34 hours of tracks onto a single Hi-MD. The recording quality isn't great, but might be useful for some situations (surveillance?) In this case, you'll need a big external USB power source.
One rechargeable USB battery is the LENMAR PPU1700B. This battery provides 1700mAh at 5 volts. That's 5 times as much power as the Hi-MD's internal battery provides. It's
available for about $30 from several online sources. Here's one:
http://www.bybb.com/product_info.php?products_id=46792 An alternative is a case for AA batteries that has a voltage regulator to keep the power output around 5 volts, the USB standard. Here's one example, cost $4.
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2970You'll also need a cable to connnect the Hi-MD recorder to the power supply, $10 or less.
Flintstone