Dan Dugan wrote:
> Zach, you wrote,
>
>
>>Given the versatality of the MP-2 power supply stage, I was thinking I
>>would like to power mine with a 6v lantern battery, but I was wondering i=
f
>>anyone would happen to know what the mA rating on one of those is. Has
>>anyone used a lantern battery to power one? Does anyone know the minimum =
mA
>>rating I can use for a 6V feed? The site says 50mA for a 12V, so I don't
>>know how that would equate to a lower voltage.
>
>
> You'll find Sound Devices very helpful with any question you have.
> Ask Jon Tatooles
>
> The MP-2 is rated to work down to 5V; using a 6V battery is awfully
> close to that. You'll be out of power when the battery is only partly
> drained. It would be better to power it from a higher-voltage source
> like 12V-worth of AA batteries. You'd get longer life and less weight.
This is somewhat dependent on the type of battery. NIMH will still be at
or above their rated voltage at 80% discharged. Meanwhile alkalines
would be well under a volt at the same discharge level. In fact
alkalines pass the rated cell voltage of a NIMH at about 40-50%
discharged for the sort of discharge rates we are dealing with.
So, using a alkaline rated at 6 volts it will reach 5 volts at around
60-70% discharged. A 5 cell NIMH pac would reach 5 volts at about 80%
discharged. The actual amount depends on the discharge current, it takes
a fairly heavy load to pull them to 1.2 volts before 80%.
When you couple the slower discharge voltage drop of the NIMH with it's
higher capacity the alkalines look much worse. And definitely, if going
with alkalines stay well away from the voltage floor in selecting your
number of cells/starting voltage.
There is somewhat of a difference in how ratings are done for Alkalines
vs NIMH. The 1.5 volts on the label of a alkaline is the voltage of a
fresh, unused cell. The 1.2 volts on the NIMH is the voltage when 80%
discharged, in other words it's a expected minimum voltage at the load
levels we are talking about. A NIMH starts at 1.3 - 1.4 volts.
Note that you should not be pushing a NIMH past that 80% mark for
maximum life of the cell.
Note that a carbon-zinc lantern battery would probably be even worse
than the alkaline, though I did not check this out.
The only other issue is that if using NIMH, because their voltage is not
dropping much, the power indicator light on the MP2 will be of little
use. When it warns you it's often only a minute or two until shutdown.
So you will have to figure out other ways to track how used up the
batteries are.
It would be nice if manufacturers worked on this little problem. It's
the same with the HHb Portadisc running on NIMH. When it drops off the
99% indicator for the battery you have 15-20 minutes at most left. I try
never to run lower than the 90% mark. The MP2 could give you similar
warning if it was set to turn when you reached 1.2 volts. Of course
alkalines would reach that awfully early. Maybe 1.2 goes yellow, then
red comes on at the alkaline cutoff.
Walt
________________________________________________________________________
________________________________________________________________________
|