In article ,
Barrett wrote:
Finally got the batteries charged, discharged and charged again. I have
managed to fully charge them now.
How long should they last for with the MFJ-259 left on continuously being
used?
The MFJ 269 manual says that it draws a maximum of 150 mA on HF and
VHF, and max 250 mA on UHF. Given the similarity of the instruments
I'd guess that the 259's current drain is similar to that of the 269
on HF/VHF.
This would suggest a useful run-time of around 12 hours for
fully-charged 2000 mAh cells, and perhaps 16 hours for the
larger-capacity 2500 mAh types. You'll get more time if the MFJ goes
into power-saving mode.
Is it normal to only get 10 - 15 minutes worth of on time before getting the
low battery warning on the MFJ?
If you're getting a flashing low-battery warning after only 10-15
minutes of use with freshly-charged cells - no, that's not good. It
would suggest to me that either one (or more) of the cells in the
battery stack is defective (or not fully charged) or that the MFJ's
low-battery detector is misadjusted.
Don't mistake the flashing low-battery warning, with the "display goes
mostly blank" sleep mode. The latter would be expected after some
minutes of non-use, while the former should not appear for hours.
NiCd and NiMH cells have a rather flat discharge characteristic - the
voltage remains almost the same across a broad part of the discharge
curve (from 90% charged to 10% charged). By the time that the voltage
starts to drop off, there's very little charge left in the cell, and
you have little warning.
Well-designed devices which were designed to run on NiCd batteries
will often have a low-voltage shutoff circuit, which deliberately
turns off the device once the voltage hits the "almost drained" knee
point of the curve (nominally 1.0 volts per cell or so). This is done
to avoid "over-discharging" some cells in the battery (continuing to
pull current through them after they're completely exhausted, but
while other cells in the battery still have some charge left).
Overdischarging can permanently damage a NiCd or NiMH cell.
I suggest that you try this: fully charge the cells. Put 'em in the
MFJ. Turn it on. Let it run until the low-battery warning appears.
Then, with the MFJ still turned on, use a digital voltmeter to
carefully measure the voltage appearing across each of the cells in
the battery, and across the full battery stack.
Under these conditions (fresh charge, little discharging, and a low
current drain) the stack ought to be reading at least 12 volts, and
probably more than that.
If you find a "freshly-charged" cell that is not reading at least 1.2
volts under these conditions, it's probably bad.
Also the MFJ-259 is new. What is the best ways to check it to make sure its
working properly?
Test a variety of known-value loads (e.g. 25, 50, and 100-ohm
resistors) and confirm that the MFJ's readings are not unreasonably
wrong.
Cut a length of coax of a known type, short the far end, perform the
"velocity of propagation" tests shown in the manual. Typically a coax
with a solid polyethylene dielectric will have a VOP of around 0.66,
while a foamed-poly dielectric cable will calculate out at around 0.8
or so.
--
Dave Platt AE6EO
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