I have had the batteries on charge in the MFJ-259Z for over a day now. The
jumper inside the unit is on for charging. The batteries were at 12.6V when
put on charge and are still 12.6V. The PSU is working fine. Because I can
run the unit off the PSU that came with it.

Any ideas on what can be wrong with it now?

It certainly sounds to me as if the charging circuit is not working,
or is not working effectively.

12.6 volts is 1.26 volts per cell, which is what I'd expect to see in
the no-load or light-load case of a NiMH with a fractional charge
(maybe half-charged?). The cell voltage will rise, as charging
continues, to as high as 1.4 volts or more (depending on charging
current).

It could be that the charging circuit simply isn't working at all.

It's also possible that the charging circuit is only delivering (by
error or design) just enough of a trickle charge to prevent an
exernally-charged NiMH battery from self-discharging. If the
battery is only receiving 10-20 mA, this would be the case - the
battery will essentially *never* recharge itself completely under
these conditions.

There's a real tradeoff with NiMH cells. Any simple charger which is
capable of recharging them in a reasonable amount of time (say, a day
or less) should *not* be left hooked up to them indefinitely - it'll
overcharge them and significantly shorten their life. Conversely, a
simple charger which *can* be left hooked up for days at a time, will
not recharge a dead battery rapidly enough to be useful.

Charger circuits *can* switch between two charging modes (fast and
maintenance) but it requires quite a bit more complexity.

So... what to do now. My suggestions:

[1] First, measure the voltage across each individual cell in the
battery. If they're all around 1.25 or so, then the cells are
probably OK, and the problem is in the charging.

If you find that most of the cells have a higher voltage, and one
is very low, then that one cell is junk and should be recycled and
replaced.

[2] Get a digital multimeter with small alligator-clip leads. Set it
on its "DC current" scale, remove the charging jumper in the MFJ,
and connect the two leads of the multimeter to the two pins that
had been jumpered together (stick a bit of paper between them to
prevent the multimeter leads from shorting). Plug the MFJ into
its wallwart (with the power switch turned off) and read the
current on your DMM. This will tell you how much current is
flowing into the battery through the charging circuit (assuming
that it's a simple linear charger and not a pulse-charger).

If the current consistently reads zero, then the charger is not
working at all.

If it's charging at a rate of below 50 mA, then it's just a
maintenance trickle-charger... sufficient to keep charged
batteries alive in standby mode, but insufficient to recharge them
in any reasonable amount of time. You'll need to do the major
charging using an external charger.

If it's charging at 100-200 mA, then it'll take a day or more to
fully recharge.

If it's charging at over 200 mA, then it'll recharge effectively
in under a day, but should *not* be left hooked up indefinitely...
it'll overcharge the batteries and shorten their service life.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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