Rik Gossard wrote:
Is there a way to extend NiMH units battery life?

Don't overcharge them, do let them run down but not below 1.1V per
cell, and correct any cells with reverse polarity promptly.

I have a dozen NiMH 3.6v (3x AAA) 800mAh battery packs
that don't seem to charging well just wondering if
I can do anything to kick a little more life into them.

I suspect they have reverse polarity or shorted cells. Reverse
polarity can be corrected by charging only the affected cell, but
charging its entire battery the usual way will never help such cells.
A shorted cell can be fixed temporarily by applying brief high-current
pulses directly to it to vaporize any conductive whiskers inside it
(charge a big capacitor to over 10V, connected it directly to the
cell. Repeat until the spark becomes less intense), but the whiskers
will grow back in hours or days, so it's better to replace shorted
cells.

I have a variable 24V DC power supply but doesn't have a varible
current control is there a way of charging them with this?

What voltage should I charge them at?

At more than 3.6V, but far more importantly, limit the current to
1/10H the mA-H rating, or in this case 80mA. With a 24V power supply
this can be accomplished by inserting a 250 ohm resistor in series.
It should have a power rating of at least 5W to prevent burns. A
completely dead battery will need about 14 hours to fully charge this
way.