Tomb,
I think most people will find a 20 hour charge a bit slow. Plus, the charger is not 100% efficient (if it were there would be absolutely no heat on the battaries), so your charge time is more like 25 hours. A 50 ohm resistor would give you 0.1C, which is what I assumed you were doing.
If you read the link I supplied above, you might come to the conclusion that .05C is too slow to properly charge the batteries. On the other hand, they are primarily talking about AA and AAA battereis, not 9V batteries, which are usually recomended to be charged at 0.1C. There's a lot of voodoo here- even more than the voodoo surrounding audio caps.
I do use a Tread for my recharging. The primary reason I have a problem with the 2 diode design is that there is no way to ascertain that the Tread is plugged in. The LED will be lit either way and will give you the "wrong answer" if you want to make sure the charger is powered up. I think that will lead to problems and confusion. I don't know about you, but I tend to have rats nests of ac wires and it's easy to have something accidentally unplugged. With a wall wart, I guess that is a moot point.
My portable Pimeta does not have a separate onboard charger LED and I am always uncertain if the thing is actually charging. Some of my DC-DC cables use Kobiconn locking plugs, but some of my Treads do not have the matching locking jacks. Although the Kobiconn locking and non-locking jacks and plugws are compatible with each other, the barrel is a bit short on the locking versions and it is very easy to lose power and not know it because the plug has to be absoutely fully inserted to make the connection. When I built my Pint I added a 2nd LED that only comes on if the wall supply is live, giving me positive verification that the thing is really charging.
If your 9V batteries are rated at 250ma then I suspect you have 7 cell batteries with a nominal voltage of 8.4V. If you use an 8 cell battery, then 24V is a bit low and you might not get a full charge. This is an important point for others and anyone using a 24V regulated supply with your charger circuit needs to consider this. The standard formula is 1.55V per cell plus the Vdrop across the regulator, plus any diodes between the wall supply and the battery. For 14 cells, as you probably use, that works out to 23.7V, allowing for a 2V drop across the LM317. Your regulator, wall supply and batteries put you right on the edge, with 0.3V of overhead.
Fixing my problem by adding a diode between the regulator and the jack could cause problems here. On the other hand, my problem is a bigger issue with a Tread or Steps and in that case it's easy to jack up the voltage a bit to compensate. I use 25.6V with my Pimeta, but I use a resistor instead of the LM317. If I used an LM317 I would have to bump that up to 27.5V to allow for the reg and my 3rd diode.
Although I like using an adjustable Tread to tweak the voltage, and that is a requirement, more or less, for resistor based charging, I didn't know that I could get a regulated wart for $14. Aside from the voltage issues, that is a much more portable solution, especially for travel.
It's amazing how complicated this can be!
