parallel connecting 24v 1500mah NIMH battery packs and how to charge
1) Is it safe to connect and discharge 10 packs in parallel?


Yes. However, make sure they have the same voltage before you connect them in parallel. Either charge/discharge to the same voltage, or connect them with a resistor to limit the current while they equalise, before connecting properly.

Given the high current capability of NiMH, it would be very wise to include a fuse in series with each battery. If one pack suffered a short in some of the cells, the other 9 could gang up on it to drive a fire-starting current through it.


  2) How is it possible to charge all 10 packs at the same time? (wall plug)


Apply a charging current from some source.


  3) Is it possible to use a 24V PMM solar panel


That's the nominal voltage of the batteries, you'll need more than 24v to drive current through them.


  4) Do i need a cutoff when voltage is getting low?


While individual NiMH cells don't get damaged even if abused down to 0v, a battery of series connected cells can reverse any single cell that discharges first, so sending it below 0v. This can happen even with an apparently adequate voltage on the battery pack. Unless you monitor the voltage of individual cells, you will need to stop discharging at a very conservative threshhold, at least 1 V per cell, to reduce the possibility of this sort of failure.

The other way to reduce this sort of failure is to balance the cells with a proper overcharge, 16 hours at 0.1C. This is why we only hear about cell balancing with LiPos, as it happens easily and automatically with lead and nickel chemistries with a simple overcharge, not possible with LiPo!