Electric Bikes - Do NiCads normally explode like this?

I've just blown up my second set of NiCads in about a year. Is there something I'm doing wrong?

Also, I now have two packs that I can't use. Can someone put the good cells together to make one good pack?

Exploding batteries are not a good thing. Too much heat and overcharging are two main culprits.

Also possible shorting

When do they explode? Charging or discharging? Yes, in many cases it's possible to repair a bad cell in a series string.

The only NiCads (r/c) I have had self destruct have been from dodgy chargers that did not shut of correctly.

You might want to investigate cell balancing .... we just charge ours at a very low rate for a long time to balance all the cells ... no problems , and only once in a while do you need to do this ...
Monitering your charges with a good meter to determine amps/volts to the battery might be a good idea..just don't exceed the batteries max charge rate ... or just buy a good peak charger, that detects the battery peak voltage and then shuts off...

I have a good charger, That batteries have both exploded (like a gunshot) on discharge. I have also observed that it seems to happen in a certain way. First the batteries are usually about 3 or 4 months old and have had almost full discharge each time. I bike 1000 kms a month and that is about 50 kms (2 charges) a day. After several months like that the batteries don't appear to have the same 'umph' they first did and seem to really be mostly discharged after about half the trip. Somewhere near the end of the trip one of the cells explodes.

Is there a place near Toronto that can rebuild these things? I'm riding about on my trusty SLA right now. Boy, are they heavy!

I think your cell's are getting out-of-balance. Nickel seems to be just as picky about this as lithium. Try looking for RC groups or 2-way radio folks in your area for help with these type packs. They've been using them a long time and will know the best ways to repair and prevent this.

The bottom-line is that multiple cell strings need individual cell voltage monitoring during both charge and discharge to alert you to when a cell nears or has reached it's critical point as a member of the string. Good news is that a pack can be balanced fairly easily but it does require some tech skills to understand and you need to do it before it reaches the condition both of yours reached.

NiCad batteries need to be balanced like any other multicell device. It appears that one or more of your cells is much less charged than the others. During discharge, the weak cell becomes completely discharged and begins to charge in the "wrong" direction due to the ability of the remaining cells to keep pumping current through it. Cell failure, overheating and eventually rupture of the vent port "loud pop" is the result. Sometimes a fire will happen.

Ballancing of cells is usually accomplished by charging the pack at a slow rate (1/10 of the capacity or less) which will not damage the cells that complete charging first.
A second method involves discharging the pack to 0 in a slow controlled manner and recharging from that point.
This second method is only applicable to NiCad batteries and is totally inappropriate for NiMh and other forms of batteries. The newest generation of Lithium/Polymer batteries are even more dangerous and several instances of houses burning down have been attributed to careless or untrained handling.