dagoinc66 said:
Originally it had 2 wires coming off it
ni-cad battery pack
that I am rebuilding with Li-ion batteries instead.
It was taped to the side of one of the inner most batteries
I've rebuilt the pack but when I put pack on a charger it doesn't begin charging it,
No pics, but I can still tell you what it is, from the above snippets of your post. It's a thermistor (thermal sensor), used in Ni chemistry packs because they're commonly charged until they reach a certain temperature then shut off, because of the way that chemistry works. Any good Nixx charger requires a connection to that thermistor to read it's temperature, and it it cant' read it it won't charge because it could cause a fire if it did. (if the cells were already hot).
You will want to read up on NiCd and NiMH batteries (there's a bunch of threads to help, just search for NiMH or NiCd or NiXX in the titles), to help you understand what you will need to do to switch a system from Nixx to a Li chemistry. It's not a straightforward cell-swap.
You cannot use the original charger for Nixx to charge Li. The one does not charge the same way that the other needs to be charged.
You'll need to buy a new charger for the Li pack, or modify the old charger by replacing it's guts with a Li charger. "safe" way to do that is with an RC charger with a balancing connector wired out from the cells in your pack.
You probably also want to add a per-cell voltage monitor (like the RC LiPo pack alarms), or even a BMS, so you will know when to stop using the tool. Nixx tools aren't designed with a cutoff, they'll drain a pack dead, whcih is ok for Nixx cells, but will destroy Li cells (and potentially cause a fire when you recharge them afterward).