NP-1's are popular rechargeable batteries for location sound applications. They were originally created for powering professional cameras and VTR's.
Today, a Lithium-ion NP-1's costs about US$ 180 to $225, depending on brand. In the Li-Ion category, chargers and batteries are typically not compatible across brands.
New cells for the NP-1's can be purchased individually for about eight dollars each. But creating a reliably-soldered series-parallel pack for an NP-1 is probably more trouble than its worth.
I picked up a couple of VariZoom brand NP-1's before realizing they are not compatible with my IDX chargers. The IDX will charge them partially before exhibiting a fault condition. So the VariZooms collected dust on my shelf.
Then I was given some IDX NP-7LS NP-1's that had been purchased but never used. About 8 years old, they would not accept a charge. So off to the shelf they go.
What follows is a happy marriage between the two brands, and a couple of like-new IDX NP-1's for me.