The explosive doesn't need to be in the battery so much as connected to it. If anything they probably wouldn't have modified the battery at all as to not raise suspicion. I highly doubt it was the battery supplier as the pager MFG, unless intimately involved in the operation, would've just slapped in any battery from the parts bin when assembling them. If you get the finished product, you can add in the explosive and controls before shipping. Even after they can be broken open, modified, then reassembled and repackaged.
I'm still going with the supply chain intercept. Knowing what I know about logistics, it's incredibly easy to infiltrate, hold up, or otherwise delay shipping to the point the pagers could've been tampered with enroute to hezbolla. Any other supplier or company more than likely wouldn't take the risk of losing sales, nor wouldn't mossad want that kind of Intel leak possible by offering.
Modern electronics have since been miniaturized. Smart batteries are one thing, there's also dumb ones coupled with charging hardware built in. By far and large though, most smart batteries are just an IC, no BMS is involved with them. The bigger smart batteries run a BMS and a switch mode charger at best, but they're usually on expensive devices only and larger ones at that. Smart batteries can't control a detonator as they're just a controller, particularly in this size and cost of a device.
