I'm talking about the context of using the phones for detonation of explosives. If it is to detonate an IED, then a regular cell phone battery might not last the week or so that they want the IED to be buried before they light it off by dialing the number. They would install better batteries to use for that purpose, IMHO.
The old batteries could probably be sold off to someone and the money kept for airfare.
This assumes though that they had nefarious intent.
If the cell phones were to be used for IEDs, you would put a cheap non rechargable battery in them. Since they were a one shot device, there is no reason to keep a more expensive rechargeable battery in them. In fact, such batteries and their recharger could be sold at a profit, and as post #83 said, you could almost get the cost of the phone from their sell.
Correction: "sell" = sale.