Car batteries, big nicads, gel-cells, piles of alkaline "D"-cells, lantern batteries... nVDC is nVDC -- although the common "car cord", with a voltage regulator built into the cigarette lighter plug, and a connector on the other end of the cord built to mate with a specific phone model, makes 12-14VDC very "user-friendly" for any cell phone. 12V battery wired to the plug, and the other end plugged into the phone, and you're there.
More than car bombs, though, I'd worry about long-duration power supplies to be used for bombs they've prepositioned. The advantage to them would be a long "standby" timeframe, so that they could "set it and forget it" until they wanted to let 'er rip. (Literally as long as they wanted to make it -- all they need to do is calculate standby MAH drain, and then rig up a battery with capacity sufficient to supply it for the desired length of time.)
There would be a pattern to this, that could be observed at the central cell sites. A phone in the tracfone category would appear on the local roaming database (it has to check in with the cell periodically to let the cell know it is there to get calls), and would stay there day after day, never ever making or getting calls.
Maybe this explains why the purchasers in Caro, Michigan were reported to have separated the phones, batteries, chargers and wrapping into separate containers. Why do this you are going to re-sell the phones for normal consumer use?