The use of rechargeable battery system is safer that traditional lithium batteries. There is no need for special handling or transport, as these are not qualified as dangerous goods.
The rechargeable batteries can be used with or without the downhole generator, depending on the length of the job. The capacity of such cell is about 40% of the lithium cell, so the batteries alone will be sufficient to power the MWD string.
The addition of a generator, will eliminate the time limit requirements. Such system can power any MWD/LWD system.
The only restriction for a rechargeable cell is the operational temperature. The rechargeable batteries can be safely used up to 100 DegC. The lithium ion rechargeable batteries can go a bit higher, but these are considered as dangerous material, same as the regular lithium ion cells.
Our downhole rechargeable battery systems are in a successful operation now for several years.