The FAA-approved Boeing service bulletin for the 787 calls for modification of the charger and battery monitoring unit to narrow the acceptable level of charge. In essence, this means lowering the maximum charge allowed and raising the minimum level of discharge allowed. In other words, it cuts the performance gain the lithium-ion technology is supposed to bring.
Even worse, the additional weight of the modification kit is close to 185 pounds. A large part of it is the stainless-steel enclosure meant to isolate the unit from the rest of the equipment in the electronic equipment bays. Each of the two 787 batteries weigh 63 pounds, versus 107 pounds for one less-performing nickel-cadmium battery on the 777.