hey hey, you havent quite got it. yes lipoly and lifepo4 packs often come with charging circuitry to limit the input charge and make sure that they arent over discharged as well. aplies more to lipoly, not so much to lifepo4 because lipoly can literally explode in slow motion and cause fires, whereas you have to be pretty 'absentminded' to cause such a problem with quality lifepo4; its much more forgiving, but not immune. this is mainly for batteries that are sold to the public for use in a variety of devices, just as a backup. but i would say that the fostex has its own charging circuit to take care of this and the battery, being that it was sold to an oem producer, it is not required by law to have the circuit, as they now are for the packs sold to the public.
no what i was saying, is that which ever chemistry it is, it will be a certain voltage and a certain capacity for example 13.2v (4 x 3.3 cells) and 2300mah (capacity) this will have been specified by fostex to get the best runtime in their specific amount of space. the energy density of batteries increases as time marches on, but in general each battery chemistry sticks to a nominal voltage per cell in the pack. for lifepo4 the nominal voltage is 3.3vdc per cell and from memory lithium polymer is 4.2v, this means that over its charge when fully charged it (lifepo4) will be closer to 3.6vdc per cell and near the end of its useful life more like 3vdc, but it spends most of the cycle between 3.4-3.3 dropping from 3.6 pretty quick and dropping from 3.3-3 pretty quickly also.
so the charger and charge manager/fuel gauge in the fostex, will be designed to charge based on the nominal, max and min thresholds of the chemistry its designed for. if you go and change that voltage or battery type, the charger will act on the wrong info, thinking the battery is full when its not, thinking its empty when its not etc and damage, or make the battery go al explody in your pocket. battery capacity is ok to increase, generally anyway, as the charger will be based on the voltage thresholds, not the capacity, it judges its actions on that and has no idea of the capacity, which is what gives you longer life anyway.
with me? sorry for the longwinded explanation; its what i do hehe. so all i'm saying is, as long as it will fit, its the right chemistry and the right number of cells that are wired in the correct way (series or parallel), it may well work even if its not the same custom made pack, but you absolutely must make sure thats all correct by checking with the manufacturer, dont take my word for it.
its all elementary anyway as i doubt very much it will be an issue you ever have to deal with.