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Well, mate I'd suggest you do a little more research on how these things are set up, particularly how lithium batteries are put together and their charge and discharge rates All these tools use basically the same cells ( same voltage / amphours ) they are just joined differently to give you different voltages, and yes whilst a higher voltage battery may offer more power that will relate directly to less run-time on the same size battery. the only way around this is to add more cells ( make the battery bigger )
So using your example a 14v battery could run eight 3.6 volt cells with two banks of four batteries wired in series , so 14.4v then the two banks wired in parallel to each other, so if each cell was 1000ma you end up with amp / bank and two banks so 2amps, to make that same set of cells 28v you'd wire the lot in series, so you end up with 28.8 volts, but you only have 1 amp. So whilst you deliver a higher voltage, you only have half the amps , and being a sparky, you would know that watts ( power ) is volts multiplied by amps,
Charge rates are governed by the power of the charger and the limitations of the cells themselves, a larger battery can be charged in the same time as a smaller battery, however it requires more power, because you are charging more cells. The limitation is only in the maximum charge rate of the individual cells, which given they are are all basically the same, it's the same. ( 1c ) The only difference being that with larger batteries ( more cells ) the balance part of the charge cycle can be longer as there are more cells to charge. Typically this will be the last 20% of the charge cycle. Lithium stuff is pretty clever, the battery and the charger "talk" to each other and monitor charge rates, which is necessary because if cell voltage drops too low, or charge rates are too high, they overheat and go up in flames.( exploding samsung phone for example )
Where you experience different charge times ( longer for a larger battery ) it is because the charger hasn't the capacity to charge the larger battery at the full rate at which the all the cells combined can handle. So as long as the the larger battery is mated to a larger charger, the charge time is the same, because its limited only by the maximum charge rate of the individual cells, which are all the same.
