In 2S battery applications with one or two replaceable cells, the battery cells can have very different voltages because replacement batteries can come from anywhere, at any state of charge. Traditional multi-cell chargers only monitor the total voltage of series-connected cells; the charger does not measure each individual cell voltage. Consequently, even though the total voltage of series-connected cells does not reach the regulation voltage, the cell with the higher voltage may overcharge due to unbalancing of the cells. Overcharging battery cells not only shortens battery life but can also cause serious safety concerns. On the other hand, the cell at the lower voltage cannot fully charge, which means that its full capacity is not completely utilized. Therefore, cell-balancing becomes essential during battery charging in this type of 2S battery application.
The BQ25887 boost charger has a cell-balancing function that charges 2S battery cells from a 5-V USB adapter. Unlike traditional pack-side cell-balancing, an integrated cell-balancing function enables the charger to balance the voltage of the two cells during charging within one charge cycle. With the integration of input current- and voltage-limiting functionality, along with an input current limit optimizer, the BQ25887 can detect the current capability of unknown adapters, enabling the device to universally charge with any 5-V adapter.