Lithium Polymer Batteries pack a lot of power in a small package. But they can be tricky to charge safely. The Adafruit LiPo Chargers all provide a charging cycle designed to safely charge 3.7v Lithium Polymer cells. But what if your project needs more than 3.7v?
Simply pumping more voltage into a multi-cell pack is risky. If the cells become unbalanced, some will end up with more of a charge than others. When put into service, the cells with the higher charge will end up working harder than the other cells. This will have an adverse effect on the capacity of the pack. Over time, it can result in damage and/or failure of one or more cells. And LiPo failures have been known to be quite spectacular.
The examples in this tutorial are based on the Adafruit USB / DC LiPo Charger. But it will work equally well with our USB LiPo Charger V1.2.
The batteries shown in the photos are 2600 mAh cells. But these examples will also work with our 1300 mAh, 4400 mAh or 6600 mAh cells.
Cells in a balanced pack must be of the same type and capacity. However, even among cells from the same manufacturing run, there can be variations in charging and discharging characteristics that can make them unsuitable as a balanced pack. If your pack shows signs of limited capacity, premature shutdown or high running temperature, stop using the pack.
Simple Balance Charger
This charger assures that the cells are ‘balanced’ at all times by charging them in parallel. When the cells are connected in parallel, any difference in charge level is equalized and they appear as a single cell to the charger. At all times during the charge, the cells are at equal charge levels.
This circuit will work with the USB / DC LiPo Charger or the USB LiPo Charger V1.2 .
To accomplish this, we must disconnect the cells from the load. We use a 3-pole double-throw switch to simultaneously disconnect the load, connect the charger and switch the cells from a series to a parallel configuration. Charger output can be taken from any of the battery terminals via soldered connection or a JST connector.
In charging mode, the cells are connected in parallel to the BATTERY IN terminals of the charger. The positive side of the Load output is disconnected during the charge.
In run mode, the positive connection to the BATTERY IN terminal is disconnected and the batteries are re-connected into a series configuration. The positive output is re-connected to supply 7.4v to the load.