The inspiration for this design came from the author’s experience with a mini model helicopter (from Silverlit). This particular model has a hand-held transmitter powered by six AA batteries which acts as a charging station in between flights to recharge the helicopter’s LiPo battery.
Even alkaline batteries become discharged relatively quickly because of the energy demands of the helicopter. Replacing the alkaline cells with six rechargeable NiMH batteries brought its own problems; the cell voltage is around 1.4 V after recharging but this quickly levels-out to 1.2 V once you begin drawing energy and this proved to be too low to recharge the helicopter battery. What is needed here is a voltage converter design small enough to fit into the space taken up by an AA battery which pumps up the voltage from the (now five rechargeable cells) up to the level produced by six alkaline batteries.
The author was not satisfied with the most simple design solution to the problem; it would be more useful if this booster cell could be used in any battery compartment irrespective of the number of cells. The number of batteries (n) would then be replaced by n–1 rechargeable cells (with one cell position taken up by the booster) giving an output voltage the same as if n primary cells were fitted.
The circuit described here can be used in applications requiring four to ten primary cells. With the booster fitted, only three to nine rechargeable cells would be required. The use of (more bulky) electrolytic capacitors with a 35 V rating would allow the booster to be used in applications of up to 20 batteries.
The author was not satisfied with the most simple design solution to the problem; it would be more useful if this booster cell could be used in any battery compartment irrespective of the number of cells. The number of batteries (n) would then be replaced by n–1 rechargeable cells (with one cell position taken up by the booster) giving an output voltage the same as if n primary cells were fitted.
The circuit described here can be used in applications requiring four to ten primary cells. With the booster fitted, only three to nine rechargeable cells would be required. The use of (more bulky) electrolytic capacitors with a 35 V rating would allow the booster to be used in applications of up to 20 batteries.
In principle almost any switching regulator IC can be used in this way. The power output from this circuit with a LT1172 regulator is around 500 mA but it can be increased to 2 A for example by using the LT1170 instead.
