Primary + rechargeable + solar in action

Sometime in May this year I set one of my mobile GPS trackers to run "full blast", i.e. with the GPS receiver permanently on.

Despite the resulting outrageous current consumption of about 7mA on average I needed to wait until October 18th for something interesting actually to happen. Before that time the tracker ran happily without touching the primary battery.

This, in itself, is cool, but having the power switch-over operation plotted afterwards takes the satisfaction to another level:

This design uses three power sources: a rechargeable battery (LiFePO4, 1100mAh), a primary battery (2x AA primary lithium to be exact, 3500mAh capacity) and a small PV panel (65x65mm, maybe 0.4W).

In order to achieve maximum power efficiency there is no power conversion in this design (apart from the DC/DC solar charger). You can clearly see how the system voltage (EFM32G VCC/3) alternates between primary and rechargeable. At no point the LiFePO4 is being allowed to discharge below 2.7V which conveniently covers most of the usable capacity and at the same time can still power all of the required electronics directly.

Primary battery voltage rises when cells are not in use. The maximum voltage is 3.6V which also is conveniently the maximum allowed for the system rail.

Improvement notes

The above "battery current" plot doesn't give a direct answer to the most obvious question: how much power was used from the primary lithium cells.

Need to work on that - the tracker does know when it is running from primary and when from rechargeable.

Need to implement two separate software counters. Probably need to keep the existing one for backwards compatibility.

Need to improve plotting template. Split and stack battery on top of rechargeable would be cool.