Cross-Needle analog meter to monitor charge time for Lithium-Ion single cell

Abstract

The Cross-Needle panel meter is an analog Standing wave ratio meter with a modified panel. It consists of two meters which are mirrored horizontally and 38mm apart. One needle indicates the forward travelling wave on a trans-mission line, while the other needle indicates reverse travelling wave. A third scale, occupying the area swept by both needles, shows Standing Wave Ratio (SWR), essentially reflection coefficient, typically of an antenna connected to a radio transmitter. This thesis reports on an adaption of this idea. The first needle shows current into a battery, the second needle indicates the voltage of the battery. There is also a third scale using contour lines which show the amount of time left until the battery is fully charged. The contour lines are calculated and drawn using analytic geometry by predetermining the rotation of the two needles. Hence the amount of time left for the battery to fully charge can be read by simply looking at where the the two needles cross and estimating from the closest contour line that displays charge time. The input current scale is expanded logarithmically with a wide range from 1mA to 10A. This scale also evenly spreads the contour lines from 10 minutes to 1 week as opposed to a linear current scale, which causes the contour lines beyond 6 hours to cluster making it difficult to observe the charge time at low charge current inputs. The battery voltage scale is offset to suit the battery voltage range. The meter has been implemented using an embedded microprocessor on a PCB that mounts at the rear of the meter housing. The prototype has been calibrated for a single 18650 Lithium-Ion battery which is a popular scenario for portable equipment. This project provides a manufacturing solution for a low cost, low power, portable Lithium-ion battery monitor which can be easily replicated.