Certainly one of my electronics pursuits is constructing radios, notably these featured in older UK electronics magazines reminiscent of Sensible Wi-fi, On a regular basis Electronics, Radio Constructor, and The Maplin Journal. Most of these radios are designed to run on a 9-V disposable PP3 battery.

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Utilizing 9 V as a substitute of the three V discovered in lots of home radios permits the transistors in these often-simple circuits to function with the next achieve. PP3 batteries are, at a minimal, costly in circuits consuming tens of mA and are—I believe—laborious to recycle. A extra environmentally pleasant resolution was wanted.

Previously, I’ve used single 3.6-V lithium-ion (Li-ion) cells from discarded e-cigarettes [1] with low cost mixed charger and DC-DC converter modules discovered on eBay. They supply a pleasant, neat resolution when housed in a small plastic field, however sadly generate loads of electromagnetic interference (EMI), which falls inside the shortwave band of frequencies (3 to 30 MHz) the place loads of the radios I construct function. I wanted one other resolution that was EMI-free and environmentally pleasant.

Answer

One resolution is to get rid of the DC-DC converter and string collectively three or extra Li-ion cells in a battery pack (B₁) with a variable linear regulator (IC₁) to generate the required 9 V (V₁) as proven in Determine 1. Li-ion cells, like all digital parts, have tolerances. The 2 most essential parameters are cell capability and open circuit voltage. Variations in these parameters between cells in sequence result in uneven charging and in the end stressing of some cells, resulting in their eventual degradation [2]. To even out these variations, Li-ion battery packs typically comprise a battery administration system (BMS) to make sure that cells cost evenly.

Determine 1 Li-ion battery pack, with 3 or extra Li-ion cells, and a variable linear regulator to generate the required 9 V.

As luck would have it, on the native buy-nothing group in Ottawa, Canada, the place I dwell, somebody was freely giving a Mastercraft 18-V Li-ion battery with charger as proven in Determine 2. The individual providing it had misplaced the drill, so there was little expense for me. Upon opening the battery pack, it was certainly discovered to comprise a battery administration system (BMS). This appeared like a super resolution.

Determine 2 The Mastercraft 18-V Li-ion battery and charger obtained regionally.

Circuit

The following step was to make a linear voltage regulator to drop 18 V to 9 V. This, in itself, will not be notably environmentally pleasant, as it is just 50% environment friendly, and any dropped battery voltage can be dissipating as warmth. Nevertheless, assuming renewable energy era is used because the supply, this may show a extra environmentally pleasant resolution in comparison with utilizing disposable batteries.

In one among my bins of outdated tasks, I discovered a continuing present nickel-cadmium (NiCad) battery charger. It was based mostly round an LM317 linear voltage regulator in a pleasant black plastic enclosure offered by Maplin Electronics as a “energy provide” field. The NiCad battery hadn’t been used for over 20 years, so this mission can be a repurpose. A schematic of the rewired energy provide is proven in Determine 3.

Determine 3 The facility provide schematic with 4 selectable output voltages—6, 9, 12, and 13.8 V.

In Determine 3, swap S1 features as each the ability swap and selects the output voltage. 4 completely different output voltages are selectable based mostly on present wants: 6 V, 9 V, 12 V, and 13.8 V might be chosen by adjusting the ratio of R2 and R3-R6 as proven within the LM317 datasheet [3]. R2 is often 220 Ω and develops 1.23 V throughout it, the remaining output voltage is developed throughout R3-R6. To get the precise values, parallel mixtures are used as proven in Desk 1.

Desk 1 Totally different values of paralleled R3 to R6 resistors and their mixed worth.

{A photograph} of the completed energy provide with a Li-ion battery hooked up is proven in Determine 4.

Determine 4 {A photograph} of the completed energy provide with 4 selectable output voltages that may be adjusted by way of a knob.

Outcomes

Crimp-type spade connectors had been fitted to the 2 enter wires, which mated effectively with the terminals of the Li-ion battery. Perhaps in some unspecified time in the future, I’ll 3D-print a full connector for the battery. With the resistor values proven in Determine 3, the precise output voltages produced are: 5.96 V, 9.03 V, 12.15 V and 13.8 V. Whereas these usually are not the precise designed values because of the usage of most well-liked resistor values, it’s of little consequence because the output voltage of disposable batteries varies over their working time and there may be in fact a voltage drop because of cables. With this energy provide, although, the output voltage of the ability provide will stay fixed throughout this time, even because the output voltage of the Li-ion drops because of its discharging.

Transportable energy

Though the ability provide was supposed for powering radio tasks, it has different makes use of the place transportable energy is required and a DC-DC converter is just too noisy, like delicate instrumentation or some audiophile preamplifier [4]. 

Gavin Watkins is the founding father of GapRF, a producer of on-line EDA instruments specializing in the RF provide chain. When not doing that, he’s happiest noodling round in his lab, engaged on audio electronics and RF tasks, and restoring classic tools.

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References

1. Reusing e-cigarette batteries in a e-bike, https://globalnews.ca/information/10883760/powering-e-bike-disposable-vapes/
2. BU-808: Easy methods to Extend Lithium-based Batteries, https://batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries
3. LM317 regulator datasheet, https://www.ti.com/lit/ds/symlink/lm317.pdf
4. Battery powered hifi preamp, https://10audio.com/dodd_battery_pre/

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