I've featured the availability of free rechargeable lithium cells from disposable devices often found lying in the street, so I thought I'd show the use of a multiple of those cells to make a power pack that can be used to power lighting or other small loads. It can be charged from a USB charger or a small 5/6V solar panel.
The module is a TP4056 lithium charger with extra cell protection. It lets you plug a USB lead straight into the pack to charge it. Very common on eBay and cheap when bought in multiples of 5 or 10pcs. It is intended for standard rechargeable lithium cells often described as being 3.7V cells - which is the mid charge voltage. The full charge voltage is 4.2V and fully discharged is around 2.5 to 3V.
When lithium cells are connected in parallel they behave as a single cell. It's important to make sure the cell voltages are very close before connecting them as otherwise high current can flow between them as they attempt to match voltages. Even a 0.1V difference can cause quite high current to flow.
The safest way to work on lithium cells is when they are almost fully discharged to around 3V or less. That way, any accidental short circuits will have low energy. Keep in mind that these are energy storage devices, and that the disposable devices for fog inhalation have quite high current cells. They are very popular with model drone enthusiasts as a source of cheap expendable cells.
Always take extra care to avoid short circuiting the cells. Cap the wires with tape when needed.
It's always useful to have a metal dish on hand in case of little incidents.
Pouch cells lack the external armour of cased cells, and should be protected against damage or shorting.
In this video I used the module to power a string of LED lights with a suitable resistor in series with them (33 ohm) to limit the current. They will run for over a day on a charge, gradually reducing in brightness until the protection kicks in and they turn off. You could also use the pack to power a small work light or other 3-4V load.
The pack can be charged while it's running LEDs, so it can act as a decorative emergency light that stays on during a power blackout.
Another approach to this might be to use the original metal tubes to hold two paralleled cells with a suitable connector to the charging module, so one module could be used with multiple power packs.
This power pack is ready for solar charging from a 5V solar panel or a 6V one with diode in series. The cells will be charged with whatever current is available from the panel, capped to the maximum current set by the modules programming resistor. Too high a charge voltage will result in extra heat from the charge chip and it may self regulate the current lower.
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