DIY 9V Rechargable Battery Replacement (850mAh @ 7.4V)

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I'm tired of buying low-capacity 9-volt alkaline batteries or overpriced lithium-ion ones, not to mention the mediocre performance of nickel-metal hydride rechargeables. So I placed an order for lithium polymer cells and connectors from China. The resulting battery has a nominal voltage of 7.4 volts, peaking at 8.4 volts when fully charged due to four cells connected in series. This gives it a capacity of 850 milliampere-hours. Since devices that use 9-volt batteries usually run them down to below five volts, the lower voltage shouldn't be an issue, as the lithium polymer cells have a cutoff voltage of about 5.5 volts. Fortunately, these packs are equipped with a protection circuit, so balancing is not necessary. To minimize cell drift, just make sure to charge them to exactly the same voltage before assembling. Before you start assembling, disassemble the 9-volt connector and clip off its sides. Then desolder all the battery leads and connect them in series and then to the connector. BE CAREFUL NOT TO SHORT THE BATTERIES! Make sure to connect the black wire of the connector to the positive terminal and vice versa, as it's reversed since it's designed for the load. Check the polarity before finishing and cover any blank connections with tape! Next, put the top part over the connector and hot-glue it onto the batteries. You can find these products on AliExpress: https://www.aliexpress.com/item/4pcs-L055-3-7V-850mAh-802540-PLIB-polymer-lithium-ion-Li-ion-battery-for-SMART-WATCH/32640381350.html?spm=a2g0s.9042311.0.0.27424c4dz6ShLD https://www.aliexpress.com/item/10pcs-9V-Battery-Snap-Connector-clip-Lead-Wires-holder/32810031927.html?spm=a2g0s.9042311.0.0.27424c4dz6ShLD If you order more, the shipping cost won't increase much, so I paid about four dollars per cell, which would be equivalent to buying one high-capacity non-rechargeable 9-volt cell if you were lucky.