How to Build Your Own Battery Tender

by Spanner Spencer
Electric power adapter image by Nikolai Sorokin from

A battery tender, sometimes referred to as a trickle charger, provides a low, constant current to an inactive battery to prevent it from going completely flat. This can substantially prolong the life of a high-power rechargeable battery, which can suffer degradation of its internal components when left in a discharged state. By employing a low-current power adapter as a battery tender, the battery's charge level will be automatically topped up whenever it begins to discharge through inactivity.

Step 1

Acquire a power adapter from an unused household electronic device, such as an old mobile phone. Check that it is rated at a 12-volt output by looking at the label on the underside of the casing. A current rating between 200 milliamps-per-hour (mAh) and 100-mAh is most suitable, though it needn't be a high-powered adapter. This will also be detailed on the label.

Step 2

Snip off the connector the end of the adapter's cable and strip half an inch of casing off each wire using the wire cutters.

Step 3

Keep the two bared wires separated, and then plug the adapter into a power outlet. Check the polarity of the voltage using the multimeter and mark which wire is positive and which is negative.

Step 4

Unplug the power adapter and solder the positive wire to one leg of the resistor and the cathode of the diode. The cathode can be identified as the leg of the diode attached to the large, black part of its body. This is the opposite end to the white stripe.

Step 5

Solder a crocodile clip to the other leg of the resistor and the anode of the diode. This is the leg attached to the thin white stripe at one end of the diode's body. Solder the other crocodile clip directly onto the negative wire from the power adapter.

Step 6

Wrap any exposed connections, including the diode and resistor, in electrical tape to the point where the wires connect to the crocodile clips.

Step 7

Clip the crocodile clips onto the battery terminals, ensuring the positive wire with the diode and resistor goes onto the battery's positive terminal.

Plug the adapter into a power outlet and switch it on to trickle charge the battery.

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