Lithium battery charger Circuit

Lithium batteries are extensively used almost everywhere these days. These batteries needed to be treated special since charging and discharging a lithium battery is quite peculiar process. For this purpose we have designed a 7.4v Lithium battery charger circuit which is capable of catering its charging needs efficiently.


This circuit starts with 220v/110v AC supply from a regular wall socket. This AC supply is stepped down using a step down transformer which converts this 220v AC signal to 24v AC. This AC signal is then rectified to DC signal. This 24v AC signal is now converted to 24v DC.

This 24v DC signal is fed into a classic 12v regulator 7812. This 12v supply is used to power up the Lithium battery charger IC LM3420. This LM3420 is a dedicated chip to charge the Lithium battery in optimum way. The output of this chip goes to 8.4v when charging a 7.4v Lithium battery since the charging voltage of a 7.4v Lithium battery should be about 8.4v.


One unique thing about Lithium batteries is that it requires different charging current based on its charge level for prolonged and efficient usage. Using same charging current throughout its charging time will lead to depreciation in its charging level as time goes. This is where the concept of trickle charge come into place.

Trickle charging employs using maximum charging current when the battery was in low charge state say like 30%. But once the charging progress battery voltage will raise. For a 7.4v Lithium ion battery the output voltage will gets close to 8.4v. This will be the charging voltage when battery inches to reach 80% to 90%. At this point LM3420 drops the output current from maximum to a point where it will be equivalent to batteries self discharging current. This is the concept of trickle charging where charging them with less charging current at  end of its charging cycle will prolong the life of battery and keep the maximum charge capacity of battery intact.

LM3420 employs this method to charge the 7.4v Lithium battery and prolong the battery life while keeping the charging capacity of the battery intact.


  1. Diode 1N4004
  2. Resistor 2kOhm, 1kOhm
  3. Voltage Regulator 7812
  4. Current Source
  5. Battery Charger LM3420


  1. This circuit is applicable with 7.4v Lithium ion batteries
  2. The current rating of transformer should be at least 1A.

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7812 without any capacitor ?? Bad idea!