# What would happen if I connect a battery which has lower voltage than output of battery charger

At this moment I'm building a charger for Li-ion battery based on constant current, constant voltage method. I have used IC LT3741 to build a charger.

The specs of my charger: 8,4 V, 2A

But I dont understand one thing:

When charging the battery, the voltage of battery will be lower than the output voltage of the charger (but of course greater than 6V.) What would happen then with the output of the charger? Because we have now the situation of two "DC-sources" which are connected parallel and have different voltage. Or am I wrong?

Two DC-sources, different voltage, connect together will not be good idea, will it?

• Luckily, real life battery has an internal resistance. – Meenie Leis Sep 20 '19 at 10:01
• DO NOT DO THIS do not attempt to design your own lithium battery charger until you have far more experience than this question indicates. Choose projects with more forgiving technologies for learning. – Chris Stratton Sep 20 '19 at 13:23
• Designing a lithium charger is like making a fine cheese, there is a delicate balance of ingredients and environment that must be maintained, otherwise you end up with a smelly mess. – crasic Sep 20 '19 at 18:45

What would happen then with the output of the charger? Because we have now the situation of two "DC-sources" which are connected parallel and have different voltage. Or am I wrong?

You are wrong. By definition, the voltage at a single point or 'node' is the same for all components connected to it, so the charger and battery will always have the same voltage when connected together.

This is why you are using a 'Constant Current, Constant Voltage' regulator rather than a straight voltage regulator. When disconnected the charger will (when properly adjusted) put out 8.4V. When the battery is connected and tries to draw more than the set current, the charger will drop its voltage to limit current. At the same time the battery voltage will rise due to the charging current. When battery voltage reaches 8.4V the charger will progressively lower the charging current to prevent the voltage from going higher than 8.4V.

The LT3741 itself is not a battery charger. You can safely use it to charge a Lithium-ion battery provided that you have mechanisms in place to handle fault conditions such as an over-discharged battery (must be charged at a lower current until reaching 3.0V/cell), charger malfunction (not limiting current or voltage), cell voltage imbalance, and excessively high battery temperature. A BMS (Battery Management System) or PCM (Protection Circuit Module) does some of that. For the rest you need extra circuitry, or manual monitoring and intervention (not recommended).

Your charger circuit should limit the current to not exceed the maximum charging current of the Li-Ion battery.

Note that this maximum charging depends on the charge state of the battery. Exceeding the recommended values stresses the battery and shortens its lifetime. If you grossly exceed what the battery can handle it might overheat and start smoking or catch fire!

You're using a DCDC converter chip as a charger, that's OK but do realize that charging a Li-Ion cell properly isn't something a DCDC converter can do on it's own. Especially for fast charging the maximum charge current needs to be controlled depending on the battery's charge level and temperature.