Charging Batteries

Chargers Control Voltage and Current To Safely Charge Batteries

Rechargeable batteries are everywhere today and provide powerful portable and cost-effective energy.

Which makes chargers ever more interesting. In this section we take a look at what exactly a charger does and how it differs from normal power supplies.

Understanding Charging

Batteries work both ways: you can draw energy (in which case the battery voltage slowly sinks), and you can feed energy (in which case the battery voltage slowly raises).

The art of feeding energy to a battery in an appropriate way is what makes Chargers unique.

Voltage Drives, Current Matters

To charge a battery, you cannot simply connect a power supply to it and hope for the best. Instead, there are two prerequisites:

  • High enough voltage: the charging voltage must be above a certain threshold. If the voltage is too low, the energy won’t flow into the battery, and nothing happens.
  • Low enough current: you must limit the current so that the battery can accept and store the new energy. If you feed too much energy into the battery, it will heat up and eventually explode.
What happens when I connect a regular bench power supply to a battery?

Three things can happen:

  • Voltage lower than battery voltage: if the power supply voltage is significantly below the full charge battery voltage, no energy would flow from the supply to the battery. Possibly the battery would start to feed power to the power supply instead.
  • Small power supply: if the voltage is high enough, and it is a small power supply with only a limited output current of maybe 1A, it could in fact work as a charger at first. Once the battery is fully loaded, though, the power supply would continue to feed energy to the battery. The battery, unable to store any more energy, would heat up and eventually explode.
  • Big power supply: should you connect a huge and powerful bench power supply to a battery, large currents flow into the battery. The battery would not be able to store so much energy so quickly, and the excess energy would heat up the battery and eventually cause it to catch fire or explode.

What a Charger Does

A charger controls voltage and current to provide just the right amount of energy to the battery.

For lithium-based batteries, here is what that means:

  • Phase 1 - Constant Current (CC): the charger first supplies a constant current to the battery: it automatically raises the voltage until the desired current flows. The battery now accepts and stores this current, and while it does so, its voltage slowly raises.
  • Phase 2 - Constant Voltage (CV): while charging, the charger monitors the voltage, and when the voltage reaches a threshold that indicates that the battery is almost fully charged, it switches to constant voltage: the voltage gets fixed and no longer raises. Now, this fixed voltage can drive lesser and lesser energy into the battery. The current falls.
  • Phase 3 - Turning Off: When the current that flows into the battery falls below a threshold, the battery is considered fully charged, and the charger stops charging the battery.

This is the general recipe. The actual constant current in Phase 1, and the actual constant voltage in Phase 2 depend on the battery chemistry and battery voltage you are charging.

Technical Requirements

Here is what any charger must be capable of:

  • Energy source: it needs to receive energy that it can pass on to the battery. The energy source can be AC or USB or a regular bench power supply
  • Power Regulator: it needs to contain a power regulator that can control voltage and current
Is a Buck converter a charger?

A Buck converter takes a range of input voltage and converts it to a lower specific output voltage. Many Buck converters can also control constant current, and if that’s the case they can be used to charge batteries.

Let’s assume you want to charge a single 18650 LiIon battery, and let’s assume this battery can accept a 5A maximum charging current. Since it is a LiIon battery, its maximum voltage is 4.2V.

When you set the Buck converter to a maximum voltage of 4.2V and also set its constant current to 5A, it will automatically follow much of the above mentioned charging recipe:

When you connect an empty 18650 LiIon cell to it, first the constant current would kick in and limit the current to 5A by lowering the voltage appropriately.

Once the battery gets charged, the voltage rises until at one point it reaches the maximum voltage of 4.2 that the Buck converter was set to. Now constant voltage limits the voltage to 4.2V, and the current would slowly decrease.

The Buck regulator would still not be a safe charger because what’s missing is step 3: cutting off the power when the battery is full.


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(content created Mar 03, 2024)