Rechargeable batteries are everywhere. From the laptops we type on to the portable medical equipment used to perform examinations or provide health record information, rechargeable batteries help to provide products and services to make our lives better. However, not many people know about what goes into recharging a battery in the correct manner, the right charger for certain battery chemistries, and where a battery management system (BMS) fits into the process.
Here at Epec, we pay close attention to how we customize our battery packs, battery chargers, and BMS technologies to ensure the longest life to your rechargeable batteries whether you are using lithium-based batteries, nickel metal hydride (NiMH) batteries, or other types of chemistries. By matching the right chargers to battery chemistries, you can ensure that the battery pack will be charged in an efficient and safe manner without overcharging, overheating, or causing permanent damage. Here are some things you should know about battery charging.
Charging Methods
Not every battery has the same charge rate or charge time. So, they require different charging methods to ensure the battery isn't undercharged or overcharged or that there is a reduction in the number of charge cycles. There are different charging methods that can be applied to certain battery chemistries.
Trickle Charge
A trickle charge is where the charger provides a very low voltage that is usually equal to the self-discharge rate of the battery. It is normally set at a rate of 0.05C and can go up to 0.01C. Trickle chargers are used to prevent overcharging when batteries are placed into storage. At times, a trickle charge can cause batteries to become too hot. In addition, the slow constant charge can cause memory effect problems for certain battery chemistries. A trickle charger can take 14 hours or more to fully charge a battery.
Rapid and Fast Charges
Rapid charges and fast charges offer higher voltages over a shorter amount of time at a constant rate. The rapid charger has a rate of 0.3C to 0.5C; a fast charger has a rate of 1C. Overheating can occur to some battery chemistries, which could result in damage, so the battery must have a cooling down period before the charge is applied again. Other batteries respond well to rapid and fast charges. Another issue with fast charges is that battery chemistries can become unstable, and this may lead to a rupture, leak or explosion if the battery is left on the charger for too long.
Step-Differential Charges
A step-differential charge is a method where a fast charge of 1C is applied to the battery. When the battery reaches a specific charge threshold, it is allowed to cool off. It is placed into a rest phase as a lower amount of charge is added. Then, as the battery reaches its next charge threshold, the charge is lowered more until the battery chemistry reaches a full charge.
Ultra-Fast Charges
Ultra-fast charges are newer charging techniques where the charge voltage will be set at 1C to 10C. These types of charges are normally reserved to specialty batteries as the batteries can be at a 70% charge from 10 minutes to about 60 minutes.
Charge Rates and Charging Times
When comparing NiMH to lithium chemistries, both batteries respond well to rapid charges and fast charges without experiencing a significant loss in capacity or a shortened service life. Knowing the different charge rates and times will allow you to get the greatest capacity in the right amount of time.
NiMH Chemistry
This chemistry can go through rapid charge, fast charge, ultra-fast charge or step-differential charge methods. Charge rates and times are:
- Rapid Charge: A charger can provide a rate of 0.3-0.5C as it will take 3-6 hours to charge.
- Fast Charge: Charger can be set at 1C charge rate as it can take from 1 hour or more to fully charge.
- Ultra-fast Charge: A charger can have a charge rate of 1C up to 10C. The battery can be charged up to 10 minutes to an hour as the state of charge (SoC) will only reach 70%.
Lithium Chemistry
Lithium chemistries can experience a rapid, fast and ultra-fast charges. A slow or trickle charge should be avoided as too low of a voltage will cause degradation and instability.
- Rapid Charge: A charger can have a set rate of 0.5C as it will take 3 hours or more to charge.
- Fast Charge: Lithium chemistries can take charge rates of up to 1C, although the maximum for safe charging can be around 0.7 to 0.8 depending on the battery pack. The charging time can range around 1 hour.
- Ultra-Fast Charge: There are ultra-fast chargers available for lithium battery chemistries. Like NiMH, the charge rate can go from 1C to 10C, although a lower charge is advisable. It can take an hour or less to charge the battery. This method is reserved for specialty batteries.
Understanding Battery Management Systems
Battery management systems provide a range of benefits to battery packs depending on the brand. Each BMS can have different features, yet all will provide you with the SoC of the battery as well as protection from overcharging and overheating to prevent thermal runaway. Additional features for some BMS technologies include state of health (SoH) diagnostics, cell balancing, cell protection, and charge control.
While the battery charger will have a sensor to measure temperature and the applied current to adjust the voltage, a BMS control provides additional benefits to increase the safety and protection of the batteries. Lithium chemistries require a BMS to monitor the charge voltage, so it doesn't become too low or too high.
Custom Charger vs. Off-The-Shelf Chargers
Many people will just pick up an off-the-shelf charger for their smaller, individual rechargeable batteries placed into toys, appliances, and home electronics. However, you need to be aware that different charges provide different charge rates and voltages that can significantly impact the lifecycle of the batteries.
The most important tip to keep in mind is that you should match the charger's design to the right battery chemistry. There is no "one-charger-fits-all-battery chemistries" setup. You should never use a nickel cadmium battery charger to charge NiMH batteries. And while it is true that you could use a NiMH charger to charge nickel cadmium batteries, it is still safer simply to use a charger designed for a specific battery chemistry.
When it comes to purchasing custom-made battery packs, always consider picking up custom chargers designed for those packs. The custom chargers will be made to support the right voltage and current as it will specify the amount of charge time that is necessary to reach 70-100% SoC.
If you use an off-the-shelf charger, you want to make sure you are matching the right charger to the battery. Look at the side of the battery or the battery package. It will state the recommended charge and the amount of time it will take to fully charge the battery. Using this information, you can then get the correct charger from the shelf that will go with those batteries.
Additional Charging Tips
Here are some additional charging tips to use for your batteries to prolong their life and to avoid issues.
- Always charge NiMH batteries and lithium-based chemistries at room temperatures of 0°-45°C (32°-113°F).
- There are times when an off-the-shelf charger will not correctly terminate the charge. Always check batteries to see if they are warm and remove them from the charger.
- Both nickel-based and lithium-based chemistries charge better using a rapid or fast charge method.
- Never leave batteries on the charger for more than one day. Remove the batteries and later top off the charge when the batteries are put into use.
