how to charge custom battery packs ？Charging lithium batteries correctly not only extends their lifespan but also enhances safety. Therefore, using the correct charging methods is very important. Different types of lithium batteries, such as lithium nickel manganese cobalt oxide (NMC) and lithium iron phosphate (LFP) batteries, may have different charging recommendations. 

 

For example, LFP batteries are recommended to be fully charged regularly, while NMC batteries may not need to be frequently fully charged. Following these guidelines can ensure the battery stays in optimal condition. To understand how to properly charge a custom lithium battery pack, we first need to know what a lithium battery pack is.

 

A lithium battery pack refers to the processing, assembly, and packaging of lithium battery cells. The process of assembling battery cells into a pack is called PACK. It can be a single cell or a series-parallel battery pack. A lithium battery pack typically consists of a plastic shell, a protection board, battery cells, output electrodes, connecting tabs, and other insulating tapes, double-sided tapes, etc. So, how to charge custom battery packs when connected in series or parallel?

 

Charging a lithium-ion battery involves using an external power source to drive positively charged lithium ions from the cathode to the anode. Therefore, the cathode is negatively charged, and the anode is positively charged.

Externally, charging involves electrons moving from the anode side to the charging source and pushing the same number of electrons into the cathode. This is opposite to the direction of lithium-ion flow internally.

 

During discharge, an external load is connected to the battery terminals. Lithium ions stored in the anode move back to the cathode. Externally, this involves electrons moving from the cathode to the anode. Therefore, current flows through the load.

 

In simple terms, during charging, what happens inside the battery is that lithium cobalt oxide releases some lithium ions on the cathode side, turning into a compound with less lithium but still chemically stable. On the anode side, these lithium ions are embedded into the interstitial spaces of the graphite molecular lattice.

 

Several issues must be considered during charging and discharging. Internally, lithium ions must cross multiple interfaces during charging and discharging. For example, during charging, lithium ions must transfer from the bulk of the cathode to the cathode-electrolyte interface. From there, it must cross the electrolyte, through the separator, to the electrolyte-anode interface. Finally, it must diffuse from this interface into the bulk of the anode material.

 

Series Charging of Lithium Batteries

Currently, series charging is generally used for lithium battery packs because it has a simple structure, low cost, and is relatively easy to implement. However, due to differences in capacity, internal resistance, attenuation characteristics, and self-discharge among individual lithium batteries, the cell with the smallest capacity in the battery pack will be fully charged first during series charging. At this point, other batteries are not yet fully charged. If series charging continues, the fully charged cell may be overcharged.

 

Parallel Charging of Lithium Batteries

When charging lithium batteries in parallel, each lithium battery should be balanced to ensure proper charging; otherwise, it will affect the performance and lifespan of the entire battery pack. Common balanced charging technologies include constant shunt resistor balanced charging, intermittent shunt resistor balanced charging, average battery voltage balanced charging, switched capacitor balanced charging, buck converter balanced charging, and inductive balanced charging.

 

Charging Considerations for Lithium-Ion Batteries

Trickle Charge (Pre-charge) If the battery’s charge level is very low, it is charged at a reduced constant current rate, typically about 1/10 of the full-rate charging described below. During this time, the battery voltage increases, and when a given threshold is reached, the charging rate increases to full charging rate. Note that some chargers divide this trickle charging stage into two phases: pre-charge and trickle charge, depending on how low the initial battery voltage is.

 

Full Charge If the battery voltage is initially high enough, or if the battery has charged to this point, the full-rate charging phase begins. This is also a constant current charging phase where the battery voltage continues to rise slowly.

 

Taper Charge When the battery voltage rises to its maximum charging voltage, the tapering phase gradually begins. During this phase, the charging voltage is kept constant. This is important because allowing charging at a voltage higher than the maximum voltage can cause catastrophic failure of the lithium-ion battery. If this charging voltage remains constant at this maximum value, the charging current will gradually decrease.

 

Cut-off/Termination When the charging current drops to a sufficiently low value, the charger disconnects from the battery. This value is usually 1/10 or 1/20 of the full-rate charging current. It is important not to float charge a lithium-ion battery, as this will reduce the battery’s performance and reliability in the long run.

 

When charging custom lithium battery packs, several issues need to be noted:

Use the charger specified by the manufacturer and ensure the charger model matches the battery model.

Avoid high-temperature charging. Do not charge lithium batteries in environments above 40°C, as high temperatures can cause battery capacity degradation.

 

Charge promptly. Try to charge lithium batteries as needed, avoiding charging only after they are deeply discharged each time, as this can greatly extend battery life.

 

Pay attention to the charging environment. The optimal charging environment temperature is 25°C. Most chargers do not have an automatic temperature control system to adapt to the environment, so most chargers are designed according to an ambient temperature of 25°C. Therefore, charging at 25°C is better. Temperature is a crucial factor to consider when charging lithium-ion batteries. Lithium-ion batteries should not be charged at low or high temperatures.

 

At low temperatures, lithium ions move slowly. This can cause lithium ions to accumulate on the anode surface and eventually turn into lithium metal. Since this lithium metal forms in dendritic form, it may pierce the separator, leading to an internal short circuit.

 

At the high end of the temperature range, the problem is generating excessive heat. Battery charging is not 100% efficient, and heat is generated during charging. If the internal temperature of the core is too high, the electrolyte may partially decompose, producing gaseous by-products. This can permanently reduce battery capacity and cause swelling.

 

For high-quality batteries, the typical charging temperature range for lithium-ion batteries is 0°C to 45°C. For cheaper batteries, the charging temperature range is about 8°C to 45°C. Some batteries also allow charging at higher temperatures, up to about 60°C, but the charging rate will be reduced.

 

It is advisable to partially discharge lithium batteries, avoiding over-discharge and frequent full discharge. Over-discharge can have catastrophic consequences for the battery, especially high-current over-discharge or repeated over-discharge, which has a significant impact on the battery. Custom lithium battery packs must avoid overcharging during charging and avoid over-discharging during use.

 

Lithium batteries connected in parallel will have a charging protection chip to protect the lithium battery during charging. Lithium battery manufacturers have fully considered the changes in lithium batteries connected in parallel and designed current and cell selection accordingly. Therefore, users need to follow the instructions for parallel lithium batteries to charge them properly to avoid potential damage caused by incorrect charging.