How to Prime Batteries

Priming a New Battery

Not all rechargeable batteries deliver the rated capacity when new, and they require formatting. While this applies to most battery systems, manufacturers of lithium-ion batteries disagree. They say that Li-ion is ready at birth and does not need priming. Although this may be true, users have reported some capacity gains by cycling after a long storage.

“What’s the difference between formatting and priming?” people ask. Both address capacities that are not optimized and can be improved with cycling. Formatting completes the fabrication process that occurs naturally during use when the battery is being cycled. A typical example is lead- and nickel-based batteries that improve with usage until fully formatted. Priming, on the other hand, is a conditioning cycle that is applied as a service to improve battery performance during usage or after prolonged storage. Priming relates mainly to nickel-based batteries.

Lead Acid

Formatting a lead acid battery occurs by applying a charge, followed by a discharge and recharge. This is done at the factory and is completed in the field as part of regular use. Experts advise not to strain a new battery by giving it heavy duty discharges at first but gradually working it in with moderate discharges, like an athlete trains for weight lifting or long-distance running. This, however, may not be possible with a starter battery in a vehicle and other uses. Lead acid typically reaches the full capacity potential after 50 to 100 cycles. 

Nickel-based

Manufacturers advise to trickle charge a nickel-based battery for 16–24 hours when new and after a long storage. This allows the cells to adjust to each other and to bring them to an equal charge level. A slow charge also helps to redistribute the electrolyte to eliminate dry spots on the separator that might have developed by gravitation.

Nickel-based batteries are not always fully formatted when leaving the factory. Applying several charge/discharge cycles through normal use or with a battery analyzer completes the formatting process. The number of cycles required to attain full capacity differs between cell manufacturers. Quality cells perform to specification after 5–7 cycles, while lower-cost alternatives may need 50 or more cycles to reach acceptable capacity levels.

Lack of formatting causes a problem when the user expects a new battery to work at full capacity out of the box. Organizations using batteries for mission-critical applications should verify the performance through a discharge/charge cycle as part of quality control. The “prime” program of automated battery analyzers (Cadex) applies as many cycles as needed to attain full capacity.

Cycling also restores lost capacity when a nickel-based battery has been stored for a few months. Storage time, state-of-charge and temperature under which the battery is stored govern the ease of recovery. The longer the storage and the warmer the temperature, the more cycles will be required to regain full capacity. Battery analyzers help in the priming functions and assure that the desired capacity has been achieved.

Lithium-ion

Some battery users insist that a passivation layer develops on the cathode of a lithium-ion cell after storage. Also known as interfacial protective film (IPF), this layer is said to restrict ion flow, cause an increase in internal resistance and in the worst case, lead to lithium plating. Charging, and more effectively cycling, is known to dissolve the layer and some battery users claim to have gained extra runtime after the second or third cycle on a smartphone, albeit by a small amount.

Scientists do not fully understand the nature of this layer, and the few published resources on this subject only speculate that performance restoration with cycling is connected to the removal of the passivation layer. Some scientists outright deny the existence of the IPF, saying that the idea is highly speculative and inconsistent with existing studies. Whatever the outcome on the passivation of Li-ion may be, there is no parallel to the “memory” effect with NiCd batteries that require periodic cycling to prevent capacity loss. The symptoms may appear similar but the mechanics are different. Nor can the effect be compared to sulfation of lead acid batteries.

A well-known layer that builds up on the anode is the solid electrolyte solid electrolyte interface (SEI). SEI is an electrical insulation but has sufficient ionic conductivity to allow the battery to function normally. While the SEI layer lowers the capacity, it also protects the battery. Without SEI, Li-ion might not get the longevity that it has.

The SEI layer develops as part of a formation process and manufacturers take great care to do this right, as a batched job can cause permanent capacity loss and a rise in internal resistance. The process includes several cycles, float charges at elevated temperatures and rest periods that can take many weeks to complete. This formation period also provides quality control and assists in cell matching, as well as observing self-discharge by measuring the cell voltage after a rest. High self-discharge hints to impurity as part of a potential manufacturing defect.

Electrolyte oxidation (EO) also occurs on the cathode. This causes a permanent capacity loss and increases the internal resistance. No remedy exists to remove the layer once formed but electrolyte additives lessen the impact. Keeping Li-ion at a voltage above 4.10V/cell while at an elevated temperature promotes electrolyte oxidation. Field observation shows that the combination of heat and high voltage can stress Li-ion more than harsh cycling.

Lithium-ion is a very clean system that does not need additional priming once it leaves the factory, nor does it require the level of maintenance that nickel-based batteries do. Additional formatting makes little difference because the maximum capacity is available right from the beginning, (the exception may be a small capacity gain after a long storage). A full discharge does not improve the capacity once the battery has faded — a low capacity signals the end of life. A discharge/charge may calibrate a “smart” battery but this does little to improve the chemical battery. Instructions recommending charging a new Li-ion for 8 hours are written off as “old school,” a left-over from the old nickel battery days.