Troubleshooting Power Bank Charging Issues: Why It Stays at 99%

Have you ever encountered a situation where your power bank's charging progress gets stuck at 99% and doesn't progress to 100% for a long timesometimes even taking 30 minutes or an hour? This common problem can be attributed to several factors, including battery calibration, trickle charging, battery chemistry, temperature, and the quality of your charging cable and adapter.

Understanding the Issue

A power bank getting stuck at 99% before it reaches 100% can be confusing and frustrating. However, several technical and environmental factors can contribute to this behavior. Here's a breakdown of some of the common causes:

Battery Calibration

The battery management system (BMS) in your power bank may not be perfectly calibrated. This can cause it to misreport its actual charge level, especially near the top of the charging cycle. Calibration issues can be a critical factor in why your power bank seems to pause at 99%.

Trickle Charging

As the battery approaches full charge, the charging current significantly decreases to prevent overcharging. This trickle charging can take longer to fill the last 1% of the battery, leading to delays. This process occurs naturally as a safety feature to protect the battery and improve its longevity.

Battery Chemistry

Lithium-ion batteries, which are commonly used in power banks, often have a built-in mechanism that slows down charging as they reach full capacity. This is due to a protective charge cut-off mechanism designed to prevent overcharging and thus prolong the battery's lifespan.

Temperature

If the power bank is warm due to usage or charging, it may slow down the charging process to avoid overheating. This can cause it to take longer to reach 100%, potentially getting stuck at 99% for extended periods. Temperature management is crucial during the charging process to maintain safety and battery health.

Charging Cable and Adapter

The quality of the charging cable and adapter can also impact charging speeds. A low-quality or damaged cable may not provide sufficient power, leading to slower charging times. Ensuring you use high-quality cables and adapters can help prevent such issues.

Internal Resistance

Batteries can develop internal resistance over time, which can affect charging efficiency. This can lead to longer charging times as the battery nears full capacity. If your power bank shows consistent behavior, it might be worth using a different charging cable or adapter or recalibrating the power bank.

Why Do Last 1% Generate More Heat?

The last 1% of charging generates more heat compared to other segments of the charging process. This is because the charging process slows down as the battery approaches full capacity to prevent overheating and damage. The increased heat can cause the charging circuit to slow down, leading to delays in reaching 100%.

DIY Power Banks

If you have a DIY power bank, the behavior described might be due to your own design and build. If you've designed and built the power bank yourself, you likely chose the chips that regulate the charging process or designed the charging algorithm yourself. Measuring voltages and currents within the charging circuit can provide insights into why the charging circuit believes it has reached 100% charge.

For a DIY power bank, the charging algorithm is crucial. One example of a Lithium-ion (Li-ion) charging algorithm is as follows:

Lithium-Ion Charging Algorithm Example

Ensure that the cell voltage is higher than a safety cutoff voltage ( V1 ). If the cell temperature is outside a certain temperature range, do not charge the cell until the temperature is within the range. If the cell voltage is below a fast-charge start point ( V2 ), apply a trickle charge to see if the voltage will come up to ( V2 ). If the cell voltage reaches ( V2 ), begin constant-current charging at a current ( I1 ), depending on the cell capacity. Monitor the cell voltage during fast charge. When it reaches a maximum voltage ( V3 ), switch from constant-current to constant-voltage charging. While holding the cell voltage at ( V3 ), monitor the charging current. It will slowly reduce as the cell absorbs the last 25% or so of its charge. When the charging current drops below a threshold ( I2 ), stop charging entirely and announce that the cell is 100% charged. ( I2 ) is typically a few percent of ( I1 ).

This is a simple algorithm that can determine 100% charge but can't provide detailed information about the state of charge at 25%, 50%, 75%, or 99%. For that level of detail, additional circuitry is needed.

Understanding End-of-Charge Detection

While devices like phones often use coulomb counting to estimate the charge state of the battery between 0% and 100%, this is only an estimate and can be inaccurate. Your power bank likely uses coulomb counting to estimate the charge states between 1% and 99%, with end-of-charge detection to announce 100% when the battery is truly finished charging.

Seeing 99% for a long time likely indicates that the battery has been fed enough energy to be fully charged but has not reached the proper voltage or is still drawing current when it should have stopped. This suggests that your battery is no longer behaving correctly and might need further investigation.

What are the conditions that would cause your charging circuit to decide that the cell is not yet 100% charged? Typically, the charging voltage and current should be at a specific threshold for the charging circuit to announce 100%. Measurements and detailed analysis will be necessary to identify the precise condition causing the issue.