A LiFePO4 battery can charge more slowly near 100% because the charger reaches its charging-voltage limit and the current starts to fall. If your display shows charging voltage and amps, look for voltage leveling off while amps gradually decrease. That pattern fits the normal finishing stage; a sudden stop or fault message needs a different explanation.
Why the charger reduces current near full
Earlier in a typical charge, the charger supplies a steady current while battery voltage rises. This is called constant-current charging. Once the charging-voltage limit is reached, the charger holds voltage steady instead. This is constant-voltage charging, and the current decreases as charging continues. That gradual decrease is the taper in a two-stage lithium charging profile.
The important change is what the charger is controlling. Earlier, it adjusts voltage to maintain the charging current. Near full, it limits voltage and allows current to fall. Continuing to force the earlier current would require letting voltage rise beyond that set limit.
Lower current means less charge enters the battery each minute. The battery can therefore still be charging even though the percentage display advances much more slowly. The taper itself does not mean the charger has lost its output capacity.
This explanation applies when the battery is approaching its charging-voltage limit. If current is low well before that point, the finishing-stage explanation alone does not establish the cause. Likewise, a gradual taper and repeated charging interruptions are different observations; do not assume both mean the battery is simply almost full.
Why the last few percent take longer
A percentage point does not take a fixed amount of time to add. Near full, the remaining amount is small, but the current delivering it is also falling. The constant-voltage finishing stage explains why an earlier charging rate cannot simply be extended to the end.
For an illustrative example, suppose 5 amp-hours remain to be added. At a steady 25 amps, that would take 5 ÷ 25 hours, or 12 minutes. At an average of 5 amps, the same amount would take one hour. These are simplified calculations, not predicted charging times: they assume the stated average current reaches the battery and ignore losses. During a real taper, current changes throughout the finishing stage.
A charger's current rating also does not promise that current all the way to full. For a 36V LiFePO4 battery, the Vipboss 36V 25A charger provides a 43.8V, 25A output. It is a suitable option to consider when your battery's charging requirements match those specifications.
Frequently asked questions
Why does charging current fall near full charge?
LiFePO4 chargers commonly move from a constant-current stage to a constant-voltage finishing stage, where the battery accepts less current.
Is it normal for the last few percent to take longer?
Yes. The remaining capacity is added while charging current tapers, so the final portion often takes longer than the earlier bulk stage.
Does a higher-amp charger remove the final taper?
No. A higher-rated charger can shorten the bulk stage when the battery permits it, but it does not remove the normal reduction in current near full charge.
