Battery Charge Time from Charger Current
Battery Charge Time from Charger Current
Once a battery’s discharged capacity and a charger’s output current are known, the recharge time follows directly — but charging is never perfectly efficient, so some of the charger’s delivered current is lost to heat and chemical inefficiency rather than restoring usable capacity.Underestimating charge time (by ignoring efficiency losses) leads to batteries that are put back into service before they are actually full, which is a common cause of premature capacity fade in rechargeable battery systems.
The charge time is t = Ah/(I·η), the amp-hour capacity divided by the charging current and charging efficiency. where Ah is the battery’s amp-hour capacity to be restored, I is the charger’s output current, and η is the charging efficiency.
Inputs
Ah_batt := 100 A·h=360000 C
I_chg := 10 A=10 A
eta_chg := 0.9=0.9
Dividing the amp-hour capacity by the charging current gives an ideal charge time, which is then increased slightly by dividing by the charging efficiency to account for losses during the charge process.
t_chg := Ah_batt/(I_chg·eta_chg)=40000 s
Results
A charge time of about 11.1 hours to fully restore a 100 Ah battery at 10 A reflects both the simple capacity/current ratio and the extra time the 90% efficiency adds on top of the ideal 10-hour figure. Using a higher-current charger shortens this time roughly proportionally, but very high charge rates can reduce charging efficiency and battery lifespan, so charger current is usually capped well below the value that would give the fastest possible charge.
Assumptions & limitations
- Constant charging current throughout (no taper at the end of charge).
- Charging efficiency assumed constant across the charge cycle.
- Battery assumed fully discharged at the start of the charge period.