# Charging Battery

#### Isochroous

##### 1 µW
Hi
I'm just trying to get a rough idea of how long it would take a standard 4 amp Bosch charger to provide a one watt charge. My understanding of amps and watts is completely lacking in this context. According to Bosch, a 50% charge of the PowerTube 625 will take 2.1 hours and a 100% charge will take 5.4 hours, so if I understand things correctly, one watt will take ~1/2 minute. Yes or no?
Since my bike has two 625 watt batteries, a 50% charge would take ~4.2 hours. Is this correct?

The formula would be WH (watt-hours) = Voltage (volts) x Current (amps) x Time (hours)

Your voltage varies from 30V for a completely discharged battery to 42V for a fully charged battery, Using 36V is close enough, The current is 4 amps because you have a 4A charger, Finally, time is 2.1 hours.

36 volts x 4 amps x 2.1 hours = 302 watt-hours.

OK, if a powertube 625 means you have 625 watt-hours, the above is close enough. However, the formula doesn't really care about the battery size. WH =volts x amps x hours. The volts and amps are set by the charge. You set the time.
.

to complicate things, lithium ion chargers initially charge with constant current, and when the charge capacity approaches full, it switches to constant voltage to nurse the cells to full capacity (CC/CV charging). At this point, the charge current will fall steadily until the battery is full. This will mean the battery will take a little longer to charge than as calculated assuming a steady current.

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