charging time calculation
- Thread starter RyanI3
- Start date
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theothertom
Well-known member
Your calculations are theoretically correct, but:
1. You didn't allow for resistance/heat losses. This will add time to your charge
2. How do you know your circuit draws 13 amps?
3. Charging rate is not linear. BMW designed the circuits in your car to slow the charging rate as you near 100% so as not to overheat/overcharge the battery.
1. You didn't allow for resistance/heat losses. This will add time to your charge
2. How do you know your circuit draws 13 amps?
3. Charging rate is not linear. BMW designed the circuits in your car to slow the charging rate as you near 100% so as not to overheat/overcharge the battery.
Its nominal gross capacity or energy is 21.6 kWh with only 18.8 kWh usable.RyanI3 said:
Where are you located? In North America, residential power is nominally 240 V. In much of Europe, it's 230 V, but there are many local variations which could be as low as 220 V.RyanI3 said:
In North America, a charging device shouldn't consume more than 80% of the charging circuit's capacity to prevent its circuit breaker or fuse from heating over time and tripping or failing. It's unclear whether your charging circuit or charging device is rated at 13 A.
Volts x amps / 1000 = kilowatts (kW), a unit of power. kWh is a unit of energy.
Charging efficiency at 240 V and 30 A has been measured to be 94% and 91% at 6 A. Assuming 92% under your situation:RyanI3 said:
18.8 kWh / 2.86 kW / 0.92 = 7.15 hours
However, this assumes constant charging power which is true only in the charge level range from 0% to ~80%. Above ~80%, the charging power decreases as the charge level approaches 100% which makes calculating the expected charging time impossible without knowing more details about the charging power taper. However, if we assume a linear taper from 80% to 100%, our charging time estimate would be more accurate:
(18.8 kWh x 0.8 / 2.86 kW + 18.8 kWh x 0.2 / 1.43 kW) / 0.92 = 8.57 hours
jadnashuanh
Well-known member
The charging rate will also depend on the temperature of the battery pack when it starts. Plus, it may require some battery heating if it is quite cold and a slow start to charging. Or, it may require some cooling before any useful charging can occur.
In the USA, the circuit for a device that can be on for more than 3-hours at a time is required to be able to provide 125% of the draw, which, depending on how you look at it, is 80% of the circuit's capacity.
In my home, my incoming voltage is often in the 248vac range, which is enough to max out my i3 with a 30A EVSE. At least in the USA, a 13A circuit should not be loaded to provide more than 13*0.8=10.4A. If the device is rated for 13A, then the circuit should be 13*1.25=16.25A (an uncommon breaker or fuse rating).
The EVSE itself will use some power, but generally, not a whole lot. It has an internal power supply and a logic board, indicator lights, plus, often a relay coil that will consume some power even in standby when plugged in. Mine only seems to draw about 4-5W, so in the scheme of things, not relevant, but real.
In the USA, the circuit for a device that can be on for more than 3-hours at a time is required to be able to provide 125% of the draw, which, depending on how you look at it, is 80% of the circuit's capacity.
In my home, my incoming voltage is often in the 248vac range, which is enough to max out my i3 with a 30A EVSE. At least in the USA, a 13A circuit should not be loaded to provide more than 13*0.8=10.4A. If the device is rated for 13A, then the circuit should be 13*1.25=16.25A (an uncommon breaker or fuse rating).
The EVSE itself will use some power, but generally, not a whole lot. It has an internal power supply and a logic board, indicator lights, plus, often a relay coil that will consume some power even in standby when plugged in. Mine only seems to draw about 4-5W, so in the scheme of things, not relevant, but real.
Not mentioned but our BMW i3 come with two maximum charging rates:
My 7.4 kW AC charger car on a full power, L2 charger, adds ~20% SOC per hour. Around 92%, the charge tapers off in the last 35-40 minutes:
The block during taper at 18:15 is when the A/C came on to cool the battery.
My recommendation is when the car returns with SOC below 60%, plug-in the car and note the time. An hour later, disconnect the car and measure the SOC. Use the change in SOC over the time interval to get your charging rate in %/min or scale to %/hour.
GOOD LUCK!
Bob Wilson
- 3.7 kW - mostly found in the EU, many early versions had this limit
- 7.4 kW - standard in North America and also found in the EU, this is the fastest, built-in charger
My 7.4 kW AC charger car on a full power, L2 charger, adds ~20% SOC per hour. Around 92%, the charge tapers off in the last 35-40 minutes:
The block during taper at 18:15 is when the A/C came on to cool the battery.
My recommendation is when the car returns with SOC below 60%, plug-in the car and note the time. An hour later, disconnect the car and measure the SOC. Use the change in SOC over the time interval to get your charging rate in %/min or scale to %/hour.
GOOD LUCK!
Bob Wilson
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4U8 Fast Charging AC (Schnell-laden Wechselstrom)bwilson4web said:
