Using MightyBoyEV - Real life at the wall kilowatt-hours.....
The true cost of using an EV is usually considered to be the actual energy taken out of the plug to make the vehicle run. So this page covers the exercise of starting with a fully charged pack, driving some distance and measuring the kilowatt-hours at the mains to completely recharge the pack. This measurement takes into account the overall charging inefficiencies.
Charger efficiency: The Canadian designed Delta-Q charger information states the following...."The high-efficiency design and near unity Power Factor combine to make the QuiQ charger extremely grid-friendly. Over 88% of power taken from the grid is converted to real power to charge the battery. This not only saves users over 30% in raw electricity costs when compared to ferro-resonant chargers, but avoids Power Factor surcharges from utilities as well".
Battery charge efficiency: The next issue is with the charging inefficiency of lead acid batteries which is largely a function of the state-of-charge (SOC) when you start the re-charging process.
This article at http://photovoltaics.sandia.gov/docs/PDF/batpapsteve.pdf states.....
A test procedure has been developed to allow the examination of battery charge efficiency as a function of battery state of charge. Preliminary results agree well with established general understanding that the charge efficiency of flooded lead-antimony batteries declines with increasing state-of-charge, and that charge efficiency is a non-linear function of battery state-of-charge. These tests indicate that from zero SOC to 84% SOC the average overall battery charging efficiency is 91%, and that the incremental battery charging efficiency from 79% to 84% is only 55%. ….….”
This is reinforced by this Powersonic graph from their data sheet for these AGM cells
As can be seen above, the efficiency drops off at higher SOC levels
In my test case below I was starting the re-charge at approximately the position of the red arrow....
So the overall charging efficiency of approximately 50% would be correct for this example.
Results are as follows:
Details of the test run - power taken from the pack (as taken from the Cycle Analyst with fully charged pack)
Distance travel = 14.8 km (or 9 miles)
MaxSpeed = 68.3km/h (limited by road speed limits in my area of 50 or 60 km/h)
AvgSpeed = 46.5 km/h
Gear Used = 4th only
Trip duration = 19 mins
MaxAmps = 310 amps
VMin = 61.5 volts (as occurred when 310 Amps pulled,,,)
Watt-Hours per Km = 82.4 w-h/km (or 135 w-h/mile)
Watt Hours used from pack = 1214 watt-hrs
Details of the full re-charging process - power put back into the pack (as read via a series plug in watt-hour meter)
In my case, the total watt - hours taken from the wall to completely recharge the pack (including pulsed "finishing" stages etc as shown below) was found to be 2400 watt - hours
So for the 14.8 km (or 9 miles) travelled, that would equal 162 w-h/km (or 266 w-h/mile) at the wall
Thus at my current "Off-Peak" rate of 7 cents per kilowatt-hr that 14.8 km would cost 1.1 cents per km (or 1.9 cents per mile)
So starting the re-charge with a SOC of approximately %80 the overall charging efficiency is approximately 50%.
Worth noting that starting the re-charge at a SOC of 40% (%60 DOD) would most likely increase this overall charging efficiency to 70 -> 75 % or approximately 125 w-h/km (or 200 w-h/mile) at the wall. So it would in fact be more efficient and cheaper (plus maybe prolong battery life?) to not shallow re-charge if possible. With small low voltage vehicles with limited range this is not very practical as you need all the energy available for normal use. The additional power costs between an overall charging efficiency is approximately 50% vs %70 is not significant at the 7 cents per kilowatt-hour rate anyway.....
About the batteries:
Below are the charging details - algorithm 42 currently programmed into the Delta-Q
(Note - This is a cyclic charger NOT a float charger and in my view better suit for these type of application)
Charge set points are:
I1 = Max - Pulse
I2 = Max
V1 = 2.41 Vpc
I3 = 1.5A
Overcharge = 110%
V2 = 2.60 Vpc
V3 = 2.35Vpc
Battery balance and pack voltages @ full charge (as measured at batteries directly - for future reference):
Battery 1 = 13.11 Volts
Battery 2 = 13.10 Volts
Battery 3 = 13.11 Volts
Battery 4 = 13.14 Volts
Battery 5 = 13.09 Volts
Battery 6 = 13.12 Volts
Total Pack Voltage = 78.2 Volts
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