Extending the Granny cable

[Chrispydoc]

worth knowing.....

 

[Kithmo]

Sorry, what I meant was, are there more or less losses (e.g. through heat etc.) on one or the other, in other words (being a tight Yorkshireman) would there be a difference in the cost to charge a fixed amount at a fixed electricity rate.

 

[JodyS21]

Sorry, what I meant was, are there more or less losses (e.g. through heat etc.) on one or the other, in other words (being a tight Yorkshireman) would there be a difference in the cost to charge a fixed amount at a fixed electricity rate.

Interesting one.
I can’t believe there can be any real world difference.
There is bound to be slightly more heat produced from all the bits&bobs at a higher current, more heat does equal wasted energy.
But, if it’s really cold, then it’s not ideal conditions for the battery, so a bit more heat as a by product maybe beneficial.
There will be X amount of energy used to do all of the monitoring etc within the car as it will need to be powered up to a certain extent when doing the charging, so you could say if charging at half the current then the car will be awake for twice as long and therefore using twice as much of this energy.

 

[Coulomb]

The granny charger is quite small for a 3kW charger but I suppose it is possible.

It isn't a charger. It's basically a relay, with some electronics to control the relay and generate a PWM signal. The actual charging is done inside the car, with the On Board Charger. The granny brick is properly called an EVSE (Electric Vehicle Service Equipment), but it's a losing battle trying to correct people on this.

DC rapid chargers are actually chargers; that's why they can charge faster than the OBC. The OBC isn't used in DC charging.

 

[Coulomb]

are there more or less losses (e.g. through heat etc.) on one or the other, in other words (being a tight Yorkshireman) would there be a difference in the cost to charge a fixed amount at a fixed electricity rate.

Slow AC charging will spend a slightly larger proportion of the overall energy (and hence cost) running pumps and other overheads. So medium charging (at a wild guess, something like 7 kW) would be the most efficient way to charge, charging the battery with the fewest pennies.

Edit: Some Renault Zoes, for example, have an unusual charging system with large overheads; they don't recommend very slow AC charging for that reason (you might get up to 80-90% of the power from the AC in swallowed up in overheads).

 

[Deleted member 2439]

My choice is to actually run my 7kW Zappi at 1.4kW. So why do I do that I hear you think? Easy, during the day the car is away at work and my solar PV system charges up my house battery system. I then work out how much stored energy I have to play with and can let the car have. 1.4kW is the slowest charge rate it will do, so I often leave it on overnight and let the car have 10 or 12kWh. Of course, this strategy only works when there’s been sufficient sunshine, but I don't charge the car every day anyway. During the winter the Zappi runs at full speed charging with Octopus Go - which I also use to charge my house battery system for use during the following day.

 

[Deleted member 2439]

Slow AC charging will spend a slightly larger proportion of the overall energy (and hence cost) running pumps and other overheads. So medium charging (at a wild guess, something like 7 kW) would be the most efficient way to charge, charging the battery with the fewest pennies.

Edit: Some Renault Zoes, for example, have an unusual charging system with large overheads; they don't recommend very slow AC charging for that reason (you might get up to 80-90% of the power from the AC in swallowed up in overheads).

I think the Zoe system you refer to actually uses the motor windings as a filter to stop any noise spikes getting into the rectification/charging system. It’s a good idea in some ways, but is presenting the resistance of those same windings in series & this wasting energy. I saw a video about this on YouTube recently.

 

[Coulomb]

the Zoe system you refer to actually uses the motor windings as a filter to stop any noise spikes getting into the rectification/charging system.

My understanding is that the Zoe uses the motor controller as a big part of the AC on-board charger, and the inductance of the motor is a necessary part of the power converter. This seems like a clever idea, making use of high-power electronics that is already present in the vehicle, and reduces weight and cost. A colleague and I considered this design when converting an mx-5 to electric; the first design used an off the shelf VFD as the motor controller. But it turned out to be be physically too big for the compact convertible.

The disadvantage of this scheme is that the motor windings are not optimised for charging, but for driving. Driving uses a lot more power than is typically available when AC charging. That mismatch results in poor efficiency when charging at low power.

 

[Tim Green]

I take the point about lower efficiency when using low power charging systems.
However, if using a granny charger on a home electricity supply you will probably be on a pretty low £/kW rate and thus still get a low cost recharge.

 

[Deleted member 2439]

I take the point about lower efficiency when using low power charging systems.
However, if using a granny charger on a home electricity supply you will probably be on a pretty low £/kW rate and thus still get a low cost recharge.

Indeed yes. 5.5p with Octopus go, or “free” using solar or stored solar. It isn’t actually free of course as I had to buy the solar and storage systems in the first place……

 

[notdodgy]

Well the charge time (from empty) of 18 hours corresponds to 3kW constant charge. The granny charger is quite small for a 3kW charger but I suppose it is possible.

Chargers are wrongly named.

The AC "chargers" - Granny and fast are not actually chargers.
They are charge controllers which switch on and off the mains electricity and signal to the real charger inside the car how much it is allowed to take from the supply.
The size only has to be big enough to contain a power relay, protection devices and a circuit board, so Granny and a 7kW are similar sizes.

I think they make them bigger so it looks like you are getting something substantial for your £500 rather than a box and £20 worth of components.
The charger in the car takes the mains power and converts it to DC at a voltage high enough to charge the battery.
(The mains 10 amps probably results around 5-6 amps going into the battery).

Rapid chargers are effectively a much bigger version of the charger that is inside the car.
They convert AC to DC and supply DC direct to the battery.

 

[RichardC]

Chargers are wrongly named.

The AC "chargers" - Granny and fast are not actually chargers.
They are charge controllers which switch on and off the mains electricity and signal to the real charger inside the car how much it is allowed to take from the supply.
The size only has to be big enough to contain a power relay, protection devices and a circuit board, so Granny and a 7kW are similar sizes.

I think they make them bigger so it looks like you are getting something substantial for your £500 rather than a box and £20 worth of components.
The charger in the car takes the mains power and converts it to DC at a voltage high enough to charge the battery.
(The mains 10 amps probably results around 5-6 amps going into the battery).

Rapid chargers are effectively a much bigger version of the charger that is inside the car.
They convert AC to DC and supply DC direct to the battery.

This is adding to my education. Before I got my EV the only thing that I knew about electricity was how to wire a plug.

Over the last week my knowledge had developed dramatically