A simple but practical and cheap EV range extender?

Electric cars, trucks, ATVs, NEVs - things bigger than a motorcycle.
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redilast   1 kW

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A simple but practical and cheap EV range extender?

Post by redilast » May 08 2018 5:47pm

So all production EV's I'm aware of use a 12V flooded lead acid battery which is charged by a DC to DC converter off the main Li-ion pack. This 12v battery powers lights, fans, accessories, and other car electronics. Some articles I read suggested that it may even be powering the heating and cooling systems. If so, an accessory battery which connects directly to the 12v battery which could be housed in the Trunk or back seat, could charge the 12V flooded battery, reducing load on the DC/DC converter and leaving the main Li-ion battery exclusively for powering the motor.

What are your thoughts? Does anyone know for certain what components of a cars heating and cooling system are actually powered by that 12V flooded battery? Is it just fans or is it actually the AC compressor and restive heating element?

I know my Ford Focus Electric can go from 75 mile range to 50-55 mile range with the AC cranked on a 110F degree day. And same goes with heat, if its freezing outside and you have the heat cranked range can drop to 50 miles or less!

An extra say 5Kwh in the Trunk which is used to power the heating and cooling systems could lead to a significant range improvement when using heating/cooling/lighting/accessories etc.

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Re: A simple but practical and cheap EV range extender?

Post by amberwolf » May 08 2018 10:59pm

YOud want to verify what your specific vehicle uses for powering those systems before going further, but if it does indeed power them off the 12v system, then you could add a pack to essentially enlarge that 12v battery, and so that pack would be the same voltage range as that battery so it can simply have a direct connection (most efficient use of the power, no conversion losses, simpler).

5Kwh is probably 80-100lbs of battery, including mounting and containment and wiring and such, depending on which cells you use. More with stuff like LiFePO4 and probably LTO.

Something to think about, also, is that that extra weight will require more power to accelerate from a stop each time, and to climb hills. One figure I found for the FFEV weight is about 3400lbs, so lets be round and say 100lbs of battery added. If I did that math right, thats about 3% extra weight, so 3% extra power needed in those situations. Probably a lot less than the power it contains that is used by the heat/cool systems, but youd need to do the math on that to be sure.

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Re: A simple but practical and cheap EV range extender?

Post by redilast » May 09 2018 2:35am

5Kw could be ~500x 18650's which would be sub 25KG just for the cells or around 55lbs.

I might try connecting my DC clamp meter to the 12V battery cable so I can see what kind of power that puppy is supplying with the AC and heat on etc. I believe the real killer of the battery is the liquid chilled water and heat for the batteries. Draws more than the inside cabin AC/heater.

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Re: A simple but practical and cheap EV range extender?

Post by Alan B » May 09 2018 6:14am

The 12V is used to power 12V items. Not major power loads. So it powers fans and electronics. The biggest load is probably headlights. For example on the A/C the compressor motor runs on the mail traction battery at 400V or whatever, but the controls and air-moving fans will power on 12V. The best heaters use the A/C as a heat pump, so again the real power comes from the traction battery.

Thus the amount of traction battery power that is used by the 12V systems is small. Making actual measurements is a great idea. However the load will be primarily on the DC-DC converter when the car is running, the 12V battery won't see the full load.

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Re: A simple but practical and cheap EV range extender?

Post by billvon » May 09 2018 11:57am

redilast wrote:
May 09 2018 2:35am
I might try connecting my DC clamp meter to the 12V battery cable so I can see what kind of power that puppy is supplying with the AC and heat on etc.
You have to put the clamp on the output of the DC/DC not the battery.
--bill von

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Re: A simple but practical and cheap EV range extender?

Post by redilast » May 09 2018 9:53pm

billvon wrote:
May 09 2018 11:57am
redilast wrote:
May 09 2018 2:35am
I might try connecting my DC clamp meter to the 12V battery cable so I can see what kind of power that puppy is supplying with the AC and heat on etc.
You have to put the clamp on the output of the DC/DC not the battery.
Why? The fans, lights, radio are all drawing power through the battery which is being charged by the DC to DC converter, the same way an ICE car battery is charged with an alternator?

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Re: A simple but practical and cheap EV range extender?

Post by redilast » May 09 2018 9:56pm

Alan B wrote:
May 09 2018 6:14am
The 12V is used to power 12V items. Not major power loads. So it powers fans and electronics. The biggest load is probably headlights. For example on the A/C the compressor motor runs on the mail traction battery at 400V or whatever, but the controls and air-moving fans will power on 12V. The best heaters use the A/C as a heat pump, so again the real power comes from the traction battery.

Thus the amount of traction battery power that is used by the 12V systems is small. Making actual measurements is a great idea. However the load will be primarily on the DC-DC converter when the car is running, the 12V battery won't see the full load.
I would have assumed that would be the case, but the article I read yesterday said the AC/heating system is powered by the 12v battery. Perhaps though it was just meaning fans and thermostat and LCD control panel etc. Are you just guessing or assuming or do you actual know that these cars use AC compressors that run off ~400v?

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Re: A simple but practical and cheap EV range extender?

Post by billvon » May 09 2018 10:21pm

redilast wrote:
May 09 2018 9:53pm
billvon wrote:
May 09 2018 11:57am
redilast wrote:
May 09 2018 2:35am
I might try connecting my DC clamp meter to the 12V battery cable so I can see what kind of power that puppy is supplying with the AC and heat on etc.
You have to put the clamp on the output of the DC/DC not the battery.
Why? The fans, lights, radio are all drawing power through the battery which is being charged by the DC to DC converter, the same way an ICE car battery is charged with an alternator?
Because all energy comes from the DC/DC converter. If you want to measure the amount of energy that is being used by the accessory system that's where you have to look.
--bill von

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Re: A simple but practical and cheap EV range extender?

Post by redilast » May 09 2018 10:24pm

billvon wrote:
May 09 2018 10:21pm
redilast wrote:
May 09 2018 9:53pm
billvon wrote:
May 09 2018 11:57am
redilast wrote:
May 09 2018 2:35am
I might try connecting my DC clamp meter to the 12V battery cable so I can see what kind of power that puppy is supplying with the AC and heat on etc.
You have to put the clamp on the output of the DC/DC not the battery.
Why? The fans, lights, radio are all drawing power through the battery which is being charged by the DC to DC converter, the same way an ICE car battery is charged with an alternator?
Because all energy comes from the DC/DC converter. If you want to measure the amount of energy that is being used by the accessory system that's where you have to look.
That doesn't make sense to me, unless there is a bypass that allows power to flow from the DC/DC directly to the accessories and not through the battery.

To give a simple example of why it doesn't make sense: If I were to take say a single 18650 battery and charge it at 1A @ 3.7VDC. If I draw 2A to power an LED from the battery, measuring the power from the charger (which is a DC to DC converter) has nothing to do with the power I am drawing. Wouldn't it be the same in an EV, again, unless there is a bypass?

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Re: A simple but practical and cheap EV range extender?

Post by billvon » May 10 2018 10:20am

redilast wrote:
May 09 2018 10:24pm
That doesn't make sense to me, unless there is a bypass that allows power to flow from the DC/DC directly to the accessories and not through the battery.
The battery is not an energy source, and no power flows "through" it. In a hybrid it only provides power to start the car (and power for lights etc when the converter is off.)
To give a simple example of why it doesn't make sense: If I were to take say a single 18650 battery and charge it at 1A @ 3.7VDC. If I draw 2A to power an LED from the battery, measuring the power from the charger (which is a DC to DC converter) has nothing to do with the power I am drawing.
Over time it would, since batteries don't charge forever. Over time the energy out of the DC/DC converter is exactly equal to the energy drawn by all the accessories.

(And if you want to replace the DC/DC converter with a battery, you care about energy over time.)
--bill von

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Re: A simple but practical and cheap EV range extender?

Post by Alan B » May 10 2018 6:12pm

redilast wrote:
May 09 2018 9:56pm
Alan B wrote:
May 09 2018 6:14am
The 12V is used to power 12V items. Not major power loads. So it powers fans and electronics. The biggest load is probably headlights. For example on the A/C the compressor motor runs on the mail traction battery at 400V or whatever, but the controls and air-moving fans will power on 12V. The best heaters use the A/C as a heat pump, so again the real power comes from the traction battery.

Thus the amount of traction battery power that is used by the 12V systems is small. Making actual measurements is a great idea. However the load will be primarily on the DC-DC converter when the car is running, the 12V battery won't see the full load.
I would have assumed that would be the case, but the article I read yesterday said the AC/heating system is powered by the 12v battery. Perhaps though it was just meaning fans and thermostat and LCD control panel etc. Are you just guessing or assuming or do you actual know that these cars use AC compressors that run off ~400v?
A/C Compressors that run from 12V exist, they are specialty devices that draw around 40-50A. This power would have to come from the DC/DC converter because the 12V battery could not handle it for very long, and all 12V power comes from the traction battery eventually anyway. This would require the DC/DC converter to have much higher power and would add the inefficiency of this conversion to the total power cost. So no EV that was well designed would use 12V for the A/C compressor. It is just a waste of power and additional DC-DC converter cost and capacity.

The 12V power is there to power 12V type equipment, and the 12V battery is there to "start" the vehicle. The 400V traction battery is cut off until the 12V powered high voltage contactor brings it online. Once the traction battery is online the DC-DC converter provides the operational 12V power. The battery is just there to get things started and handle any short term current surges above the capacity of the DC-DC converter. The 12V battery is charged from the DC-DC converter. The 12V will power the control logic, temperature sensors and fans, headlights, stereo and so on.

Not all articles are precise or correct. Reading a few more articles will yield a better perspective. A quick search shows that Tesla uses HV on the compressor, and I have read about the Leaf using HV as well. Even the Prius uses a high voltage compressor since the engine is not always running.

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Re: A simple but practical and cheap EV range extender?

Post by redilast » May 11 2018 3:48am

billvon wrote:
May 10 2018 10:20am
redilast wrote:
May 09 2018 10:24pm
That doesn't make sense to me, unless there is a bypass that allows power to flow from the DC/DC directly to the accessories and not through the battery.
The battery is not an energy source, and no power flows "through" it. In a hybrid it only provides power to start the car (and power for lights etc when the converter is off.)
To give a simple example of why it doesn't make sense: If I were to take say a single 18650 battery and charge it at 1A @ 3.7VDC. If I draw 2A to power an LED from the battery, measuring the power from the charger (which is a DC to DC converter) has nothing to do with the power I am drawing.
Over time it would, since batteries don't charge forever. Over time the energy out of the DC/DC converter is exactly equal to the energy drawn by all the accessories.

(And if you want to replace the DC/DC converter with a battery, you care about energy over time.)
Sorry, you are not making sense, and I believe you are incorrect. DC/DC converter literally dumps power to the 12V + and - terminals of the battery. These same 12v battery + and - connections have cables to the cars chassis which then powers accessories.

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Re: A simple but practical and cheap EV range extender?

Post by Alan B » May 11 2018 7:41am

The DC/DC converter, 12V battery, and 12V loads are all in parallel. While the car is started the 12V battery supplies the 12V to the electronics and the high voltage contactor. Once the contactor makes connection between the traction battery and the DC-DC converter the converter then generate 12V power and this recharges the battery and supplies the 12V loads. Current flows into the battery to recharge it. At any time the 12V demands temporarily exceed the capacity of the DC-DC converter the 12V battery makes up the shortage, but over time this is recharged by the DC-DC converter.

The 12v battery is lead-acid, so it takes more current to charge it than it delivers (peukert effect, inefficient charge cycle). On average this makes the amp-hours into the battery greater than the amp-hours out, it consumes power. It is only a source of power on a short time scale. On the long time scale it is a consumer of power. The only significant source of power is the traction battery. The energy capacity of the 12V battery is about 400 watt hours compared to the energy capacity of the traction battery at 20 kilowatt hours or more.

The exact same cycle occurs in an ICE vehicle, the only difference is the alternator takes the place of the DC-DC converter.

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Re: A simple but practical and cheap EV range extender?

Post by billvon » May 11 2018 10:11am

redilast wrote:
May 11 2018 3:48am
Sorry, you are not making sense, and I believe you are incorrect. DC/DC converter literally dumps power to the 12V + and - terminals of the battery. These same 12v battery + and - connections have cables to the cars chassis which then powers accessories.
Right. So if you measure power to/from the battery, it will be zero over time, although you will see surges when the car starts (i.e. power flows out of the battery) and surges when the DC/DC starts up (i.e. power flows into the battery.)

So you have an average of (approximately) zero energy. Will you then conclude that the car requires zero accessory energy?
--bill von

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Re: A simple but practical and cheap EV range extender?

Post by dustNbone » May 11 2018 2:06pm

It makes perfect sense that they'd use 12V for fans, climate controls, etc as those parts are most likely based on off the shelf automotive stuff that's not specific to an EV.

Heaters and A/C compressors are normally accessories of the ICE package, the compressor driven directly off the engine and the heater using engine coolant through a heat exchanger.

Those parts won't work (or at least not well) with an electric drivetrain, so they are substituted with electric parts, and the most efficient way to do that is with parts designed to run off the traction pack voltage

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Re: A simple but practical and cheap EV range extender?

Post by redilast » May 18 2018 4:49am

billvon wrote:
May 11 2018 10:11am
redilast wrote:
May 11 2018 3:48am
Sorry, you are not making sense, and I believe you are incorrect. DC/DC converter literally dumps power to the 12V + and - terminals of the battery. These same 12v battery + and - connections have cables to the cars chassis which then powers accessories.
Right. So if you measure power to/from the battery, it will be zero over time, although you will see surges when the car starts (i.e. power flows out of the battery) and surges when the DC/DC starts up (i.e. power flows into the battery.)

So you have an average of (approximately) zero energy. Will you then conclude that the car requires zero accessory energy?
Completely incorrect just as I originally guessed. Just took a clamp meter and tested the car under various load conditions. It draws 25A upon startup (when some pump is running, presumably some hydraulic pump for breaks/power steering). Then settles down to around 20A with headlights on. This is current measured from the 12v battery ground using a DC clamp meter. Even while charging it seems to use 5A to power a coolant re-circulation pump. When the AC or heat is on it draws a few more amps because the fans are probably 12VDC fans.

I'm aware that the DC-DC converter is probably sending power to the battery at the same time, so most of the power may be coming off the DC-DC converter, but as I originally stated, the power is coming off the battery terminals, I never said that the battery was being discharged. The DC-DC converter works most likely very similarly to the way a ICE car's alternator charges the battery and powers accessories through the battery, but without cycling the battery.

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Re: A simple but practical and cheap EV range extender?

Post by jonescg » May 18 2018 9:58pm

Yeah when you look at the major loads in an EV, they probably stack up like so:

Driveline (inverter, motor): 100 kW+
Aircon: ~6 kW
Heater: 2-5 kW
Pumps_fans: <1 kW
Headlights: ~100 W

So anything that draws more than 2 kW is best run from the traction pack. At 12 volts, 2 kW is 167 amps - far too much for the auxiliary battery and DC/DC converter. However if powered from the 355 volt traction battery, you're looking at a 5 or 6 amp load. Easily managed by contactors and/or some solid state electronics.

I recently bought two of these for a couple of car conversion projects:
Wurz AC compressor.jpg
Wurz AC compressor.jpg (225.24 KiB) Viewed 1110 times
As well as a water heater for heating the cabin (mainly for demisting the windsheld):
water heater.jpg
water heater.jpg (142.62 KiB) Viewed 1110 times
Both are powered by the 355 V DC bus, but controlled electronically on the 12 V system.

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Re: A simple but practical and cheap EV range extender?

Post by billvon » May 18 2018 11:12pm

redilast wrote:
May 18 2018 4:49am
Completely incorrect just as I originally guessed. Just took a clamp meter and tested the car under various load conditions. It draws 25A upon startup (when some pump is running, presumably some hydraulic pump for breaks/power steering). Then settles down to around 20A with headlights on. This is current measured from the 12v battery ground using a DC clamp meter.
Then you are measuring it incorrectly. Batteries are not an energy source; they simply store it.
--bill von

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Re: A simple but practical and cheap EV range extender?

Post by redilast » Jun 13 2018 10:35pm

billvon wrote:
May 18 2018 11:12pm
redilast wrote:
May 18 2018 4:49am
Completely incorrect just as I originally guessed. Just took a clamp meter and tested the car under various load conditions. It draws 25A upon startup (when some pump is running, presumably some hydraulic pump for breaks/power steering). Then settles down to around 20A with headlights on. This is current measured from the 12v battery ground using a DC clamp meter.
Then you are measuring it incorrectly. Batteries are not an energy source; they simply store it.
"The power is coming off the battery terminals, I never said that the battery was being discharged. The DC-DC converter works most likely very similarly to the way a ICE car's alternator charges the battery powers accessories through the battery, but without cycling the battery."

Is it that hard to understand? It's like you are having difficulties reading or something.... I never said the batteries are the primary energy source. I said though that power is flowing from the DC-DC converter and hitting the battery terminals, and from the battery terminals the same conductors show current draw to accessories. That doesn't mean its charging it and cycling the battery constantly.

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Re: A simple but practical and cheap EV range extender?

Post by billvon » Jun 14 2018 9:53am

redilast wrote:
Jun 13 2018 10:35pm
I never said the batteries are the primary energy source. I said though that power is flowing from the DC-DC converter and hitting the battery terminals, and from the battery terminals the same conductors show current draw to accessories. That doesn't mean its charging it and cycling the battery constantly.
You said "This is current measured from the 12v battery ground using a DC clamp meter." Kirchoff's Law says that if the current is flowing through the ground cable to the frame, then it is flowing through the battery. And if you are seeing a more or less steady 20 amps then that is a constant discharge (or charge, depending on the polarity of the current flow.)

I suspect you were measuring something else, because the other part of the post above is spot on - the DC/DC supplies ~20 amps to the battery bus, and then accessories draw that current from the battery bus, with little to no current sourced or sunk from the battery. In that case the current measured on the battery ground terminal would be close to zero.
--bill von

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Re: A simple but practical and cheap EV range extender?

Post by Punx0r » Jun 14 2018 3:44pm

I think there's some confusion here as to whether the dc-dc converter across the battery terminals (and so uses the battery ground strap) or is connected elsewhere to the car's wiring, with a separate ground.

The first scenario will show the current from the dc-dc flowing through the battery ground strap (even if it's not flowing INTO the battery), the second wouldn't.

That said, in both scenarios if the current drawn from the 12V system exceeds the output of the dc-dc then the battery will begin discharging to make up the shortfall, so you would see at least some current on the battery ground strap.

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