Replace your car battery with capacitors!

[redpoint5]

Originally Posted by Enki

If that's peak draw, what about overall consumption during start? I'm not sure what peak draw on my Mazda is, but total power consumption to start is somewhere near 570 amps.

I need to find a way to measure this. My clamp meter only measures min/max, and real time, I believe. There isn't a cumulative option. The meters I bought for cumulative draw have a 100A peak, so I can't use them.

Is that 570 amp seconds you measured? If so, that equates to only 158 mAH (milliamp hours). How did you measure cumulative draw?

[Cobb]

I think you need to check your starter with a V-O-M meter and make sure the armature isnt shorting to ground.

[redpoint5]

Originally Posted by Cobb

I think you need to check your starter with a V-O-M meter and make sure the armature isnt shorting to ground.

I've heard that several times on here. I assure you that 290 peak amps isn't unusual. The Camry draws 255A, and the Cummins 5.9L peaks at over 400A, which is higher than my gauge can go. The motorcycle peaks at 80A, and it only has a 0.6L engine.

A motor at dead standstill IS basically shorted to ground. The copper windings in the armature have very low resistance, so when it's connected to the battery, a huge spike in current flows through them until it gets spinning. Once spinning, the reverse EMF induces a resistance and the amps quickly drop down. The faster the motor spins, the more resistance is induced and amperage is reduced.

The battery only has to sustain the ~300 amps for the fraction of a second it takes for the motor to begin spinning. After that, it might fall to 1/4 of that.

There is a reason batteries need to be rated at 400+ CCA. The initial surge is tremendous.

That said, I have a pretty sweet SuperCap/LiFePO4 setup that I'm going to install right now. I'll update my thread with my findings.

EDIT: Some comments from another forum:

A starter motor from a normal 2 Litre petrol car draws between 60 and 200 Amps when turning over the engine when the oil is warm and thin.

Under winter conditions, this current can easily double when the oil is thick.

Diesel engines have a very high compression ratio e.g. 22:1 and require more powerfull starter motors. On average they draw between 300 and 500 Amps for average sized car engines 2 to 3 litre while on truck engines the current could easily reach 1000 Amps at initial turn over... The resistance in these circuits are between 0.1 and 0.01 Ohms.

I think that 300 amps is a better estimate for that car.

I've heard 70A @ no load and 400A locked rotor @ 4v across the motor. Maybe 250A amps while cranking?

FYI- A stopped motor that has not yet begun to spin IS a locked motor.

The 12 V starter motor for a V8 4 litre diesel takes a peak of around 1000 A.

[oldtamiyaphile]

Originally Posted by redpoint5

I've heard that several times on here. I assure you that 290 peak amps isn't unusual. The Camry draws 255A, and the Cummins 5.9L peaks at over 400A, which is higher than my gauge can go. The motorcycle peaks at 80A, and it only has a 0.6L engine.

A motor at dead standstill IS basically shorted to ground.

The battery only has to sustain the ~300 amps for the fraction of a second it takes for the motor to begin spinning. After that, it might fall to 1/4 of that.

There is a reason batteries need to be rated at 400+ CCA. The initial surge is tremendous.

FYI- A stopped motor that has not yet begun to spin IS a locked motor.

This is exactly what I was getting at. "Peak" is really a misleading term and actually depends on the resolution of the meter being used. A meter that samples at 1000Hz will record a much higher "Peak" than one that samples at 100Hz. Electronic devices have 5, 10, 20 second etc current ratings.

IMO, these high peak values are for such a short amount of time that small LiFe packs are quite capable of handling them. I have one of those 1/5 scale R/C buggies that I converted to electric power it runs the same LiFe cells as my 1:1 Fiat and draws 'peak' currents of 7500W (measured by the onboard data logger) and this happens dozens of times per charge.

That's over 400A

[Cobb]

Peak vs continuous.......

This is why some of the guys who play loud bass use more batteries than an ev and use multiple alternators that are rated at to to 400 amps each for the power supply. Granted caps can help with burping, you need to refill the caps for a continuous drone or bass line.

[Enki]

To measure the total consumption, just use the farad rating of your bank combined with start voltage and end voltage.

For instance, if you have a 100 farad bank, and the voltage when started has dropped 5 volts, you've used 500 amps to start the car.

[Enki]

Also the problem with caps in a car audio system is that when they draw down, the overall load on the system goes up considerably. They reduce flickering, sure, but the overall performance of the electrical system doesn't change much unless you have a rather large cap bank to supply the demanded loads without dropping voltage too much.

1-10 farads isn't going to cut it if you think about how the farad rating works vs what the sound system's burst draw is capable of pulling.

[redpoint5]

Originally Posted by oldtamiyaphile

This is exactly what I was getting at. "Peak" is really a misleading term and actually depends on the resolution of the meter being used. A meter that samples at 1000Hz will record a much higher "Peak" than one that samples at 100Hz. Electronic devices have 5, 10, 20 second etc current ratings.

IMO, these high peak values are for such a short amount of time that small LiFe packs are quite capable of handling them. I have one of those 1/5 scale R/C buggies that I converted to electric power it runs the same LiFe cells as my 1:1 Fiat and draws 'peak' currents of 7500W (measured by the onboard data logger) and this happens dozens of times per charge.

That's over 400A

Good points.

The peaks are still good to know even if they are just ballpark figures and only for a brief moment. Batteries are still rated in burst draw, and for my small 4.2Ah pack, that figure is 168A. I don't think starting my car is good for the battery given this rating. LaserSaber blew the same pack by trying to jump start his lawnmower.

Regardless, my main concern isn't discharge due to starting, but the charge current from the alternator, especially in sub-zero conditions. My battery is rated at only 8.4 A charge at normal temperatures. An alternator would easily exceed this after sitting for a while and then starting a car.

Ultimately I plan to keep the ultra capacitor in the engine bay for starting the vehicle, and relocate the LiFePO4 battery to inside cabin. This will help regulate battery temperature and make it easy to take it indoors to balance charge. Eventually I want to do an alternator kill switch and charge the battery from the grid.

Originally Posted by Enki

To measure the total consumption, just use the farad rating of your bank combined with start voltage and end voltage.

For instance, if you have a 100 farad bank, and the voltage when started has dropped 5 volts, you've used 500 amps to start the car.

I started with about 13v in my 350 F series capacitor bank (58.3F series capacitance). After starting, I hit a minimum voltage of 9v. So, I used 233 amp seconds in just under 2 seconds of cranking the starter.

[serialk11r]

Thanks for measuring the peak amperages, that's data that's hard to find. Seems like 6 350F caps is better used for slightly smaller engines. My 1ZZ probably cranks reliably at 12V with those, so I think I'm going to go with 350F caps when my battery goes. Can't wait to get this giant hulk of a battery out.

[oldtamiyaphile]

Originally Posted by redpoint5

Regardless, my main concern isn't discharge due to starting, but the charge current from the alternator, especially in sub-zero conditions. My battery is rated at only 8.4 A charge at normal temperatures. An alternator would easily exceed this after sitting for a while and then starting a car.

That's a fair point, I went with 16.8Ah capacity for my 875cc Fiat but my biggest enemy has been self discharge while parked, I've put a solar panel on the garage roof to solve than one now.