[ayasbek]

Hi all,

This is my first post. I recently purchased a voltsrabbit - 1979 VW rabbit with 96 volt DC kit from Electro Automotive. The price was right and the car needed some love to get back on the road. The pleasant surprise was that the batteries are still ok (or so I believe). They are US-125 lead acid 6v batteries. They are about 5 years old (I think) and hold a charge well. I think they were very lightly used and always kept charged. Everything seems good. The car drives fine and the range is looking to be about 15 miles (have yet to really push the car). My daily round trip is 5 miles which I do twice a day.

I recently made an arduino based amp-hour meter to replace the broken amp meter in the car. To my surprise the meter works! But then I noticed that the car is using about 600 watt-hours per mile. I thought I had messed up in programming the arduino so I put a kWh meter on the outlet that the charger is plugged into. I got almost the exact same result - 6.55 kWh to recharge from a 11.3 mile trip. I am fairly certain that this is from full charge to full charge. This is in town stop start driving.

So now I am stumped. How do I figure out why a vehicle is so horribly inefficient? There is no obvious brake drag, the individual battery voltages all look good. Fully charged the pack voltage is about 104 volts. Even the instantaneous amperage readings indicate something is not right - cruising around at 30 mph draws around 200 amps! What do I do?

Is it really the batteries? (I may be deceiving myself because I really want to believe that they are ok). Would old batteries really cause such high amperage draw?

The controller is a Curtis 1221 and the motor is an Advanced DC 206-03-4001 (8"). Could a funky controller or motor be the cause?

Any help is appreciated and if this post is in the wrong place please let me know. Thank you.

 

[Ziggythewiz]

I'd say you need a third opinion. Especially for a short trip the charging efficiency of lead is far worse than for driving, so if you're seeing 6.55 from the outlet you're probably using 4-500 wh/m. It could be as low as 300 though.

At the least I'd recommend using a DMM to measure something like brake or parking lights to get a good current reading that you could test with the arduino.

Also, what's the weather like and what gear do you drive in?

 

[onegreenev]

Lead sucks when cold. Totally. Lithium sucks too in the cold but not so much. I'd double check the battery, motor connections, brakes, Alignment. Stock tires suck amps and lessen the distance. Check wheel bearings and replace and regrease if needed. Your beastie is old and abused and needs to be loosened up. Might check the brushes too on the motor. Maybe the bearings too.

I would question your arduino Amp hour counter. These have been a bit of a bugger to build.

 

[CrazyAl]

A couple of questions:

1. What gear are you running in?
2. What is the final drive ratio?
3. What RPM is the motor running at?
4. Are you going up hills?
5. What sort of driving are you doing? eg. driving up hills, stop-go etc?

 

[Duncan]

Another question for you
- 200 amps at 30mph?
Is that motor current or battery current??

 

[piotrsko]

1: your batteries are OLD, and probably below 1/2 life. Unless you have forklift batteries (you don't) it is time to think about replacing. They really don't like sitting, even fully charged, because the sulfation process never stops. I expect to have lost 15% this winter because I didn't drive the Ranger.

2: unless they are at 70 degrees and above, range/capacity is gonna really suck, much worse than in the summer. Figure perhaps 20% decrease for every 10 degrees below 80. At 40 Degrees the Ranger just makes it out of the driveway and to the top of the street.

3: with the aerodynamics of a polished brick, the Ranger runs at about 430 wh/mi, but I have some obscene hills. On one uphill, I pull 160 v and 380 battery amps for 2.5 miles out of my 1/2 ton 192v pack at 55 mph. The pack is rated 25 KWH, and I get probably 26 miles flat and level on the freeway at 70% SOC. I don't know, I haven't EVER been able to run flat and level for the whole trip. Your 6.5 KWH for an 11 mile trip is what I plan on formy use.

If you are currently OK with the range, run them until everything gets re-loosened, and you start getting low voltage warning as you arrive at your charging point. you MAY get a small increase in capacity.

 

[CrazyAl]

1: your batteries are OLD, and probably below 1/2 life. Unless you have forklift batteries (you don't) it is time to think about replacing. They really don't like sitting, even fully charged, because the sulfation process never stops. I expect to have lost 15% this winter because I didn't drive the Ranger.

Maybe Inox Battery Conditioner might help dissolve and restore the lead acid batteries. Although I have used it on one of my lead acids, I don't have scientific proof of its effectiveness, but might be worth a try. I did use it when the battery was fairly new and I still have the battery.

 

[lou-ace]

I would be really suprised if your really getting 600ah/mi from your rig. the method of calculating energy use in an EV can not be confused with the method of calculating say gas milage. charged - energy use + recharge and the difference IS NOT the same as something as static as gas milage. first of all with lead acid batts. ( especially old batts) there is loss in both charging and discharging of the batts that is very dependant on the age of the batts, temperature C rate, ect. there is also the pukert factor for your type of battery. To truely determine the ah/hr rate you are using you really need an Ah/hr meter. do a good search and you will find a number of ah/hr meters that are reasonable. Do check your drive drags, ( its easy to lift one wheel at a time and make sure it turns easily in neutral to find the binding break ect), get a good meter and switch to lithium for better preformance. good luck.

 

[ayasbek]

Wow, thank you all for the suggestions. To answer some questions:

The weather is great here in California - always between 45 F (cold mornings) and 70 F (mid-day). The drive is all stop and start. No hills. I drive between 25 and 35 mph in mostly second and shifting up to 3rd on the longer - faster sections.

The arduino amp-hour meter is connected to the shunt and measures the mV drop across it. This only measures the current going to the motor. It does not monitor charging current. How do I tell the difference between motor current and battery current? I am pretty sure the shunt is between the controller and the motor.

I don't know the drive ratios or the rpm of the motor.

I am pretty confidant that the tires are low-rolling resistance, the alignment and bearings are ok and that there is nothing obviously not right (unless you consider the old batteries).

So even if the on-road energy use is not 600 wh/mi The car is still using this much energy in charging/re-charging once all the losses are factored in.

I will poke around some more soon and report back if I find anything odd.

Thanks again.

-Alex

 

[Ziggythewiz]

The weather is great here in California - always between 45 F (cold mornings) and 70 F (mid-day). The drive is all stop and start. No hills. I drive between 25 and 35 mph in mostly second and shifting up to 3rd on the longer - faster sections.

45 F is not great for batteries. With lead you can lose half your capacity at that temp, and the sag will make you pull much higher amps than if they are above 70. My bug uses 20% more juice in 3rd compared to 2nd so that can take a hit too.

 

[Siwastaja]

The arduino amp-hour meter is connected to the shunt and measures the mV drop across it. This only measures the current going to the motor. It does not monitor charging current. How do I tell the difference between motor current and battery current? I am pretty sure the shunt is between the controller and the motor.

To monitor energy usage from motor current, you need to simultaneously monitor motor voltage too (because it is lower than the battery voltage), both integrated at high frequency or low-passed by an analog RC filter.

It's easier to monitor and integrate battery current (going from battery pack to the controller) and voltage.

My bet is that you don't have one single reason for such high figure. Your motor current metering probably gives you results too high. If you get similar results from wall energy meter, it's because of the poor charging efficiency of old lead acid.

Update to LiFePO4 as you need to do it anyway soon, at the same time you need a working Ah counter in the battery line to work as a "fuel gauge", and it can be used to estimate the efficiency. And, charging efficiency for slow charge will be somewhere near 99.9%.

 

[dougingraham]

The arduino amp-hour meter is connected to the shunt and measures the mV drop across it. This only measures the current going to the motor. It does not monitor charging current. How do I tell the difference between motor current and battery current? I am pretty sure the shunt is between the controller and the motor.

So even if the on-road energy use is not 600 wh/mi The car is still using this much energy in charging/re-charging once all the losses are factored in.

You are measuring the motor current. The controller combined with the motor acts as a switching power supply which steps down the battery voltage and acts as a current multiplier. You need to be measuring this on the battery side of the controller. If you were reading watts it would be nearly the same on either side of the controller but amps is not. Amps will always be lower on the battery side unless the motor controller is at 100% on.

With Lead Acid I can believe you are seeing much higher numbers at the wall because there are losses in the charger and in the batteries. Of course the wall is what matters because you pay someone for that. Switching to Lithium gets rid of most of the charging losses in the batteries but there is still some losses in the charger itself.

Move the shunt to the battery side of the controller and the AH/mile will go down some. It won't change the wall side of things. To do that you will probably need to switch to Lithium unless there is actually something wrong with the car. With Lithium I am seeing about a 20% difference between the wall and the car which is almost all in the charger. On cold days this goes up to about 30% because the sag in the batteries requires more current to do the same work.

 

[kennybobby]

Saggin' out the batteries...

... cruising around at 30 mph draws around 200 amps!

i think your meter is reading fine and is consistent with the charging data. Your rig is not efficient due to worn out batteries.
Can you put a volt meter on the battery pack and measure it while under driving load? i'm guessing that you will see the pack is sagging down to about 47 to 58 volts depending upon whether you are in 2nd or 3rd gear, and your motor is only turning at about 2200 to 3000 rpm at the sagged voltage. That kit was rated at 65 mph top speed and 60-80 mile range--how does that compare with what you can get out of it now? Those batteries are advertised to provide 75 amps for 140 minutes--think they will do that now?
You could securely jacked up the vehicle so you can run the wheels at 30 mph and measure the current and voltage without the road load to get another data point for comparison.

 

[ayasbek]

OK. I moved the shunt so that it is now between the batteries and the controller. Everything now looks fine! The current draw is reasonable (around 100 to 150 amps when cruising, and around 250 amps when accelerating). On my 12 mile trip today it averaged 450 watt-hours/mile. Not great, but understandable given the age of the batteries, the stop-start driving, and speeds between 25 and 35 mph. By the way, this is calculated by multiplying the amp-hour reading by 96 volts (the nominal pack voltage). I have also been watching the volt meter while driving. Fully charged is about 104 volts while driving the voltage sags down to mid 80's and returns to mid 90's. It will go down to mid 70's for a second when accelerating hard. I did get it to dip to 68 volts for a second by pulling off uphill in second (skipped first gear).

I plan to soon drive the car until it runs out of juice to see how many amp-hours I can pull out of the pack. I will post that here when done.

I know I NEED lithium batteries (and incidentally I need to rob a bank) but the current lead pack is looking like it is going to work for a few more months.

Thanks to everybody for your suggestions and ideas. I have no idea why the person who built the car put the shunt between the controller and the motor. It would never have occurred to me to look. Thanks again.