# "Fuel Gauge"



## exchaoordo (Aug 4, 2010)

I hope someone can help me with a foolish question: how do I know how much charge is in my batteries? I've just bought a converted 1971 Bug done with a kit from Wilderness EV. It has a voltmeter and amp meter and a capacitive charger. I'd like a simple gauge that will tell me how far till empty -- or if someone can just explain how it's done. The Volt Meter reads 72, so I know I can drive, but for how long?
Thanks in advance for any help.


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## Coley (Jul 26, 2007)

With the bug sitting still and the voltmeter showing 72 volts, you should have a fairly good charge (80volts would be better).

Now watch the amp meter when you step on the accel pedal.
While slowly picking up speed, you should see about 130 amps.

While moving at 30 mph on flat road, you should see 69 or more volts and 65 or so amps.

Step on it hard and see what the readings are:

Less than 65 volts and less than 150 amps= not far to go until recharge.
More than 67volts and 180 amps= 4-5 miles of charge left.

What you really need to do is fully charge the batteries and then run out a few miles to see how the initial response is.

Then stop to let the gauges got to full volts and 0 amps.

Record this volt reading and then run back home and see what they read on the return trip.

If a lot worse by the end of the run home, that is your range.

The better the volt reading at the end, the better.

Also do this using 30 mph, 40 mph and 50 mph to see how much the range drops with the higher speed.

You will soon know what your "fuel level" is by glancing at your gauges.

(distance and speed may vary according to your battery amps ratings)

This about what I get with my Gels. Don't know what you have for bats.


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## madderscience (Jun 28, 2008)

The most effective "fuel gauge" in an EV is a kWh (kilowatt hour) meter. basically, it tracks how much energy you have used as you drive. 

Once installed, you will still need to initially figure out how much energy is usable in your pack. This does take some expirementation especially when you aren't familiar with the car (yet)

Your first estimate of usable energy is simply a matter of doing the math on your batteries. You didn't say what you have except that it is 72V, so for this example I'm going to assume it is twelve 6V golf cart batteries. So nominal capacity would be about 72V * 200AH or about 14.4kWH. However, due to peukert, and DOD considerations (see wiki) you really only get to use about half of that. So in this example, the most energy you could realistically figure on using is about 7kWh. If these are lead acid batteries, then to be even more conservative, knock another 20% off for every year of age on the battery pack after the first year. In other words, if the pack is 3 years old, subtract 40% of the 7kWh to get 4.2kWh usable. 

To get from kWh to range, in your case you can probably assume about 200wh/mile presuming the car is properly aligned and rolling easily on high pressure tires. So if you have 7kWh usable you could get 35 miles nominally.

Next, by starting with a full charge and a good theoretical estimate of what should be achievable, use a technique something like what coley described to figure out when the batteries are "empty", (and knowing you won't be able to go much past the number you figure above) drive til your batteries are discharged to the point described by coley. Take note of how many actual miles driven and kWh you have used, and you will know that that is "empty" and about how much driving that actually is.

Once you've got it all figured out though (for example say if the batteries started sagging at 6kWh) then you watch your kWh meter and you'd know when you've used 3kWh that your are halfway down.

You will need to occasionally reassess usable energy based on various factors like ambient temperature and the age of your battery pack as well. Presuming nothing fails catastrophically any changes will be fairly gradual as the pack ages, so you shouldn't get any surprises.

Good luck.


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## Guest (Aug 9, 2010)

72 volts is nominal and not fully charged. Your charger is anemic. You should have 76 volts.


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## tinrobot (Aug 26, 2009)

Sure you could go to more sophisticated methods such as measuring actual power consumption, but for lead acid batteries, voltage is a pretty decent indicator of charge. Lead acid has a fairly linear discharge curve, so as the battery discharges, voltage drops proportionally. I have a digital voltmeter on my car. Works fine.

For a 72 volt system, it should be a little over 76 volts when fully charged. 72 volts would be about 50%. Much lower than 72 and you'll need to recharge. For maximum battery life, keep it above 50% charge. Voltage should be measured when the batteries have rested for a few minutes at least.

Towards the bottom of this page is a discharge chart for a 12v flooded battery. Just scale it up proportionally for your pack:

http://www.windsun.com/Batteries/Battery_FAQ.htm


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## EV-propulsion.com (Jun 1, 2009)

here's a pretty accurate chart for resting voltage for each 12 volt battery. Multiply by 6 in your case for voltage readings and their corresponding state of charge.
State of Charge 12v battery voltage
100 % ----------------12.7+
90 --------------------12.50
80-------------------- 12.42
70 --------------------12.32
60 --------------------12.20
50 --------------------12.06
40 --------------------11.90
30 --------------------11.75
20 --------------------11.58
10 --------------------11.31
0 ---------------------10.50

Mike
www.EV-propulsion.com

sorry tin, I see you linked to it already!


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## exchaoordo (Aug 4, 2010)

Thanks to everyone for the advice. It sounds like I need to do some charging for starters and starting doing some math. Much obliged!


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## EVfun (Mar 14, 2010)

10.5 volts for a 12 volt lead acid battery (1.75 vpc) *under load* is fully discharged at that rate. No 12 volt lead acid battery should ever find itself under 11.7 volts resting or damage has almost certainly been done to one or more cells. It will take a few minutes without a load for a battery to recover to that level and that is OK, but it is very important that returns to nearly 12 volts.



EV-propulsion.com said:


> here's a pretty accurate chart for resting voltage for each 12 volt battery. Multiply by 6 in your case for voltage readings and their corresponding state of charge.
> State of Charge 12v battery voltage
> 100 % ----------------12.7+
> 90 --------------------12.50
> ...


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## TheConverted (Jun 20, 2009)

One of these may help;

http://www.evworks.com.au/index.php?product=INS-EVW-FGD

Clever and cheap.


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## exchaoordo (Aug 4, 2010)

First, thanks again to all of you who replied to this post. I've been living with the car for a few weeks now and have some data under my belt.

There is one thing that I got some differing opinions on: what is a fully flooded battery pack? I've got a 72v system made from 9 8v Eveready golf cart batteries and I think they were pretty new because they seem to be taking a bit more charge now. But one person said I could get up to 80v reading for a full pack whereas the standard number of 12.7 per 12v flooded comes to 76.2v for a full charge.

So, how high can I go? And, what are the dangers of overcharging by a little bit, a lot, regularly???
I'm using a Capacitance Charger that came with the car. I'd like to get a smarter charger, something I can plug in and forget. Any opinions on that score would be great too. Thanks again all!


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## Guest (Sep 10, 2010)

The voltage can be upwards to 80 volts while charging but when you stop charging your voltage will drop then sink a bit lower after sitting for awhile. 76.2 is right for a full 72 volt pack. You won't have a long distance pack but it will do for short around town driving. I started with 72 and currently use 96 volts. The charger you have is anemic but you still won't get more than the batteries can actually hold. I had mine up to 78 volts once but that took a real long time and I used a charger that kept the power up and it boiled my batteries. Don't over charge. 

Quickcharge has a Select-A-Charger on board charger that will do you just fine. Or you can get one of those old Lester Chargers with a timer and use those. I used a 36 volt Lester converted to charge a 72 volt pack at 15 amps. I ended up buying a new timer on ebay after a year but it worked perfect. Can't really go wrong with one of those. For floodies either will be fine. 

Pete


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