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Flooded Lead Acid Calculations - Range/Performance

7684 Views 24 Replies 9 Participants Last post by  pflipsen
A consolidation of notes from Kitty Rodden's presentation to the Peninsula Chapter of the Electric Auto Association, by David Coale.

What battery should I use in my EV?

This is the age old question; what battery should I put in my EV? There is no standard answer to this, it depends on what you want out of your EV. The old rule of "thumb" if you will, is the more lead the greater the range and the poorer the performance due to the increase in weight. It is also true that the higher the voltage the better the performance (acceleration and top end speed). This is of course also dependent on the type of driving and on the controller. Since these are fixed for any given EV, these generalizations are not too far off. Two designs illustrate this:

Long range: 96 to 120 volt system using 6 volt deep cycle batteries. This will give you lots of amp hours and weigh 976 to 1420 bls. This type of battery pack should last two to three years depending on the type of driving.

High performance: 96 to 144 and up using 12 volt batteries. If you use a starting battery your car will be light (400 to 700 bls.) and quick and your batteries will last about 3 to 6 months or less. If you use deep cycle batteries (12V) your batteries can last as long as one and a half to two years but the car may be heavier depending on the battery.

So what is the range in each case? Isn't that what we want to know; how far will my EV go? Answering this will help you decide what battery you will use.

To find out how far a particular battery pack will take us, we need to know how much energy is in the pack and how much energy the EV uses per mile.

Range = Energy in pack / (energy used per mile)

The energy in the battery pack (wired in series) is the amp hour rating times the pack voltage. The amp hour rating is how may amps a battery can supply over a given time. Most batteries are measured over a 20 hour period. This is a standard that is used to compare batteries with, and can be found in the specs. on most batteries. The 20 amp hour rating has to be adjusted for EV use. The faster one draws current from a battery the less capacity there will be. This is due to the chemistry of the battery and the internal resistance. Therefore the capacity of a battery at the 20 hour rate is more then the capacity at the 1 hour (EV) rate. The following is a table from Trojan Battery Company showing the conversion factor for finding the X hour rate given the 20 hour rate:

Conversion of 20 hour rate to X hr rate.

X hr Conversion
rate factor

1 .57 = Most commonly used value
2 .67
3 .74
4 .77
5 .82
6 .84
7 .86
8 .87
9 .89
10 .91
20 1.00

Note: these values will be a little different for each battery.
One can see from the table above that as you drive faster the range will decrease due to reduced available capacity. This does not take into account wind resistance which also increases with speed.

Now we need to know the energy or watt hours per mile that your EV gets. The table below lists some common EVs and their watt hours per mile rating.

Impact = 130 W-hr/mi (AC drive very good aerodynamics)
Metro = 160 W-hr/mi (PM drive good aerodynamics)
Metro = 200 W-hr/mi (DC drive good aerodynamics)
Truck = 350 W-hr/mi (DC drive poor aerodynamics)

This table was developed using several cars for each category traveling at highway speeds (60 mph). The numbers reflect the efficiency of the cars listed. With the following information the range equation for a smaller type of EV on the freeway would be:

RANGE = (20 amp/hr rating) X (.57) X (pack voltage) / (watt hours per mile of the vehicle)

Note: The above is the range @ 100% DOD, Divide that amount by 2, to get a 50% DOD range on your battery pack, I would try and not go lower than 50% DOD in order to maximize the life of your Flooded Lead Acid batteries. AGM/Gels might be more forgiving.
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Pretty accurate, my current battery pack calculates out at this:

210ah x .57 x 78 = 9336.6 pack energy
divided by 250 whr = 37.35 miles

I typically only drive 20 miles per charge, and that is only at city street speeds (35mph), therefore I'm not at the 50% DOD of my pack in that 20miles. I don't drive highway speeds much, since this conversion is only with a 78volt pack. It's pretty hard on the batteries, controller, motor driving the the car at 65 miles and hour for long.

Someday, I hope to take the EV Technician course in Oklahoma, so I can legally build a higher voltage EV. Just don't have the time and money to dedicate to the class right now.
Yep, in Oklahoma (The only state that I know of that has this law) says you have to hold an EV Technician Certificate. It's part of the "Alternative Fuels Act" they dreampt up for vehicles that run on Propane, Compressed Natural Gas, and Electric. Of course this is not a FREE certificate. Word is, someone blew themselves up with an improperly built propane conversion, and therefore the AF act was born, which some yahoo politician (probably oil bribed -IMHO) included EV's in it.
Yep, This is why I purchased a 100amp load tester at Harbor, I've had batteries that show a good charge, yet once under load, they just poop out, and won't pass the grade. These testers aren't expensive, and if you test your batteries in the pack, once a month or so, you'll hopefully find out issues while they are small, and possibly under warranty.
Yes it does monitor the batteries, but I'm beta testing for Ken, and therefore am using other equipment and tests to confirm what it's telling me. So far, all has been good for the PakTrakr. This little unit is a pretty kewl device. I would like the interface you are working on for the carpc, it would be really nice to get it to work on an ol' PocketPC that would mount nicely on the dash of the car. I have a Jornada 625 pocket pc, with serial inteface if anyone's up to the challenge...... lol
KiwiEV, be carefull of the batteries you use, and make sure they are truely: DEEPCYCLE batteries. I first attempted to use a DeepCycle/Starting battery that showed CCA amps on it. They lasted 2months and gave me a 11mile range. I woke up and went to true DEEPCYCLE 6vdc Golf Cart batteries, and have much better performance/range now. I'm using some Telco AGM batteries for my motor field pack, and they do a really nice job in that particular application. You certainly don't want to replace batteries very often. Being you're in frozen country, if possible consider placing the batteries inside the vehicle where you could preheat the car with a heater that turns on when you're charging the main pack, keeping the interior of the car and batteries nice and toasty. You should be able to find a thermostatic controlled heater that runs off your house ac wiring, same as your battery chargers, that can do this for you. Once you're driving the EV, the internal resistance of the batteries, and current draw will help keep the batteries warm most likely until you get to your destination. Then plug in to recharge, and your heater will keep the batteries and interior of the car toasty once again.

Now remember, batteries tend to produce hydrogen and oxygen during the charge cycle, so you need to becareful to keep those gases purged out of the vehicle if there's a possible chance of any open arcing of any device during the charge cycle. Your heater could be either an electric blanket over the batteries, an oil based heater, even just a simple high wattage infrared heat lamp.
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