You will find articles organized by categories, along with recent comments, along the right hand column of the website. If you are just getting started with the idea of converting a gas car to electric be sure to check out Your First Electric Car
Welcome and enjoy!
The Battery Dilemma · 10 April 06
Now that the weather has started warming up and my other projects are winding down I’ve cracked open the garage door and started measuring Eve for her spring wardrobe.
I know, I’ve been putting it off: stalling, dawdling, jerrymeandering, going to the thesaurus to look up alternatives to procrastination just so I don’t have to actually think about them.
Batteries. Love ‘em or hate ‘em, ya gotta have batteries in an electric car.
At some point you have to make a decision so that metal can be cut, frames welded, boxes built, and wiring run in preparation for the big day when you heft a dozen batteries into the EV.
I made a vow that I’d try something besides the basic lead-acid for this conversion. AGM, NiCAD, Gel, NiMh, Lithium…something. I’d like something that, a) doesn’t need to be watered, and b) lasts longer. Things like high surge power, ability to mount on its side are bonus points.
The main problem is sticker shock. That and finding out if the damn things can even be purchased. Seriously. Company A comes out with a series of press releases for their newer, better, cheaper, safer, more powerful battery. Nobody has them for sale. Inquiries to the company either go unanswered or are met with a meaningless form letter, “Thank you for your interest. You don’t seem to be a big company…good bye.”
So we work with what we have, it takes enough time to figure out existing products, no reason to waste days trying to chase down ghosts of future tech.
What do we need from a battery? Here’s what I know:
- Voltage – well, duh! Seriously, the higher the voltage your overall system (higher volts typically mean better performance) the more batteries you will need. If you also want range then you’ll want big batteries, i.e. 6 volt deep cycle, which means even MORE batteries. Eve will run at 144vdc: either twelve 12v, eighteen 8v, or twenty-four 6v batteries.
- Current – even if you’ve never bought a battery for your car you’ve probably seen the ads on TV: MORE COLD CRANKING AMPS! Meaningless. The battery in your car has to turn a small electric motor (starter) a few times a day, sometimes when things are cold and creaky but there’s no way the battery can pull those kinds of amps for more than a minute or two. Our electric car batteries need to pull 300 to 500 amps from time to time and around a hundred amps most of the time.
- Amp Hours – think of this as a measure of work a battery can do. Like gallons of gas, amp hours gives an approximation of how long you can run at a specific current draw before dropping to 10.5 volts (considered “empty” for a 12v battery). Amp hours, like anti-depressants, come with all kinds of fine print and caveats: actual amp hours can be negatively effected by temperature, rate of discharge, jostling, bad thoughts, past charging or lack thereof, sudden elevation swings, and unfair atmospheric pressure. Do not taunt the amp hours, keep away from children and adults, do not expose to sunlight, keep out of darkness, store in a cool dry place, your mileage may vary…
- Cycle Life – while amp hours try to quantify how much work you can squeeze out of a battery the cycle life is how long you should expect them to put up with this abuse. Typically with lead acid deep cycle batteries it is stated as the number of times you can drain 80% out of the battery. I’ve noticed that most of the AGMs express this as 50% cycles and really don’t suggest going down to 80%, so buyer beware. What may not be obvious is that you can get more cycles by not draining as deeply but far fewer if you regularly over-drain and/or abuse the batteries. Which says to me: get more than you need.
- Cost – it’s more complicated than finding the cheapest. There’s typically a reason why one battery is cheaper than another. Yes, you can buy a starter battery pretty cheap at *Mart but it’s not even close to the performance of a deep cycle battery. All deep cycle batteries are not created equal either.
That’s a bunch of things to evaluate and it’s only the tip of the iceberg. Do you drive in cold weather? Knock 30% off the expected capacity. Do you drive like a grandma?1 Bump up the life cycles. Do you live in steep, hilly country? You’ll want more current and capacity. Are you a racer? Get out the checkbook…
1 an unfair generalization: Grandma Halstead drove faster and crazier than most folks I know
Years ago I created reports from databases that a VP would use at company meetings. I noticed a particular trend, which I’ve seen at other companies, with other VPs and Presidents: creative rounding.
Whenever things were dire and they were trying to boost moral they’d creatively round UP the numbers. Instead of saying that sales were $170,000 they might say “nearly a quarter million.” The same number in a sales meeting, meant to get them off their butts and selling, might round DOWN to become “barely cracked a hundred thousand, what’s up with you people?”
We do the same thing. I tell my wife that something’s only ninety bucks, while she points out that $99.99 is really a hundred bucks hiding under a penny.
You don’t want to to too much creative rounding when choosing batteries. This isn’t horse shoes or hand grenades, if you short change your EV with cheap batteries it will catch up with you eventually. Remember, there isn’t any free energy. On the other hand the consensus of the EVList is that most folks kill their first battery pack; maybe buying a cheaper first pack has some merit?
Enough yacking, let’s do some battery math.
range = energy in pack / energy used per mile
energy in pack = 20 ah rating * .57 * voltage
energy used per mile = 160 W-hr
It’s not perfect, but it’s a formula that we can use with all of the batteries. Ideally we would love to have the 1hr rate for all batteries, but battery specs are frustratingly inconsistent and typically the best we can get for all of them is the 20hr rate. The 0.57 is a number that folks use to approximate the 1hr rate given a 20hr rate.
Also you need to draw a line in the sand and decide on a rate of consumption for the formulas. In this case the 160 watt hours per mile is what the EV Calculator shows for driving 35mph with my setup and a 1% incline. Note, the range calculation is based on power only and doesn’t take into account the weight of the battery packs, incline, or other outside factors.
cost per mile = battery cost / miles in pack
miles in pack = range * cycle life
battery cost = # batteries * price
miles per lb = range / battery pack weight
battery pack weight = # batteries * battery weight
Toughest thing in this bunch, and vital to some of the more interesting calculations is the cycle life. There’s a few battery charts with this information, some anecdotal evidence from EV’ers, but this is an elusive number to find. 6volt deep cycle batteries are in the neighborhood of 700 cycles. 12volt batteries from the same companies are typically half of that, mostly because there’s less plate material and lighter construction. AGMs are generally in the 300 cycle range, some less, some more, and some claiming numbers that nobody has had a chance to prove yet.
Remember that AGMs (and some other advanced batts) report the cycles for 50% DOD (depth of discharge) while the heavy old lead acids are 80% DOD ratings. One rule of thumb is that only discharging 50% for those 80% rated batteries increases their cycle life 50%. So a 300 cycle lead acid Trojan could clock in at 450 cycles if you treat them well…but that also means less range.
In order to compare apple to apples we probably should use the same DOD and bump up the cycles for the 80% DOD batteries by 50%. I’ve done this as best as I can in the spreadsheet below.
A few more numbers that I think are interesting.
cost per year = battery cost / years
years = miles in pack / estimated yearly miles
ten year cost = cost per year * 10 + ($300 * (10/years - 1))
The last one is something I just made up. Let’s figure that most of us will try to keep the EV running for ten years. If we had a battery pack that lasted ten years then it’s just a matter of popping them in and enjoying the next ten years. On the other hand if we bought the cheapest batteries imaginable and they only lasted a year we’d be replacing them ten times over that period (assuming we didn’t smarten up). Since time is money, even for zealots, I’m factoring in a $300 time and trouble tax for each time the pack gets changed. Seems reasonable.
Ready? Here’s the pile-o-numbers!
Click to see a more readable version, you can download the spreadsheet here. Lot’s of caveats here…LOTS, and I suspect that I’ll have to redo this a number of times so check back from time to time. Also, I’ll try to make the base spreadsheet available to anyone who is interested, drop me an email.
The main caveat is the dang cycle numbers. Jeesh, those are as hard to find as unicorns or decent politicians (which one is imaginary is left as an exercise to the reader) and a chart like this doesn’t help. The next near-fable is the price. You’ll find a battery for, say, $100 on one website and then the same battery for almost $300 on another! Which one is the scam? Be sure to contact me if you have updated numbers for any of these.
I didn’t shop around much when I bought batteries for the first EV. After a couple of days poking around this time I’ve found that the prices are highly variable. Mention that you are going to buy a dozen batteries and the special pricing comes out. I hope that this holds true with all of the battery sellers.
Now that we’ve run some numbers, what have we learned? Well, no surprise that the cheapest approach in the long run is still with the stalwart lead-acid deep cycle batteries from Trojan and US Battery. The chart is showing estimate life expectancies of over ten years for the six volt packs…that’s assuming I don’t do something stupid like forget to water them, have a charger go bad and cook them (it’s happened), or get a bad batch. All of which could happen to the other batteries.
If you glance over at total weight column you’ll see the downside: fifteen to eighteen hundred pounds of batteries! Mon dieu, celui est beaucoup de pile! (I’m taking a french class with my wife)
Yeah, forget that. Maybe if I was converting a small pickup.
If I were to stick with basic lead-acid I think the Trojan J150 is the way to go. It has the same internal design (plate size, etc..) as the T-105 but in a 12v package. A thousand pounds for a twelve pack of them, that’s a couple hundred pounds heavier than the SCS225’s I used to use, but over four hundred pounds lighter than the T-105s. They are taller by a little over an inch but my old battery boxes were constructed to handle taller batteries
The main feature of the J150 is the longer cycle life, always the achilles heal of most 12v deep cycle batteries. With an 80% DOD cycle life around 700 vs 300 the initial extra expense pays off over the long run with J150’s costing $1,200 less over ten years.
Still, I’d rather not go with flooded lead-acid if I don’t have to. I don’t need much range: daily driving around 10 miles, 20 on long range days. Also, except for climbing the hills to our house, I don’t need high performance, although it would be nice to have a little pep when showing the car off.
An interesting battery is the EaglePicher Horizon 12D2000. It’s an AGM with a unique form factor, as you can see: 30” x 4” x 5”. No need for watering, which means they can be put in inaccessible locations like the groove down the middle of the car where the muffler pipe used to run and the indent formerly occupied by the gas tank. They can’t lay on their side, but upside down is OK.
You pay extra for all of that convenience. The manufacturer suggested price of $329 (I haven’t looked for a dealer or quantity pricing yet) makes the initial cost more than double that for the J150’s. Yet it has a high cycle life of 1200 @ 50% DOD and a good ah rating (I converted the C/3 rate to C/20). Another thing going for it is a pack weight 100lbs lighter than my last pack and if you use the “jerry-factor” it comes to the same relative cost over a period of ten years (less if I get a quantity discount).
Next candidate is the Dynasty DCS-100L. The DCS-100L is also an AGM and has a more traditional battery form factor. When I first started researching batteries I ran across a cycle life for this battery of 600 cycles, but I can’t seem to find that spelled out in any of the company’s current literature.
There’s an EVList post from 2002 that lists the 50% rating as 1200, which is the kind of thing we like to hear. Form factor-wise they are close to the SCS225 size, an inch less in length, with about the same weight. Not much else to say, maybe I can find some EV’ers with first hand experience?
I’m not as familiar with the Deka MK 8G31 Gel cell batteries (PDF here). Gels claim a higher cycle life than AGMs. From what I’ve read they aren’t able to supply as high of an on-demand current, like what an EV racer would want.
I’ve downloaded their PDF on VRLA Gel and AGM batteries, for some late night reading.
Which brings us to Lithium (forget NiMH, I think Chevron has pretty much killed/monopolized this choice). Thundersky out of China is the lithium of choice for EVs it seems, at least that’s the one I’ve heard the most about. I exchanged email with them last fall and got the prices shown in the spreadsheet (600+ bulk purchase, add $5/ea to the smaller batts for smaller orders).
The most attractive thing with Lithium is the energy density and cycles. In a 210lb package of batteries I can get all the range I need and they’ll last for well over ten years. Heck, Eve would weight in at under 3,000lbs. They’re the only batteries that cost less to run for ten years than it does to purchase them (which speaks more to my spreadsheet skills than reality!)/ ’:^)
The downside is that it’s going to cost big bucks for that luxury. Not as much as Valence or Kokam, but almost $6,000 just to outfit the car with a lithium battery pack. I’d be in good company though.
That’s where I’m at with batteries: a spreadsheet full of numbers, browser full of bookmarks, and a folder packed full of brochures and specs.
The easiest thing to do would be to go with lead-acid: cheap, I’m familiar with them, and no need for a battery management system. I’m really resisting the urge to take the path of least resistance. My inclination is to see what kind of quantity price I can get and go with the Horizon AGMs. There’s a certain attraction to being able to put the batteries wherever I want, optimize the center of gravity, and hopefully get ten years or so out of the pack.
Of course I’m always open to the possibility of a battery manufacturer or distributor sending a pack this way in exchange for site advertising! Oh, and, a, technical evaluation…
Update: As Ryan notes, I’d forgotten to mention the additional cost of a BMS. You HAVE to have one for lithiums and it’s probably a really good idea to have one for the AGM/Gel batteries as well. From reading Lee’s posts about his BMS it sounds like everyone, lead-acid included, could benefit from a decent BMS.
Update 4/10/06: thanks to everyone for sending email, leaving comments, or for feedback on the EVDL. I’ve updated the chart with a few numbers passed on by folks, added a battery recommended by Dave Brandt. I’ve also put the spreadsheet online for anyone wanting to download it and check my math (promise not to laugh!).
Update 4/21/06: A few folks have written (thanks!) to let me know that most Lithiums these days have a life expectancy. In other words even if you have enough cycle life to theoretically run your EV off of lithiums for 14 years the batteries themselves may only last four or five years. They have also cautioned against believing cycle life claims, this is especially true considering the newness of the tech and limited real-world experience.