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EV Batteries · 29 September 05
Now that the engine and most of the extra parts are out of Eve I’m starting to think about next steps. Right at the top of the list is figuring out what kind of batteries to use, how many, and where to put all of them.
On the first EV I went with lead acid and the more that I look into the types (and prices) of batteries these days the more I’m starting to think that I’ll be using lead acid again.
Ken Norwick and I exchanged emails two weeks ago and his advice was to stick with lead acid until the proliferation of Hybrids starts to drive down the prices and increases availability of the more advanced battery technologies. Bang for the buck, lead acid still rules.
Over the course of the next week or so I’ll be looking into batteries and kind of blurting out observations, opinions and maybe share some links along the way. There is so much information it’s really hard to pick a point to start (or end).
Lead acid is probably the best you can get in the price performance category. Proper deep cycle batteries provide plenty of range and long cycle life, which correlates to more years of service. The rule of thumb has always been LEAD==RANGE: more lead, more range.
If you have the room go with 6 volt batteries, like the Trojan T105 or US Battery US125. These are heavy duty deep cycle batteries with 600 or greater cycle life (number of 80% discharges). Failing that some of the manufacturers offer 8 volt deep cycle batteries.
Our first EV used 12 Trojan SCS225, which is a 12 volt battery. I used them lightly and treated them pretty well and could just about squeeze 3 years from the pack. Of course I had a short commute, longer commuting over the same terrain would probably have shortened the life span.
Quite often I hear of folks heading down to the nearest *Mart and picking up marine batteries or, even worse, car batteries. That’s a good way to waste your money and time. A car battery is designed for short bursts of power and will quickly fail after regular EV use. The deep cycle batteries used for an EV (sometimes called traction batteries) are designed for frequent, deep draining, down to 80% capacity.
Marine batteries lie somewhere between a true deep cycle and a car battery. My EVs second set were marine batteries ordered from a nearby distributor. I saved about $6 cheaper per battery, a whopping $72. They barely lasted a year…no savings at all.
So what’s the deal with batteries, why is this so tough?
Battery chemistry is neither exact or entirely predictable. Two identical battery cells may, over time, behave differently. In an ideal world you would have one battery with one cell and it would generate all of the voltage you need. Instead, lead acid battery cells generally produce a little over 2 volts and you need six of them just to make a 12 volt car battery.
Those six cells in the 12v battery are like sextuplets: they may look the same, but underneath the freckles and identical smiles are slightly different chemistries. After a year or so one of the sextuplets starts going to bed late and waking up even later, missing out on breakfast, eating doritos late into the night, and generally straying from the pack. Same thing with battery cells (except the dorito part)...who knows why they do it, but the end result is that over time the six cells are no longer equal.
I’m not sure what to do with the stray sextuplet but for EV batteries we perform an equalization charge from time to time. This is done by charging the batteries, like we normally do, and then continuing the charge for an extended period. Slowly the weaker battery cell gets recharged, while the good cells just sit around and boil off their frustrations. I suppose it’s like taking the sextuplets to Church.
While lead acid has the price/performance edge and is time-tested technology it also requires regular maintenance. The water levels need to be checked, specific gravity measured, and the occassional equalization performed. Every couple of weeks or once a month, depending on your usage.
Is there a better way? Maybe…
I’ve only read about these and each time they tempt me to cross over to the “waterless” side. AGM batteries are a form of sealed lead acid using what is called a “glass mat” for storing the electrolyte. Since these batteries are sealed and don’t need maintenance they can be located practically anywhere.
AGMs have low internal resistance, which translates into quicker charging and discharging without generating as much heat. The downsides, near as I can figure, are that they cost 2 to 3 times as much as lead, don’t like to be discharged as deeply, and you need special circuitry to charge them properly.
Being sealed they shouldn’t be overcharged because it would cause heating and venting of the gasses, breaking the seal. To insure that the AGMs live a long, healthy life specialized circuits are attached to each battery which monitor the charge voltage across the battery and when it is full, bypasses current around the battery to protect it.
So what’s not to love? A little extra cost for batteries, some bucks for safety circuits, but no watering or fussing.
Let’s do some fuzzy math. The SCS225 battery has a 20 hour rating of 125ah and costs $85. An Optima has a rating of 100ah and costs $140. Let’s say that with a limit of 80% depth of discharge (dod) the SCS225 is really more like 102ah, while an Optima which prefers 50% dod can only do 50ah. Right off the bat we need to buy twice as many Optimas to equal the amp hours of an SCS225, $280 vs $85 per battery or $3,360 vs $1,020 to equip the EV.
Not only that but I’d have to find room for twice as many batteries and buy 24 regulators. Suddenly watering the batteries every once in a while doesn’t sound so bad.
Of course my data, math, and assumptions could all be flawed. I’m basing the price/capacity off of this chart and it may be out of date. I’m also guessing that both of these batteries are otherwise equal and have the same, relatively low, cycle life (~200).
There are other AGM batteries out there and some might be more suited to an EV. The Dynasty DCS-100L looks interesting. I haven’t done much research but the higher cycle life is attractive. If I plug it into the EV Calculator page it also looks good.
This is the stage of the conversion where you really ought to know what it is you need. Like I’ve mentioned I don’t need much range. In fact buying batteries that provide 50-70 miles of range would be a waste of money. I drive 8 miles round trip most days, maybe 20 miles if I run an errand. If we take worst case winter driving with 1/3 less capacity an average range of 30 miles should work.
Equipping your EV for more range means higher cost, a heavier vehicle, and more work trying to jam extra batteries into the chassis. If you don’t actually NEED the range then you are wasting energy hauling unneeded weight around (although your wallet will be lighter).
One reason I’m not giving up completely on the AGM idea is flexibility in layout. The other night I was under the car looking at what it will take to remove the gas tank and realized that its old location might make an ideal spot for a pack of batteries. Only about seven or eight inches deep, which means they’d have to be on their sides. It’s a great spot for weight distribution and center of gravity.
I realize that I haven’t covered some of the other batteries: NiCad, NiMh, Lithium, and the other acronyms bouncing around battery world. If I come across sufficient information about these (along with pricing) I’ll write up a new article just for them. If you have any links or information please feel free to drop me a note.
While we are thinking about batteries I’ve put together a little online Battery Layout Tool.
Tell it how many battery boxes you want, how many batteries to put in each box, along with what size and type of battery you’ll be using. The tool then creates an interactive page to let you experiment with the layouts. You can resize the boxes, drag the batteries and boxes around the page, and when everything is just right print out (or screen capture) the page.