Tech Talk #1 -- Batteries

"Hey Bill, I want to pick some batteries for this conversion I'm doing on a '72 VW super beetle. Just when I thought I was getting down the learning curve nicely, I find out there is more to it than I thought."

"OK Connor, let's try and figure out some things. The very first thing is what battery voltage to use. There are some people who have been doing really inexpensive conversions on VWs with 48 or 72V. On the other hand, we have some friends that have a car just like the one you want to do. They have 12, 12V 100Amp Hour gel batteries (144V) in their car. The 48V volt guys say they can go 45 miles per hour with a range of 40 miles. This is kind of hard to believe. Our friends with the VW here in town get about 20 miles and they have a larger, 9 inch motor from Advanced DC."

"Boy Bill, with all those batteries I sure would have thought their car would have gone further."

"Let's begin at the beginning. The 12 12V 100AH batteries give us 12 x 12V x 100AH = 14,400 watt hours of capacity. Since most people are used to dealing with Kilowatt hours, like they get on their home electric bill, we can divide watthours by 1000 to get Kilowatt hours or 14.4KWH.

One of the properties of lead acid batteries is that when you pull more current from them, their rated capacity appears to go down. This effect was discovered in the late 1800s and a formula for calculating this was published by a German researcher named Peukert (pronounced POY KERT). This effect was dubbed the Peukert Effect and his equation is published all over the Internet. The capacity can go down quite a bit! In my experience, it works out to about 40%.

To make matters worse, you really can only discharge the batteries about 80% before you begin to damage them."

"How about you pencil some of this out for me."

"OK. Let's start with our 14.4KWH pack. If we give Mr. Peukert his due, our pack is down to 0.6 x 14.4= 8.64 KWH. Now, let's not let the battery get more than 80% discharged, so our capacity is reduced to 8.64 x 0.8 = 6.91 KWH."

"That's not at all what I expected. So how far do you think my car would go on these batteries?"

"The first thing you have to do is make an estimate of how much energy it will take to move the car a mile. Sort of like the Miles Per Gallon in a gas car. I once had a Geo Metro that weighed about 2400 lbs with the same type of batteries. It took about 250 Watt Hours per Mile (WH/Mi). Your VW weighs 2000 lbs as it is. Let's say that the gas engine weighs 150 Lbs, and a 9 inch motor weighs about the same. So, that part is a wash. The batteries weigh 71 lbs each, that makes 852 pounds of batteries. Since your car would weigh in at 2800 lbs and mine was 2400, let's estimate that your car will use 290 WH/Mi.

To get the range we simply divide our battery capacity by how much energy we use per mile. In this case, 6910 Watt Hours (it's easier to use watt hours so everything is in the same units), divided by 290 Watt Hours per mile, or 24 miles. Our friends who have the electric VW beetle, tell me they get around 20 miles of range. So the calculations are pretty close."

"I had hoped for more range, what can I do?"

"Stuffing more or bigger batteries in that car is probably not an option. Our friend's conversion already has batteries in the front trunk, under the rear seat and in the rear engine bay. And it is already pretty full.

What would you think about lithium batteries?"

"Bill, I have heard that the lithiums are like 5x the cost."

"You know Connor, how technology is moving forward. The lithium batteries are not as expensive as they used to be. But, let's look at how much it would cost to do your car both ways and you can decide.

Using the 12V, 100AH gel batteries, your cost would be $236.25 x 12 or $2835.00. This will get you about a 20 mile range.

One other thing I didn't get to mention is that when you load lead acid batteries heavily, like accelerating a car, the voltage goes down. That's why we put 144V of batteries (12 x 12V) to run a 120V motor. It's not unusual to have the battery voltage sink to 120V or below when accelerating, especially when the batteries are not fully charged.

Lithium batteries do not decrease in voltage when heavily loaded. They also only suffer less than a 2% drop in apparent capacity. Which for our calculations, we can neglect. Take that Mr. Peukert! This will allow us to use a 60AH battery pack in lithium instead of 100AH and get the same range.

To compare apples to apples, the first thing we have to know is that lithium batteries come as individual 3.2V cells versus the familiar 12V packs that lead acid batteries come in. No problem! We just have to use more of them. For our 120V system, we need 120 divided by 3.2 or about 38 cells. These cells cost $96 each and 38 of them cost $3648.00.

" $3648 versus $2835, that's almost 800 bucks! Still a big gap my friend."

"Let me finish. The price of the batteries was just the first thing. The lithium cells only weigh 5 ½ pounds each. That makes the whole pack weigh just 209 lbs versus 840 lbs. This will bring the weight of your car down to about 2200 lbs, which will lower the power consumption to maybe 225 WH/Mi. This will increase your range by 5 miles or so. Because of the lower weight, you may be able to use a smaller motor, like an Advanced DC 6.7 inch. This will save you about $400.00. Now the gap is only about $400.00.

People usually only consider what a battery costs going in, not what it costs over time. A typical lead acid gel battery will last about 1000 charge and discharge cycles, or about 3 years. A lithium battery will last 3000 cycles or about 10 years. "

"OK, I'm almost sold on the idea of the lithiums. What about them catching on fire? "

"There are two main types of lithium batteries on the market: Lithium Polymer (written LiPoly) and Lithium Iron Phosphate (written LiFePO₄ ). As you know lithium is quite reactive, and like magnesium can catch fire. The electrodes in the LiPoly batteries can get into thermal runaway if they are overcharged or over discharged. Normally, there is a complex bit of electronics, called a protection circuit to prevent this. The LiPoly batteries were the first lithium batteries that were widely available. It was these batteries that caused the fires in the laptops and caused the fire in our building. The LiFePO₄ batteries on the other hand, are vastly more tolerant of abuse and have electrodes that have a flash point well above what they would ever see in any kind of normal application.

"But 25 miles is just not enough. What can I do about that?"

"The good news is that the battery packs from lithium batteries are about half the equivalent size of the lead acid. They also come in different sizes. We discussed 60AH cells. Other sizes are 90, 120, 160, and 200. And they are priced by the AH. So a 120AH cell is twice the price of a 60 AH. Just pick how far you want to go and you can price it accordingly.

That should be at least enough to get you started."

"Until I have some more questions, that is."

"Well, if something comes up, just call me at 928-600-3925 or email me at bschlanger@connect-tech.com."

"Thanks Bill."

"No problem. I gotta go. I'm driving my electric Land Rover home. It's spaghetti night!"