Monday, February 7, 2011

I'll be back... in 8 hours

Some of my readers have wondered why I have been off the blogosphere in the last few months. The reason is that we brought a house and the move from the apartment to our new place has been a bit of a time sink.

First we went through the four stages of home buying:

Stage 1: What the &%#@ do you mean they accepted our bid? I thought you said we were lowballing?

Stage 2: When you use words like "downpayment", does this involve us giving you a check?

Stage 3: I assume roof's are like batteries? Meaning, when you say it is at the end of its life, there is still 80% left, right?

Stage 4: Keep repeating after me: "Owning is better than renting" and, please, stop asking "why?"!

Then we realized that owning a house also meant owning things like leaf blowers and lawn mowers! So, when I saw that there was a battery-powered lawn mower, I jumped at the chance to push my favorite technology forward.

I was looking forward to using my expertise in batteries to maintain and extend the life of my lawn mower for many years to come.

I was not particularly looking forward to mowing the lawn, but owning a battery-powered mower seemed to make up for that.

Until I realized that the top-rated battery-powered mower uses lead-acid batteries.

Lead-acid!!! really! How old school can one really get.

My first thought: Start a battery company to make Li-ion batteries for lawn mowers.

Then I started thinking about this some more. There must be a catch here. So I started digging into what it was.

Let us do some math: The battery for this lawn mower cost ~$270/kWh. That is one expensive lead-acid battery. Presumably, it is a deep-discharge battery, and so it is better made than a car battery.

And there is probably a significant markup.

Did I mention that the mower was on sale for $300! More like a mark-way-way-up.

Considering a typical Northern California growing cycle, this mower will probably be used ~50 times a year (once a week). These deep-discharge batteries can probably go a few hundred cycles. So I'm thinking 4-5 years easy.

But the bigger problem is going to be the calendar life. Sulfation can kill these cells.

Having said that, the battery is probably going to be at the top-of-charge pretty much through its life (think about it: Mown for 1 hour. Keep it plugged in all week). And remember our rule for lead-acid batteries: Keep them charged. So I think we can expect to get ~3 years from these cells.

Lets do some math for a Li-ion battery. I bought one a few weeks ago. This battery will probably last me 3 years and get me ~300 cycles or so. So it has similar specs to the lead-acid battery.

The Li-ion battery, on the other hand, cost me a whooping $2300/kWh!! No... really. This is what it cost me.

Now... knowing Apple, a new word has to be coined for their level of markup. But still this is one expensive battery.

At this price, a Li-ion lawn-mower-battery would have cost me $800!

All you MBA-types are probably cringing because you all know that cost and price are very different from each other and that the price is dictated by what the market is willing to pay (my wife is an MBA and she gave me this spiel).

Granted. So let us do a cost-differential comparison. This comes out to be ~$50/kWh more expensive for a Li-ion cell. Using this, the cost of a Li-ion battery for this mower would be higher than the lead-acid battery by... the price of lunch!

Not at Chez Panisse. But at the LBNL cafeteria (same quality, but at a much lower price?)

So why not use a Li-ion instead. After all it is almost 5x the energy density.

The mower that I have been talking about in this blog is a push mower. So the battery does not need to drag itself. All it has to do is turn the cutting blade. And space is not a big constraint. The mower's size is dictated by the size of the blade anyway.

So why bother using a new type of battery when you don't really see much of a benefit?

Frankly, although I have not looked at the life-cycle, I'll take a bet that it is probably better for the environment to use a lead-acid battery considering how much of this lead is recycled. In comparison, all you recycle in a Li-ion is the high-value metals in the cathode. The rest, literally, goes down the drain.

Maybe the math will change for a self-propelled mower. Or if the weight of the mower is an issue. But I don't see any reason to jump to using a Li-ion battery for the mower I was looking at. This business plan does not appear to have much legs.

So what did I do? I got the corded version of this mower. It cost me $100 less.

I guess the price that I'm willing to pay to push my favorite technology forward is less than $100!

In my guilt I decided to do my part for the technology by buying a Roomba.

I'm not sure if you guys know about this amazing robotic vacuum. It is pretty interesting to watch. It has sensors that make it slow down when it approaches objects, detect dirt, and prevent it from falling off of stairs.

It is not particularly good at vacuuming. But it feels like it is cleaning the floors and isn't that what's important!

But here is the kicker. The one I have has a Ni-MH battery.

I suppose I should be thankful it is not a lead-acid, but Ni-MH? COME ON!

The last time I checked, the cost of Li-ion and Ni-MH batteries were pretty comparable. And the energy density of Li-ion is 2-3x greater. And remember that in this machine (unlike the mower) the vacuum has to drag its battery along. And space is a big deal. The smaller the footprint, the smaller the space it can vacuum.

So why use a 20th century battery for a 21st century machine? Strange.

A web search revealed that Li-ion Roomba's have apparently died prematurely. If this is the case, then iRobot (the company that makes the Roomba) needs to change battery suppliers.

The vacuum I got goes through 2 rooms and then runs out of juice. It could have finished my house in one charge if it had a Li-ion of the same size. Or you could have a better vacuum on it so that it actually picks up dirt instead of moving it around and still only vacuum 2 rooms. You get the point.

When I first saw this machine, I thought that all the things on Terminator (the movie) were coming true. Jokes apart, it really is a pretty decent robotic cleaner which does indeed find its way around. You can, pretty much, set it and forget it.

But then I realized that we had nothing to worry from the machines as long as battery technology evolves the way it has been in the past.

When Arnold Schwarzenegger's character said "I'll be back" on T2, he (it) actually meant "I'm running out of battery and I need to go find an outlet and charge for 8 h. I will then come back to look for you for the 1/2 hour my battery lasts. Wish me luck".

So much for the machines taking over.

If you constantly complain about how batteries are not evolving fast, did you ever consider for the second that maybe we are out to save the world in our own way?



  1. You should have gotten the lawn-roomba,

    granted three grand is a little higher than your $100, but c'mon look how cool that is.

  2. I have a saying about home ownership: "Owning a home means doing for free what you wouldn't do for money!". You'll understand soon enough if not already. There's nothing like the feeling of walking into Home Depot and saying "no thanks, I know where everything is..."

    That being said, try a push mower, they're not that hard to push, give you exercise, never have to be charged, and you can hang them on the wall easier....

  3. One of my favorite parts of the Terminator movies is of course when the Fuel Cell starts malfunctioning, he pulls it out, throws it, and it explodes like a A-bomb.

  4. $3000 for a mower! After you buy a house in the Bay Area your concept of what expensive is changes, but still $3000 is a lot! It is cool.

    I was going to say about the Fuel Cell terminator that Arnold will be lugging around a very large hydrogen storage unit so that he can power himself! Makes for a very uncool image. Moreover, consider the cost of a Fuel Cell, I suppose the machines will bankrupt themselves! Then again, the human body generates more electricity than a 120V battery so if the Roomba sees the Matrix movie then we are dead :-)

    Not to start any controversies, but I've always been amazed at the fascination the general public has about Fuel Cells. The word(s) invoke a certain sense of something very energetic and I wonder how much movies like the Terminator aided in that. I suppose when you have them on space missions its easy to think of them as being something amazing.


  5. The instruction with all of our Roombas, yes we have 3, plus a Scuba, say leave the battery connected to the charger when not in use. Now the battery does seem to reach a point of charge where the charging light goes out, but does your rule (unplug your battery) apply? With different models, we have at least two or three different Roomba batteries, but I don't know what technologies they are.

    I don't expect a Roomba to clean a room, but what they do is keep a room cleaner than it would be if we were to manually vacuum (and we have 3 cats). Our house is dramatically cleaner since we brought the Roombas to bear on it.

    Forty Five years ago in Australia my parents owned a battery powered reel mower (one that uses a series of helical blades that move against a fixed blade, so that is not a typo), it dragged along a very heavy body and a catcher for the grass. After the 3rd or 4th battery, we replaced it with a gas powered rotary mower. It's taken a long time for battery powered mowers to re-appear.

  6. Colin: For both Ni-MH and Li-ion batteries pulling the plug is a good idea. In Ni-MH, at the beginning of life this is not a issue, but as the battery fades, this will be important. So, pull the plug.

    Lead-acid batteries need to kept plugged in; but I think we can be confident that your Roomba does not have a lead-acid.

    I do agree with you that the Roomba does do a decent job. But 3!! Lot of machines running around!


  7. You didn't save a hundred dollars on your purchase, you started a payment plan. Every week you pay PG&E for the privilege of mowing with electricity. You MAY come out ahead over the life of the mower vs. the battery model, but you still didn't save $100.

  8. "You didn't save a hundred dollars on your purchase..."

    As opposed to what? goats?

  9. ""You didn't save a hundred dollars on your purchase..."
    You have to charge the battery! If you need, say 1 KWh, to cut the grass then it does not matter if I use a corded mower and pay PG&E or use a battery powered one and then pay PG&E to charge the battery.

    Maybe you are thinking that you can charge off peak while you use the mower on peak or something, but I can;t imagine this being a big difference. I don;t think I will be mowing at peak!


  10. Welcome back,Venkat.Its really difficult to choose good battery pack. I am thinking for cordless tool packs for my projects.Thanks for good point on lead acid
    green credentials. for outsider is counterintuitive.

  11. Awesome article! I find it great that someone publicly speaks out his mind on technologies that we encounter everyday and its pros and cons. The Battery vs AC issue, I like everything plugged to the wall, unless strictly necessary. Keep the blogs coming!

  12. You've raised some really good points about lawn mower batteries. I really appreciate your doing the numbers and looking in to how economical these things really are. Great job!

  13. "The Li-ion battery, on the other hand, cost me a whooping $2300/kWh!! No... really. This is what it cost me. "

    I hope you realize there are much less expensive lithium cells available. The LiFePO4 cells in my EV cost me less than $400/kWh. They should also last much more than 3 years/300 cycles.

    I do agree that the plug is the better way to go if possible. I'm working on an electric snow blower that I'll plug in instead of using batteries. Much more cost effective.

  14. Probably a wise decision to skip the battery powered mower, but you are betraying the confidence of your followers ;-).

    I just finished puplling apart my less than year 2 old battery powered mower (craftsman 48V). The battery indicator would drop to critically low after about 10 minutes. Upon closer inspection, one of the SLA - 8AH cells seems to have vented, corroding the terminals. Seriously considering replacing the SLA's with Headway lithiums (& risking my wife killing me).

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