Sunday, February 28, 2010

Battery Rules.

I’ve been reading Michael Pollan’s new book titled “Food Rules” and thinking about his Haiku on food which goes: “Eat food. Not too much. Mostly plants.”

Succinct, but loaded with a lot of complexity. Inspired by this, I decided to try my hand at a (kind of) poem for battery health. But first, the way to extend the life of a battery depends on the battery type. Lets start with the battery we use everyday in our laptops and cell phone- the lithium-ion battery. The rules read,

Don’t charge them too high

Don’t swing them too wide

Keep the temperature low to extend their life.

Lithium ion batteries are not all equal. A battery from A123 will be different from one from Panasonic. So use these rules only for your cell phone and laptop batteries.

In the previous blog post, I told you why you don’t want to charge them too high in voltage (answer: side reactions). Higher the voltage, higher the fade. Turns out that if you swing the state of charge too much (i.e., charge and discharge the battery completely each time) the life decreases significantly.

This is because the battery materials expand and contract on charge/discharge (by as much as 10%). This constant “breathing” results in the particles cracking. As a matter of fact there there is data that shows that if you swing the battery to, say 3% (like in a HEV) you can get 300,000 cycles (yes, you read that right). But if you swing them all the way, you only get 300-1000 cycles. So you can charge and discharge them a lot, but you cant let them swing too wide.

Turns out that while a small swing in the state of charge is good for life, its wrecks havoc when trying to estimate the state of the battery. So as a good rule of thumb, every so often (say 2 months), discharge the batteries completely and then recharge them back up to make sure the software can reset the battery capacity and predict run-time better.

Finally temperature. Temperature is a boon if you want to make things faster (reaction rates increase with temperature). But remember those side reactions? The rate of these reactions also increases with temperature and they accelerate the capacity fade. Hence the recommendation to keep the temperature low.

As a matter of fact, I’ve seen recommendations that ask you not to keep your cell phone in your pants such that the battery is close to your skin (which would be, assuming you are normal, at 37 C). The 15 C higher temperature compared to the ambient (if you are in California) will kill the battery fast. Folks around the equator- sorry.

Turns out that if you go down to freezing temperatures, you get other problems with lithium batteries, but that for another blog post.

This is it. Three simple rules for a long lasting lithium-ion battery. Forget all the rest of the stuff that you hear about keeping it on the top of charge, get the juices flowing e.t.c.

Question is, where do these myths come from? Like I said before, the rules change when you change the battery chemistry. So rules from one battery chemistry get applied to another and what you have is confusion.

So here is a second (kind of) Haiku that tries to capture the different batteries we typically encounter or have encountered. This reads,

Keep your lead-acid’s charged

Let the Ni-Cad’s completely discharge

Lithium-ion and Ni-MH? Somewhere between

Lets get to the first rule. For a lead-acid car battery, the failure mechanism is called sulfation, where the discharged material undergoes a phase transformation after which it can’t recharge. Remember the time you left the glove compartment light on, the battery died, you got it jumped a couple of days later, and was told to buy a new battery? Yup, that was sulfation. Happens everytime the lead-acid battery discharges. Hence the rule that we should keep the lead-acid battery charged.

But, keeping a lead acid completely charged also leads to other problems (like grid corrosion) which would be lessened if you let the voltage decrease a bit. Moreover sulfation takes a few days. So one could do something complicated like let the battery charge, and then let it discharge a bit, but come back the next day and charge it back up before sulfation kicks in.

But let’s not make life very complicated, shall we. Just keep the battery charged, it will last 6 years, then get a new one. If you are an enthusiast, contact me and we can talk about an optimal charging scheme. And if you are one of those using the lead-acid battery for deep discharge-cycling then you are in real trouble. This rule is not going to help you. Some companies are now claiming to have solved this problem. Maybe they can help you.

On to the next rule.

Remember the mythical memory effect? Picture this (true) scenario: father calls his electrochemist son asking why the cordless phone battery was not holding charge; son admonishes father to get off his cheap lifestyle and buy a new battery already; father is convinced that son doesn't know what he’s talking about; father contacts second son who decided to use google and discoveries memory effect; second son asks father to discharge battery completely and try using it; father reports success and decides to disown electrochemist son. Amateur battery enthusiasts are the bane of my life (especially if they are family).

That was a Ni-Cad battery. if you don’t discharge these batteries completely and charged them back up, they seem to not remember that there was actually some capacity left. I have never experienced this myself, but there are many reports on this subject. Hence the oft repeated mantra- let the battery discharge completely before you recharge. Its specific to Ni-Cad batteries. See how different batteries are different?

Remember when the nickel-metal hydride (Ni-MH) battery came out and everyone said there was no memory effect? Turns out they jumped the gun. Ni-MH also appears to have a memory, albeit not as bad as Ni-Cd. There is some controversy as to why it actually happens and there is a thinking that overcharging these batteries causes memory. I used to work on this system and have overcharged these batteries quite a bit, and have not seen the memory effect, but... maybe I have a golden touch. Without getting into details, its best to pull the plug in these batteries when they are charged. Even if you don’t believe in memory, this also helps prevent drying up of the battery because of hydrogen evolution and venting.

As for discharge: completely charging and discharging a Ni-MH battery is not a good idea (the metal hydride particles also expand/contract and can crack if you do that), but then again, there is some data that suggests that there can be a memory-type effect if you don't discharge them completely.

So what should you do? I’m going to recommend that you don't worry about memory and try not to swing the state of charge too much. Every so often (say 2 months) discharge the batteries completely. May be a bit hard to do in practice, but one can try. Remember that the Toyota Prius has Ni-MH batteries and you never completely discharge them. They work fine for 10 years without any problems from the memory effect.

Hence the rule to keep the state somewhere between the lead-acid and the Ni-Cad for the Ni-MH battery. In many ways this is the same as the lithium-ion battery.

Lets hope this post does not end up adding to the myths that are already out there!

So here’s to Haiku-ing your way to better battery life.


  1. Thanks a lot. I've never been able to sort out which rule applied to which battery type before.

    Is there any significant difference between the different Li-ion chemistries?

  2. Love the blog, this kind of information is long overdue and to bring poetry to it makes it that little bit better.

    Can we have a haiku devoted to the storage of batteries please?

  3. Then what about the new Nissan Leaf EV? Won't the charge and discharge cycles be "swinging them wide"? So the battery will have to over sized so as not to discharge too deep?

    P.S. I love your writing style!

  4. What an awesome and well written blog. I am visiting from where we are very interested in battery chemistries and technologies. One feature I'd like to see is a way to contact you or a form to submit questions or topic ideas.

    Here are a few:

    1. Do a post on battery myths (e.g. keeping them in the refrigerator makes them last longer)
    2. Explain what the cut off points are (in volts) for rechargeable. For example, for a 1.2v NiMH AA, how far would it need to drop before I should recharge it? How far before it's too late/dangerous?
    3. An overview of Li-ion cells and the caveats and challenges with properly charging them (voltage, cut off, etc), risk of fire and explosion, etc.

  5. Venkat SrinivasanMarch 6, 2010 at 1:39 PM

    Tom- There is some difference, but not a whole lot. In batteries like the one from A123, the voltage of the cathode is constant, so you can't take the voltage too high. So in this battery, things are better.

    On the Nissan leaf: I suspect that some games are being played. I suspect that they probably limit the voltage at the higher end. In general, you can discharge them to the end of discharge, its the charge end that is the problem.

    sygyzy- I will ask and see how to add a question form. I'm a bit web challenged (although I'm really good at surfing!). Thanks for the questions. These are helpful for future ideas for blogs. I probably should do a "battery myths" posting at some point.

  6. Venkat,
    I just stumbled on your blog via EcoGeek. I wanted to say thanks very much for an informative and well-written blog! I've already learned a lot from reading all your posts to date (and all your thoughtful answers to the comments), and I'm looking forward to future posts. Good writing, good information, applicable, understandable, great work. Keep it up!
    If you have time, would you consider a post on storage of the various types of batteries? I.e. how to storage times and conditions affect a battery's performance on that particular charge, and on the battery's long-term performance.

  7. @ Venkat - Love your blog - just found it and will follow along to learn more about batteries.

    We all appreciate your efforts to do the impossible as in educate the general public on a technical topic!

  8. Just to clarify, when you write, "As a matter of fact there there is data that shows that if you swing the battery to, say 3%..." do you mean to unplug my computer until it reads 3% or until it reads 97%? I'm asking because it warns me to plug in when it gets around 15%.
    I just bought a new battery for my MacBook. I'd been advised to use the battery all the way down before plugging it in--advice I followed religiously while writing my dissertation. Then when it obviously needed replaced, the "Genius" at the Apple Store advised me to keep it plugged in all the time. He said the battery was designed to do 300-some cycles, and mine had done 500-some.

  9. what are the unsafe features of lithium batteries in a role over smash up? When they become shorted out, metal to mwtal opposite post on a quick discharge, do they overheat and explode or put off unsafe gas oders to breath? What safety tests have been made on this product? Under all conditions and can the public have access to the results? such as on fire-underwater-exstream heat or cold or vibration? I would sure like to know for scientific prrposses.Thanks, awaiting comeback.

  10. Venkat-I have repeatedly been replacing my cordless drill batteries for all the WRONG reasons! Thanks for the education.

  11. Ok, so here is my attempt at a Haiku for you:
    Charge middle. Swing in-between. Low Temperature.

    Absolutely love the blog - came across it when researching why my Ni-MH batteries in my baby monitor kept running out!

  12. Hi, I would also like to see a post on battery myths as comment by "sygyzy" stated

    1. Do a post on battery myths (e.g. keeping them in the refrigerator makes them last longer)

    My company's biggest problem is the up keep of the batteries and the cost to replace them frequently, everyone has a different solution to maintaining them and most are just myths.

    My problem is knowing what is myth and what is not.


  13. May I quote some of your content on my blog and ecommerce site? Many of the products I offer have either internal or external li-ion batteries and I would love to use this to explain to them how the batteries work and the best ways to get the most out of them.

    Thank you,
    Jeff Monson
    Owner - JLM Merchandise
    Ref: Battery powered covert video cameras

  14. Great thread dear! I truly enjoyed all the discussion. Thanks!

  15. Well, I presume you all should be vaguely familiar with industrial battery. And yes I am referring to batteries like what is used in your cars and trucks. Now it can get rather complicated without an advanced degree in chemistry and electrical engineering so I am going to attempt to explain it the best I can.

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