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.