Battery Magnitude

Recently, there was news on a new formulation of lithium-ion battery using a Si-Graphene anode that would provide for 10 times the charge in 10 times the power capacity that current lithium-ion batteries in the same size (and I’m assuming the same weight). Now the standard three to five years disclaimer applies – in that it has to be brought out of a lab, commercialized and they have to figure out how to mass-produce them and not lose any of their stand-out characteristics.

But it didn’t immediately seem apparent to most observers that it would be useful because of rapid capacity fade – the battery would only last 150 cycles before it had only 50% of it’s original capacity, although that is still five times current battery capacity. Traditional lithium-ion batteries last 300 cycles to 80%, lithium-polymer (the battery in your iPhone and Mac laptop) last about 1000 cycles to 80%, and lithium-titanate batteries can last 5000+ cycles to 80%.

The difficulty with the Si-Graphene battery is managing the user experience. If a user were to go through their entire battery in a day, every day, in 5 months they’ll only have half the capacity. So the device developers have to oversize the batteries but artificially clamp the energy storage to keep heavy users from destroying the battery in a short time frame.

Consumer Electronics? Sure…

Putting this battery into a smartphone to replace the current lithium-polymer battery would let average users go 10 days between charges. However every 250 days (25-30 charges) the user would notice they’ve lost a day of use before it was time to recharge, so from 10 days to 9 days. Will users be upset that they lost a day? Will they even notice? Or will they beat down the door of the store where they bought it demanding an exchange on a perfectly good product? How can we avoid this? By artificially limiting battery capacity.

If we were to limit the battery capacity artificially to a value that the 80th percentile user will have after 2 years of usage, we can save the trouble of users noticing their battery doesn’t last as long as it once did. I’ll assume this number is 75% of capacity, that is the 80th percentile user will go through enough battery capacity in two years that will cause a 30% capacity fade (this also factors in an increase in usage due to the bigger battery). So the phone will be setup so the average user can go 7 days before hitting the 25% warning and then recharging. Using 4Wh/day the user will go through 36.5 full cycles per year, which represents a 12% capacity fade per year. After two years, the capacity fade will be 24%. So after two and a half years, the average user will start to experience the battery holding less energy, and probably notice it around the end of year three – about which time the user will need to buy a new phone anyways as this one will be long out of warranty. An 80th percentile use will probably start to experience capacity fade earlier, around 18-24 months. A 95th percentile user is likely to do crazy stuff with their phone like stream audio all day and go through one cycle per day, and run into capacity fade in 6 months. This last case could actually be accounted for in software – if the phone notices its being used heavily it could ask the user to plug it in while engaging in the heavy activity, or just nerf capacity in software in the name of getting out of the warranty period without having to replace the battery.

Below is a chart of a traditional Li-Polymer batter (5.3) and a new Si-Graphene battery (53). You can see that the new battery has much larger capacity, it also fades much quicker. If you were to limit the Si-Graphene battery at 40Wh (40) capacity, the battery would get to two and a half years of average use before the user experienced any capacity fade.

The downside to this approach over traditional batteries is that users might increase their phone use and suck down more battery power per day knowing they have a lot more energy available to them, which is all the more reason to artificially limit capacity in the name of having the battery last long enough to have a useful device for 2-3 years.

The same results from the smartphone situation above would also apply to tablets. Laptop computers would probably see more agressive artificial capacity restrictions, as users usually run out of battery on the laptop before they are done with whatever they were doing (like doing internet research and writing a blog post about batteries ಠ_ಠ), so the issue of using more energy per day and higher annual cycle counts would apply.

Electric Vehicles? Not so fast…

If the approach of limiting battery capacity in the name of extending its life sounds familiar, it should, as it is how the battery in the Chevy Volt is managed. So what would happen if you applied this to the battery in the Volt? Not much difference, and probably an increase in cost.

If you recharge the Chevy Volt once a day, 365 days per year, it is equivalent to 237 full battery cycles per year (10.4kWh used for 35 miles, 16kWh capacity), and the battery type they use is expected to have a life of 1500 cycles (without any depth of discharge reduction bonuses). But if you were to drop-in a replacement battery with this new technology (assuming same size, weight, etc), you’d have a 160kWh battery. Now that doesn’t mean you drive 10 times as far, rather you just use an increasingly small portion of the battery, specifically an initial depth of discharge of 6.5%, and a rate of 25 full cycles per year. By the end of year 10, or 250 cycles, the battery would have degraded enough where it will start to run into problems storing and producing enough energy (assuming they can last that long from a calendar standpoint). This doesn’t appear to be a significant change from the current battery regimen, where the battery is warrantied for 8 years or 100,000 miles (12,500 miles per year). The only benefit to using the Si-Graphene batteries would be the increased power output – a Volt’s 9-second 0-60mph times could improve dramatically, along with faster recharging times.

What would help

The problem with this is that batteries are predominately priced in $/kWh, which would make the above scenarios prohibitive. The fundamental question is would it be more appropriate to charge by mass and volume? Does it cost 10 times the amount of making traditional batteries to these batteries? I don’t think it will. They might be able to charge more than the highest end batteries, but the $/kWh would need to be discounted compared to other types of batteries that have higher cycle lives. The best figure to use when it comes to battery prices is $/lifetime kWh, or the amount of energy a given battery will output until its no longer usable for the specific application (e.g. smartphone, EV, etc). A battery might cost $700/1500 kWh lifetime, and it might not matter that its 1kWh of storage for 1500 cycles or 10kWh for 150 cycles for certain applications – assuming other factors are held constant (volume, weight, safety). In fact, the latter configuration helps in applications where power demand is high (e.g. a car).

So the most basic thing to help these batteries would be an increase in cycle life. Even a relatively small increase in cycle life would dramatically impact the usefulness and increase the impact these batteries can have.

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