Lithium battery lifespan part 2, a new battery checks in

The end of last year, I posted my first entry comparing the lifespan of a LiFePO4 battery to the claims made by the manufacturer. That entry relied on data from 525 cycles on the battery. Since then, I have continued my testing and added a second battery and testing rig to the mix. Let’s check in on the testing of both batteries and an interesting trend developing with the first battery as it crosses 1,000 cycles.

Currenlty, the original WattCycle 100ah battery I started with has 1,100 cycles on it. I have also added a Blue Heron 100 amp hour Extreme series battery to the testing. That battery now has 500 cycles on it. Both WattCycle and Blue Heron make pretty heady claims about the life of their batteries at 5,000 and 6,000 cycles respectively. Frankly, I’ve always been skeptical that batteries will last that long and my testing hasn’t done anything to tamp down that skepticism. However, I also don’t believe batteries need to achieve anywhere near the number of cycles they claim to provide practical usage and excellent value compared to their cost.

Current results

As we look at the data so far there are two things that jump out at me. First, the WattCycle’s rate of capacity degredation has slowed dramatically. If you click the “Projected Lifespan (recent)” tab above, you can see how much the more life the recent tests indicate. Because that battery is the very first one I’ve cycled continuously, every piece of data I glean with it is new.

So, currently I’m trying to understand trends with a dataset of one which is dangerous. But, despite that danger, I am wondering if this is indicative of a behavior in which degradation is faster in the early portion of the battery’s life and slows over time. When comparing the degradation from the very first (over rated capacity) tests to the tests a few hundred cycles in, we see a pretty big drop.

The second thing I notice is that the Blue Heron data is quite volatile. I am very careful with my methodology in these tests and control external variables as much as possible. That includes discharging and charging the battery in the exact same manner throughout the tests. Additionally, the every 100 cycle capacity tests are performed in the same manner with the same West Mountain Radio CBA V tester each time.

The test chart from West Mountain Radio’s CBA software confirms that the overall discharge pattern looks correct. It started at the same voltage, didn’t display any anomalies during the test and tapered with the same curve as the other tests. But, that test also showed less energy was stored in the cells that previous and subsequent tests. With confidence in the methodology, all I can do is report the results.

The table above breaks down projections of expected cycle life based both on all tests and the delta between each test. Note that there is no projection based on recent data for the Blue Heron at 500 cycles. That’s because of the increase in measured capacity observed between 400 and 500 cycles. That’s an unexpected result and I don’t see a way to make cycle projections based on it. Both batteries show quite a bit of volatility in their projections based on recent data. Perhaps that estimate isn’t as useful as I hoped. I’ll keep watching it and may drop it eventually if it proves low fidelity.

Understanding the actual cost of batteries

One of the reasons I undertook this testing is to understand some of the financial parameters of LiFePO4 compared to other battery chemistries. The most obvious comparison to draw is to lead acid. To support that comparison, I’ve begun cycle testing a Group 27 deep cycle battery. Lined up right behind that battery is a 6-volt, GC2 deep cycle battery. I hope to have something to report on the results of that testing in a month or two.The lead acid batteries will likely last many fewer cycles, capacity retention will fade faster, and overall they will store and return much less energy.

I expect the lead-acid testing will take significantly longer per cycle. LiFePO4 batteries both accept charge quickly and show minimal effects from relatively high discharge rates. The cycle testing I’m doing now both charges and discharges the batteries at 0.5C or half their capacity. So, each charge and discharge cycle takes a little about 5 hours including a 30 minute rest period. Lead acid can’t be discharged nearly as quickly without significant capacity impact.

The VRM detail above shows what the SmartShunt sitting in front of the WattCycle has observed over the last 1,100 cycles. One interesting observation is that the total charge and discharge energy suggests the battery has been operating at 93.7-percent efficiency. That’s pretty impressive and another aspect in which I expect to see a big contrast with lead-acid.

Without performance results from lead acid testing, we have some holes to fill in. But, we can start to get a picture of the cost of energy as stored and delivered by a battery. There are quite a few energy efficiency benefits to LiFePO4 owing to the chemistry’s fast acceptance of charging and high efficiency in accepting that charge without turning much into heat. But, for the sake of these comparisons, I’m not going to worry about those benefits. We are just going to look at how much energy a battery delivers versus the cost of the battery.

As we look at the cost per kilowatt hour, I want to mention that this metric ignores nearly all other aspects of these batteries. For example, the Watt Cycle 100ah Mini is as stripped down of a LiFePO4 battery as you will find. It has no Bluetooth communications, no Victron Communications, no heating, and supports significantly lower limits than the Blue Heron battery. This comparison focuses only on the ability of the batteries to store and deliver energy. While that is a battery’s primary purpose, modern, high performance electrical systems often utilize significant other features worth considering.

What jumps out at me about this comparison is how much more costly each kilowatt hour of energy is when delivered by a lead-acid battery. The very short cycle (and typically calendar) life of a lead-acid battery means that every cycle and every kilowatt hour of energy comes at a much greater cost. Let’s first compare ratings where we see that even an inexpensive lead acid battery delivers energy at more than 16 times the cost of the WattCycle battery and more than 4 times the cost of the Blue Heron. We don’t have enough lead acid data yet to fill in any sort of projection, but we do see that comparing the cost per kWh is not favorable for lead-acid. That’s not surprise since the expected cycle life is so much shorter.

Figuring out what this all means

I have done a lot of testing on LiFePO4 batteries recently. Some of that testing has been in support of some consulting that I’m doing and I’m still figuring out how to write all of that up. But, the theme of nearly all the testing I’ve done is the degree to which the cells determine the performance of the battery. That might sound obvious, but, when we compare lithium-ion batteries to lead-acid, the big difference is the presence of the BMS. So, there has been a lot of focus on the BMS. I think that focus was emphasized with some early batteries with BMS design or reliability shortcomings. However, the testing I’m doing now suggests even relatively low-end BMSs are very solid and not likely to be a source of issues.

What has become very clear is that in proper operation, the BMS fades into the background and the performance of the battery really comes down to the performance of the cells. A BMS performs two functions and if all goes well, only one is active under normal operations. The first function the BMS performs is to act as a gatekeeper and safety device. It will shut things down if voltages, temperatures, or currents get too high or (for voltages and temperatures) too low. A properly installed and functioning battery shouldn’t hit any of those values. The second thing a BMS does is ensure the cells stay balanced. There are a few ways they accomplish that, and that’s a topic for another day.

Many of the ratings we see on the side of the batteries we buy come directly from the cell manufacturer. If a cell manufacturer rates their battery for 5,000 cycles, that’s what the battery manufacturer will put on the spec sheet. If the cell manufacturer says the battery can handle 1C continuously, the battery manufacturer will size the BMS appropriately (hopefully) and put that spec on the sheet too. But, like so many specs, many of those ratings are based on laboratory conditions. In the real world, the cells might not be held under the same compression or subject to the same temperature. So, our actual specs are likely to lag those on the sheet.

For the vast majority of boaters, 3,000 cycles is a sublime lifespan for batteries. It is generally acknowledged that LiFePO4 batteries should last a minimum of 10 years. At 3,000 cycles and 10-years, that is 300 cycles per year. The test cycles I’ve subjected the batteries to have been to 90-percent depth of discharge, which I believe exceeds typical use on a boat.

With a sample size of two batteries, I’m not ready to draw too many conclusions. But, I do find it very interesting that the less expensive battery is performing as well or better than the more expensive one. This cycle testing stresses the cells far more than the BMS and electronic components. So, perhaps WattCycle’s cells in this battery carry a little bit longer than those Blue Heron is using.

The only way to draw better conclusions is more testing. So, that’s just what I plan to do. I’ll start with the lead-acid testing but will also add additional LiFePO4 batteries to the mix.

P.S.

I’m using some new visualizations on this article with both the charts and tables. I’d love to hear any feedback on how easy they are to understand and what can be improved. Thanks! Ben S.