Thank you for your input and clear explanations with sources! It is greatly appreciated, discussions like this is why these forums are helpful not only to our users, but us as well.
I am the engineer in charge of the design and testing of our battery, and I also have the responsibility of setting the general guidelines and specs of how to use it and our charger, having written the store pages and all documentation.
The endless sphere forum is a great resource with a lot of knowledge, I spent many hours there reading through discussions and various sources back when we were designing and testing the first prototypes in the summer of 2016.
I’d like to explain some of our logic in the recommendations we make, as with batteries there is never a simple right answer that is set in stone for every use case.
@corndog, I know you know most of this and not all of this is directly in response to you post, but I figured I would make this as clear and thorough as possible as this is visible to anyone here. A good opportunity to lay it all out.
To my knowledge, lithium ion/ lithium-polymer charging profiles are essentially the same, save for the end voltage difference of 4.1/4.2 V. The charger still goes through the same CC-CV modes, it just stops at a different point.
To answer you question:
Essentially, yes, we decided on 4.2 V because that is the number in the datasheet. If the manufacturer, Samsung, has technical specifications that state the cell is designed for charging to 4.2 +/-0.05 V and shows that it has an acceptable lifetime while being charged to this voltage, then I feel that that is appropriate to reflect that in our technical specifications .
I am aware that reducing the voltage range at both ends can significantly benefit cycle life, however, this is a contentious question, as its a sliding scale in a big messy grey area.
To really nitpick, then technically the best possible charge/discharge cycle for a lithium ion cell is to charge it to 3.8 V and discharge to 3.7 V. Of course this isn’t practical, I just want to use that as an example to illustrate the point that battery performance/lifetime trade off is a sliding scale, of which this is one extreme.
Thank you for the references you posted, I read through them and learned a few things. I did not realize that last tenth of the volt made such a big difference to lifetime. This graph from here sums it up nicely:
I agree with and acknowledge the conclusion that reducing charge voltage to 4.1 V or even less can more than double a cells cycle life. However, most every resource was referring to cells used in electric car/e-bike or other EV batteries, and using them as examples. I don’t feel this is directly comparable to our battery, for a number of reasons:
An electric car battery is extremely expensive and is perhaps the majority of the cost of the vehicle. Our battery is a couple hundred dollars and less than 10% the cost of the BlueROV2.
An electric car battery is difficult and potentially impractical to replace, requiring specialized equipment and skills. Our battery can be swapped in a few seconds by simply opening the enclosure.
An electric car battery is (hopefully) designed to last for at least 10 years, while undergoing at least some level of charge or discharge every day. Our battery, depending on the user, may be used at various levels, but I think its safe to say the longevity and cycle demands are less than that of an electric car. The most demanding users will likely have more than one battery to spread the load evenly with as well, which is not possible with an electric car.
Using our recommended charge/discharge cycle of 4.2-3.0 V, and assuming an average discharge current of 4-5 A per cell, I think it would be safe to estimate a lifespan of at least 500 cycles. (Compared to the official Samsung data of 300 cycles with far tougher conditions)
This is on the order of $0.50 per cycle, which I feel is reasonable, but this is definitely open to discussion.
At one extreme, I myself have responded to emails from a few users that have over discharged their batteries, and when asked how it happened, they stated that they knowingly did so, with full knowledge of the damage that would occur. Their reasoning being that as commercial users, it was far more important to complete an inspection job in less time than stop short and swap a battery. Their and their clients time was more important than a couple hundred dollar battery, a situation that I had never previously considered.
My point is that there is a wide range of users of the BlueROV2 and our battery with extremely varied skill levels, use cases, and power demands.
Unfortunately, we can’t post a 20 page essay on battery physics with graphs and charts in our technical documentation for Jen/Joe user, we need to keep our documentation clear without being overwhelming for someone who just wants to get going using the vehicle.
Ultimately, we have to choose a suggested set of specs to put in the technical specifications that we think is a good balance for most people. In order to fit most users needs well, we decided that a general guideline that resulted in a 500 + cycle lifetime while maximizing vehicle performance (thrust) and run time over the course of a cycle was an appropriate balance. This is all assuming the average users wants/needs.
Since this an ROV with a battery that isn’t that expensive and won’t undergo that many cycles, plus is easy to replace and most people want longer dive times, we more or less went with what the cell data sheet says. We made the change of setting a more conservative discharge voltage as we know that is the point where most users are likely to make mistake.
I am open to thoughts/suggestions regarding this from everybody:
Do you think the current set of specifications is appropriate given the information here?
Or would it still be clear and not cause confusion if we had two sets of specs, one the same as the existing one for max vehicle performance/run time and another for max battery cycle life?
Do you think expecting a 500 cycle life out of the battery is appropriate?
A resource I would like to share is the lygte 18650 Comparison Tool. Different 18650 lithium-ion cells can be directly compared at various discharge rates to see how they behave under load, I found this invaluable when we were deciding how to refine the ratings.
Thanks for prompting me to get this out there @corndog.