Limiting charge to 80%, Why (or why not?). Some JuiceNet features

There is some debate out there whether charging to 80% (as opposed to full) gives you better battery health in the long run. Various factors include a potentially dodgy battery chemistry in the first generation of LEAFs, a class action lawsuit in the US, and general theories on lithium battery technology.

However, the following factors make me believe that in general, the 80% setting is not necessary for battery longevity. It may make a difference, but should not be a significant factor for more recent models. When you charge to a full 100%, the actual cell voltages get to around 4.1V, which actually corresponds to around 90% according to typical lithium battery discharge curves. (A really full battery should hit 4.2V). So if you aimed for 80% charge on the dashboard, you're actually more like 70% or so. Heat and time still seems to be larger factors, from most sources I've seen. Another factor that I think plays a huge role is how aggressively you cycle the batteries, which means a.) Quick charge use and b.) aggressive acceleration and regen. Of course, if you look far enough there'll always be anecdotal evidence to the contrary, but most general information out there seem to be relatively consistent.

So, why would we want to charge to 80%? Actually, after some further testing, we've started going towards 90%... For us, there are the following reasons:

1.) Given our typical commute of less than 20km per day, we do not need the full capacity of the battery

2.) Perhaps more importantly, having a full charge limits the amount of regeneration available. Since we do mostly city driving with a lot of start and stops, and it is not always feasible to coast towards every stop, being able to regen means that in the long run, we will be saving energy.

3.) Another energy saving factor is that since there is a fixed charging overhead of around 300W (to run the cooling pumps and other charging circuitry), charging beyond 80% or so becomes significantly less efficient due to the charge rate slowing down as the cells are being topped up and balanced fully. Basically at that point, you continue to consume ~300W but the % gained by the battery slows down significantly. Having said that, the process of cell balancing is important and should be done once in a while. Our current habit is to do a full balance every 3 or 4 charges, which works out to be around once every two weeks (depending on weather and driving needs).

So, how do we do it?

Without a smart EVSE, one can simply set the car's charge timer to a slightly later time than the actual departure. This will hold off charging when you plug the car in and only start X hours before your departure time. X is calculated based on the power of the supply you're plugged into, and ranges from about 5-7 hours on full Level 2 (240V 27A) to 14+ hours on Level 1 (110V 15A) for a typical charge. This requires some guesswork which gets better with experience as you have to figure out how much exactly in advance you set the time to hit the charge level you need. However, since we already invested in a JuiceBox, it's a lot more convenient to use the built in features which are accessible from the smartphone app and web consoles. Here's what the main charging screen looks like:

As we've set the vehicle to a 2014 Leaf, it knows that 24kWh of energy is a full battery. From there it allows you to set a target value for each charge session. Above we see the small triangle on the gauge set to 60%. What this will do is limit the amount of energy piped to the car to 24*.6 = 14.4kWh.

It should be noted that the amount of energy that is metered through the EVSE will not be exactly what the battery gains, due to a variety of loss factors and estimation of actual charge levels on the dash. In our experience, what you ask the JuiceBox to give to the car, compared to what the car actually gains, is actually 5-10% more when aiming for a target in the 80-90% range. I expect the value to be closer when the charge level is in the more linear voltage vs energy level range (i.e. below 80%).

In addition to the charge limit, we can also apply the "Time Of Use" feature to start charging at a later point in time:

Here in Quebec we don't have time of use billing so the consumption rate is same any time of the day, but this scheduling can be used to control when your car is fully charged, similar to the car's built in charge timer. The convenience once again is that you can set this from anywhere via the app or web portal.

All in all, a "smart" EVSE is probably not essential, but for people who like to look at data, it's probably a great compliment to the telemetry data that you get with the SV/SL models!