Yet another 100% charging question

[canbonbon]

Ah, I assumed the buffer was above and below! yes I've settled on 95% and will do the rest in the morning, will less than an hour between getting up and leaving though.

Yup I think that's what I'll do!

Ah apologies, I missed that bit, be interesting to see the health on hire cars, when I rented a Tesla before it was on 100% when I picked it up and they advised me to charge it to 100%, this was in Iceland I think the average temp was around -10c at the time, it did spend a lot of time preheating the batteries though. I would assume most people renting an EV will just charge it to 100% and they will be constantly used and run down.

Good point. I guess the rental cars might provide some actual data on how charging cars to 100% and then run them down to closer to 0% (in a rinse and repeat cycle) impacts the battery life of a car. Granted rental companies don't keep cars for more than 3 years, still it might provide a good insight. But even Uber/Lyft drivers do the same thing. They charge the car to 100% and then use it all day then do it again the next day. From what I have read they try to charge to maximum if they can.

 

[HitchHiker71]

So I feel I need to hijack this tread as I'm doing my pre trip dilemma of charging to 100% or not knowing it will be sitting for about 7 hours at 100% ...

The battery has a few Kwh hours of capacity as a buffer right? so realistically charging to 100% should be fine?

I've got a phev that's nearly 10, now I haven't been driving it much due to well the tesla is so cheap to run we use that, however I did a 350 mile trip a few weeks ago and it brought the max stated charge back to an indicated 31 miles (same as when it was new) now I've been doing lots of short trips and basically just driving on the battery range for a while, very seldom the engine kicks in, in doing so the range dropped to 24 miles on a full charge, that big trip was constantly charging a bit and dropping down to as close to 0% as it lets itself before charging it back up, this seems to cause it to do some sort of calibration in my mind and restores it. Now that phev has 8kwh useable out of 11kwh capacity battery, in my mind this extra capacity does 2 things, it prevents damage to the cells at 100% state of charge and it prevents the battery going to a true 0% as there is always a bit kept charged. I'm not sure if this is true or not but in my head makes sense, would this be the same kind of logic for an EV?

So charging to 100% and leaving it for a while, will that kind of help the BMS determine its capacity a bit better? in my mind running it down to close to 0% then doing a 100% charge is going to let the car guess its capacity better? Or am I just thinking its nonsense ?

Every time I try to find a decent answer all I get is opinions never backup with anything. much like my own antidote above.

Just use the Off Peak charging feature in the app and set the Off-Peak end time to when you're going to leave in the AM - this will calculate the required charge time to go up to 100% such that it'll complete the charging session shortly before your scheduled leave time and eliminate the need to have to manage anything manually for your charge session before your road trip starts. You can also set the precondition departure time for that same leave time so the vehicle is warmed up and HVAC set when you leave. Then just set the max charge percentage up to 100% the night before. This is what we do every time we have a road trip planned. The BMS doesn't need "time" to determine capacity better - whether it's at 100% for 15 minutes or 15 hours makes no difference - 100% capacity is 100% capacity whether at 100% capacity for 15 minutes or 15 hours.

 

[father_of_6]

My phone says my battery is at 43%, but I'm almost certain it's actually 43.4% or 42.891%... lies... LIES!!!

I totally get that my post is worth making fun of (which a few folks did) but my point stands.

@canbonbon - I know this post is aged and you're probably far over it, but I just want to circle back and say that I wasn't trying to make fun of you or your post, just trying to "make fun" in general.

It's true though, these small numbers are like rounding errors... also the energy in the battery isn't like pennies in a jar or even water in a jug where it can be somewhat easily measured. The physics of measuring battery capacity are complicated and tough to get exactly right.

The battery management system (BMS) has to check the voltage of all the cells and guestimate (based on the battery type), what the remaining capacity of the cells are based on that voltage. The voltage varies based on capacity, but it does so on a curve that doesn't always tell the exact story.

This is why Tesla recommends charging LFP batteries to 100% weekly - because the voltage of LFP batteries remains fairly constant in the middle range, and only increases or decreases at the top and bottom ends respectively. So Tesla's solution for accurately measuring the LFP capacity is to charge it to 100%... the BMS sees that magic voltage number at the top and then can tell it's full.

A few miles here or there on the guessoneter truly is just noise. You've probably already driven enough at this point to experience this yourself. Also just want to add that I'm not an electrical engineer, just repeating back what I've learned from reading these forums.

I hope the car is treating you well!!