In the top right corner of this tool, I would like to plot the estimated charging times for the configured cell/pack, based on a chosen charge amp value. (I might add a toggle option for a “balanced charge”)
Does anyone have or know of a relatively simple algorithm to plot a nice charge curve? So the user can see for how much charge they would get after a certain amount of hours.
I hope this makes sense. Plenty of battery boffs out there
That’s the first time I’ve ever heard of this, and I’ve spent a lot of time reading and learning about battery chemistries and behavior. What’s your source?
From all I’ve seen, li-ion cells have a coulombic efficiency when charging of better than 95% (generally closer to 99%) (see this paper), and the charge-discharge energy efficiency is similarly good, this paper shows 88% for LTO batteries.
The charging voltage vs capacity curve is non-linear so there’s no simple equation you can use. You could probably work out something good enough by checking out charge curves for a few popular cells at a couple (or more) charge current levels,
Pretty much all the chargers we use follow the same CC/CV algorithm, which means that from 0 to ~80% SOC or so, the charge time is linear, and pretty close to 1C=1H. From there on up, depends on the exact battery chemistry and conditions, not so much on the charger.
What that means is that in most cases, for a 9Ah battery, a 3A charger will take a bit under 3 hours to reach 80% or so, and then the last 20% can take up to an hour ish. You could approximate the last 20% as maybe around CR/3, with CR being the CC charge rate.
That’s a rough ballpark curve-fit based on what I’ve experienced, so take it with a generous pinch of salt - no hard data here. But it’ll at least get someone within a factor of 1.5 or so, which is often good enough to know “hey, I want a faster charger” or not.
If you really want to get deep into it, I recommend doing this specialization, think it still free if you don’t want a certificate
It’s heavy content but get into deep details and you will learn how to model and simulate any cell, maybe overkill for what you want, but even if you just skim through it and don’t watch all classes it still good
And if you will watch and do everything, again, highly recommended, do yourself a favor and watch the classes at 2x speed
You have forgotten to take efficiency into account. Torque per wheel should be motor torque x gear ratio x efficiency. (If we assume that entered efficiency number is purely from the drivetrain. Which it usually is, when you compare belts vs gears or something) Otherwise seems perfect! Thank you so much!
That graph on the right side is cool as well; really useful for picking motor kv.
Another graph you could add is the one @fessyfoo made which shows max torque at standstill, and top speed. I found it really helpful when comparing high voltage boards to standard ones.