@Barkuti, sorry about criticizing your manners on that deleted thread the other day. I eventually did recognize the irony, but by then the thread was gone.
Hi
@Barkuti: don’t worry I am an engineer and numbers are also more or less my daily bread. The question is however in my opinion much less a geometric progression but much more: which values will people really want to set in the end and which ones will they miss.
In my opinion there are a 2 cases:
-a battery which shall be charged normally with lots of time. Nobody would care weather it takes 10 or 14hrs. For this reason I believe that this many steps on the lower end are not really necessary. (Below 100mA I don’t get the necessity anyways but as I said that might be due to my lack of knowledge about chemistry)
-quick charging a battery because of a hurry. Here many steps make sense because you would want to go to the max value the battery can take.
So in my opinion it does make lots of sense to have 2500mA for batteries which should not get more than that but I don’t think there are batteries that cannot take 250mA but can take 200mA. Even if there are batteries like that it will just take a long time to charge them and quickcharge will not be a topic.
For me the right way to look at this is not some mathematically nice calculations but rather some thoughts about what people will want to use based on the majority of batteries out there.
Maybe my thoughts about concrete values are wrong concerning but I think your basic idea of mathematically looking at the problem is not the best way and will make people jump over the same values (which h then almost never uses) each time at the same time missing some values which did not fit in the mathematical pattern
Regards
What MCU will be used? (I assume you were implying that porting GitHub - stawel/cheali-charger: cheap lipo charger to whatever is being used would increase the cost too much.)
No worries Joshk, 
all is fine.
0K daniu, let's see.
I think my approach is a “best overall”, and this is because I am being equanimous or impartial with the potential target use of the charger. I understand one could think there are many values at the lower end, but this is understandable if we think there are some pretty small cells in the market like 10180s with just a hair above 70 mAh, 10440 LiFePO4s with ≈200 mAh or little more, and even 280 mAh rated 14250s with “enough capacity” (funny), and even very small prismatic cells but… but 0K that's none of our business here. The small values were because of pretty small batteries, but well, let's do away with the two lower values, 35 and 50.
Concerning the “quick charging a battery because of a hurry” I understand you may be aiming here to stick to the maximum charge current whatever OEM lists in its datasheet. But I cannot agree with this in a default charge currents list. There are thousands of cells in the market which this charger can handle, and if I am to list every single maximum charge current value in every datasheet (or close) the list would be very long. I also don't think it is wrong to go slightly (or even moderately) above whatever maximum charge current OEMs list in their datasheets, and with my list of currents with the given progression any cell would find a value within ±20% of its rated maximum; which in practice is a very small charging time difference because of the CV phase, anyway.
In any case, as I've said before multiple times this charger must feature a fully editable list of charge currents (via advanced menu or however), so no-one complains. 
Perhaps we can just be sure it has a programming header 
Then we can each fiddle all we want.
Here’s how the iMax B6 does it:
The lowest is 0.1 amp and the arrow buttons add or subtract 0.1 amp. You can hold the arrow for quick repeating. I can do a video of the quick repeating if we want, it even transitions into repeating swiftly. Do you want to charge or discharge at 1.6 amp? No problem.
im in
Yeah, my Nitecore SC4 does pretty-much the same thing, in 0.1A increments from 300mA to 3000mA. It’s still a PITA everytime I put in a cell and don’t want to charge it at the default 2000mA. It’s the only thing I hate about that charger.
OTOH, my Liitokala 500 has just 4 settings to scroll through: 300mA, 500mA, 700mA, and 1000mA. Much easier! Though, I think they should dump the 700mA setting, which serves no purpose.
I noticed some AA NiMH got pretty hot at 1000 mA, started to use 700 mA instead. For me 700 mA fits perfectly.
I’d definitely rather pay a little more and get almost infinitely adjustable charge rates as in any of the small to large hobby balancing chargers have incl the iMax B6 above. I’m not into “counting seconds of my life away” while taking the few seconds to program in the exact current I want. A custom charger should be as controllable as possible. That’s the idea of a custom charger. If you want 4 charge currents to choose from, buy an off the shelf charger.
I totally agree about losing seconds of your life. It’s never “just a few extra seconds” to those of us with other things on our mind. Whatever it is needs to be quick.
Feature: keyboard to precisely input whatever parameter. It may sound weird but if the charger is done with touch screen it should or will be a sure thing.
Concerning the 0.1 A increments thing, that is probably big on the low side of the list and small on the high side. If done that way, the stepping should be fine enough, but I think this is cumbersome. It is best to scroll along a list of (12) presets, with a relatively fast “press and hold” scrolling through values. If you don't like whatever default values you go to the advanced menu and edit them with down to the milliamph or centiamph granularity. Together with charging slot memory to keep the last used current and voltage for li-ion/LiFePO4, this solution is pretty good in my honest opinion.
When you say “AA cell” do you mean AA NiMH or a 14500 Li-Ion?
Panasonic allows fast charging their AA NiMH with as much as 2000 mA for standard Eneloops and 2550 mA for Eneloop Pro, so that’s basically 1C:
As for 14500 Li-Ion, some cells specify 1500 mA quick charge current, for example this Vapcell:
Yes, when I say AA I mean NiMh or NiCd. When I say 14500 I mean LiIon.
For your Panasonic example, 2 amp was listed on my list, so you’re covered:
0.2 (ideal for AAA NiMh/NiCd batteries and 18650 “recovery”)
0.5 (ideal for AA NiMh/NiCd batteries, 14500 li-ion, or “heater” 18650)
1.0 (ideal for testing 18650 or charging cheap 18650)
2.0 (a nice fast charge for quality 18650, 21700, or 26650)
Just to play with some numbers, an Amazon Basics AAA has a capacity of 800mAh, and 0.2 amp (0.25 C) gives you a charge time of 4-5 hours. An Amazon Basics AA has a capacity of 2000mAh, and 0.5 amp (0.25C) also gives you a charge time of 4-5 hours. This is quite ideal as I understand. Not to say you couldn’t charge that Panasonic at 2 amp if you wanted to brag 
I guess I just don’t understand why you said 500mA is the limit for AA. It’s clearly not.
As far as I know, the smart limit for charging an AA battery without knowledge of its datasheet is 0.5 amp.
I have a Panasonic charger that came with an Eneloop kit, and it charges at 2000mA (for AA). The cells get a little hot at that rate, though, so I usually use a slower charger. 500mA - 1000mA is ideal.
Very interested in this.
A general rule of thumb for charging current for NiMH is 0.5C. That would be ~1A for a AA, but I like to use 700mA because the cells stay cooler.
You don’t want to charge a NiMH with too little current or the charger could miss the -dV/dt peak and overcharge the cell. I wouldn’t go less than 0.25C.
Thanks for sharing, it’s interesting. What’s your rule of thumb for NiCd? Do you have a link to a source, or is it based on your experience?
My knowledge is just based on my experience with reclaimed NiCd and MiMh cells. But I may be being too conservative on what an acceptable temperature rise is.