I have never discharged my lipo’s under 37V and get 12miles of range consistently (medium to full throttle). It’s generally accepted that you don’t want discharge lipo’s under 37-36v. It decrease’s the life span of the cells.
I’n not sure what kind of board or battery you have, but that range is very low. Nobody I know uses 36 or 37V as a cutoff UNDER LOAD. Maybe you are talking about the voltage the cells recover to. Not relevant for the vesc settings.
Uh I really don’t think so man. I’ve always fully discharged from 4.2 to 3.2V (under load). Discharging to 3.7 would literally halve my range lol. Use quality cells that don’t have ungodly voltage sag and you can easily discharge further. Vesc settings will kick in to prevent voltage sag (cutoff starting at lets say 3.4V, hard stop at 3.2) is more than effective at protecting cells.
Also, you really don’t need individual cell monitoring (assuming there isn’t a hardware failiure). You shouldn’t be using cells that cause you to worry about voltage drift from a single discharge. High quality lipos are just as reliable as 30Q’s.
Also, in talks to cell manufacturers I’ve been told 4.2-3.2 can be guaranteed 500 discharges (before capacity hits 80% of initial), which would last me 2 years easy. So I’m not too worried about lifespan.
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Please list them up, and also the brand/model for the 500 cycle ones.
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He’s probably referring to garpehes. I’m using 90c 5600mAh garpahene. IMHO I don’t think it’s a good idea to recommend below 3.7 dischrage on their first build especially if it’s a charge only set up. you don’t wanna have a runaway discharge on a lipo. So better to be safe than have a fire on hand. Again, just my opinion. But Do as you wish. I’m not going to discharge my garpahenes under 3.6 at most which is plenty of range for me. Hell I only charge my boards once every 3 days.
This is resting voltage but there’s not much energy available under 3.7v
There are a few other similar tables, most agree there’s not much left under 3.7v.
Granted it’s different model to model, but I’ve found from reading a lot of places, that RC lipos are full at 4.2v, 50% around 3.85v, and for all usable energy, empty around 3.7v. These are resting voltages, not under load.
There’s a few good reasons to be conservative with the cutoffs:
- there’s a “knee” in the discharge graph, where when you get to almost empty, the voltage starts to drop very quickly. The actual knee will be at a different point for every cell in the pack. If you get close to the average “knee” point for the pack as a whole, some cells will be well above their knee, while other cells are most likely quite a bit below their knee and dropping fast.
- there’s not much energy to be squeezed out when the pack is near empty, at high currents. And as voltage drops, drawn current will increase (watts = volts * amps)
I’m ignoring HV stuff for sake of brevity.
HK Graphenes are kind of a category buster, but still they’ll be happier if you don’t abuse. I’m interested to know if there are other high quality lipos that can cycle more than 300 times and have very high discharge.
IMO the best advice, stay above 3.7v in all cases.
EDIT Say you can get 500 cycles going from 4.2v to 3.2v. If you usually don’t empty your packs, would you not prefer to get 1000 cycles going from 4.1v to 3.7v? I think it maybe be sensible, if you have packs that can take it, to set cutoff at 3.2v, and try to never ever hit the cutoff.
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Personally use 3.7 as soft cut off per cell and 3.4 as hard cut off, agree it depends on the cell but good to start with conservative values then see how much your battery actually sags under load using a bluetooth module to get your data and then can adjust accordingly (can also check voltage “drift”/difference between cells after discharge and can check internal resistance of individual cells if your charger does it or can get a separate meter if you’re interested enough in checking individual cell performance).
Also, to note here I don’t typically actually run the batteries down to “empty” where I actually hit the cut offs they are there just in case, but usually I’ll recharge at about the 3.7-3.8V range back up to full (believe best option is actually to go a bit less than 4.2V unless your batteries are rated for a higher voltage as deucesdown said, but since I charge two lipos separately it’s just easier to let them both fill up and balance).
I should also not run them below 3.2V I have had several hot and swollen batteries when I discharged to around 3V.
Cutoff around 3.4V looks fine, at 3.6V there’s only like 10% capacity left in the battery so it doesn’t make sense to discharge them furder.
Those multistar packs must give you more than 500m. From my experience: I had a 10S1P 10C 5.2Ah multistar pack and got a range around 12km (the esc pulled max 20A with a low voltcutoff 3.2V)
Those packs gave around 4V sag when pulling 20A so I shouldn’t pull more amps.
Which is exactly why I specified that these voltages I talked about were under load. You can’t continue to discharge a lipo with a high load when it’s resting at lets say 3.4V.
Setting your vesc to cut off at 3.7 will grant you literally zero range. Any cell is going to sag. If you’re at 4V resting per cell you won’t be able to ride up a hill if you set your cutoff at 3.7V.
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OK,
So to go in the middle (kind of) Ill go with 36v soft and 34v hard cut off.
Im struggling to find the battery current max for these batteries, i spoke to HobbyKing people and asked and he said it was 375? Which even i know this cant be right. Any advice on this part would be great as i think ive got the rest of it sorted. The HK site says the motor current max is 65 so thats fine but its just the battery settings im stuck on here.
Thanks in advance
Woahh wait, 34V and 36V does not work out for those 4S batteries. Did you mean 3.4V and 3.6V?
How many of those batteries did you get? Are you using 3 in series? Or two in series?
Yea sorry 3.4 and 3.6v. just shows how important a . is lol
Running two 4s in series
Okay, then your cutoff should be 27.2V and soft cutoff should be 28.8V
Since it’s a 5000mah cell, you can at most have 5A of Regen current. (That would be a 1C charge current, fairly standard)
So battery min should be -5A. You’ll have fairly weak breaks at higher speeds, maybe bumping up to -7A would work but only if you aren’t satisifed with -5A
Battery max is pretty subjective for these. The C rating is definitely very inflated and isn’t representative of actual performance.
I’d say 50A battery max would be acceptable and relatively conservative for these cells. Anyone with experience want to chime in?
Also, if you’re doing a dual drive, make sure to halve the battery amp ratings (-2.5 and 25) so that each vesc adds up to your total max values. The voltage values should all stay the same whether it’s a dual drive or a single though.
Amazing. Thanks for your help
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I think these modern lipos have a 5C max charge rate, so you can probably go to -25A total, if you needed to.
The cut-off value depends on your current draw. High quality lipos have very little sag compared to Li-Ion Just to give you guys an example here is a graph from my RC helicopter
Notice that at 120A current draw (battery amps) my 2x6s 5000mAh in series (hence 12S 5000mAh) drops from 47.75V to 43.95V. That is around 0.3V per cell. Now imagine a few more of those cells in parallel, the battery would not break a sweat. My mean current draw is way above 30A, something very unlikely to happen on an esk8. Having a low cut off in an application like that would result in too much bogging to make it enjoyable. Now if I am flying with low rpm cruising slowly I can lower my cut-off, however I have made it a rule to never - ever discharge a lipo below 25% capacity (around 3.75V resting voltage) and I have batteries 10 years old that are still usable. Oh and as far as charging is concerned, I charge mostly at 4C and in some rare cases at 8C (that is 40A for the 5000mAh batts)
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Which packs you use again?
Using two of these in series
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I don’t remember exactly which ones I used at that particular flight as I have 1 set of these (the 6S version):
and 2 sets of these
Photos taken from the web, as I don’t have any photos with me right now.
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Depending on how you are charging your batteries… If your charger tells you the capacity it charged in mAh. Run some settings and check how much it capacity was put back into your batteries to get them back to full charge. I would aim for between 70-80% of their rated capacity.
Then tweak your voltage cutoffs until you have a good feeling for where 70-80% is. The voltages you end up with will depend a lot on things like ambient temperature and discharge rate (how aggressively and fast you ride).
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