Preparing for my first DIY build and the question always comes to me, since 4.2V is the top voltage, what’s the best discharge for a Lipo and what will give the best usage without going below or leaving more power that can be used?
Discharge rate refers to the rate at which you discharge the pack. That is, how much current you are drawing from the pack when it is being used. It has nothing to do with the voltage you discharge the pack down to. That is the cutoff voltage or the low voltage cutoff or cutoff point or something similar.
We also need to identify whether we are taking about the voltage under load, when the battery is being used, or when the battery is not being used and is at its “resting voltage”.
You can bring a LiPo, if being used hard, down to 3.0V/cell and it can bounce back up to 3.4V/cell or even higher depending on how hard it was being discharged. Which voltage you are talking about makes a big difference, especially when setting up an ESC.
Going down to a lower voltage is not a problem when it’s the voltage under load, which is what the ESC looks at. What it bounces back up to depends on your pack and how hard it is used.
There is no “best”. It all depends on your preferences and priorities. If long overall cell life is important you might want to stay above a resting voltage of 3.3V or higher. Many suggest around 3.6V. If the longest riding time possible is your priority then you would want to discharge down to a lower resting voltage. But then you might be sacrificing overall battery life for that.
You have to choose which is more important to you and then choose a resting voltage that reflects that choice.
What voltage under load you set your ESC to depends on your pack. You’ll have to see what voltage your pack rises back up to after a typical ride down to the low voltage cutoff point and then adjust that cutoff point to give you the resting voltage you chose earlier.
You should probably ask about voltage cutoff start and voltage cutoff end values. If you search this forum you will find a really helpful thread where everyone has posted their preferred voltage cutoff values.
Agree with mooch, should update your title to “cutoff voltage” instead of discharge rate since the poll you give indicates thats what you’re looking for.
I wouldn’t recommend going below 3.6v resting voltage, as that hits diminishing returns on capacity vs cell longevity, at least for me. I believe lipos are less susceptible to voltage sag than the typical cylindrical cells, even taking into account their usually inflated C ratings. I’m just talking out of my ass on that point tho so i could be wrong.
I would recommend looking at this table for lipo cutoff values.
Definitely true for many LiPo’s.
I agree, stopping at 3.6V resting can give longer cycle life, if no other abuse is occurring, than stopping at lower voltages.
I go down to 3.0V under load which typically brings me to a 3.3V-3.4V resting voltage. I haven’t noticed any difference in cycle life between that and using higher voltages but I really baby my packs when charging and when in storage (especially in storage) so that may help to prevent a short life, making up for what loss in cycle life I experience due to my lower cutoff.
The internal resistance of a good LiPo cell is typically lower than a good round cell so I agree, you will get less voltage sag with LiPo’s. The tradeoff is that LiPo’s have increased volatility, shorter cycle life, and are a lot more fragile. LOL…there is always a frakkin’ tradeoff with cells! But for raw performance a good LiPo cell is verrrrry hard to beat!
What was the difference in cycle life you were seeing with 3.3V resting voltage versus what you are using now?
I didn’t really see any but was babying my packs during charging and storage. Always been curious how much of a difference that really makes but just not curious enough to spend the money and weeks to months of cycle life testing to find out.
I don’t have enough data to quantify.
However everytime I ran my lipos that low I noticed bigger imbalance between cells and the voltage sag was very noticeable when drawing high currents in my big rc helis (>100A from 5000mAh packs).
I’ve been told that 3.6V, you still have a lot more juice left in the battery that hasn’t been used. Then again would that damage the cells if I go lower. Maybe, based on what I’ve seen explained here. For my setup, I’m not planning on upgrading till later on so I do need the batteries to especially last for a long long time. That’s one of the reasons why I made a poll, to see what’s the safest cutoff voltage