Lowest recommended voltage before charging

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cthetoy   100 µW

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Lowest recommended voltage before charging

Post by cthetoy » Oct 23 2018 3:34pm

I know we recommend to charge up to 90% or 4.1v (single 18650) to get better cycle life

What about the recommended lowest voltage taken into the effect of voltage sag? Some say 30% but not sure what voltage is 30%

What is 30% in terms of 18650? Is it 3.6 volts?

What is 0%? . I know 0 volts is not 0 percent. While 0volts=0% is true in some way its not true in ebiking battery capacity terms

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wturber   10 MW

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Re: Lowest recommended voltage before charging

Post by wturber » Oct 23 2018 4:12pm

On my LGs, 30% remaining is at about 35.5 volts based on my discharge testing. 0% is technically 27.5 volts, but as a practical matter, it is 33.5 volts since there is only 5% capacity left at that point. The remaining capacity per volt drops dramatically after about 34.5 volts.

This seems fairly typical, but each batter will have its own behavior. You can often look up the spec sheet for a particular cell if you want to know more about your particular battery.
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adcockj   100 µW

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Re: Lowest recommended voltage before charging

Post by adcockj » Oct 23 2018 9:35pm

I don't look at individual batteries, so in terms of my 4x13 48v battery...

I generally follow the 53v-43v rule for the safe usable portion of my battery. That makes it easy because I look at 53v as the 100% mark (though I know it's actually around 90% of maximum charge which saves battery life) and 43v as the 0% mark (though I know it's really at about the 25% point). Plus, the battery drops very quickly below this.

By using this system, each volt represents 10% of my usable range. So,if my battery reads 46v, I know I have used about 70% of my usable range and have about 30% left. I generally get around 5 miles/10% (volt), so I have about 15 miles left.

While the battery might read a little higher than 53v when I start, that surface voltage quickly drops off. Also, by making 43v my "0" point, I have a little buffer at the bottom.

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MadRhino   100 GW

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Re: Lowest recommended voltage before charging

Post by MadRhino » Oct 23 2018 10:02pm

Lico chemistry does suffer much more being discharged low than charged high. You should avoid that any cell goes lower than 3.7v.

Charging above 4.1 does not wear a cell, if it is put to discharge right after. It is resting high that is wearing them. Ideal sleeping voltage is between 3.8 and 3.85v and that, is the main factor of life expectation in terms of voltage control.

The main wearing factor is temperature though. Every time a cell goes too hot, it does wear very quickly.
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dogman dan   100 GW

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Re: Lowest recommended voltage before charging

Post by dogman dan » Oct 24 2018 8:23am

I think what really matters on discharge, is the load on it when it gets low. As you get down towards 10%, internal resistance goes up, and the same load it handled nicely when full will make the cell get hot.

And as said above, heating up the cells is really the thing you need to avoid.


Charging to less than 4.2v can make sense for other reasons, like not having to have the pack perfectly balanced. I liked to charge to 4.15v, so I could more safely use a dumb bulk charger. But still, even at 4.1v, good idea to limit the time it sits full, to a reasonable degree of convenience. I liked charging to about 3.9v to store overnight, though that is still a bit high. When I needed them, then I could top up quicker than if I stored at 3.8 or less. If not expected to use for a week though, I'd leave them at 3.7 or so, where I stopped. If really low, I'd top up to 3.7 or so.


A pack with a bms though, it might be worth it to just charge it full, and let it balance, then store. Particularly for a long storage, where a bms might kill one or two of the cells that power the bms.

Whatever voltage is 30%, lowering the load when you get below it is good thinking. What is that voltage? that can depend on your starting voltage, whether due to age and inability to hold 4.2v anymore, The load on it, or just what you charge to in the first place.

You can find any percentage by first testing total watt hours. To make the math easy, lets call it 1000wh real world capacity, by test. Discharge out 700 wh, and you have 30% left in this example. Turn off, and let voltage recover, like for 30 seconds or so. now you see your 30% voltage. Apply load again, and now you see your 30% voltage under that load.

Other than as said, don't overheat your cells, and you can stop worrying about what the voltage is under load. But as that discharge gets very low, like the last 10%, keeping the cells cool means a discharge that does not sag much.

30% stopping point is not some magic number that saves a battery imo. But if your load is constant, then that may well be the stopping point that avoids heating up the cells, which definitely kills them. how fast depends on how hot they get.

Bottom line is simple, keep em cooler, by varying discharge rate as they get low.

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Re: Lowest recommended voltage before charging

Post by AHicks » Oct 24 2018 8:34am

In radio control, we use 3.0 volts as the drop dead don't discharge any further voltage. Below that and the consensus is there's a good chance of cell damage. From a more practical standpoint, if you look at the available power curve you can see there's very little capacity left below 3.7-3.8v (chart drops like a rock at that point), so not a lot of sense going any lower (on purpose anyway).

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Re: Lowest recommended voltage before charging

Post by DrkAngel » Nov 05 2018 1:05am

Used to be Lithium-ion cells were effectively empty at a static 3.6-3.7V.
New formulations, however, shift this "empty" point to 3.4V or even 3.2V. It is necessary to investigat manufacturers discharge graphs.

Image

Optimally, cells should not be discharge beyond the "cliff" point where voltage drops suddenly.
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StuRat   100 W

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Re: Lowest recommended voltage before charging

Post by StuRat » Nov 05 2018 5:20am

Interesting. Have you got more info on that graph?

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Re: Lowest recommended voltage before charging

Post by DrkAngel » Nov 05 2018 9:27am

What "more info"?
Graph displays voltage at minimal discharge, which is near to static voltage, which shows, effectively, optimal usable voltage.
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cthetoy   100 µW

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Re: Lowest recommended voltage before charging

Post by cthetoy » Nov 06 2018 8:23pm

So if we used 3.4v as a voltage low point before recharging that means for

10s 36v pack : 34v before charging
13s 48v pack: 44v before charging
14s 52v pack 47v before charging
20s 72v pack 68v before charging

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DrkAngel   100 GW

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Re: Lowest recommended voltage before charging

Post by DrkAngel » Nov 07 2018 7:09am

cthetoy wrote:
Nov 06 2018 8:23pm
So if we used 3.4v as a voltage low point before recharging that means for

10s 36v pack : 34v before charging
13s 48v pack: 44v before charging
14s 52v pack 47v before charging
20s 72v pack 68v before charging
Remember ... the 3.40V for the Samsung 22p cells is the static voltage, the resting voltage several seconds after the throttle has been released.

The active voltage varies greatly by:
throttle position;
battery size;
controller amperage,
temperature ... etc.

"Active voltage" (throttle engaged) might hit ≤3.00V to match "static voltage" (throttle released) 3.40V.

But, recommend not fully discharging to the (cell specific) 3.4V point and highly recommend not discharging below 3.4V.
Deeply discharging is similarly damaging as fully charging.
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