# Battery Charge% and altering Li Ion Charger Voltage

#### iONic

Joined Nov 16, 2007
1,656
It is often suggested to store Li Ion batteries in a cool location at "60%" charge. But how does 60% charge equate in voltage? At what voltage should they be charged, 3.00V and max charge voltage should be never more than 4.20V.

4.20V - 3.00V = 1.2V
1.2V * 60% = 0.72V
Thus is 60% charge is 3.72V ???

On another note: many entities are saying (especially in the case of EV cars) that you should charge to 80% and not to 100%. 3.96V
So can a LiIon charger be made to charge the battery to 3.96V or in that ballpark? Could one just add a diode(dropping 0.3V) on the end of the charger do the trick?

#### Irving

Joined Jan 30, 2016
3,548
The relationship between voltage and State of Charge (SOC) isn't linear. A typical chart might be as below, but it will vary from manufacturer to manufacturer.

The only correct way to determine SOC for storage is to fully charge, then partly discharge at 1C rate.

#### crutschow

Joined Mar 14, 2008
33,358
The only correct way to determine SOC for storage is to fully charge, then partly discharge at 1C rate.
That seems to be overkill.
I would think charging each cell to about 3.87V would be close enough, as I think 60% charge is just a rough figure as to how best to store the batteries.
Hard to believe there's a significant difference in battery life between storing them at say from 50% to 70% charge.

#### iONic

Joined Nov 16, 2007
1,656
no ideas on making a Li Ion charger charge only to 75%-80%. would a diode rated at the maximum charge current work!?
Or is there a better way.

#### nsaspook

Joined Aug 27, 2009
12,309

#### Jim_PDX

Joined Nov 13, 2019
15
I use a charger that has a special "storage mode". It will take an existing Li Ion battery and either charge or discharge it until it reaches 3.7V. This is what it considers a good voltage for storage. I originally built my own device to do this, but then found this product which does the same thing but for 4 batteries at once.

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#### Audioguru again

Joined Oct 21, 2019
6,447
Lithium-Ion battery cells are sold at 3.7V because that is the storage voltage and they might be in the store for years.

#### nota33

Joined Jul 9, 2023
25
my understanding is they live more cycles if not charged full or discharged

what the exact best top value or dis change level or storage is variable by maker size style ?

#### Audioguru again

Joined Oct 21, 2019
6,447
my understanding is they live more cycles if not charged full or discharged

what the exact best top value or dis change level or storage is variable by maker size style ?
Today there are a few different chemistries for Li-Ion batteries including Lipos:
1) Old Lithium-Cobalt, minimum 3.0V to 4.2V maximum.
2) New LiFeP04, about 2.8V minimum to 3.6V maximum.
3) New LiGraphene maybe 3.2V minimum to 4.5V maximum.

If they are discharged to a voltage too low then they are probably ruined.
If they are stored fully charged then their capacity and number of cycles are reduced.

#### Irving

Joined Jan 30, 2016
3,548
If they are stored fully charged then their capacity and number of cycles are reduced.
True, and the effect varies by chemistry and other parameters such as charge & discharge rates. The old LiPo had a lifetime of 500 - 1000 charge/discharge cycles, whereas LiFePO4 is measured in 1000's, typically 3- 4k. Smaller batteries are worse than bigger ones; phone batteries rarely last much more than 3y if charged 100% daily (and yes, everybody's phone is an exception to the rule!) but large solar packs are good for 5 - 8y or more.

My 180Ah wheelchair LiFePO4 pack (8S1P) is 5y old now, gets charged at 40A twice a week to 100% but rarely drops below 20% - I tend to keep it above 3v as far as possible because overdischarge is more damaging, and they exhibit maybe a 1 or 2% capacity loss since new. But charging to 100% is of little consequence really as they are never 'stored' for any length of time; they are always working!

#### bassbindevil

Joined Jan 23, 2014
798
Imax B6 hobby chargers can charge to "storage" voltage, which I think is about 3.6 volts. IMHO every (electronics) hobbyist needs one of these; they'll charge or discharge every common battery chemistry.

#### MrAl

Joined Jun 17, 2014
10,909
It is often suggested to store Li Ion batteries in a cool location at "60%" charge. But how does 60% charge equate in voltage? At what voltage should they be charged, 3.00V and max charge voltage should be never more than 4.20V.

4.20V - 3.00V = 1.2V
1.2V * 60% = 0.72V
Thus is 60% charge is 3.72V ???

On another note: many entities are saying (especially in the case of EV cars) that you should charge to 80% and not to 100%. 3.96V
So can a LiIon charger be made to charge the battery to 3.96V or in that ballpark? Could one just add a diode(dropping 0.3V) on the end of the charger do the trick?
Hi,

One of the things about storing at 50 percent charge (not 60 percent) is for safety. Less stored energy means less potential for fire or worse. I suppose there isn't a huge difference between 50 and 60 percent though.

The main reason for charging to less than 100 percent, other than for storage, is for the increased longevity of the cell. As far as i know, the life of the battery goes up fast with charge terminating voltages less than 4.20 volts (standard Li-ion cell). Even going down to 4.15 volts is beneficial, but going down to 4.10 or 4.05 or 4.00 makes the cell last even longer.
Also, charging at a current less than the maximum for the cell also extends the life of the cell.

Some Samsung phones have a built in "battery protection" feature you can turn on or off. You can turn the fast charging off so the cell charges more slowly and thus extend the life of the cell, and you can set the maximum charge to 80 percent and that also extends the life of the cell.
I believe all manufacturers should be including this in their phone designs.

This reminds me of the days of the notorious NiCd cell that appeared in many devices years ago. They were charged with around 100ma which is great, BUT, they were allowed to charge 24 hours a day 7 days a week with no instructions to do otherwise. People would of course leave their devices plugged in all day long so that the device was ready when they needed it. Too bad though because after a year of that abuse the cells started to die off and not charge much at all after that rendering the device useless. One such device was a small portable vacuum cleaner, and another a portable drill.

We've learned a lot in the years after that and now things have turned to other battery technologies anyway, but the newer ones need time to evolve too.

#### Irving

Joined Jan 30, 2016
3,548
This reminds me of the days of the notorious NiCd cell that appeared in many devices years ago. They were charged with around 100ma which is great, BUT, they were allowed to charge 24 hours a day 7 days a week with no instructions to do otherwise. People would of course leave their devices plugged in all day long so that the device was ready when they needed it.
One of the reasons for that is that a NiCd and NiMH cells are tricky to determine end-of-charge. There's a phenomenon where, after charging at a constant current, the perceived battery voltage rises then falls and that's the end-of-charge point. Cell temperature also peaks. The issue is that it varies wildly from manufacturer to manufacturer so building a fast charger that works across all cells is quite hard. Much cheaper to trickle charge through a moderate value resistor which is 'fine' for a cordless phone, even if it kills batteries within 18mo to 2y.

#### MrAl

Joined Jun 17, 2014
10,909
One of the reasons for that is that a NiCd and NiMH cells are tricky to determine end-of-charge. There's a phenomenon where, after charging at a constant current, the perceived battery voltage rises then falls and that's the end-of-charge point. Cell temperature also peaks. The issue is that it varies wildly from manufacturer to manufacturer so building a fast charger that works across all cells is quite hard. Much cheaper to trickle charge through a moderate value resistor which is 'fine' for a cordless phone, even if it kills batteries within 18mo to 2y.

View attachment 298116
Hi,

Oh, this was in the days before termination methods like Delta T or Delta V or Delta Schmelta came about, maybe 40 years ago or more. They just charged and charged and charged and charged them to death. All they had to do is tell the purchaser to "Not charge continuously", or "Unplug after 24 hours", or something like that.
For these devices the lifetime was just 1 year and that was because the battery cells began to die off so no more charge retention.

Maybe about 15 years ago I bought a portable drill Ryobi. It came with not one but two battery packs. Both packs were dead within 6 months. That was not because of overcharging, but because they used cheap cells in their products. However, after I studied the charge regimen I found that they were using the same charge method which was no termination whatsoever. I replaced the cells at \$40 USD but I also rebuilt the charger using a microcontroller and special algorithm I had developed, and I got 5 years from the new cells after that. The new charge algorithm didn't even need any Delta BS in it. It just used the idea that every day the cells discharge by a small amount and if you replenish that plus a little more you have cells that are always charged but don't have to be charged continuously. After those 5 years I went to a lead acid gel cell.