How to charge Li-ion battery without going extreme voltage.

[mnikhilesh]

I am building a 14S Lithium-ion battery pack. I will be using a 4.35V max voltage 18650 Samsung cells. To increase the life of cell I will restrict the battery pack to charge each cell only to max 4.0V instead of 4.35 volt. By this way I will loose about 20% capacity and I expect battery life to be extended substantially. Now my problem is I am not very clear how I should charge my battery pack. Normally chargers are designed for charging constant current to 4.35 V and then keep this voltage in constant voltage phase so that charging current decays to very small amount like 0.1C. I can custom order my charger so should I make it so that my battery charges in constant current phase till 4 volt and then switch to constant voltage at 4V and wait the current to decay or should I only charge it with standard charger (4.35V CC) in constant current mode only and once the voltage reaches 4V let BMS cut the charging process.

 

[dnmun]

this is not true. full charge for lipo is 4.2V and the balancing chargers and BMSs use that voltage to fully charge the pack.

the only evidence that has presented about life cycles with respect to charging voltage was from the lecture the guy gave that was on the tesla users forum.

he observed that the time spent at full charge was directly related to reduction in cycle life. leaving it undercharged at 4V may not extend cycle life if you leave it charged at all times. then if you only charge it up to about 75% SOC, you end up with the cell voltage dropping into dangerous territory almost as soon as you start discharge.

when the cell is transporting current with lower cell voltage then there are fewer mobile ions to transport charge so the cell heats up faster and causes damage to the cell.

it is best to leave it uncharged, above 3.5-3.7V until you need to use it and then fully charge the pack to 4.2V and use it immediately. cell voltage will drop immediately when you use it and then it spends no time at full charge and you also do not have to carry 50% larger battery to have the same amount of power available. with no impact on cycle life.

 

[dogman dan]

Lots of discussion about various ways to extend lifespan.

My thoughts are definitely only opinion, but gleaned from lots of others opinions here.

I think it's true, what really matters most is not storing in hot temperatures fully charged. Also, undercharging will not guarantee a longer lifespan, if it leads to very deep discharges every cycle. If your pack is plenty big, good idea. If a bit on the small side for what you need, better to fully charge to 4.2v, but not till just before the trip.

So a good strategy is one using a charger with multiple choices for end of charge voltage like the one EM3ev sells.

This charger will charge to a very conservative storage voltage, or to a slight undercharge, or a full charge by simply flipping a switch. What I do now much as possible, is leave the pack discharged, or half charged for multi day storage. If I need to ride early in the morning, I'll charge to 4.0v the night before.

Only when going for the longest rides do I charge to higher voltage. I did tweak down full charge to 4.15v, because I'm bulk charging cheap hobby lico. I wanted another .5 v of wiggle room for bulk charging a battery that is dangerous to overcharge.

On the rare occasions that I actually balance the pack, I will charge it to 4.2v for the balancing. Mostly, by practicing a low c rate discharge, and stopping most rides at around 3.5-3.7v per cell, my cells remain balanced enough for safe use.

 

[dnmun]

this guy disappeared too but i was trying to get him to understand that 4.2V is fully charged.

the manufacturer put up a warning to not charge to a voltage above 4.35V. so this guy mistook that to mean he should charge to 4.35V.

so now he will go ahead and charge to 4.35V because he also decided he did not need the advice since he knew everything anyway.

maybe someone can find that lecture again, but it was pretty easy to interpret his graphs. his grad students were the ones going to work at tesla in the new giga plant so i think he knew what he was doing.

chemical reactions do depend on temperature.

the reaction that is driven by voltage is the reaction of the electrolyte with the carbon of the electrode. this is the reaction that produces the SEI. i think his graphs were using an index of (1-N) as i recall, where N is the number of cycles.

so charging to less than full charge, but leaving the pack always charged to 4.15V (since this seems to be the current favorite number) could be as damaging as charging to full charge only when used. that was the point of his lecture.

i interpret that to mean that it is best to leave the battery uncharged until needed, and then charging to full charge before use so you can scrub off the voltage immediately in discharge.

i read in another review, the battery article in the chemical engineering journal that someone (pdf?) posted up, that when the lipo is discharged and left below the 3.5V level then the matrix of cathodic material shrinks and collapses which causes physical damage to the cathodic material and shortens cycle life too.

so the idea is to keep it from being stored in an over discharged state but not charged up to full charge until needed.

 

[wineboyrider]

I have 12s 15ah pack that I have been using for over a year now and have been following these rules basically. I only charge to 4.10 and only charge to 4.2 for long rides. I leave my pack discharged in the morning and slam 10 amps in it before I ride to work and then trickle charge at work, ride home for lunch trickle charge until I go home for work. This equates to 12 miles a day almost and I have been doing this pretty routinely I'd say at least 3 times a week since around December of last year. Sometimes I do it 5 days a week and others once a week. But, the battery pack resistance is still very low and it still doesn't sag hardly at all under load!
I wish my CA hadn't broke, but the amount of watt hours and miles this vetted Lipo pack has given me is incredible. This pack should last a lot longer than my other lipo experiences...LOL

 

[dogman dan]

Agreed. 4.35 is not the voltage he needs to balance at. 4.2v. If charging to 4v or whatever, it won't be perfectly balanced at that voltage, 4.2v for the balancing charges.

If you have to keep the battery fully charged basically all the time, then a hot garage just makes it even worse. Bad enough what the battery endures out on the road. Store in a relatively cool place if possible. And not fully charged.

Trouble is, if you need that bike early in the morning to go to work or whatever, you might have to charge it the night before. So then the next best compromise is to undercharge the night before, inside where it's room temp, and then top up in the AM.

Best would be to half charge, or leave uncharged till a few hours before the ride. But then you keep getting caught with no charge before an unplanned ride.

Since I have a lot of batteries, what I practice is keeping most of it stored at room temp at least half discharged. One 5 ah pack is kept charged all the time. It can be used anytime for a quick run for the dogs, or a run to the nearby dollar store, veggie store, or fast food nearby.

Does never charging past 4v work? I don't know. Others report some good results. But the experiment also included other practices said to help lengthen cycle life. So shallow cycles and low c rate discharges may be what really works.

I do feel it's better to charge to 4.2v immediately before the ride than to discharge below 3.5v. I think the science is correct, that it's time fully charged and higher temps that cause the damage for lithium batteries.

So limit the time at full charge and the time in the heat as much as is practical to do for your climate and situation. If charging to 4v, limit the time it's "fully charged" to 4v too! If practical for you do do so.

One option for doing that might be a charger on a timer, (with safe chemistry lithium) so it starts your charge in the night, rather than finishing before you go to bed. The risk here is that the timer doesn't work.

If you have lithium cobalt, RC lipo from an RC hobby store, NEVER sleep while a pack is charging, and ALLWAYS store in a fire bunker.

 

[Matt Gruber]

there is some confusion on
"charge to"
so i bought a single cell charger, and sure enough, 1 hour after the charge it was 4.20v.
BUT to get to that, it charges as high as 4.28v during the charge.
.
Also saw some presentation from panasonic/sanyo i think, that showed how each generation of 18650 has a higher final charge, now their best cells are 4.35 and going higher in the future.
so be sure to look up the data sheet on the EXACT cell discussed.