Max and Min cell voltage/charge cycles

R

rtgunner

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Jan 14, 2018
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I'm wondering what folks are using as their target max and min cell voltage to maximize the number of charge cycles they can get out of their batteries.

My daily loads and solar recharge is keeping my cells between 3.9V charged, 3.6V when the sun comes up.



Thanks in advance. :)
 
Max/min voltages are chemistry dependant, as is nominal voltage, you might want to confirm what cells you are using.
 
The majority of builders here that are using standard 18650 Lithium Cobalt cells, are using a range of 4.1V - 3.3/3.4V. There are a few that are 4.0V - 3.4V ranges. And I think there's 1 who is going to 3.8 as the top end.
Granted, most of these builders have literally 1000's of cells in their builds, if not over 10,000 for several.

If you can only manage to make a 40p of your desired voltage series, then you may want to push your cells to the higher point of 4.1 max. If you can have 80p or more, especially over 120p, then you can probably be ok going with 4.0V as the top.

Note: the parallel arrange isn't necessarily for a single pack. it's cumulative of all strings that are connected in parallel in some way. meaning if you have 2 strings of 14s40p, you electrically have 14s80p
 
i'm running 15s40p. I was wondering how long you can extend the life of the cells by not fully charging to 4.2 and and discharging to 3.2 or lower. how often are folks having to replace recycled cells in packs that have reached end of life to to exceeded charge cycles. I'm assuming the chemistry of the battery gets to a point where replacement is needed.

I have 25Kwhonline currently and another 10kwh in the works.
 
There's plenty of DOD against expected cycle data around, but hi/lo endpoints aren't the only way to maximise pack longevity.

Under/over temp control - per cell discharge rate - rate of charge etc etc all contribute to pack lifespan.


image_akmukc.jpg
 
roger that... have seen these curves... was just wondering what folks are actually charging to and what that implies for the replacement of cells in their packs. Obviously if someone has 100kwh of storage, 20kw of generation, and only uses 10 kwh a day on loads the battery voltage won't budge much.
 
The way most of us run our powerwalls, I would expect to see 10's of 1000's of full cycles. Notice, I said "full", so that'd be from 4.1 - 3.2

But since most of the time there isn't a full cycle, the life should drastically increase in cycle count. That's why none of us have had any videos replacing cells that were at the end of their life cycle.
Granted, there have been a few where the cells became self-dischargers. Altho, this may be more to having put a slighly discharging cell in service to begin with than with a cell that started to go on its way out afterwards.

But like Sean mentions, there's a lot of factors. And for the most part, most of us don't even come close to the edges of those factors. Kinda like being on a plateau and never getting near the edge but staying near the center ;)
 
Yep, and if folks are using new cells implementing high and low limits to maximise pack life is sensible, with packs made using unknowns I'm not sure there's much point in just doing anything other than just running the packs from 3v to 4.15v - monitoring as you go.

There's not much capacity low down but every little helps I guess.
 
cool thanks for the input. I have 10 kwh of new 21700's that I'll be putting on line soon so the suggestion that used cells of unknown cycle count and charge regime makes sense.

As fall and winter comes my daily production will increase. guess I'll dial up the charge level on the Watchmon... - Thanks :)
 
as per link that not2bme shared above,

"... Most Li-ions charge to 4.20V/cell, and every reduction in peak charge voltage of 0.10V/cell is said to double the cycle life. For example, ..."

Personally, all cells in my 'warm storage' IE ready for pack building, are stored at 3.93V ... and if theres any option, this is the [NASA preferred] voltage I like to hover around.

Even my 'tiny' 10s12p electric skateboard battery is kept below 4.08 if I can ...

In my limited experience with 'big' packs, if I find myself needing to push them higher, its a great indication that the battery doesnt have enough overall capacity.

And I have experienced the catastrophic damage that follows over discharge (under load). It is INCREDIBLY annoying and time consuming to repair this cell level damage. I take the low end cutoff far more seriously now! Going high is very rare for me, as I have far more loads than sources of [offgrid] charging.
 
Rad said:
I am planning on:
min 3.30
max 4.10
Click to expand...

Hahah, sounds like we switched from batteries to a muscle car :p :cool:
 
DCkiwi said:
as per link that not2bme shared above,

"... Most Li-ions charge to 4.20V/cell, and every reduction in peak charge voltage of 0.10V/cell is said to double the cycle life. For example, ..."

Personally, all cells in my 'warm storage' IE ready for pack building, are stored at 3.93V ... and if theres any option, this is the [NASA preferred] voltage I like to hover around.

Even my 'tiny' 10s12p electric skateboard battery is kept below 4.08 if I can ...

In my limited experience with 'big' packs, if I find myself needing to push them higher, its a great indication that the battery doesnt have enough overall capacity.

And I have experienced the catastrophic damage that follows over discharge (under load). It is INCREDIBLY annoying and time consuming to repair this cell level damage. I take the low end cutoff far more seriously now! Going high is very rare for me, as I have far more loads than sources of [offgrid] charging.
Click to expand...

are your large packs 24V? I went 48V to keep current draw low
 
rtgunner said:
DCkiwi said:
[...]
In my limited experience with 'big' packs, if I find myself needing to push them higher, its a great indication that the battery doesnt have enough overall capacity.
[...]
Click to expand...
are your large packs 24V? I went 48V to keep current draw low
Click to expand...

yes, my 'high voltage' system is 48V nominal. (13s)

but I also run a fairly decent sized (~200AH) 4s (15.5V ish) setup so I can run a bunch of radios, lights, chargers ... all straight off of the DC, without any buck conversion. this gets far more use than my 13s/48V stuff, as I dont normally invert to AC while grid power is available (not enough PV to justify this).

I use the min/max voltage of these packs to manage load/capacity growth. The dropout of the linear voltage regulators (12V, mainly for the Opus's) is ~14.5V. So for the 4s setup needs to stay above 3.65V at ALL times. I dont try hard to keep it in balance (looking forwards to my impending Batrium purchase!), so I get nervous if the battery goes above 16.20V, IE 4.05V / cell. So I keep it within 3.65-4.05V and hope for many many cycles.
 
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