Wednesday, November 13, 2024

Re: [electricboats] Cells (modules) are dying.

If you're ever bored and have nothing better to do, watch these two videos to understand the what and why of lithium battery degradation, and the difference between the two. 
 
 
 
 
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Tuesday, November 12, 2024

Re: [electricboats] Cells (modules) are dying.

The 10yr lifespan might be a bit skeptical---I'm still driving my 2011 THINK City car (85kmiles) on its 1st battery pack with date codes of 2010 --- ie. 14 years+ old cells.  I've never had an issue with any of the cells, BMS or modules in the 12 years I've driven the car (knock on wood).  But then I have been pretty lucky.  The furthest I've driven on a charge was about 81 miles maybe 11 years ago.  Then the other day I drove 66 miles and still had about 7 miles range left before the car would shut me down…

I have also been powering my 26' boat with 10 of the same module assemblies for about 10 years now with no issues or degradation that I've seen.  Early on, with the 1st module to power my boat I came out to the marina after being away for a month and found the module had dropped from near 48v to 3vDC.  My heart sank (I had paid about $1000 for it at the time).  I had forgotten to physically disconnect it from the powered-off charger, which drained the module over time.  Not panicking, I pulled it out, took it home and found it was already back at 13v.  Slowly I charged it up to the 30v minimum and then charged at a good charge rate to 48v.  It absorbed rated capacity and all cells tracked.  I was pretty lucky.

A Nasa study on lithium chemistries found that contrary to popular opinion, the cells don't just die when you take them below, say 2.5vpc (e.g. lithium ion).  Rather, they found no irreversible damage until the cell voltage dropped below something like minus 0.4v (guessing).  Essentially this meant that any single cell that under load can only drop to zero volts won't get permanently damaged.  The problem comes when we string these little buggers in series.  In that typical case, even a single module of, say, 12 cells in series runs a significant risk that at least 1 of those cells will reverse voltage if a load is connected up that drains the module (or pack).  Less risk if the cells are all balanced and have nearly identical capacity.

There are ways to eliminate the risk, but all add parts, volume, connections and cost.

 

-MT

 

From: electricboats@groups.io [mailto:electricboats@groups.io] On Behalf Of gsxbearman via groups.io
Sent: Monday, November 11, 2024 3:40 PM
To: electricboats@groups.io
Subject: Re: [electricboats] Cells (modules) are dying.

 

I think that the charging to 100% is only detrimental to NMC batteries, not LFP. But you are correct that all the batteries have about a 10 year lifespan (max for NMC, min for LFP).

The thing to remember about the cycles is that the cycle rating is for degradation down to 80% of original capacity. This is still a very usable battery if this happens before your 10 years ends. Most of the NMC batteries are rated around 2000 to 3000 cycles and LFP around 4000 to 8000 cycles.

The 80% to 20% use/charging is to prolong the cycle life of NMC batteries because the NMC's get most of the charging/discharging degradation in this first and last 20%. The cycle rating is so large on LFP's that there is no need to prolong the cycle life because they will age out before they reach 4000 cycles. And they don't seem to suffer near as bad degradation in these First and last 20% ranges.

From everything I have read the main detriment to life expectancy is rate of charge/discharge, but with a huge caveat that this rate of discharge/charge is relative to the discharge/charge rating of the battery. So discharging a battery at 2C or 3C that is rated for only a 1C discharge would lead to premature degradation. But discharging that same battery at 0.5C would actually make it last longer.

Float voltage is indeed different on any Lithium batteries, and there shouldn't be constant voltage applied. My Victron solar charger disconnects the charging when the batteries are full (programmable) and then periodically checks the voltage and if it drops reconnects the charger. I can also program what voltage the charger converts from "bulk" charge (fast) to "absorption" charge (slower). I have mine set to do up to about 85% on bulk mode, then on up to 100% on absorption mode.

On a boat you should always follow the "rule of 3rds". 1/3 out, 1/3 back, 1/3 in reserve. I'm going to repeat the most important part KEEEP 1/3 IN RESERVE.

So while it's not advisable to regularly go below 33% on your boat batteries, it will not hurt your LFP's if you do.

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Re: [electricboats] Cells (modules) are dying.

If you want to read some in depth writing about lifepo4 batteries, here are two sources from which I have gleaned some useful information. 
this is an article that talks about cycle life and storage parameters etc. but go up one level
 
anton
 
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Monday, November 11, 2024

Re: [electricboats] Cells (modules) are dying.

   Anton, yes the big no-no with lithium chemistry batteries is to leave them high or leave them low. Certainly charge them to 80% and discharge to 20%. Or target a little narrower band (70%-30%) if you want your batteries to last 20 years or more. An occasionally wider band (100%-10%) is helpful with many BMSs (the edge of the performance envelope has more obvious voltage changes, and some % of BMSs only do "top-balancing"). Just don't leave them high or low. A minute is inconsequential, an hour adds up, a month would shave years off their lifespan, and a year would likely kill them. The time spent at naughty State Of Charge levels (the ballpark of over 95% or under 5%) savages battery lifespan. And certainly no trickle charge... that just leaves them high all the time.
   I would be very reluctant to drain lithium chemistry batteries all the way to zero. If it keeps me alive, I'd do it, but I'd have to assume it kills or cripples them within minutes or hours.
   And yes, you are in the ballpark with cycle count. Each manufacturer probably counts cycles different, even to the point of not counting any cycles if you stay between 60% and 40% (guessing, because the chemistry is so stable at these voltages in room temperature).
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Re: [electricboats] Cells (modules) are dying.

[Edited Message Follows]

I think that the charging to 100% is only detrimental to NMC batteries, not LFP. But you are correct that all the batteries have about a 10 year lifespan (max for NMC, min for LFP).

The thing to remember about the cycles is that the cycle rating is for degradation down to 80% of original capacity. This is still a very usable battery if this happens before your 10 years ends. Most of the NMC batteries are rated around 2000 to 3000 cycles and LFP around 4000 to 8000 cycles.

The 80% to 20% use/charging is to prolong the cycle life of NMC batteries because the NMC’s get most of the charging/discharging degradation in this first and last 20%. The cycle rating is so large on LFP’s that there is no need to prolong the cycle life because they will age out before they reach 4000 cycles. And they don’t seem to suffer near as bad degradation in these First and last 20% ranges.

From everything I have read the main detriment to life expectancy is rate of charge/discharge, but with a huge caveat that this rate of discharge/charge is relative to the discharge/charge rating of the battery. So discharging a battery at 2C or 3C that is rated for only a 1C discharge would lead to premature degradation. But discharging that same battery at 0.5C would actually make it last longer.

Float voltage is indeed different on any Lithium batteries, and there shouldn’t be constant voltage applied. My Victron solar charger disconnects the charging when the batteries are full (programmable) and then periodically checks the voltage and if it drops reconnects the charger. I can also program what voltage the charger converts from “bulk” charge (fast) to “absorption” charge (slower). I have mine set to do up to about 85% on bulk mode, then on up to 100% on absorption mode (3.6V per cell for me).

On a boat you should always follow the “rule of 3rds”. 1/3 out, 1/3 back, 1/3 in reserve. I’m going to repeat the most important part KEEEP 1/3 IN RESERVE.

So while it’s not advisable to regularly go below 33% on your boat batteries, it will not hurt your LFP’s if you do.

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Re: [electricboats] Cells (modules) are dying.

I think that the charging to 100% is only detrimental to NMC batteries, not LFP. But you are correct that all the batteries have about a 10 year lifespan (max for NMC, min for LFP).

The thing to remember about the cycles is that the cycle rating is for degradation down to 80% of original capacity. This is still a very usable battery if this happens before your 10 years ends. Most of the NMC batteries are rated around 2000 to 3000 cycles and LFP around 4000 to 8000 cycles.

The 80% to 20% use/charging is to prolong the cycle life of NMC batteries because the NMC's get most of the charging/discharging degradation in this first and last 20%. The cycle rating is so large on LFP's that there is no need to prolong the cycle life because they will age out before they reach 4000 cycles. And they don't seem to suffer near as bad degradation in these First and last 20% ranges.

From everything I have read the main detriment to life expectancy is rate of charge/discharge, but with a huge caveat that this rate of discharge/charge is relative to the discharge/charge rating of the battery. So discharging a battery at 2C or 3C that is rated for only a 1C discharge would lead to premature degradation. But discharging that same battery at 0.5C would actually make it last longer.

Float voltage is indeed different on any Lithium batteries, and there shouldn't be constant voltage applied. My Victron solar charger disconnects the charging when the batteries are full (programmable) and then periodically checks the voltage and if it drops reconnects the charger. I can also program what voltage the charger converts from "bulk" charge (fast) to "absorption" charge (slower). I have mine set to do up to about 85% on bulk mode, then on up to 100% on absorption mode.

On a boat you should always follow the "rule of 3rds". 1/3 out, 1/3 back, 1/3 in reserve. I'm going to repeat the most important part KEEEP 1/3 IN RESERVE.

So while it's not advisable to regularly go below 33% on your boat batteries, it will not hurt your LFP's if you do.

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Sunday, November 10, 2024

Re: [electricboats] Cells (modules) are dying.

If you read too much about lithium batteries it just makes you paranoid!
I am not a chemist, so take this with a grain bag of salt, but from what I read I understand that lifepo4 and NMC batteries have a calendar life, which is negatively affected by storage at higher states of charge and higher temperatures but also a cycle life which is negatively affected by charging to higher voltage and discharging to lower voltage. and of course number of charge cycles. What constitutes a charge cycle is hard to pin down. 100% to 0 would definitely be one, but 70-30, then 90-50, then 65-36, etc.? I imagine 100%-0% counts as 1, but does 70-30 counts as .4 of a cycle? I doubt it’s linear and likely only quantifiable by extensive testing and only valid for that battery types size etc. 
The thing that makes me paranoid is that I read somewhere that if current is pushed into a lithium battery, even a tiny current, even if the voltage doesn’t go very high, if done long enough,  it will overcharge and ruin the battery.  That is to say there is no safe “float” voltage. Once charged, stop charging until some energy has been used. So it seems to me that traction use, where the battery is charged then disconnected and used, is a simpler situation, but solar charging must be setup carefully so as to not hold the batteries at a high state of charge. 
I have decided that the best policy is to avoid holding at high voltage ever, but just use as necessary and convenient and not worry too much. Have a large enough battery that you don’t have to use 100% of its capacity every day. 
With lifepo4, if not held at high voltage with solar, service life should be decades if rarely drawn down below 20%. 
anton
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