Can I get away with 95-100% DOD?

[krby]

I'm planning on a portable power station, using LFE batts. The case will be to keep it charged most of the time, then take in on camping other other outdoor trips, and then use it for the occasional power outage to keep the fridge and a few other things running. I expect to cycle it _maybe_ hundreds of times over the course of several years, at which point I may want to replace the batteries anyway with whatever the latest tech is.

Given my expected low cycle count, when I'm sizing for capacity, I think I'm ok assuming 90% discharge. What about 95 or 100%? I'll likely be assembling this myself from cells.

 

[Steve_S]

I would think that really depends on the battery & BMS as the BMS rules. Batteries like Battleborn, SimpliPhi etc and all other drop in types have internal BMS' you can do anything with (they leave safe margins). If your building your own pack then you can choose a BMS that you can configure and tweak out to squeeze more. As for the percentage you can push cells to, also depends on manufacturer, the type & exact chemistries, there's a lot of variations, making for brain pain !

I was given a site today that discusses various BMS, battery manufacturers lists and more, fairly handy really, might be worth a gander as there is nice clean tables for comparisons etc.. http://liionbms.com/php/index.php

Hope that helps.

 

[krby]

Thanks for that link! That makes searching for one, *much* easier.

So let me rephrase the question a bit: Ignoring the BMS, with the typical CALB or Sinoply cells, or maybe BattleBorn drop-in (which claim 100% DoD) am I ok actually sizing based on 95-100% of the watt-hours listed in the published specs? 24V*50Ah = 1200Wh. I know I'll get fewer cycles out of it, but does that mean 3000-5000 cycles turning into 1000-2000? Or does it mean "maybe 100 cycles if you're lucky)

Unfortunately, it seems like very few of these LFE batts or cells list usable watt-hours

 

[offgriddle]

The general rule of thumb is, if you keep lifepo's from rising above 80% maximum charge and falling below 20% minimum charge, they will last until the second coming.

 

[krby]

The general rule of thumb is, if you keep lifepo's from rising above 80% maximum charge and falling below 20% minimum charge, they will last until the second coming.

Agree, I'm aware of how to treat them well. What I'm trying to figure out is if I keep them at 100% charge almost all the time, and discharge to 95% or 100%, what is a ballpark number of cycles I could expect? I'm trying to build a portable "generator" using LFE instead of a gasoline generator, so size matters. I'm right on the edge for 80-90% and going to 95% or 100% means less size, weight, and cost. If I can still get several hundred cycles out of them. BattleBorn claims 100% DoD on their spec sheet so it got me wondering how CALB or Sinoply cells would react.

 

[SolarRat]

...Ignoring the BMS, with the typical CALB or Sinoply cells, or maybe BattleBorn drop-in.....

Unfortunately, it seems like very few of these LFE batts or cells list usable watt-hours

Dude, stop and do a lot research before wasting money. Calb or Sinopoly cells have as much to do with Battleborns as a chunk of lead has to do with Interstate Battery Co. If you take an unprotected lithium cell below a certain voltage you may never bring it back...throw it away.

All batteries "list" usable watt hours, you just have to know how to read and calculate it.

You can quickly destroy a ton of money in batteries... so do yourself a favor- stick around, absorb some boring info you don't want to hear, and research a bit.

 

[SolarRat]

If you want a battery you can abuse like a red-headed stepchild...there is really only one choice....good old flooded lead acids.

 

[krby]

Dude, stop and do a lot research before wasting money. Calb or Sinopoly cells have as much to do with Battleborns as a chunk of lead has to do with Interstate Battery Co. If you take an unprotected lithium cell below a certain voltage you may never bring it back...throw it away.

Ya, I guess I came off as comparing those directly. I didn't mean to. What I was trying thinking (and failing to communicate) was: "BattleBorn seems to be reputable and claims 100% DoD. For these cheap ass cells I wouldn't expect the same nearly number of cycles out of them if I do that but if I do run a cheap cell down to 2.6V (using Sinoply as an example, their spec sheet says 2.5min voltage) then various LFE discharge curves I have seen say that's somewhere around 95% DOD. If I get 300 cycles out of the cells doing this, that might be ok for my use case.

If the folks here tell me that I can do that, what I will do is find a BMS that will let me go that low (I'm not going to run without a cell balancing BMS) and then do some testing to set a low voltage disconnect to the whatever the voltage ends up being under load when I've pulled out 95% of the battery.

To be really clear, being able to do this doesn't make or break my project. I can almost certainly get away only going to 90% discharge, maybe 80%. My point in asking was that for what I'm planning, I expect maybe several hundred cycles over 3-4 years at the most, likely not even that. I am hoping to get a ballpark for what this level of discharge will do to the expected lifetime of the cells so I can decide if that trade off is worth it to me.

All batteries "list" usable watt hours, you just have to know how to read and calculate it.

Would you tell me how? Or point me to a place that will? I'm seeing Ah and Volts. That gives me a theoretical max capacity. Maybe by finding the minimum voltage and then looking at discharge curve for typical LFE and I can and idea how much of that is usable? Or is there some other method?

 

[Will Prowse]

Agree, I'm aware of how to treat them well. What I'm trying to figure out is if I keep them at 100% charge almost all the time, and discharge to 95% or 100%, what is a ballpark number of cycles I could expect? I'm trying to build a portable "generator" using LFE instead of a gasoline generator, so size matters. I'm right on the edge for 80-90% and going to 95% or 100% means less size, weight, and cost. If I can still get several hundred cycles out of them. BattleBorn claims 100% DoD on their spec sheet so it got me wondering how CALB or Sinoply cells would react.

Well, SOC during storage and cycle life matter a lot... but the ambient temperature is equally important. Keeping it in storage or charging when it is 85 degrees Fahrenheit (29.44 °C) or higher, makes degradation speed up a lot. Keep those battery cells nice and cold if you want them to last for ages.

There are battery studies for lifepo4 online, for charge cycle life vs temperature vs SOC etc. Keep them around 20%-80% SOC, cold, and charge them slowly, and you are set.

and 100% DOD with drop in replacements is determined by the BMS. Not the cells. The characteristics of this chemistry are consistent regardless of what battery you buy. Sinopoly has a slightly different electrolyte but thats about it. Not that different at all. DOD for advertising batteries is for rating cycle life at a specified temperature.

If you want to use LFP, buy a nice bms and match your cells. Buy them new.

 

[SolarRat]

Ya, I guess I came off as comparing those directly. I didn't mean to. What I was trying thinking (and failing to communicate) was: "BattleBorn seems to be reputable and claims 100% DoD. For these cheap ass cells I wouldn't expect the same nearly number of cycles out of them if I do that but if I do run a cheap cell down to 2.6V (using Sinoply as an example, their spec sheet says 2.5min voltage) then various LFE discharge curves I have seen say that's somewhere around 95% DOD. If I get 300 cycles out of the cells doing this, that might be ok for my use case.

Sorry if I came off as jumping on ya, went out and drank too much tonight and ...anyway I gotta take it one paragraph at a time...

Battleborns have an internal BMS. Their 100% claim means you can take it down to 100%, but 'behind the scenes' the bms only allows you ~80% of the cell's charge. So you are never fully draining the cells, but BB is telling you you are. It's a fail-safe for dummies.

As far as what you call "cheap ass cells", Sinopoly, Calb etc....they are actual, good batteries. Battleborn is a company that throws even "cheaper ass cells" into a box, adds a $2 BMS, and charges 5x the sum of the parts to newbies. People will learn that someday, until then I'm just the grumpy old guy that hates everything.

 

[SolarRat]

If the folks here tell me that I can do that, what I will do is find a BMS that will let me go that low (I'm not going to run without a cell balancing BMS) and then do some testing to set a low voltage disconnect to the whatever the voltage ends up being under load when I've pulled out 95% of the battery.

To be really clear, being able to do this doesn't make or break my project. I can almost certainly get away only going to 90% discharge, maybe 80%. My point in asking was that for what I'm planning, I expect maybe several hundred cycles over 3-4 years at the most, likely not even that. I am hoping to get a ballpark for what this level of discharge will do to the expected lifetime of the cells so I can decide if that trade off is worth it to me.

I've taken many lithiums past their limit, even when manufacturer's "safeties" were in place. They never fully recover, if they recover at all. If you are asking "how far can I push it", I'll ask you "how deep is your wallet".

It's a whole lot cheaper to upsize your battery by 25% than it is to replace your entire bank every few months.

 

[SolarRat]

Would you tell me how? Or point me to a place that will? I'm seeing Ah and Volts. That gives me a theoretical max capacity. Maybe by finding the minimum voltage and then looking at discharge curve for typical LFE and I can and idea how much of that is usable? Or is there some other method?

You got volts and AH which will give you watt hours. From a reputable battery company that should be all you need to calculate what you need to calculate. As far as "usable"? Forget all the BS that has been spewed throughout all forums and youtube videos.

Lead acids are said to have half the "useable" capacity of lithium batteries. Bullshit.

For ideal capacity and longest life, lead acid batteries should be used from 100% to 50% state of charge. Lithiums, under the same "ideal capacity vs longest life" scenario should be used from 80% down to 20%. I agree with both. Lead acid gives you 50% of the rated AH, lithium gives you 60%.

But actual run-time depends on the C-rate you are drawing and how well the battery can handle it. But always, the higher the C rate the quicker the battery will die, the shorter its life

 

[Steve_S]

Battleborn is a company that throws even "cheaper ass cells" into a box, adds a $2 BMS, and charges 5x the sum of the parts to newbies. People will learn that someday, until then I'm just the grumpy old guy that hates everything.

Seriously ? Got any valid tests, video's or photo's to back up such a statement ?

 

[Aussiecroc]

I want to know whats in a plastic box before I buy it. Companies that have big warrantees just charge more to cover a % of customers that will bother to use it. Some of these boxes have over 300 small cells in them with over 600 small spot welds. A lot to go wrong. Some might not even be lifepo4. Few will tell you whats inside. I want to know there are 4 biggies in a 12 volt.

 

[Solarfun4jim]

Well, SOC during storage and cycle life matter a lot... but the ambient temperature is equally important. Keeping it in storage or charging when it is 85 degrees Fahrenheit (29.44 °C) or higher, makes degradation speed up a lot. Keep those battery cells nice and cold if you want them to last for ages.

There are battery studies for lifepo4 online, for charge cycle life vs temperature vs SOC etc. Keep them around 20%-80% SOC, cold, and charge them slowly, and you are set.

and 100% DOD with drop in replacements is determined by the BMS. Not the cells. The characteristics of this chemistry are consistent regardless of what battery you buy. Sinopoly has a slightly different electrolyte but thats about it. Not that different at all. DOD for advertising batteries is for rating cycle life at a specified temperature.

If you want to use LFP, buy a nice bms and match your cells. Buy them new.

@Will Prowse As regards "There are battery studies for lifepo4 online, for charge cycle life vs temperature vs SOC etc. Keep them around 20%-80% SOC, cold, and charge them slowly, and you are set. ", what would be your 'ballpark figure' for an ideal 'cold' temp, if maintaining these batteries inside a sealed insulated containment environment? Obviously the trade off is ideal temp versus energy used to maintain said temp. Using all your experience Will, could you give us a temp figure to aim for to maximise cycle number?

 

[krby]

Ok, so for many drop-in batteries, if they claim 100% discharge, I might be able to trust that if I trust the brand. They're likely to have configured their BMS to cut-off at some level their testing indicates I get ~100% of the stated Ah within some margin of error they're willing to stand behind with a warranty.


I'm not looking for "how to best take care of these cells" I'm looking for "if I abuse them a bit should I expect 300 cycles? or 5?"

If I build my own from cells, I guess should be looking at a chart like this

and then defining "abuse them a bit" as 3.0V per cell under 0.25C load.

If I set my BMS or LVC disconnect at 3.0V (roughly 9-10% SOC on that chart). Can someone give me a ballpark on how many cycles I can expect? I know this isn't ideal conditions, but I'm trying to figure out if it is likely to ruin the cells in 10 cycles or not. Assume I store the battery pack inside where it won't hit 80deg and that I charge at no more than 0.5C. For my use case, fast charging isn't needed and 2000-5000 cycles aren't needed.

I understand that finding that exact cut-off voltage depends on the voltage drop based on the current draw, but I know my worst case draw so I could plan and test for that.

At this point, I guess this has become more theoretical than anything. It seems the easiest solution is to size on 80% discharge and buy accordingly. The common less-expensive non-configurable BMS' seem to have much lower points than above, so then I need to decide if I want to monitor that battery and rely on myself to keep it at 80% having the BMS' limit as worst-case backup or if I install a disconnect/protect device and set it where I think the 80% mark is based on testing.

 

[Sojourner1]

Where did you get that chart? If you print that out, roll it, you have TP.

 

[krby]

From here: https://www.solacity.com/how-to-keep-lifepo4-lithium-ion-batteries-happy/

Why, is it wrong? If so, will you point me at one that is better?

 

[Sojourner1]

From the paragraph just above the table.

" Below is a table that shows battery Voltage for a 12 Volt battery pack vs. Depth-Of-Discharge. Take these Voltage values with a grain of salt, the discharge curve is so flat that it really is hard to determine SOC from Voltage alone. Small variations in load, and accuracy of the Volt meter, will throw off the measurement.

Next if the "resting" voltage is 14.0v that's overcharged. 13.2-13.3v would be resting, but is resting 1 hr, 6hr, 12hr, 24 hr after charging?

There isn't one that would be accurate too many variables from a controlled environment to real world use because LFP has such a linear voltage.

One thing you'll see after researching is there are a lot of opinions.

 

[electric]

If I set my BMS or LVC disconnect at 3.0V (roughly 9-10% SOC on that chart). Can someone give me a ballpark on how many cycles I can expect? I know this isn't ideal conditions, but I'm trying to figure out if it is likely to ruin the cells in 10 cycles or not. Assume I store the battery pack inside where it won't hit 80deg and that I charge at no more than 0.5C. For my use case, fast charging isn't needed and 2000-5000 cycles aren't needed.

If you charge to 3.5V and discharge to 2.9V, then you are operating in approx. 10%-95% SOC window and your cells should give you up to 5000 cycles, assuming no freezing and no overheating ( up to 40 degC is fine for very long life ).
Capacity will slowly fade to 80% of original after 4-5 years of use due to calendar aging. Voltage will start to sag a bit more due to aging, but if your C rate is low you won't notice it much.
I still have CALB cells working fine at 70% original capacity after 10 years of careful use.
Also, CALB and Sinopoly cells are excellent products, no worse than what's inside BattleBorn in any way.
Any cell with UL 1642 certification is a great product.
CALB is still my first choice after 10 years in this business and trying everything there is on the market.

 

[Solar Surfer]

If you charge to 3.5V and discharge to 2.9V, then you are operating in approx. 10%-95% SOC window and your cells should give you up to 5000 cycles, assuming no freezing and no overheating ( up to 40 degC is fine for very long life ).
Capacity will slowly fade to 80% of original after 4-5 years of use due to calendar aging. Voltage will start to sag a bit more due to aging, but if your C rate is low you won't notice it much.
I still have CALB cells working fine at 70% original capacity after 10 years of careful use.
Also, CALB and Sinopoly cells are excellent products, no worse than what's inside BattleBorn in any way.
Any cell with UL 1642 certification is a great product.
CALB is still my first choice after 10 years in this business and trying everything there is on the market.

I just installed 16 CALB 100ah cell’s in parallel, series to make a 12v 400ah battery, needing guidance on charge controller bulk and float parameters, using outback 60 charge controller.
Would also like best bms system to use in this configuration.
Thanks

 

[gnubie]

~
I still have CALB cells working fine at 70% original capacity after 10 years of careful use.
~

Did you take any special precautions such as those highlighted in the document referenced in this thread?

 

[Deleted member 783]

The general rule of thumb is, if you keep lifepo's from rising above 80% maximum charge and falling below 20% minimum charge, they will last until the second coming.

So 60% use? Only 10% more than LA?

 

[gnubie]

Think of it as being hypermiling. It's possible but for most people it's just not something done as normal operation is more than adequate.

 

[Stupid Monkey]

Ideally you would double the capacity, only leave charged at 80% to avoid capacity loss, and only use down to 20% rather than use 100-0% and kill the batteries

From link above ....


To sum up, for long and happy LFP battery life, in order of importance, you should be mindful of the following:

1. Keep the battery temperature under 45 Centigrade (under 30C if possible) – This is by far the most important!!
2. Keep charge and discharge currents under 0.5C (0.2C preferred)
3. Keep battery temperature above 0 Centigrade when discharging if possible – This, and everything below, is nowhere near as important as the first two
4. Do not cycle below 10% – 15% SOC unless you really need to
5. Do not float the battery at 100% SOC if possible
6. Do not charge to 100% SOC if you do not need it