I’m continuing to see a weird behavior - now my inverter is unable to charge a single battery to over 95% (setting is 6 to 100), is stops at 95% and constantly uses some low wattage like 80w, and bat voltage slowly drops, sometimes percentage also drops to 94 and charges again to 95.
Tried to set battery as “user” - bulk/float 52.4 - it charges to 52.4 and then i see that battery is not charging anymore - SOC leds are all off (was not the case for “lithium” with bms connected via CAN - they were on but not blinking at 80-90w). If at this time I connect CAN - it shows 100%, but voltage slowly drops as well as percentage
Some of the behaviour sounds perfectly normal.
The 80 watt power consumption is normal, for just being turned on, not even providing AC output.
Discharge to from 95 to 94% due this power usage is expected also, just as it is that it will recharge to 95% again.
S15. 15 x 3.65v = 54.75v
With a user setting of 52.4v, the battery won't ever reach 100%.
What ever the inverter or the BMS will tell you about SOC, that information isn't accurate not exact.
Please visit battery University website (google is your friend) and study about lifepo4.
There are just 2 things for sure.
3.65 volt is considered 100% SOC
2.0v is concerned 0% SOC
Considered.. Not in reality.
Charge to 3.65, let it rest for a few hours... It's under 3.65, and can be topped up to 100%.
In other words, it wasn't fully charged.
Discharge to 2.0v, and let it rest for a few hours. It will "bounce back" to something like 2.5v, and can be discharged to 2.0v again..
In other words, it wasn't fully discharged.
To make it more inaccurate:
You can charge above 3.65, even to 4.2v without damaging the battery.
You can discharge below 2.0v (with some damage)
How?
It was 100%, aka full and 0% , aka empty!
And I'm not even starting with the BMS, the device that controls your cells.
Not only won't it have a perfect accurate reading of the voltage, it might be set to stop at 95% charge.
Lead acid was simple
You charge it to 14.4v, and it stabilized at 12.7v, 100% charge.
10.5v "empty"
DOD (depth of discharge) 40% of you wanted many cycles (hundreds)
Oh.. and temperature.. at 30c.. you lose 75% of cycle life..
Simpel because it s*cked.
No matter what you want, they won't last long, and give a terrible efficiency.
And they last longest / are most happy at 100% charge.
Unhappy below 60%
Lifepo4 is different.
It doesn't care that much about temperature, it does NOT like 100% charge, and doesn't mind discharge to even 5%.
Where lead acid a voltage would give a reasonable estimate of the state of charge, lifepo4 doesn't..
At 3.65 and 2.0v, those are "known"
(And at start not correct 100 and 0%)
All measurements between 2.8 and 3.4 volt are a really rough indication of the SOC.
Expect 20-30% error margins (!!)
And then there is the nice part ..
Was the battery in use?
Charge or discharge?
As being charged will give higher voltage reading, discharged a lower reading.
It S*cks.
Not accurate at all.
For the people who need to know exact SOC, within 5% error margins, shunts and special AM meters are needed.
They count the amount of kWh entering and being drawn out the battery.
5% error margins is probably too optimistic.
Not only does the battery not give 100% back of the energy put into it (and this slightly different per pack / installation / environment temperature and number of cycles), the shunt isn't really accurate
In time this 5% error will increase
And needs to be recalibrated.
For a few thousand dollars you can get accurate equipment to measure.
It is smart enough to compensate for the variables and the measurement is within 0.0x% margin.
And this is with DIY, all parts under your own control.
Many pre-build sets undercharge the set.
They pack 280Ah cells and sell it as a 260Ah battery.
Lifepo4 doesn't like being kept at 100%.
While 4.2 should not damage a perfect Cell, most affordable are "B choice", not perfect.
Easy solution, don't top charge!
Why try to find the absolute maximal performance when that increases the risk of failure significant?
Heck, even a Tesla car doesn't top charge, or fully discharge.
Or does an software update suddenly change the hardware battery installed??
Lead acid: if you like them to last as long as possible, DOD 40% and keep as much as possible at 100% charge.
Lifepo4: if you like them to last as long as possible, DOD 80%, and keep them as much as possible at 90% charge.
(So keep between 10 and 90% SOC)
Roughly between 2.7-2.8 and 3.4v
Roughly.
And depending on the factory that made the cells, the age, the temperature, matched internal resistance etc etc etc.
This 2.7/2.8 and 3.4 is not exactly 10 and 90%
It can be 5 or 15% at 2.7/2.8v
It can be 85 to 95% at 3.4v
Expecting that you are able to set between 6 and 100% SOC...
Yes, possible.
With a few thousand dollars on Investment on the right equipment that is calibrated.
Simple volt meter?
While being discharged? (Even slightly)
Ballpark estimate of roughly xx% of SOC..
Crystal Ball, fortune teller,
Like the weather forecast
Or perhaps the fuel meter in your car.
You know it's full when you can not fit an other drop of gasoline in the tank.
You know it's empty when the meter is at 0 (probably for the last 50km or so) and the engine stops (note, no other technical problems, only run out of fuel)
Between this, you hava a rough estimation of the amount of fuel left.
You can make an estimation on how much km more you can drive.
You don't know exactly.
And even with a dipstick...
Is the car horizontal?
What is the temperature? As gasoline has expansion and contraction with different temperatures.
And then the road, trafic, and the way you drive. Humidity....
The list is long to be able to accurately estimate the about of liters in the gasoline tank, and even harder how much you can drive!!
A slow discharge of lifepo4 will provide more kWh from a single charge then C0.5 or higher discharge.
The Ammeter and shunts mentioned before... They measure the kWh (ampere and voltage)
Even those "suck", for that simple reason, how fast do you discharge?
How fast do you charge?
Always the same?
Or fluctuations?
You get the idea...
Knowing SOC on a percentage, that's expensive.
Estimation with 10% error margin is normal.
To prevent damage to cells the rule of thumb is not to charge beyond 3.65, not discharge below 2.5v
Also, to stop the charge at 3.65.
No absorption (what would take many hours)
In 20% of the time, 80% of the battery is charged
Result, to get the last 20% it takes 80% of the time, or 5 times as long as the first part.
Realistic, solar, "Fully charged" from 07.00 to 15.00 (or so)
Say 5 to 6 hours.
To really fully charge, you would need an additional 25 hours of sunlight.
Reaching 3.65v isn't fully charged, not 100% SOC, not maximal capacity.
And that is OK.
Having 280Ah, being able to on normal use, use 230-240Ah is a LOT better then lead acid, were one would be able to use 110Ah.
Besides, staying in this 80% will increase the number of cycles significantly.
In other words, if you need to use more then the 80% on a day to day basis (or probably even when you reach this number) you don't have enough storage capacity, and need to extend your battery array.
94-95% is perfect.
Already kinda high.
Don't try to reach the 100%
You probably won't even be able to, due Factory protections and time restraints.
And even if you are able..
The cells that are a little less perfect can get you into trouble.
