GUIDE to properly Top-Balance and Charge a LFP Battery: Part 1
- Thread starter shvm
- Start date
Look,
If I've guessed correctly, next in line for 'made up things' would be asking for sources on how the Resting OCV of a fully charged LFP Cell is around 3.37 V. (Float)
Your 'proofs' basically amount to 'reinventing the wheel' asking how steady state OCVs are derived from (current going in/out) dynamic values provided in the cell datasheet. If you want to know the sources on where numbers like 3.65 and 2.5 are coming from, ask cell manufacturers not me.
Sorry for the late reply.Something like this ?
or maybe this ?
But I'm not seeing it on my end.
I might come across as incredibly dumb, but how do I use them for reacting to other's posts?
Its : then fp together and : with fp2 for the other one.
and Its just like a smile one which is : and ) together making
Kenny_
Solar Addict
You have to top balance them then insert into current thread.
Sorry I’m so late to the party, I’m also an engineer that is obsessed with finding all the facts one can.
Where did we leave the simplified version of how to love on our expensive batteries?? I understand the 3.5v but get hung up on the solar aspect of varying C rates.
My system is 920ah and 1200w of panels. On a great day I’m near 0.1C which makes me want to set the termination point at 3.5 - 3.55 and achieve 96%-97% charge. But on a rainy, cloudy day, I’m down to 0.02C which exceeds chart parameters for even the low setting of 3.5V.
Now you may say “But Pyrate, at 0.02C you’ll never recoup the 130ah you used during the night” to which I’d say BUT - the day may start sunny, push the batteries to nearly charged, then the clouds come in and I’m slowly charging them beyond their threshold.
So I pose this question to the hive: using the given pieces of the puzzle, 920ah, 1200w, 0.1C to 0.02C, what’s a proper charge-to voltage, or is this a case where I need to add another control function to the mix?
Where did we leave the simplified version of how to love on our expensive batteries?? I understand the 3.5v but get hung up on the solar aspect of varying C rates.
My system is 920ah and 1200w of panels. On a great day I’m near 0.1C which makes me want to set the termination point at 3.5 - 3.55 and achieve 96%-97% charge. But on a rainy, cloudy day, I’m down to 0.02C which exceeds chart parameters for even the low setting of 3.5V.
Now you may say “But Pyrate, at 0.02C you’ll never recoup the 130ah you used during the night” to which I’d say BUT - the day may start sunny, push the batteries to nearly charged, then the clouds come in and I’m slowly charging them beyond their threshold.
So I pose this question to the hive: using the given pieces of the puzzle, 920ah, 1200w, 0.1C to 0.02C, what’s a proper charge-to voltage, or is this a case where I need to add another control function to the mix?
But your loads will take that power, no? If you don't have loads, turn off at 3.45V or so.
I have never had a full battery, no loads, and 0.02C coming in. You just go back to float once you hit 3.5V per cell, and only charge again once the battery drops below the float value.
Already explained in post #51 in this thread.
GUIDE to properly Top-Balance and Charge a LFP Battery: Part 1
I have read the forum's consensus on how it approaches LFP charging and balancing. I can immediately recognise why it's wrong because I have applied both to my very own battery and witnessed the correct behaviour. That's common sense according to me. It's better because it can easily tested to...
diysolarforum.com
BTW, it has been already implemented physically for CANBus capable devices.
Here's what it looks like:
C++:
if (id(current).state > 0) { // Current can't be negative for calculation of 'charging' cut-off voltage
float cutoff_current = (id(battery_ah).state * 0.05 * (cell_bulk_v - 3.375) / (3.625 - 3.375));
float cutoff_voltage = (3.375 + 5 * (id(current).state / id(battery_ah).state));
// Stop Charging
if ((id(current).state < cutoff_current) & (id(max_cell_voltage).state > cutoff_voltage)) {
// End Of Charge
id(charge_status) = "Cutoff";
}
// Start Charging
else {
// With the "Auto Charge Current Control", the current is reduced automatically if "max_cell_voltage" approaches BMS OVPR
// Absorption : Max Cell V. > Cell Absorption V.
else if (id(max_cell_voltage).state > (cell_bulk_v - 0.005)) id(charge_status) = "Absorption";
// Bulk
else id(charge_status) = "Bulk";
}
}
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Okay, then let’s change the parameters so my question becomes valid.
I can set my Victrons to turn off at a set voltage, I’m looking for that voltage. If I set them at 3.5 or 14v for my system, they will continue to provide charging until the batteries reach 14v. If that charge is a low amount ( my parasitic loads vary between 9 -15 amps) say 25 amps, I’m sending 9-15 to run fans, fridges and such, and 16-10 is going to the batteries until they reach 14v or the BMS shuts them down, right?
I’ve read not to worry about fast charging as the voltage will spike long before the cells are truly full, but the slow charging is what sneaks past the guard and ruins the cells.
Maybe 13.8v is a happy medium, where during low load and fast charging I’ll get 95% full and during slow charging I’ll not overcharge and overheat the cells before the Victrons shut down.
Or am I looking at this wrong?
Let me add: I went and reread #51 which addresses charging at various C levels, but doesn’t address charging at all those varying C levels during one day. While it’s possible to follow a charging protocol based on a C rate, I have an infinite number of C rates going all day long, from 0.25C down to 0.00000001C.
The battery manufacturer tells me to charge at 14.6V with a 0.02C tail current. For my 920ah that’s 18.4 amps.
The battery manufacturer tells me to charge at 14.6V with a 0.02C tail current. For my 920ah that’s 18.4 amps.
ScrotusGobbleBottom
Solar Enthusiast
Isnt 3.45 the magic number for 100% with slow charging and zero detriment? I am pretty sure you are right!
I usually charge to 3.45 and once every 5-6 months charge to 3.55 for balancing.
That’s the question, and we really won’t know if our batteries last 30 years but could have made it 35, or if we added 5 years to them. I guess nobody really knows.
What I do know is I’m going against Victron’s LifPo profile, LiTime’s settings, and going with this group’s advanced thinking approach.
If the batteries can be charged to 3.5V (14V) no matter what charge rate is being allowed, then I’ll set the chargers to 14V and let them run. Some days will get less charge and some days more, but I’m not going to babysit batteries every day.
If it is beneficial to charge them every 6 months until the BMS shuts them down, then I’ll do that also.
I’m not looking to change minds or offer suggestions, I’m looking for someone to tell me how to set my solar chargers and let them work.
I’ll see if I can attach a snap of my solar settings, and the history graph of what these settings do. I get a big hockey stick when the charge voltage gets above 13.6. It climbs rapidly to 14V or 14.2V, whatever I have set.
Attachments
ScrotusGobbleBottom
Solar Enthusiast
That is totally normal and exactly why we dont need to go that high. Simply put, there isnt much energy there so not a big point to do it.
ScrotusGobbleBottom
Solar Enthusiast
I would charge to 13.8 and not think about it. Thats what I do.
ScrotusGobbleBottom
Solar Enthusiast
I dont believe there is any need for this. The idea is to charge to a voltage that the BMS starts balancing that is in the hockey stick curve of the charging process. If you dont have that info or its not available, dont worry too much. Even balancing at 3.5 yields very limited gains. If you can get and keep all your cells at 3.45 you will be just fine.
What makes you think it doesn't?
It is valid for instantaneous values of current. You can poll the value of current 10 times a second or 10 times a minute for this loop. It is up to you. It will not fail.
You said you've a victron unit?
Does it supports CANBus? Have you got a JK BMS?
If yes, you can put together $10 in parts (ESP32 + SN65HVD30 + 4 Pin 1.25mm pitch JST XH connector) that will solve this problem forever.
It will automatically handle the charging for you.
As it has for me for last two weeks.
My BMS units are sealed within the batteries and not accessible in any way.
I’m not looking to program something to monitor charge rates and volts and make changes 100 times a second so I can get the batteries to some unknown perfect charge.
I’m looking for the voltage or combination of voltage and absorption time that will do a pretty good job of keeping my batteries healthy at charge rates between 0.25C and 0.00001C.
I don’t think the battery or charger manufacturers are instructing end users to employ the best charging techniques available to them. I think they are picking a charging curve that’s readily available to a majority of the charging units on the market and depending on the BMS to stop the charge. Leaning more on the marketing side than the engineering side of battery usage.
You can utilize the tail-current feature of your Victron hardware. Set a voltage like 3.475 V/Cell for bulk and 3.37 V/cell for float and corresponding tail current. It is better than any absorption timer.