LiFePO4 Voltage Chart?

MisterSandals

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I think if you get equalizing cell voltage on charges above 3.325 v per cell, one could safely go up to 13.6 v on a 12v LifePO4 battery bank. If solar is available for that higher charge, why not go there is ... my two cents way of thinking :+) ... (from just glancing at this tread)
Yes, I am with you on charging higher. My conservative charge is 13.8V. I was just opining on how I interpreted by battery SoC based on voltage.
My RV sits in the driveway too often but I will turn off charging an "store" the battery at 13.2V and turn the solar panels on (cutoff switch) when i see 13.0V. Things don't move very fast so checking on it once a week and charging for a day here and there is pretty easy.
 

Brewman

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Would I be correct in thinking that for a 24V LiFePO4 battery bank at 80% capacity the voltage should be around 26.8V
 

smoothJoey

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Would I be correct in thinking that for a 24V LiFePO4 battery bank at 80% capacity the voltage should be around 26.8V
Yes, sort of.
Voltage at rest is different than voltage during charge or discharge.
The more current you push/pull the more pronounced the effect.
It also matters where in the system you test, as the effect is amplified by resistance.
Luckily you have an overkill solar BMS which does current accounting and voltage measurement where it matters which is at the battery.
 

smoothJoey

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Got it, check as close to the battery as possible.
Your use case is reasonably deterministic so it matters less where you measure.
After the bms has done ~2 cycles you can just discharge at your representative rate and set the voltage triggers to whatever corresponds to the depth of discharge in amp hours as reported by the battery.
That means attaching the sense leads to the busbars is fine.
 
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Brewman

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So cycle more then 2 times, discharge to 90% of total amp hour rating, take voltage measurement and that is your 90% voltage point.
 

Range

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Yep, you're not missing anything. All these things you mention are why I'm in my 7th month looking for a BMS that I like.


I like 3.0 and 3.45 and am getting roughly 90% capacity. I need to be conservative too, because I am the BMS.
Same here as I v become a living bms. I love to check my LifePo4 charging and discharging myself. I run on a small load though.
 

Barold

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An I ask a potentially stupid question - my BMV712 is showing a battery voltage of 13.20V but the SOC is 90%. According to some of the charts here that would should be closer to a 70% SOC. It's shunt based so all the loads and charge sources are going through the BMV.

Am I missing something?
 

MisterSandals

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Am I missing something?
I think this thread has run its course and purpose with the OP so continuing this thread repurpose...

Hard to say without knowing anything about your system and how you've used it.
Have you cycled it a few times to the BMS has an idea what the full range of your batteries is?
What are some of your charge parameters?
What does your battery look like and how is it wired? (pics are good to spot anything unusual)

A FWIW, i have a BMV712 that i used daily for a month in a greenhouse/nursery. I was doing a lot of testing of different battery packs and rearranging cells. It became an extra burden to deal with and i was mostly just looking at the battery voltage LEDs that were readily visible. Hopefully others with better experience have some more to add but i'm sure they'll need
 

curiouscarbon

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An I ask a potentially stupid question - my BMV712 is showing a battery voltage of 13.20V but the SOC is 90%. According to some of the charts here that would should be closer to a 70% SOC. It's shunt based so all the loads and charge sources are going through the BMV.

Am I missing something?
No worries about "stupid question", a counterpoint is "stupidity is achieved by never asking" :)

Sharing at least 1) the voltage your charge controller is set to, and 2) the voltage your BMV712 is configured to recognize as 100% SOC, may help people help you. That's the info I need to give you a good answer.

As you might be aware, the BMV712 detects 100% SOC based on battery bank voltage being above a specific value for a specific amount of time, if I'm not mistaken. Please share the time setting part too if you can.

LiFePO4 battery voltage is quite constant over the middle part of the state of charge. But it's still workable!
 
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Barold

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No worries about "stupid question", a counterpoint is "stupidity is achieved by never asking" :)

Sharing at least 1) the voltage your charge controller is set to, and 2) the voltage your BMV712 is configured to recognize as 100% SOC, may help people help you. That's the info I need to give you a good answer.

As you might be aware, the BMV712 detects 100% SOC based on battery bank voltage being above a specific value for a specific amount of time, if I'm not mistaken. Please share the time setting part too if you can.

LiFePO4 battery voltage is quite constant over the middle part of the state of charge. But it's still workable!
Thanks. The solar charge controller has the 14.2V absorption and 13.5V float. I noticed right now (nighttime) with an 11A draw (no charging right now) the bmv is showing a 12.79V with a 96% SOC. So perhaps the Voltage under load will be lower than the actual true SOC. The consumed amps since the last full charge today is 15Ah
 

curiouscarbon

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Thanks. The solar charge controller has the 14.2V absorption and 13.5V float. I noticed right now (nighttime) with an 11A draw (no charging right now) the bmv is showing a 12.79V with a 96% SOC. So perhaps the Voltage under load will be lower than the actual true SOC. The consumed amps since the last full charge today is 15Ah
I see. Thank you for those details. If you're talking about the 280Ah cells, then 15Ah is ~5.3% on paper so 96% suggests 4% so only 1.3% paper offset. Seems good at that state.

There's a transfer function for every cell, that changes over time... (input: Amps) (output: Volts drop)
I have no experience with 280Ah cell Amps vs Volt Drop mapping function firsthand.. 11A on 280Ah cells is 0.04C so very little voltage drop should be expected.

If the battery isn't charged to full regularly enough, the BMV might get out of sync. Hope some of this helps :)
 

Barold

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I turned on the ac charger this morning to try to fully charge the battery. Now an odd occurence: the Voltage and the amps from the 40A charger fluctuate on the BMV app display ( 13-23A and 13.98 to 14.4V. The charge setting is 14.4V.
I dont use the ac charger often as the Solar usually gets us there but wondering if the cells are not up to the task. Will capacity test them over the winter to see if there is a cell that is off.
(i took a video of the bmv screen but i cant seem to upload it to share)
 

Steve_S

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The Amperage should remain constant CC (Constant Current) until the VPC setpoint is reached at which point the Amps Taken should start to reduce under CV (Constant Voltage). End-Amps / Tail Current is calculated as ###AH X 0.05C. ( 100AH X 0.05C = 5A )
When charging at CC, once EndAmps/TailCurrent is reached the (between 90-95% gross Capacity Full) charger should then switch to CV to finalize charge.

* ? * Gross Capacity ?
LFP Gross Capacity Voltage range is 2.500-3.650.
LFP Working Range Voltages are 3.000-3.400, with the Nominal Voltage being 3.200Vpc
(Vpc = Volts per cell)
LFP Grade A+ can safely operate between 2.900-3.500 Vpc.
Voltage Cliffs:
- Lo Voltage Cliff Fall begins at 2.800 to 2.500 (0% SOC) and represents about 2-3% of total GROSS Capacity
- Hi Voltage Cliff Climb begins at 3.475/3.500 and up and also represents about 2-3% of total GROSS Capacity
GROSS CAPACITY is measured from 2.500-3.650.
-1) Example: 280AH A+ Cells (Matched & Batched) will typically test out at 290AH Gross
(these will not deviate greatly, typically no more than 30mv overall)
-2) Example: 280AH Bulk Cells (Voltage & IR matched only) typically test out between 270-280AH GROSS *a
These will also often have "Runner Cells" which will reach Hi/Lo Volt cutoff before the other cells, they will deviate by up to as much as 1mv per AH of storage Capacity.
 

Barold

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The Amperage should remain constant CC (Constant Current) until the VPC setpoint is reached at which point the Amps Taken should start to reduce under CV (Constant Voltage). End-Amps / Tail Current is calculated as ###AH X 0.05C. ( 100AH X 0.05C = 5A )
When charging at CC, once EndAmps/TailCurrent is reached the (between 90-95% gross Capacity Full) charger should then switch to CV to finalize charge.

* ? * Gross Capacity ?
LFP Gross Capacity Voltage range is 2.500-3.650.
LFP Working Range Voltages are 3.000-3.400, with the Nominal Voltage being 3.200Vpc
(Vpc = Volts per cell)
LFP Grade A+ can safely operate between 2.900-3.500 Vpc.
Voltage Cliffs:
- Lo Voltage Cliff Fall begins at 2.800 to 2.500 (0% SOC) and represents about 2-3% of total GROSS Capacity
- Hi Voltage Cliff Climb begins at 3.475/3.500 and up and also represents about 2-3% of total GROSS Capacity
GROSS CAPACITY is measured from 2.500-3.650.
-1) Example: 280AH A+ Cells (Matched & Batched) will typically test out at 290AH Gross
(these will not deviate greatly, typically no more than 30mv overall)
-2) Example: 280AH Bulk Cells (Voltage & IR matched only) typically test out between 270-280AH GROSS *a
These will also often have "Runner Cells" which will reach Hi/Lo Volt cutoff before the other cells, they will deviate by up to as much as 1mv per AH of storage Capacity.
Tbh ive never observed how this charger worked in the past. I suspected it killed my AGM batteries but coild not prove it. Now that the battery is reaching the top i am more concerned to see the Voltage range up to 15v. Im not so certain about this charger now. (But i don’t use the AC charger as the solar has worked fantastically.

 

Steve_S

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Charging Profiles for Lithium Based Batteries IS CRITICAL and not interchangeable with Lead battery chargers really. LFP ONLY takes CC & CV and Never Ever Equalisation or Desulphation modes. So many get fooled by the "Drop In Replacement" shtick because they do not understand.

For a "12V" LFP Battery the MAX Voltage is 14.6 Volts (3.650Vpc) which is UNHEALTHY ! they should MAX at 3.500Vpc or 14.0V.

The 2nd, 3rd & 4th links in my signature will answer many more questions and issues.
 

Barold

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Charging Profiles for Lithium Based Batteries IS CRITICAL and not interchangeable with Lead battery chargers really. LFP ONLY takes CC & CV and Never Ever Equalisation or Desulphation modes. So many get fooled by the "Drop In Replacement" shtick because they do not understand.

For a "12V" LFP Battery the MAX Voltage is 14.6 Volts (3.650Vpc) which is UNHEALTHY ! they should MAX at 3.500Vpc or 14.0V.

The 2nd, 3rd & 4th links in my signature will answer many more questions and issues.
The charge profile is set to LFP (14.4V) but i have been suspicious of this charger (cristec 40A) i think i will be looking to swap out for a victron charger inverter next season/over the winter.
I will also check the connectors to ensure loose connections are the cause for fluctuating voltage readings.
The charging is settling to 13.73V now at ~3.8A
 
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