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THE ART OF 10 - 90% (Lifepo4)

Vi s

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THE ART OF 10% TO 90%

☀ Sunny hello to everyone ☀

Would be nice and useful if we could collect some methods how to keep ones cells within 10 - 90% SoC.
Since application, circumstances and understanding differs, methods differ.

I start things up, I am going to share my method and journey to maximise the cycle life (till 80% DOD) of my cells.

Application and setup: 18V 60W panel, 20A mppt Lumiax Win500 SC, 12.8v 53Ah lifepo4. Powering a small hut in the forest, daily estimated average consumption around 250Wh.

My cells and tools:
4 x 53Ah EVE LF50K 3.2V Lifepo4 cells, 0 cycles yet (according to datasheet more than 3500 cycles till 80% DOD)
1 x JBD 4s smart BMS 60Ah w/ Bluetooth
1 x original imax B6 mini professional battery charger 60W
1 x Vokpro VP1009 true RMS multimeter

How I determined the voltage of 90%SoC while charging, discharging and at rest.

Step one - top balancing cells:
1. Charged whole pack with 3.5A (0.066C) till 13.6v then stopped
2. Next charged each cell separately with 3.6V CV and 1.5A till amps dropped to zero.
3. After about 30min rest charged again each cell with 3.6V CV and 1.5A till amps dropped to zero (less than 35 mah could still be squeezed in)
4. Assembled back to a 4s pack

Step two - noting down voltage readings at 90%SoC while discharging, charging and at rest:
1. Discharged with 48W load:
At 90%SoC, cells were at 3.310V average
At 85%SoC, still 3.310V average
2. 45min resting at 85%SoC = 3.333V average
3. Charged with 48W:
At 85%SoC = 3.345V average
At 90%SoC = 3.368V average
4. 2 hours resting at 90%SoC = 3.340V average

Step three - noting down voltage readings at 10%SoC while discharging:
1. Yet to be done but according to the official chart it is around 3.1V while discharging with 0.2c.

Step four - programming charge controller and BMS:
1. BMS:
1.1 overvoltage = 3.5V
1.2 release = 3.33v
1.3 undervoltage = 3.1V
1.4 release = 3.2V

2. Charge controller:
2.1 overvoltage = 13.5V
2.2 release = 13.32V (to prevent micro cycling)
2.3 undervoltage = 12.6V
2.4 release = 12.9V (to prevent micro cycling)

Step five - observing and adjusting:
1. Yet to be done

What's your application and setup, how do you stay within the manufacturer recommended 10 - 90% window? What are your observations, tips and tricks?
 
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I think you will find voltage a poor indicator for LFP.

Coulomb counting with a shunt would be best to stay at 80% use of capacity IMHO. Victron SmartShunt etc.

You have a well thought out start for sure.

I operate more trial and error and am surprised at how well my older stuff (5 yrs + ) is still holding up despite my complete ignorance at the start of my journey.

Step 5 is where you continue to learn and adjust accordingly .

Unfortunately that’s where I started ?.
 
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I think you will find voltage a poor indicator for LFP.

Coulomb counting with a shunt would be best to stay at 80% use of capacity IMHO.
Certainly voltage is no exact science but a convenient one. Further everything in the bms and SC is controlled by voltage so one has to use it. I understand, it's an estimation but if you know your cells and ever collected the voltage readings at the soc levels you are interested in then it becomes a fairly accurate indicator under similar conditions. My bms for example counts the incoming and outgoing energy. Those readings look pretty accurate if I compare them with the readings of my imax. So yes I have to do some math (how many ah are 10% of 53ah = 5.3ah) and gather some info once (discharge 5.3ah off my full battery and then read the voltage, discharge again 1 or 2 ah, then charge those 1 or 2ah back and read the voltage at 90%soc while charging again and finally while at rest) but that's quite fun to do IMHO. I also looked at those coulomb counters but thought that's overkill for my small pack and aim. I just aim to prevent a premature death of my cells, means I just aim to prevent "too full and too empty". Whether it's 87% or 94%, 8% or 18%, no issue for me personally, goal still achieved. ?
 
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how do you stay within the manufacturer recommended 10 - 90% window? What are your observations, tips and tricks?
the top is difficult to achieve just using voltage. The practical solution is to charge with a target of 13.8 or 14 volts and stop once the target is reached. This will charge to 90 to 98% of capacity depending on the charge current. Float if used would need to be 13.4 or less to avoid charging.
The low limit voltage of 12v is a useful starting point. If you have a significant load, like high power inverter, the low limit may need adjusting down to say 11.8 volts.
Comments on your BMS and charge settings. The charger settings should be 'inside' the BMS settings. The BMS is the final protection l suggest you set the BMS to the accepted protection levels, 3.60 to 3.65 top volts and 2.5 to 2.75 bottom volts.

Mike
 
Battery voltage is effected by the charge or discharge rate even if you measure at the battery terminals.
>95% of the time my discharge rate is between 10-15 amps so initiating and terminating charge based on voltage works fine.
I control my rv style converter with a Victron smart battery protect and an iot relay https://dlidirect.com/products/iot-power-relay .

Having a relatively low and consistent discharge rate help but having sufficient charge rate also helps.
 
the top is difficult to achieve just using voltage. The practical solution is to charge with a target of 13.8 or 14 volts and stop once the target is reached. This will charge to 90 to 98% of capacity depending on the charge current. Float if used would need to be 13.4 or less to avoid charging.
The low limit voltage of 12v is a useful starting point. If you have a significant load, like high power inverter, the low limit may need adjusting down to say 11.8 volts.
Comments on your BMS and charge settings. The charger settings should be 'inside' the BMS settings. The BMS is the final protection l suggest you set the BMS to the accepted protection levels, 3.60 to 3.65 top volts and 2.5 to 2.75 bottom volts.

Mike
Yes it all depends on the charge current. In my case 90%voc was already reached at 13.48v.

Thanks for the suggestion!
I decided to set it at 3.5v and 3.1v to prevent too full and too empty in case the SC fails to do so. SC stops even lower than that. I also don't use float just cut and release for top and bottom. I only have to prevent micro cycling so release should be below resting voltage. Example, 13.5v is ~90%soc in my case so I set cut there. Then the cells settle at around 13.36v. So I set the release voltage below that, say 13.33v and observe if that prevents micro cycling.
 
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