A must read for anyone with LFP batteries

Hello all,
I’ve had and have been using Bluetti products for almost a year (It’ll be a year in March) and I will say, the most frustrating thing about them is the LFP (Lithium-ferro-phosphate) battery. The battery is awesome, the SOC indicator is not. Part of this frustration comes from my knowledge (or lack thereof) of the charge/discharge curve of LFP batteries and there being no good way to measure SOC (State of Charge) in LFP batteries. If you are noticing that your SOC indicator is off this article is worth reading as it will help you understand what is going on with your system, specifically the SOH (state of health) of your battery. LiFePO4 Battery Discharge and charge Curve - BRAVA
I know this is a different battery manufacturer, but the charge and discharge curve should be the same. Batteries are a big investment and the heart of the system. I found this article interesting because of my desire to protect my investment so it can serve me for years to come. I’m sure you’re in the same boat.


As stated in the manual, it is recommended to fully charge and discharge the batteries once in a while to keep the battery indicator (SOC) accurate.

Nice video explaining the process and reasoning here:


That video was helpful. Last time I did this I didn’t wait until the unit powered off rather, I just waited until it was 0% and I went straight into charging. Plus, I have been doing this with my AC200Max and the B230 expansion battery connected at the same time. I have never been able to get them synced closely. A couple of questions, Is the battery charge/discharge indicator a function of the AC200Max and its processor exclusively? in other words if I just do the discharge/charge cycle with the AC200Max only will that be enough? Or is the charge/discharge indicator a function of each individual battery’s BMS? So, I would have to do each battery individually?

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I cannot answer with 100% certainty, but I would think that you would need to cycle with the expansion batteries connected. I say this because there is a charge/discharge control logic in the AC200max (and in the AC300) that communicates with the BMS. If you notice or suspect that the SOC indicator is off on one of the expansion batteries, you would have to cycle with the battery connected to re-calibrate.


@TheQuickFox thanks for the video.
First question: is it important that discharging starts from 100% or could be let’s say 42% (current level of my AC300/B300) - but from that on with a constant rate ?
Second question: what you described regarding the modular systems like AC300/AC500. If the battery is at 0% it MIGHT not connect to the main module for immediate recharging. So the optional approach via car charging is kind of work around. correct?

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I followed the great video and did the discharge. It still misbehaved once more so I will just monitor this or see if you have any other ideas. It seems to be a problem on cloudy days where not much goes into the AC200 max from my solar. But of course in January that will happen. It does not appear to be a battery problem as the discharge times seem to indicate that is ok, but buying a kilowatt meter to be sure.

Charge was at 0%

Started charging at 8.15 462w
8.30 7% charged
8.40 10% charged
8.50 14%
9.00 18%
9.10 22%
9.25 28%
9.30 32%
9.35 100%

(1 hour 20)

Went to bed and charge was on 74% Was on ECO and switched off automatically

05/01 Went to bed was charged at 69%
On Eco
06/1 Was 0%
Charged with charger at 450 w from 7.40

At 7.52 it was 8%
At 8.06 it was 12% AT 461W4
At 8.45 it was 27% at 462w
At 9.05 it was up to 38% at 470w
At 9.14 it was up to 100%

(1 hour 25)

So I decided to do the advised discharge to nil which took 3 and a half hours then some more time before switch off at 0%

31/12 Went to bed was charged at 93%.

On ECO so would have switched off

1/1 was at 0%

Started charging at 7.40 from 0% at 460w
At 8.25, it was 20%
At 8.45 it was 26%
At 9.00 it was 32%
At 9.27 it was 45% (Getting quicker)

At 9.29, it was 100%!!

1 hour 40 minutes

I should say I have had this 4 months and this only started happening recently. Also I only used it for minor things like a few cups of tea in a 1200w kettle. Nothing significant. And I have a B230 which I have not yet discharged connected to the unit. did not even think of that…

My understanding from speaking with tech support multiple times is that you just need to fully drain and then charge to 100% WITHOUT ANY LOADS (AC and DC outputs OFF).

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I’ve had SOC display hiccups and I realize that part of the problem is the nature of LFP batteries… I also keep a written record of usage (for both AC200Max and AC300+batteries). It’s too bad the app doesn’t keep records of all the incoming and out-going information. I find the batteries have a hard time staying calibrated and balanced (when multiple expansion batteries are hooked up) when they are being discharged and charged via solar simultaneously. Just what I have observed.

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thanks @MontAnna . My question was if drain should start at 100% battery level udn from that on down to 0%
Or is that not important and drain can start from somewhere in between (e.g. 42%) - but at constant rate (as outlined in the video)

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When I contacted bluetti tech support last year about a SOC jump in my AC200Max they said “or voltage jumps, you can discharge the AC200MAX to 0%, then fully charge it to calibrate the SOC.” They said basically the same thing for the AC300 B300 combo. They never said I had to charge to 100% before doing a discharge to 0%. I would assume that means you do not have to charge to 100% before doing a full discharge and full recharge (to calibrate).

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So I think I finally have my unit properly calibrated. I have the AC 200Max and a B230 expansion battery. I could never get them to drain or charge in sync. They were always far apart in their charge percentages. I disconnected the B230 so my AC200 Max was stand alone. Discharged the AC200 Max to zero then recharged it to 100 (so it should have been properly calibrated at this point). Then I put a 12V load on my B230 while still disconnected and drew it down to about 50%. Then I reconnected the AC200 Max and B230 and put a 1400W load on them until they were both zero and shutoff. I then hooked up my wall charger to the B230 and began charging. It never showed an input charge on the AC200 Max. probably because I was going through the B230. But both units fully charged at an equal rate over the entire charge cycle. At this point I dont care if they discharge a little out percentage wise. I was just happy to see them charge perfectly in sync.

I am trying to hammer out a battery imbalance issue with my AC300+2 B300 batteries. I don’t want to have to plug into the wall, I would rather charge via PV. However, after reading your reply, I plugged the battery that always has the lower SOC (and by lower, I mean that one battery will display 90% and the other will display 30%) directly into the wall via bluetti’s charging brick. There were some SOC jumps but in the end it charged from 32% to 100% and that took about 2000 watts (on a B300 battery 2000 watts is about 2/3rds of the capacity). My inquiring mind wants to know why the AC300 seems to discharge one battery at a faster rate than the other then charge the battery with the higher SOC faster than the battery with the lower SOC. It’s really weird.

You would think that the 2 batteries would be in parallel with each other and share the load equally and therefore discharge equally. But if they discharge at different rates that can’t possibly be the case. The processor in the head unit must just see them as 2 individual batteries each and determine the best way to utilize them. For them to be in parallel the voltages would have to match. Maybe that’s the reason.

I am no battery expert but my understanding is consistent with what you said. If they were in parallel they would discharge equally. I also know from reading that a LFP BMS doesn’t just look at voltages but there are a number of computations going on and that many use coulomb counting. “The Coulomb counting method measures the discharging current of a battery and integrates the discharging current over time in order to estimate SOC State of charge (SoC) is the level of charge of an electric battery relative to its capacity.” -The State of Charge Estimating Methods for Battery: A Review
I think the key phrase there is over time. With my two batteries one is older than the other (that is, I bought one in May 2022 and the other in November 2022 and have been using them constantly). Since one has been used for a longer period of time, hypothetically, if the B300 battery uses coulomb counting, my older B300 would have months more of metrics to go off of to estimate it’s state of charge. I hope that makes sense. I did an individual capacity test and they (the two batteries) are close enough that I am not worried that the problem is diminished capacity.
Thanks for chiming in. I am learning from each person who responds.

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