@jumper thanks for the insightful reply.

No, sadly my LFP batteries are the "black box" type - there is no user interface of any kind and no feedback from the battery BMS is user accessible. I have to rely on the inverter's SOC reporting which is not ideal.

In my charge curve examples, when charging happens after a load shedding period, the reported SOC quickly climbs to 95% (a guess by the inverter I'm sure) and sits there for an hour or two. When it hits 100% is roughly when that peak starts to drop. It's never ever lingered at the bulk phase (so I never get that flat mesa).

I've just tweaked my charge settings, let me see how it behaves going forward. I've pushed bulk up to 28.8 (from 28.4), and enabled equalization at 28v every 5 days, for a max of 30 min.

I also tweaked the "back to charge" and back to "discharge values" - back to charge from 25V to 23V, back to discharge from 27 to 28V.

Doing a manual equalization now:

  

I'll keep an eye on the charging curve over the next couple of days and report any changes.

Edited December 6, 20232 yr by jbroo

37 minutes ago, jbroo said:

No, sadly my LFP batteries are the "black box" type - there is no user interface of any kind and no feedback from the battery BMS is user accessible. I have to rely on the inverter's SOC reporting which is not ideal.

Nice clean UI you've made there, well done!

I reckon with these black box type batteries you'll need to keep an eye on the charging current when it hits bulk voltage and see how long it takes to drop to a few amps and then set your equalization time based on that. The CV part of the graph should be about the same time always, no matter how low the battery was discharged, it's the CC phase that'll be longer and the inverter does that part just fine, it just needs a bit more time at CV which can be mimicked with bulk charging time, equalization or increasing the float voltage, depending on the inverter.

I also tweaked the "back to charge" and back to "discharge values" - back to charge from 25V to 23V, back to discharge from 27 to 28V.

Doing a manual equalization now:

Just my 2c. Using a back to charge at 23V your are already at the falling of the cliff during discharge so not much time before cut out. 

Also battling to get above 28V and using a back to discharge of 28V means not being able to use any battery power from 23V up to 28V.

54 minutes ago, jumper said:

Nice clean UI you've made there, well done!

Thanks! I'm using HomeHabit for the dashboard, reading MQTT data from SolPipLog running on my Pi 1B. The history graph is my own software, since neither HomeHabit or SolPipLog have this option. I'll share a download link in a new post as soon as I've finished final tweaks on it.

1 hour ago, jumper said:

you'll need to keep an eye on the charging current when it hits bulk voltage

This is a problem - my inverter has this data point but always reports 0. Likely a bug. I can only ever see discharge current, not charge. Even the kak WatchPower software does not show it.

29 minutes ago, Scorp007 said:

Just my 2c. Using a back to charge at 23V your are already at the falling of the cliff during discharge so not much time before cut out. 

Also battling to get above 28V and using a back to discharge of 28V means not being able to use any battery power from 23V up to 28V.

I agree, except I'm not using solar, so this should not matter. Batteries are used for backup only on mains failure. On the low voltage side, if I believe the battery specs, I should be fine: 

 

@zylinx Apologies, I don't mean to hijack your topic 🫣 But this info is still relevant.

43 minutes ago, jbroo said:

This is a problem - my inverter has this data point but always reports 0. Likely a bug. I can only ever see discharge current, not charge. Even the kak WatchPower software does not show it.

That does present a bit of a problem. The only solution would be a shunt or a cheap clamp meter, but they aren't so cheap, maybe you can borrow one to do the measurements and then you can just keep the settings. With a shunt you could tie the readings into your dashboard and they will be a lot more accurate, but not sure it's worth it on such a small setup.

11 hours ago, jbroo said:

I agree, except I'm not using solar, so this should not matter. Batteries are used for backup only on mains failure. On the low voltage side, if I believe the battery specs, I should be fine: 

 

@zylinx Apologies, I don't mean to hijack your topic 🫣 But this info is still relevant.

Even if the cut off is only 9V a LiFePO4 is totally discharged at 10V and I doubt it's good for longevity. 

12V is normally 10% SOC. 

18 hours ago, Scorp007 said:

12V is normally 10% SOC. 

Quite correct, yes, thanks.

That then begs the question, why are we all so desperate to charge at 14.4V and higher?

Image source

1 hour ago, jbroo said:

That then begs the question, why are we all so desperate to charge at 14.4V and higher?

I charge to 56.4V (14.1V) because I have to in order to get the BMS to reach 100%. I tried lower voltages, but it would either not top balance or sit forever at 96%. I've seen a lot of problems come from not allowing the BMS to hit 100% where SOC drifts and batteries drop from 40% to 0% and shut off and need re-calibration. If my BMS was connected with inverter comms it would be charging to 57.8V (14.45V) as per the manufacturers setting, so I'm happy with 56.4V.

@jumper are you running LFP batteries?

Edit: stupid question. Of course you are. 🙃

Edited December 7, 20232 yr by jbroo

Yeah I would like to charge at 14.1V but I alas I tried messing with the settings and nothing seems to help. 

 

As soon as it reaches around 13.7V it cuts charging regardless of the set bulk charge voltage. 

 

Also I noticed the charge current is not available in QPIGS command. But I see it's far less than advertised. 15A charge rate actually puts out 8A and 25A puts out 16A.

 

I kind of suspect it could be like @jumper mentioned the BMS cutting the battery for a split second when it reaches 100% SOC, tripping the charge algo off.

As I watched it very carefully on the BT BMS app and as soon as it reached 100% the inverter stopped charging. Not sure if it was coincidence. The BMS does not disable charging  though I can still manually charge it up to 14.4V+ manually if I give it the voltage. I will hook up my oscilloscope next time and see if the battery does something weird to cause the inverter to exit charging prematurely or if it's just the inverter itself prematurely entering float.

 

it's not that I'm desperate to charge at 14V or higher. I just want the settings to work as intended.

Edited December 7, 20232 yr by zylinx

5 minutes ago, zylinx said:

Also I noticed the charge current is not available in QPIGS command.

Yup. QPIRI also has nothing, nor do other commands.

 

6 minutes ago, zylinx said:

As soon as it reaches around 13.7V to cuts charging regardless of the set bulk charge voltage.

So here I noticed when I bumped up my bulk charge V to 28.8 from 28.4, it cut out at a higher point (27.84 vs 27.5ish). But the drop after is very quick - no lingering at absorb.

15 hours ago, jbroo said:

Quite correct, yes, thanks.

That then begs the question, why are we all so desperate to charge at 14.4V and higher?

Image source

I for one have no problem to charge only to 13.7 while under charging condition and the batteries settle to 13.4V when charging stops. I don't cycle a lot. Currently only for LS and then a pedestal fan 24/7. Normal discharge is to about 13V the next morning. 

Only 11 cycles in 5 months. 

Edited December 8, 20232 yr by Scorp007
Added stats pic

yeah I've come to see the benefits of charging to lower SoC's and even more so floating at far less that the sticker recommendation. 

But as I say again at this point I'm just more curious about why its behaving this way even if its benign. 

It seems very coincidental that literally within the second the internal BMS ticked onto 100% SoC the inverter miraculously stopped charging. despite the inverter and BMS having no communication link, and the internal BMS still allowing a charge if voltage were presented. 

Edited December 7, 20232 yr by zylinx

14 hours ago, jbroo said:

@jumper are you running LFP batteries?

Edit: stupid question. Of course you are. 🙃

I'm actually running both... I have my 3kVA 24V running on LA which is where I first noticed the premature float bug, that inverter needs the float pushed up near bulk as it sometimes goes straight to float after bulk and sometimes it only charges to float and the firmware is ancient so has no equalization.

I also have a 5KVA King which I'm running on a 5kW shoto with seplos BMS and on that one I use the bulk charging time set to 120mins at 56.4V and equalization every 5 days at 57V just to make sure it hits 100% as I have solar only and bad weather might mean the battery doesn't get fully charged... I want to avoid SOC drift. This is where I noticed that the BMS switches on and off when it's top balancing causing the current to go to 0 and the mppt will ramp down to around 200W while it does this. Once the BMS hits 100% it disconnects the cells from the DC bus and they settle, so the inverter voltage doesn't matter at that point anymore.

13 hours ago, zylinx said:

The BMS does not disable charging  though I can still manually charge it up to 14.4V+ manually if I give it the voltage.

It is possible to raise the voltage once the BMS is at 100%, but it is likely not actually charging the cells, merely raising the DC bus voltage on the BMS.

2 hours ago, jumper said:

It is possible to raise the voltage once the BMS is at 100%, but it is likely not actually charging the cells, merely raising the DC bus voltage on the BMS.

Here's an image to illustrate my point. There is a difference between the bus voltage and battery voltage once the battery hits 100% and disconnects the charging circuit from the DC bus. It will only allow charge into the cells once the SOC hits 96%, which can be set in the BMS.