Hello,
I just curious about something and if anyone can suggest a fix.
I’ve noticed that when I added an extra MPPT controller to an additional solar array that when there is heavy demand on the solar, eg, A/C unit drawing and or water heater eg 1- 3.5kW, the battery voltage drops slowly below the re bulk voltage threshold I set it to, which is 0.4 volts as you would expect. (total of 21kWH of lithium). So if the float voltage is 54.4 and it drops below 54 volts for more than a minute, the charge cycles start again, including absorption before going back to float. I’ve noticed it doing this about 3 or 4 times today with each absorption cycle running for about 10 minutes.
I’ve tried setting tail current to 1 amp and also turned it off, but this obviously ends the absorption cycle not starts it. I reduced the re bulk voltage offset to just above the nightly minimum it gets to, which is about 53.2 volts and this seemed to correct the issue, but it’s way outside what the battery manufacturer recommends, which is 0.4 volts (54 volts).
Interested to know if this is normal and can’t be helped on these sort of days with intermittent cloud cover or is there a parameter that can be changed to enable only one fresh charge cycle to be performed daily.
It’s just having the battery voltage pumped up to 57.6 volts multiple times per day is a bit concerning and seems to be not the best way to run things as cell balancing like this is clearly not required at this frequency.
This is normal there is not much you can do. The best thing to do is lower the charge voltage. I charge to 56.5V. You can try 55V or lower if you can. The problem you really have is losing solar going from the absorption voltage down to float. It does this by turning off the charge controllers. If you are grid tied the energy is dumped into the grid. You can set a time once a week or month and make sure you get a good balance by making sure the loads are small and the sun is out.
Thanks. I was thinking I’ll have to live with it. I’ve set the float charge voltage to the lowest set by the manufacturer at 54.4. I guess the key is trying to reduce big loads too often. If only the 1 minute timer of the re bulk voltage offset could be adjusted. I might drop it a bit more and see how it goes.
I do all my big loads at night. My battery is sized so I only need 70% SOC at night. The last month I have only hit 100% 3 times.
If you have a Cerbo GX and want to use node red might be able to do something. Maybe set a count for how long the absortion phase has been running and diasable the charge controller when the voltage gets high after already absorbed.
Let me know if you want to have a look a that might be useful for someone else as well.
Hi there,Ive got more or less the same problem with my multi rs solar:Once the main charge is finished,the mppt’s shut completely off until float is reached.Regardless if there is a (heavy) load or not.I’m off grid and would like to have my batteries left alone as long as poss…Maybe this would be right for a future request?NOT to switch off MPPT’s when any load is registered-without charging the batteries,by all means…Help please!
You could always increase the rebulk offset… Since that is what tiggers the rebulk. And you clearly don’t want that.
The other way is to out a node red flow on and cap the voltage in DVCC during certain times of the day (then obviously removing the cap at other times)
If the manufacturer recommends a full charge (to balancing once a month) it is easily do able.
Thanks for that. Had a look at the red node thing. It looks a bit beyond me, but certainly an option if this frustrates me more. My system is completely off grid and yes have the Cerbo GX. I’ll try and do as you suggested and run as many loads as I can when the solar is off or low first thing in the morning if battery capacity allows. Its a pity that Victron don’t program the MPPT to do absorption just once per day. I swear before I did the last firmware update, the controller only ever did it once per day regardless of what happened during the day with clouds and varying loads.
Much appreciated.
I’m still working out the pros and cons to increasing the re bulk voltage offset so that it waits for the battery voltage to drop further than 0.4 volts. Eg, setting to 53 volts instead of 54. To me it seems to make sense to have it just above the normal nightly voltage at it’s lowest point so that it only triggers a cycle reset once per day (unless a large sustained load is applied with little or no solar during the day). Obviously there is a reason that the battery manufactures specify 0.1 - 0.4 volts, which I’m guessing is so the system can charge up during the day if the solar input gets too low and will run a bulk cycle to get it back to float voltage again perhaps?
Even better would be to have the re bulk voltage counter set above 1 minute so small short drops in voltage don’t trigger the cycle to bulk again.
I am on the grid but the problem I have is a 5000w export limit per a phase. I have the capacity to charge my battery before 10am everyday but also have 2x 5000W grid-tied PV inverters that export 5000W between 10am and 3pm. When I first set it up the battery would be full at 10am be full all day until the afternoon when the sun goes down. We would attempt to use most the power during day, but it is inconvenient apart from some load shedding automation.
The battery has a cycle capacity of 8000 in 12 months we have recorded 156 cycles. I figure we should just use it, I think the damage caused by keeping it at a high SOC will be worse than the cycle life. Time will tell.
From all the studies I have read it shows cells degrade when cycling, but not when held at SOC (almost any soc) and if held at almost any voltage per cell in its range.
But that being said its thousands of cycles on lifepo4 cells. I just use mine however. Between load shedding for indefinite amounts of times and weather you end up however you end up. Have run over 5 years and not seen any degradation in capacity, the only thing we have been careful about is temperature.
Got a few friends who hammer theirs full to empty everyday and have been running years now (also being careful anout temperature) and also they haven’t seen any changes in capacity.
That being said i have seen cell failure in batteries, still convinced its because of poor bms management. (And the cells range from pylons pouches to winston thundersky.)
I am going to start hammering mine when we get paid for exporting during peak times, but at the moment, we still get good money for exporting during the day. This is what my usage looks like over a month. I’m adding 12-17 kWh of solar per day and using that overnight. I start charging at 10 AM to soak up the excess grid-tied solar.
I did do two fast charges where I put over 30 kWh in a day due to being at 5-20% SOC for too long. We have some good sun now, so I’m hitting 100% SOC in the evening every day this week.
I have killed a couple of pouch cell batteries. I believe this happened because they were 2P6S, and the BMS was disabling charge at 3.8V, which caused them to fail. I am now running JBD BMSs with prismatic cells.
Oof ouch.
Looks like it is certainly an issue for some of us. A good solution would be to just have it disabled going into absorption more than once per day. I’m sure the programmers have come up with doing it this way for some very good reasons I’m not aware of, but I’m yet to think of one. Going back into bulk and then stopping at float voltage to me would be a good solution, but without going into advanced configuration using third party software, it seems not an option.
wow, I can see lots going on with that graph and the batteries certainly aren’t even close to going into absorption and cell balancing very often. I don’t export one electron, so it sits on float most of the time once the batteries reach 100% in the mornings. Can’t say I miss the grid at all either. With the Australian Govt being obsessed with renewables, the cost of electricity for those on the grid is skyrocketing.
Hi @lxonline ,charging is one thing,using then the battery is another…If you are on off grid ( at least) you want to keep your batteries charged and then left alone,as long there’s solar power.I am left know with the situation,that batteries getting charged and discharged in a continuous manner .Altering the float,maybe to a higher one,is no option either,as it unnecessary stresses lifepo4 cells-to keep them at high level ‘charge’. I think it is time for a feature request : keep home loads been fed from solar,as long as solar is available …thanks
As an off gridder as well, i understand.
One user here Ludo has his set to a long absorption, that keeps the battery up is he sets a long absorption time.
The two things that age cellas are temperature and cycling. Keeping your battery from cycling reduces stress (which is what you want to do), holding them at higher voltages (such as the recommended absorption) also does not stress the cells out.
All the studies i have read and personal experience have shown the above to be true. There are several links here on the forum for them and a few statements from manufacturers such as pylontec. {I have battery banks in the field that are over 8 years old now so not short term experience.}
ok.thanks,in the meantime ,I’ve gone around with node red,to set the relays and with those the heating element of the boiler ,only in base of soc and battery discharge…
I understand your point ,maybe with a feature update it could be left to the user to decide,whether they want to keep the float at a higher point and therefore not having the batteries charging and discharging in a continuous manner .I’ve read an article ,where keeping them at a higher float and therefore as a higher soc will stress them…If I find the article ,I shall forward this to you…Anyway I think a feature update should be considered ,maybe:thinking:
Nice Sunday to everyone ,Frank
Ah here we are
battery aging .pdf (1.9 MB)
It’s mainly focused on lithium ,but I think we can agree,that the cell chemistry is similar…
It’s under the effects of aging: A high Soc comes with a low anode and a high cathode potential .The low anode potential is known to accelerate SEI growth,therefore a high SOC will accelerate cell degration.
I’m not quite sure,either how fare heat comes into question,with keeping it at a high SOC…
Anyway in the matter of doupt…
Best regards,Frank
So at the end of the day,keeping pv power on regardless of the SOC,would have two benefits(at least)
a) switching the cerbo relays for the benefit of a hot water heater,would be far more easy ,because the only parameter you would need,would be the pv power itself
b) The batteries would be left alone,as long there is enough pv power.
I urge again,please for a feature request,to have this at least looked into,
Frank
thanks Frank for all that info. I’ve got it working pretty good now. Hooking up via the VE Direct instead of the VE Can rectified the first issue of no absorption. Using the large loads early in the morning or at night stopped the repeated cycling due to large loads with clouds.