January 6Jan 6 Hi All,I’m hoping someone on this forum can provide advice… I’m experiencing a strange problem where, just after battery SoC has reached 100%, PV voltage is still OK but PV current drops to zero.Some background on my setup. Synapse 5.0K+ inverter and Synapse 4.8kWh battery with 10 x 420W panels in a 2S5P configuration. System is managed by Solar Assistant.According to the datasheet & label on side of inverter the MPPT specs are as follows:Min Solar Voltage 40VdcMax Solar Voltage (Voc) 145VdcMPPT Voltage Range 60-115VdcRated Current 80AFirmware version 71.97Model name KING-5000Model number 35Solar panel spec is as follows:420W monocrystallineMaximum Power Voltage (Vmpp) 42.50VdcMaximum Power Current (Impp) 9.89AOpen Circuit Voltage (Voc) 50.50VdcShort Circuit Current (Isc) 10.54AInstallation is 10 x panels in a 2S5P configuration. This should give me a theoretical:Total max power of 4200W (slightly over the 4000W rating, but well within the 15% over-rating rule of thumb)Open Circuit Voltage of 101Vdc (well under the rated max of 145Vdc)Max Power Voltage of 85Vdc (within the 60 – 115 Vdc range)System has been working without problem since August 2023.Last week something strange happened…Solar generation started as normal in the morning. PV voltage and current increased and batteries started charging. Once the batteries reached the required SoC the system switched to SBU as it should.Once the batteries reached 100% SoC the strange behavior started.PV volage is still OK, but PV current drops to zero.There are a few spikes of PV current, but not for very long.PV current then drops to zero and stays there. PV voltage is still OK.After a while the battery obviously discharges to the level where Solar Assistant switched back to SUB / Grid mode (as it should)I was out of town so couldn’t do much investigation.The next morning solar generation started as usual and has been working OK for the last few days….until today when it happened again.It’s almost like the MPPT can’t find the max power point and then rather just shuts down.Has anyone experienced this before?The really strange thing is that it did something similar yesterday (I only noticed it today). There are the same PV current drops just after the battery reaches 100% SoC, but then it continues as normal after. Other days don’t have these PV current drops.Any help of advice would be greatly appreciated! Edited January 6Jan 6 by HandySmurf Firmware version added
January 6Jan 6 Author Additional information. I quickly popped home to see if I can see any obvious errors, start troubleshooting etc.I pulled all the PV fuses in the combined box. PV voltage dropped as expected. After a minute or so I started reconnecting (closing fuses) one string at a time. PV voltage increased as expected. Shortly thereafter I could hear a distinct click coming from the inverter (sounded like a relay engaging) and then the PV current started to increase. Battery started charging (ie system operating as usual)Ignore the change in load - that's just me switching off the pool pump.PV voltage dip when I pulled the fuses. PV voltage and current increase shortly after I reconnected.Don't read anything into the system is only generating +- 1.5kW PV -> that's due to the limiting the maximum battery charge current / power to 30A.Once the battery reached 100% SoC the SCC started behaving strange again (not the dips below), but then stabilized.I'm on firmware 71.97. I was under the impression the SCC firmware issue described below was resolved on 71.94 and above.What is also strange is that the system has been working 100% OK for the last 2+ years. Edited January 6Jan 6 by HandySmurf
January 7Jan 7 This is the freeze bug that the axperts are known to have. The kings seem to be particularly problematic. My company has had the factory send us adjusted firmware for the SCC but we were never able to eliminate the issue at some sites and had to change inverter models. As far as we could tell the problem is triggered by the inverter seeing the battery voltage rise quickly over the bulk charge voltage so the solar power is dropped to 0V and the MPPT tries to reset and match solar power with load. If this happens too often it shuts down in an 'error mode' and only resets when the panels are turned off i.e. the MPPT voltage drops to 0. The sharp voltage "knee" of lithium cells when cells are full plays a large part. Often the BMS has gone into a protection mode because of a high cell which means the inverter suddenly has to dump energy. Looking at your data I would guess you have cell balance issues. The triggers seem to most often be: 1) Cell imbalance triggering BMS to go into alarm and disconnect from inverter. Terminal voltage shoots up as a result. 2) A load switching off when the battery is close to the bulk voltage. The SCC doesn't ramp down in time and the voltage rises quickly/BMS goes into alarm 3) A sudden shift in MPPT power caused by a cloud leaving the sunlight and ramp down doesn't happen fast enough.Are you using communications between the Synapse battery and the inverter? The Synapse's are rebranded pylontechs and coms should be possible. The BMS should ramp down charge current when almost full and prevent this happening.Methods that may assist: 1) Reduce battery charge current so the battery has longer time to balance 2) Reduce bulk charge voltage down 0.2V-0.5V so the battery doesn't sit right at the top end of cell voltages, it should still have +/- 95% stored charge
January 7Jan 7 Author Thanks for the information and help @Shadders ! It's appreciated.It has happened 3 times in the last week. Each time just after the battery reaches 100% SoC. Load remains fairly consistent during this time.There isn't direct communication between the inverter and the battery. I use Solar Assistant to manage the system - Solar Assistant sits between the Inverter and the Battery.Max Charge current is currently set to 30A. What would you recommend I reduce this to? 20A?I'm only able to adjust the Battery float charge voltage (currently 53.0V) and the Battery absorption charge voltage (currently 53.2V). I don't see the Bulk charge voltage.Looking directly on the inverter parameter 26 is set at 53.2V so it seem like Solar Assistant uses Battery absorption charge voltage also for Bulk.What would you recommend I adjust these voltage to?Battery float charge voltage to 48.8V and Battery absorption charge voltage to 53.0V?Additional information regarding cell balancing. I can see the following information on Solar Assistant.I've seen some fluctuation, but the difference between highest and lowest cell voltage stays under 50mV (typically 30mV or under as seen below).Would this indicate an imbalance or is this normal? From what I found in previous posts this seems OK. Edited January 7Jan 7 by HandySmurf
January 8Jan 8 21 hours ago, HandySmurf said:Max Charge current is currently set to 30A. What would you recommend I reduce this to? 20A?20A should be okay but 30A isn't terrible. it would be better if you could get the communication between the battery and inverter done so that the BMS can dynamically vary the current. Typically it will drop the requested charge current down to 1 or 2A when the battery is almost full21 hours ago, HandySmurf said:I'm only able to adjust the Battery float charge voltage (currently 53.0V) and the Battery absorption charge voltage (currently 53.2V). I don't see the Bulk charge voltage.Looking directly on the inverter parameter 26 is set at 53.2V so it seem like Solar Assistant uses Battery absorption charge voltage also for Bulk.What would you recommend I adjust these voltage to?Battery float charge voltage to 48.8V and Battery absorption charge voltage to 53.0V?Absorption is bulk charge yes. I'd recomment you try float to be 52.7-52.5V and absorption 53.0-52.8V....https://powerforum.co.za/topic/4925-best-charge-settings-for-pylontech/#findComment-197229 might be helpful22 hours ago, HandySmurf said:I've seen some fluctuation, but the difference between highest and lowest cell voltage stays under 50mV (typically 30mV or under as seen below).Would this indicate an imbalance or is this normal? From what I found in previous posts this seems OK.These are pretty good. Pylontech max should be 3.54V per cell and looking at your values you are close to this, it's probably only 1 cell right as you first reach 100% that is going over the alarm threshold. Dropping the absorption should help. Edited January 8Jan 8 by Shadders spelling mistakes
January 8Jan 8 Author Thanks @Shadders I'll give those voltages a try. I changed to the following FYI:I've read though various posts to find the correct pin-outs, but there's a lot of contradicting info. I understand I need to change the battery DIP settings to 1-0-0-0 to change baud to 9600, restart the battery and then change parameter 5 on the inverter to PYL.Axpert King 5000 to UP5000Can you please assist with the cable pinouts? This is what I found:I also found the following: Edited January 8Jan 8 by HandySmurf
January 8Jan 8 On 2026/01/06 at 1:25 PM, HandySmurf said:Hi All,I’m hoping someone on this forum can provide advice… I’m experiencing a strange problem where, just after battery SoC has reached 100%, PV voltage is still OK but PV current drops to zero.Some background on my setup. Synapse 5.0K+ inverter and Synapse 4.8kWh battery with 10 x 420W panels in a 2S5P configuration. System is managed by Solar Assistant.According to the datasheet & label on side of inverter the MPPT specs are as follows:Min Solar Voltage 40VdcMax Solar Voltage (Voc) 145VdcMPPT Voltage Range 60-115VdcRated Current 80AFirmware version 71.97Model name KING-5000Model number 35Solar panel spec is as follows:420W monocrystallineMaximum Power Voltage (Vmpp) 42.50VdcMaximum Power Current (Impp) 9.89AOpen Circuit Voltage (Voc) 50.50VdcShort Circuit Current (Isc) 10.54AInstallation is 10 x panels in a 2S5P configuration. This should give me a theoretical:Total max power of 4200W (slightly over the 4000W rating, but well within the 15% over-rating rule of thumb)Open Circuit Voltage of 101Vdc (well under the rated max of 145Vdc)Max Power Voltage of 85Vdc (within the 60 – 115 Vdc range)System has been working without problem since August 2023.Last week something strange happened…Solar generation started as normal in the morning. PV voltage and current increased and batteries started charging. Once the batteries reached the required SoC the system switched to SBU as it should.Once the batteries reached 100% SoC the strange behavior started.PV volage is still OK, but PV current drops to zero.There are a few spikes of PV current, but not for very long.PV current then drops to zero and stays there. PV voltage is still OK.After a while the battery obviously discharges to the level where Solar Assistant switched back to SUB / Grid mode (as it should)I was out of town so couldn’t do much investigation.The next morning solar generation started as usual and has been working OK for the last few days….until today when it happened again.It’s almost like the MPPT can’t find the max power point and then rather just shuts down.Has anyone experienced this before?The really strange thing is that it did something similar yesterday (I only noticed it today). There are the same PV current drops just after the battery reaches 100% SoC, but then it continues as normal after. Other days don’t have these PV current drops.Any help of advice would be greatly appreciated!I wiil add my 2 cents here. @Shadders advice is spot on about Voltronic type,clones in the sense of Pv braking in sudden load or dramatic pv(cloud-edge) changes in the upper knee charging( absorption voltage) of lfp batteries.( Only Scc that is spot on, don't overshoot is Victron MPPT'S)@HandySmurf What i have observed in your SA graphs posted there is a problem with the actual cell performance of your battery module not so much your bms or solar charging. I do not believe there is much difference using inverter settings or Bms control in the sense of basic control. I don't think bms communication is going to solve your problem. You have a 16 cell 1C 51.2v battery charging way below the OVD of your battery Bms. If you look at the discharge load( 0.4C) vs battery voltage the sudden drop in battery voltage is not inline with a 1C cell. Your battery should be capable of handling 100A discharge for at least 1 minute without drastic voltage sag.
January 12Jan 12 On 2026/01/08 at 7:08 PM, TaliaB said:You have a 16 cell 1C 51.2v battery charging way below the OVD of your battery Bms. If you look at the discharge load( 0.4C) vs battery voltage the sudden drop in battery voltage is not inline with a 1C cell. Your battery should be capable of handling 100A discharge for at least 1 minute without drastic voltage sag.This is a rebranded UP5000 not the UF5000 so it is a 15 cell 48V battery. With a nomimal cell voltage of 3.3V/cell this works out to 49.5V which is consitent with the displayed graphs and within specification of the battery.The underlying issue I have fairly certain is the BMS going into alarm because of a high cell. To help the battery balance adjusting the inverter settings and using dynamic charge current control will assist with this.
January 12Jan 12 On 2026/01/08 at 12:58 PM, HandySmurf said:I also found the following:This is the correct pin out to use. Pin 3 on the inverter should go to pin 8 on the battery and likewise pin 5 on the inverter to pin 7. DO NOT connect any other pins on the battery side to any wires. We have found that if the battery side connector has wires connected to pins 1-3, even if they are not connected at the other end the coms will not work.
January 14Jan 14 Author Quick update. After reducing the charge voltages the error hasn't re-occurred. Happy days.Next step is to get comms going between the inverter and battery. I want to do this in any case as it seems to be the best long term solution.There are still random instances where PV generation drops to zero (even though it is sunny), but so far generation has always resumed again shortly after. I haven't been able to identify a pattern. Sometimes this happens with load change, but other times not (example below). It would be great to get this resolved, but it doesn't really seem to be a significant problem as generation recovers shortly thereafter. Unless I'm missing something?Thanks for all the help!
January 14Jan 14 1 hour ago, HandySmurf said:It would be great to get this resolved, but it doesn't really seem to be a significant problem as generation recovers shortly thereafter. Unless I'm missing something?If it recovers that's all that is required to keep the system going. I'm glad it is ticking along better.
January 14Jan 14 1 hour ago, HandySmurf said:Quick update. After reducing the charge voltages the error hasn't re-occurred. Happy days.Next step is to get comms going between the inverter and battery. I want to do this in any case as it seems to be the best long term solution.There are still random instances where PV generation drops to zero (even though it is sunny), but so far generation has always resumed again shortly after. I haven't been able to identify a pattern. Sometimes this happens with load change, but other times not (example below). It would be great to get this resolved, but it doesn't really seem to be a significant problem as generation recovers shortly thereafter. Unless I'm missing something?Thanks for all the help!When it happens after load change. Is it when drawing a load or when switching off a load?
March 2Mar 2 Author Quick update on this. I managed to get comms going between the inverter and battery. I can now see the charging current changing dynamically depending on state of charge (lower SoC higher current and vice versa). Charging voltages were also changed when I switched over to PYL.Error with the solar generation stopping completely has gone away. Happy.Thanks for the support!
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