Hi Fellow Solar-nauts,
I installed a new solar system a few months ago consisting of 10 x 560W Jinko (model JKM560N-72WL4-V) panels connected in series, an all-in-one 6KW charger-inverter from MPP Solar (PIP6048MT) connected to 2 x 48V 200AH LFP batteries (Blue Carbon) connected in parallel.
The system has basically been running OK, with one exception, i.e. the DC current output from the panels into the MPPT charger maxes out at only 5A or less, even in direct sun. It does not seem to matter if the battery pack voltage is less than 50V or higher. The DC voltage output from the 10 panels ranges from 450-470V, which suggests that the panels are connected in series ok, since the rated DC voltage of the Jinko panel is 49-50VDC.
This model Jinko panel is rated for a max DC output 13A, so I’m puzzled as the possible causes for this low current output. I’m wondering if ‘current backflow’ where the current from one higher voltage panel is ‘flowing backwards’ into a lower voltage panel? I assume such an issue would reduce the overall current output of the panels? Would inserting diodes in-between each panel help?
Another thought is that the problem may be with the MPP 6048MT charger circuit? Is there some way to check the MPPT charger ‘upper cut-off’ setting?
This low DC output issue is not yet critical, as the continuous load is currently less than 1KW, however, I plan to add some pumps and fans (the system powers a large-ish greenhouse) in the near future, so I want to resolve this mystery to avoid running down my batteries under the future heavier load.
Any advice, suggestions from Marty, Jason or any of the more experienced members of the community would be much appreciated. Thanks!
@chewface, if your string voltage is correct, then it sounds to me like the MPPT is not functioning correctly. It is not tracking the correct MPP voltage for some reason. If you have a DC clamp meter, then you could do a short circuit current test to test the string Isc.
The MPPT range of the PIP6048MT is fine for a string of 10 panels, but the Voc limit of 500V it is not suitable. You could easily go over the 500V limit on a cool sunny morning when the panels are below 25degC. I wonder if you have already exceeded the 500V limit, and perhaps this is causing the problem.
What is the string operating voltage (Vmp) when it is running at 5A?
Thanks for your reply.
I think you may be right about my string voltage being intermittently too high. The MPP6048 has unexpectedly shutdown several times over the past few months showing an “F07 Overload” alert. I thought this meant that the AC current being drawn from the inverter was too high, which was another puzzling issue, as the current rating for the appliances connected on the AC side of the inverter typically totals less than 1 Kw. I’ve been hunting for an intermittent AC short circuit as the culprit, but maybe it was an over-voltage issue…
I am thinking about reconfiguring my 10 panel string into 2 x 5 panel strings. My only concern is that the rated max current output of this panel is 13.4A, which would mean that 2 strings in parallel could produce a max output of 26.8A. The rated max input current for the MPP6048 is 27A which is very close to the max potential current output of the 2 strings.
So my question is whether (potentially) ‘going over-current’ is ‘better/safer’ than ‘going over-voltage’? If both are equally bad then I guess the best/least worst solution is to remove one panel from the 10 panel series string? Your thoughts/suggestions?
Your advice and comments are much appreciated.
In my experience, overvoltage is far more likely to occur and more damaging than over-current. I agree, I think dividing the strings into smaller strings of 5 panels is a good idea. You need perfect conditions to produce maximum current which is rare, but maximum voltage (Voc) can easily exceed 50V on any cold day.
I believe the overload alert would have been due to the high string voltage. You are lucky it did not destroy the unit.
I re-wired my 10S panels into 2 x 5S strings. However, I noticed that the amount of current from the 2 parallel strings dropped significantly to less than 0.5A (solar array voltage was displayed as 230-240V on the MPP6048) compared to another separate solar system with 9 x 450W panels in-series, which was generating 2A under exactly the same conditions.
I suppose this makes sense since my understanding is that one of the advantages of MPPT chargers is that they convert higher voltages into charging current. However, since I’m interested in maximizing the charging capacity of my LFP batteries, maybe it would be better to revert to a single string with only 9 panels in order to produce more current? Your thoughts?
Thanks again for your guidance.
Hi Jason & Interested Solar-nauts,
Since re-wiring the panels into 2 x 5S strings, I’ve been experimenting with the new solar config and discovered something interesting that seems to explain why my panels sometimes generate such low current.
If I completely shutdown the system i.e. disconnect the DC breakers to BOTH the solar array AND the batteries, as well as turn of the inverter and then turn everything back ON, the current displayed flowing into the MPP6048 increases significantly i.e. from 2.5A to 11+A (it’s bright overcast today). I did the same thing yesterday when there was full sun and the array current output jumped from approx 4A to 20+A !!
My hypothesis is that because the MPP6048 does NOT communicate directly with the BCT (Blue Carbon) LFP batteries (there is no comms port avail on the BCT) that unless the battery voltage drops close to the low voltage cut-off setting (which is set to 48V), the MPP6048 ‘thinks’ that the battery is not yet sufficiently discharged to initiate a new bulk charge cycle and thus only allows float charge current to flow to the batteries.
This theory seems to make sense, as I noticed that on the other solar system, installed on our main house,(which uses 2 x MPP5048MKX charger/inverters and 200AH 48V BCT batteries) that I also have to ‘manually force’ a new ‘bulk charging’ cycle, if the battery voltage does not drop below 51V (same 48V low cut-off voltage setting), by disconnecting the DC breakers to the solar array and then reconnecting the array. The array output current jumps from 2-4A to 11-25+A (there are 25 x 450W panels in this array).
I guess this is the downside of buying charger/inverters and batteries that don’t talk to each other. Not a biggie for me, as I am retired and have the time and interest to check our solar systems multiple times a day but could be a hassle for others with less time or patience.
Anyways, hopefully this may be of interest/help to others who are planning on buying solar chargers and batteries.
BTW, The system did not shutdown from an ‘Overload’ yesterday when there was full sign, so the reconfiguration into 2 parallel strings seems to have solved that problem. Thanks for your help.
Good work troubleshooting, it sounds like you have identified the problem. However, I think the true problem is that the MPPT algorithm is not programmed for lithium batteries. Unlike lead-acid, lithium LFP batteries have a very ‘flat’ discharge curve. Self-managed lithium batteries with no comms connection to the inverter work perfectly fine when they are programmed correctly for LFP. For example, Victron MPPTs have no problem working with any type of lithium battery and can be programmed to rebulk at any level you choose.
Can you update the firmware in the inverter and MPPT? This is one of the reasons why I generally do not recommend the MPP series all-in-one inverters. I know they are much more affordable, but the software is not very good.
Agree with your assessment of the MPP software not being very good. There is are settings for lithium batteries, but I learned (the hard way) that if I select the setting for “3rd party lithium battery” the system generates a “loss of comm’s” alarm and shuts down. So the only setting that works (sort of) is the “self defined” battery type. However this requires setting both the ‘High’ (bulk charge) and ‘Low’ (switchover to utility) Voltage cut-off values. This comes with its own set of issues such as having to ‘manually force’ the bulk charge cycle if the battery voltage does not fall ‘close’ to the low voltage cut-off setting. I asked MPP CS what is the ‘trigger voltage’ for initiating a new bulk charge cycle, but the CS guy wasn’t able to tell me. They also did not offer me any software updates.
To be entirely objective, both the MPP 5048MKX and 6048MT charger-inverters are quite robust and I have not (yet) experienced any serious operating outages. My 5048-based off-grid system went live in May 2022 and Nov for my 6048 system. Given their affordable price point, they represent good value for DIY solar-ists like myself who are on a budget and are willing to ‘manually manage’ their battery charging.