Hi, I have a Victron MPPT 7510 with a [KEPWORTH 24V 100Ah LiFepo4 Battery, Rechargeable Lithium Batteries with 100A BMS, UP to 4000+ Deep Cycles]
History: This was originally running with another manufacturers MPPT and the same load. The load would run 2 to 3 days easy in cloudy weather. When charging off of mains with the charger that came with the battery it would measure 27.75V at the terminals.
There was a solar charge issue so, based on the solid reviews of Victron I switched the MPPT out. Using the default Lifepoe4 selection in the Victron app the battery is only charging to 26.57V.
I emailed the battery manufacturer with no success as there is limited information for the battery available. The one piece I did find states the following:
Capacity: 100Ah, Nominal Voltage 25.6V, Max Protection Voltage: 29.2V, End Voltage 20V BMS: Yes.
The battery was new and it has been in service for about 6 months now. I’d like to nudge it closer to 27.75V. Any recommendations. Other that preventing a fire I’m okay risking it’s overall lifespan.
The default settings for lithium should charge to 28.4V and then go to float at 27.0V when the battery is full.
When you say the battery is charging to 26.75V, where is this measured, at the battery terminals, the battery BMS or the MPPT battery output.
If the MPPT battery output is only getting to 26.75V then it is likely you do not have enough solar to get it fully charged, look at the current flow and power compared to expected. As the system has been disturbed have you made sure your solar panel wiring is connected OK.
If the MPPT battery output is getting to 28V then you may not have connected the battery cables up properly when you put the new MPPT in and you have a high resistance causing voltage loss.
I appreciate the response. The 26.75V is the reading of the MPPT via the app. There are two (2) BougeRV 24V N-Type 16BB 100W Bifacial Solar Panel that have an open circuit voltage of 35.7V each.
I’ll back track and check the resistances and verify all the connections. The load is connected to the load terminals of the mppt and they are set to via the “User def. algorithm 1” to Load switch low voltage level 23.3V and load switch high voltage level of 25.90V. As a side note; even with the load off the system indicated a top charge of 26.75V for the battery.
The first sentence is unclear, the 26.75V is the battery reading via the app and the 2 solar panels are in series showing over 50+ volts on sunny days even with shading. The graph over 2 days shows it reach 26.75V where it looks like it floats with a short valley going back to 26.75V at which point it slowly drops from the evening on. If it was a solid sunny day it would make it to the next day without the system going offline.
Seeing as you have removed an mppt and replaced it with the 75/10 Victron, your cabling (or more specifically the terminals) might not be as tight or well fitting as with the last controller. Even if you had not changed controllers, the advice would still be to recheck the terminals on the MPPT - its a comically common issue.
Also, your data (26.75v) is from the app which means its measured by the Victron at its own terminals, so run this check;
- Check your voltages at every step from the MPPT to the battery - report back on what voltage the MPPT terminals are, what the breaker terminals are, what the busbar terminals are, what the battery terminals are, etc. Basically test every exposed piece of metal from the MPPT to the battery.
- Note what the current was when you tested.
- If the first reading (at the mppt terminals) does not match the app’s value, either your meter, or the MPPT, are wrong and have an offset - this is rare, but from memory there is a calibration you can do on the MPPT.
- If any of these points are more then a few mV different than the previous one, investigate that connection. Some installers polish their lugs to make sure they get a super-low resistance connection. Pay particular attention to the mppt terminals - depending on whether you use ferrules or not, and depending on the number of conductors in your battery cable, you can get a wide range of final resistance values here, and they are harder to get right than a lug.
- 28.4v (the default LiFePO4 setting in the app) and 26.75v (the actual measurement) are out by 1.65v - this is very high. Once you get this down to <0.1v, you can move on to other tests.
Don’t rely on the panel voltage to tell you much - that only shows the panel (or array) isn’t open-circuit.
A panel can easily be sitting at its full Voc, but with a mostly broken wire inside it will generate almost no current.
The fact that it worked before replacing the previous MPPT should be discounted, because the reason for the replacement might have been related. Assume the panels are bad, and re-commission them;
- Do an open-circuit voltage test - this should equal the panel Voc x 2 if your sunshine is decent.
- Do a short circuit current test - if you don’t get Isc (short circuit current) X 1 (because your panels are in series) then you have a bad panel, bad MC4, or bad cable, or bad isolator. This could be the cause of the fault, or could be an additional fault.
You don’t say why the previous mppt failed, so we have to presume it might not have been the only fault.
Some quick check of the numbers - 2 x 100w panels = 200w, at 24v = ~4A, so your controller is well within its current specs even if you were to consider a bifacial boost of 40%.
Your Voc is a little tight - STC Voc of your panel is 35.7v, x 2 = 71.4v, on a controller with a Voc limit of 70v.
You are relying on not getting STC conditions, and never having a cold clear morning.