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chbrandt avatar image
chbrandt asked

MPPT Going to Float too Early

So a bit about my system first. I’m using a 50ah LiFePo4 battery with an Overkill Solar BMS and the 75/15 VictronConnect MPPT charger. I have a 120w solar panel that, earlier this morning, was pulling 100 watts as needed. I’m using this mostly for camping and occasionally charging/running things in the back yard and driveway at home. My system is most certainly not hooked up on a daily basis, as it seems many are here in the forums.


Prime example today, I’m camping and after running my battery down some last night, I plugged it in this morning and it quickly charged up by 11am. Since then, I’ve been charging devices, running a small fan, etc. and the batter is now down to 83%. Since it’s already in Float, I can’t for the life of me get it to go BACK into either bulk or absorption and my battery just continues depleting. I’d like to be able to use it a lot more today, and tonight, but without being able to keep it topped off, I have a problem. I’m attaching two screenshots showing the side by side view with my BMS view on the left and my MPPT on the right, plus the settings screen on the MPPT. I’m pretty new to this, and the only setting I’ve really changed from the default LiFePo4 profile is to bump the Absorption voltage up to 14.6, as that’s ideally what I’d like to get charged up to. Please help! Thank you!

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MPPT Controllers
eea30cbc-a7b4-4c0d-8834-da1189979e30.png (744.2 KiB)
a92f5a6b-a21e-47a4-a08b-0aaba80b71e9.png (764.1 KiB)
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squowse avatar image squowse commented ·
Those BMS have an annoying habit of setting the SOC based on voltage. You have to take it from full to empty then back to full again to initialize coulomb counting. Easiest way to trigger 100% and 0% is to let it trip the BMS on cell over and undervoltage.


This may not be your problem but it's worth knowing.

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wkirby avatar image
wkirby answered ·

There is a discrepancy between your BMS' reported Voltage 13.19V and the MPPT reported Voltage 13.49V. This suggests that the battery is not connected to the MPPT for some reason, an open fuse or a bad connection.
As a test, disconnect the PV and see what battery Voltage the MPPT will read then, I suspect it will drop close to 0V if my theory is correct. f so, then you know you have a bad connection, open fuse, or the BMS is not allowing charge. Although, it looks like the BMS is allowing charge by the blue switch.

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chbrandt avatar image chbrandt commented ·

I’ve wondered about that discrepancy since I set this system up a couple months ago. I did the test, and you were right, it did exactly as you predicted. I had about 44 feet of wire between the MPPT and the battery, whose resistance accounts for some of that difference. I went ahead and moved the MPPT to the end of the wire nearest the battery connection and now have only about 4 feet of wire (12 gauge), so much less resistance. When charging at roughly 3A, I‘m still seeing a difference of .1V between BMS/MPPT, and when I’m charging up around 6A it doubles to .2V difference. My best guess is that these differences are being caused by resistance in the Anderson connectors I’m using.


So, the question becomes, with my current setup, how can I better configure the MPPT to charge the batteries up all the way as expected? Should I change the absorption voltage to 14.8 and count on the BMS to protect the batteries? I’m struggling to see what other settings could be changed to potentially account for the reading differences. Thanks so much for the assistance! I’m attaching a picture to show the setup here in the Colorado mountains; my battery is built into my telescope and isn’t visible here.

20220529-145023.jpg

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chbrandt avatar image chbrandt commented ·

@WKirby Curious if you have any additional guidance. Thanks!

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