Charge starter battery from 48V bank via MPPT 75/12 - Error 28 on separate MPPT 150/45

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eyacht asked Mar 27, '23

Charge starter battery from 48V bank via MPPT 75/12 - Error 28 on separate MPPT 150/45

Hi all.

Yesterday I installed a new MPPT 70/10 to charge lead-acid a start battery from a lithium 48V bank (540Ah). I connected the start battery to the 12V battery terminals and checked the auto-detect in the VC app - all was fine. I then connected the load terminals to a 12V load buss and finally connected the 48V power to the PV terminals. All seemed fine and the start battery went through bulk-abs-float no problem.

This morning I got an Error 28 alarm on one of my other MPPTs - this one has 2x24V panels in series going into the PV terminals and the output goes to the same 48V buss that the new MPPT takes power from. PV on the old MPPT was over 80V - the other 3 MPPTs for my solar set up read from 55V to 67V as normal (72 cell panels). This should be easily handled by a 150/45 MPPT.

Seems odd that it was fine at installation (late in the day & not much solar input) and then caused an error when the sun got higher in the sky this morning. For completeness, there are actually two solar MPPTs in this circuit - the other one, connected to the same buss and set up the same way, seems fine with no errors and normal voltage, currently around 63V.

Any thoughts on what might be happening?

Also wondering whether the Load Output should be set to Always Off in this configuration since the intent is not to use it to help with normal loads - it's just being used as a charger. Is that what this setting does? And if so then is this the same as just not connecting it to the load buss?

Thanks in advance.

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Answer 1 · 2023-03-28T06:16:44Z

kevgermany answered · Mar 28, 2023 at 06:16 AM

First thought is that you're using the wrong unit to charge the starter battery, an Orion would be suitable.

Does the over voltage go away when you remove the starter charging MPPT?

Check the high PV input voltage with a meter, if it's normal, then you may have a faulty MPPT that's coincided. Could also be a cabling error.

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eyacht commented · Mar 28, 2023 at 04:10 PM

Hi Kev.

If you mean an Orion DC/DC converter, the tech specs specifically say not to use them for charging; they do not have a charge profile, whereas the MPPTs do.

I undid the installation yesterday afternoon and reset the MPPT that had the Error 28 fault and everything seems back to normal again, so it's definitely "something" about the installation but I just can't figure out what's wrong with it.

I did check the PV with a meter and confirmed the app's reading. It is now back to mid-60s but the history shows it does occasionally spike to 80V, however, it was reading a pretty consistent 80-ish volts rather than a spike.

I'm wondering if I shouldn't just tap into the panel output rather than put my 48V bank into the PV terminals but spiking to 80V would likely damage my 70/10 MPPT.

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