MPPT Limit Max Abs V after Temp Compensation in extreme cold?

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ghostrider asked Jan 29, '20

MPPT Limit Max Abs V after Temp Compensation in extreme cold?

I recently got my MPPT + BMV w/ Temp Sensor configured to match my battery specs. Some are:

Abs Voltage: 15.00V
Temp Compensation: -37.8 mV/*C
High Voltage Alarm: 15.3V
- Conservatively set...I think 16V is the operating limit of most of my 12V devices

Its winter here and it's bright(ish) but cool today in Michigan (33*F)... I noticed my High Voltage alarm had triggered and it was Abs charging at 15.5V off the panel!

I thought something was wrong and then realized that Temp Compensation is likely the culprit. Did some math and sure enough, I hadn't considered what Temp Comp will do at our frequently low temps.

I'm ok with the temp comp doing its job, but what worries me is that through some math I've determined if the temp drops below 20*F and the sun comes out, it will be ABS charging at 16.0V and I'm in danger of frying a device!

Is it possible to set a max voltage in the MPPT that it's not allowed to exceed no matter what the compensation computes to? It's not uncommon for temperatures to reach 0* here when its clear skies. That would be 16.6V output!

Or..do I need to add a voltage regulator before house loads to prevent from frying my electronics during Abs charge??

Edit: Should also add that this is a lead-acid battery so I should be good to charge down to -4*F but that would be an Abs charge voltage of 16.5V!

MPPT Controllers feature request temperature compensation

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ghostrider answered · Oct 06, 2020 at 07:20 PM

Sounds like this is best presented as a feature request - marking an answer as such...unless something better comes along.

Formal Feature Request:

Add input value to the charge profile of "Max Battery Voltage" which will never allow a Bulk/Absorption to calculate above it.

Or...

Instead of adding an input, update the temperature correction function to never apply correction values calculated to be greater than the High Voltage Alarm setting.

Both solutions would prevent a High Voltage Alarm due to its own applied corrections.

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ghostrider commented · Oct 06, 2020 at 07:25 PM

Just realized - Blue Smart Chargers (IP67, etc..) don't have an alarm function but are also susceptible to the same issue.

Sounds like an input value added to the battery profile would be the best solution to protect devices with all kinds of chargers.

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Answer 1 · 2020-01-29T19:24:44Z

Murray van Graan answered · Jan 29, 2020 at 07:24 PM

Hi @Ghostrider. I don’t think it’s possible at this stage to set an absolute max voltage on the MPPT. I must say though that if I read it correctly your temp compensation seems high - normally it’s somewhat between 2.5mv and 3mv per cell per degree C. For a 12V battery with 6 cells that would then be between 15 and 18mv per degree C, and yours is double that. Please make sure the temp compensation is set up correctly.

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ghostrider commented · Jan 29, 2020 at 07:38 PM

Probably easiest to show my math by linking to another thread where I think I identified a bug in the temp compensation units...the thread already shows my math on how I got to that value. Please, please let me know if you see an error! Much appreciated!

https://community.victronenergy.com/questions/38942/possible-unit-bug-in-victronconnect-temp-compensat.html

*Note: This is for a bank with two Exide Group 24 Marine Deep Cycle batteries

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ghostrider commented · Jan 29, 2020 at 07:44 PM

This is also the example they gave in the manual that I've used as a confirming formula...

Example: at 95oF and 14.2 volt set point Corrected Voltage = 14.2 - (((95-77)/18) x (0.38)) = 13.82volts

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Murray van Graan ghostrider commented · Jan 29, 2020 at 07:57 PM

I think it’s best we look at the data sheet of your batteries, do you have it?

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ghostrider Murray van Graan commented · Jan 29, 2020 at 10:24 PM

http://www2.exide.com/Media/files/Location%20Data/Battery%20Charging%20%26%20Storage%20Specs%20%2011_13_15.pdf

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Answer 2 · 2020-01-30T16:21:00Z

Murray van Graan answered · Jan 30, 2020 at 04:21 PM

Hi @Ghostrider. The spec sheet says 0,38Vdc for every 10 degrees C, the VictronConnect app is set per degree C. So divide it by ten, your temperature compensation should be set at -0.038vdc per degree C

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ghostrider commented · Jan 30, 2020 at 07:31 PM

Actually the app is labeled millivolts per degree C (mV/*C) .. so I think its the same value to say -0.038 V/*C and -37.8 mV/*C (with rounding)

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Murray van Graan ghostrider commented · Jan 30, 2020 at 08:09 PM

You’re correct - and then it’s set up as per the data sheet. In this case (the batteries have a high temperature compensation value) it would then be normal to see those values. Do you have an inverter after the battery or dc devices? If dc, maybe a buck boost dc to dc converter could help keep the output stable?

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