Impact of BMS disconnect on MPPT controller

[andrzej0nl1ne]

I have read that the MPPT controller should be disconnected from the solar panel before disconnecting the battery side, in oder to avoid damaging the MPPT. How does this factor into the LFP config when the BMS disconnect upon a full charge?

Should a sacrificial LA battery be included?

Can the load connection on the victron MPPT controllers be used as a safety valve?

Is my original assumption an old wives tale?

[Pete7]

Can't say its ever happened to us as we have a lead acid battery to take the strain in the event of a BMS shutdown. However, you will find that hybrid systems are frowned upon and its a regular occurrence to hear "you can't". Meanwhile lots of us are quietly just getting along just fine

How about the ElectoDacus, I think it can control a relay so could cut the power to a relay in the panel to the MPPT.

Given the number of Victron MPPTs out there, I think there would lots of comments on this if it was a problem.

[wholybee]

Rod Collins has written of an incident where an mppt failed. It had a high voltage series array, and on failing the full voltage of the array was shorted through the mppt controller. The high voltage was more than the FET based BMS could tolerate, so those FETs failed, and the full solar voltage was put on the LFP cells, destroying them.

The recommendation was to limit the voltage of series installations to reasonable levels by not putting too many panels in series. He suggested that if the LFP manufacturer allows 4 12v batteries in series, then limit solar voltage to ~50V, regardless of how many batteries you actually have in series. If the LFP manufacturer allows 2 batteries in series, limit to ~30V.

https://www.proboat.com/2022/07/sola...ller-failures/

Will Prowse has done testing of the Victron MPPT controllers (using lower voltages) and was not able to make one fail from a battery disconnect. Sorry, I don't have that link, but he suggests that a BMS disconnect should not be an issue.

Edit:
The BMS should NEVER disconnect. So, the first thing is to make sure that it doesn't.

[andrzej0nl1ne]

Read Rod and watched Will, both great sources. My 400 to 600 watt solar array is at the low end of scale so not a challenge. Hope the old wives tale is the correct assessment, as that simplifies the config. Basically charge the 206 ah SOK directly via solar and no problem when the BMS disconnects on full charge. Alternator and shore charging is via Fireflies and DC to DC. Thanks for your imput.

[smac999]

A bms does not open when the battery is full. Only if there is a major error or failure.

The mppt will go to float when the battery is full. It will not disconnect.

[secessus]

Originally Posted by andrzej0nl1ne

I have read that the MPPT controller should be disconnected from the solar panel before disconnecting the battery side, in oder to avoid damaging the MPPT.

Disconnecting the CHARGE circuit doesn't mean necessarily mean disconnecting DISCHARGE circuit (powering the mppt), even on single port BMS.

Originally Posted by andrzej0nl1ne

How does this factor into the LFP config when the BMS disconnect upon a full charge?

In normal configurations the BMS would disconnect on overcharge, not on full charge.

[andrzej0nl1ne]

On the Bluetooth app associated with the SOK BMS on my 206ah LFP battery, there is a "C MOS" and"D MOS" indicator. The user guide defines the C MOS indicator, when grayed out, as disconnecting the battery due to full charge. My interpretation of their writeup is that once charged the BMS disconnects the battery from the charging sources to prevent overcharging.

I hope this is a correct interpretation as I'm depending on it to protect the battery.

Since I'm a low draw user, this SOK LFP will frequently reach full charge. I understand the importance of not keeping it at full charge, and will work to that goal.

My concern prompting my original question was driven by my intent to charge the SOK via my victron MPPT controllers. I was concerned that once the SOK reached full charge and the BMS did its C MOS thing, that this may damage the MPPT. That concern appears to be misplaced, given the comments so far. I plan to use this approach unless better guidance surfaces. Thanks all for your input.

[wholybee]

Originally Posted by andrzej0nl1ne

On the Bluetooth app associated with the SOK BMS on my 206ah LFP battery, there is a "C MOS" and"D MOS" indicator. The user guide defines the C MOS indicator, when grayed out, as disconnecting the battery due to full charge. My interpretation of their writeup is that once charged the BMS disconnects the battery from the charging sources to prevent overcharging.

I hope this is a correct interpretation as I'm depending on it to protect the battery.

Since I'm a low draw user, this SOK LFP will frequently reach full charge. I understand the importance of not keeping it at full charge, and will work to that goal.

My concern prompting my original question was driven by my intent to charge the SOK via my victron MPPT controllers. I was concerned that once the SOK reached full charge and the BMS did its C MOS thing, that this may damage the MPPT. That concern appears to be misplaced, given the comments so far. I plan to use this approach unless better guidance surfaces. Thanks all for your input.

Does the manual indicate the voltage the C MOS activates and stops charging? This is (or should be) always set at a higher than full charge voltage. Typically, 3.7Vpc or more. It is NEVER a good idea to rely on this to stop charging. The manual probably only addresses this for the case of a lead-acid to LFP conversion where the chargers were not changed and left at lead-acid settings. Don't do that, even though many cheap drop-ins promote that you can.

Set the Victron MPPT voltage to lower than the C MOS activation. The default settings for LFP in the Victron MPPT controller are probably fine. To be more conservative, you can set the voltage as low as 13.8 (3.45Vpc), and make sure you have an absorption time of about 20min per 100Ah if you do that, so the cells have time to balance. But most likely, just use the default Victron settings, and the MPPT will stop charging before the BMS stops it.

[andrzej0nl1ne]

SOK thoughtfully included explict settings as below. I will implement those.

Settings for Victron SmartSolar MPPT Charge Controllers:

Battery Voltage: Set depending on system, normally 12v.
Max Charge Current: OK to set as high as desired, but no more than 50a per battery
Charger: Enabled
Battery Preset: User Defined
Expert Mode: ON
Absorption Voltage: 14.6v
Float Voltage: 13.8v
Equalization Voltage: 14.6v
Rebulk Voltage Offset: 0.40v
Absorption Duration: Fixed
Absorption Time: 15 Minutes (This allows for proper cell balancing)
Tail Current: Disabled
Equalize Current: 0% (For safety we don’t want equalization at all.)
Automatic Equalization: Disabled
Equalize Stop Mode: Automatic (On voltage)
Maximum Equalize duration: 0 Minutes
Manual equalization: DO NOT CLICK START
Temperature Compensation: Disabled
Low Temperature Cutoff: Disabled (unless you add a temp sensor, the BMS has this protection built in)

[tanglewood]

Originally Posted by andrzej0nl1ne

SOK thoughtfully included explict settings as below. I will implement those.

Settings for Victron SmartSolar MPPT Charge Controllers:

Battery Voltage: Set depending on system, normally 12v.
Max Charge Current: OK to set as high as desired, but no more than 50a per battery
Charger: Enabled
Battery Preset: User Defined
Expert Mode: ON
Absorption Voltage: 14.6v
Float Voltage: 13.8v
Equalization Voltage: 14.6v
Rebulk Voltage Offset: 0.40v
Absorption Duration: Fixed
Absorption Time: 15 Minutes (This allows for proper cell balancing)
Tail Current: Disabled
Equalize Current: 0% (For safety we don’t want equalization at all.)
Automatic Equalization: Disabled
Equalize Stop Mode: Automatic (On voltage)
Maximum Equalize duration: 0 Minutes
Manual equalization: DO NOT CLICK START
Temperature Compensation: Disabled
Low Temperature Cutoff: Disabled (unless you add a temp sensor, the BMS has this protection built in)

Wow, those seem like very high absorb and float voltages (3.65vpc and 3.45vpc)

[lmxr]

Originally Posted by andrzej0nl1ne

I have read that the MPPT controller should be disconnected from the solar panel before disconnecting the battery side, in oder to avoid damaging the MPPT. How does this factor into the LFP config when the BMS disconnect upon a full charge?

For some (older) MPPTs this is true. Victron SmartSolar are designed to handle this, though (see Victron example schematics, there's also a thread on their Q&A forum).

That said, it is not considered good practice (following ABYC and ISO) that the BMS disconnects at full charge, and if it disconnects, it should warn before doing so and disable any chargers.

[hjohnson]

Originally Posted by smac999

A bms does not open when the battery is full. Only if there is a major error or failure.

The mppt will go to float when the battery is full. It will not disconnect.

Bingo. The BMS protection should be fully operational and never actually operate. If the OP’s bms is regularly hitting the HVC limit, something is seriously wrong, or the MPPT is not configured correctly.

The only time mine has ever operated is when I deliberately set the alarm limits artificially low in order to test things. Even then, it didn’t actually trigger initially, but that is because I run an integrated system so the BMS just commanded my charge sources off before it hit the cutoff.