how to keep batteries under 80% soc with victron mppt and cerbo gx? Can I just run a really low float voltage like 13.2 or 13.3v?

[aentrop]

Currently I have off grid system with 1200w in panels and 800Ah 12v Valence XP LiFeMgPO4 batteries (6 batteries). I have victron mppt, cerbo gx, and shunt.

I have plenty of solar power at the moment so my batteries drain to 70% each night and then refill to 100% every day and I still have a couple hours of sunlight left after I get to 100 % SOC. I'd like to shift this so that my batteries will drain to 50% and then refill to 80% and then stop each day and still be able to use that last couple hours of power to produce just enough power to run my active loads only without charging the battery past 80%.

Can I do this using the Victron equipment? My first idea is to lower the absorption and float voltages down to 13.3v. This way my devices can still draw power from the panels. Any issues this might cause by running such a low float voltage? Is there a better way to control the max daily SOC?

I do plan to charge the batteries from 80% on up to 100% once a month in order to keep them balanced.

 

[sunshine_eggo]

Fully charge your battery.
Sync your shunt to 100%
Discharge to 70%
Charge until you achieve 80% SoC
The voltage and current you see when you hit 80% is your absorption voltage and tail current to be used in the MPPT.
You will have to determine what float voltage works for you. Even at 13.6V, it will creep up to 95%.

Easier method is to simply charge to 13.8V, which will get you ~98% charged, BUT it does it at a lower current and a lower voltage, thus reducing charging stress on the cells and likely increase cycle life.

Your plan may result in significant cell imbalance that will thwart you on your monthly charge to full.

Do the Valence include BMS? IIRC, some versions of the Valence batteries require an external BMS for proper protection.

 

[Johncfii]

It is very difficult to consistently charge lithium batteries to less-than-full charge by carefully choosing Absorption/Float voltages and tail currents, if your loads and solar radiation are not constant. And those are not constant for most of us. For months I tried to find a satisfactory set of charge voltage and tail current settings that would accommodate less-than-100% charging under full sun, part sun, heavy load, light load, and no load. It was especially frustrating to see current being drawn out of the batteries in the afternoon, after the batteries reached “pseudo-full charge”, and while the sun was still high in the sky,

Below is a link to a description of the solution that I adopted, and previously posted. It has been working perfectly for me for more than a year now in daily use. I’ve never operated a Victron MPPT charge controller, but my quick scan of the manual leads me to think that you can use the method I describe. Feel free to send me a PM if my explanation of how to implement it is not clear. It is a bit complex and long to explain.

A SCC “Hack” For Charging Lithium to Less Than 100% State of Charge

This is a modification to my solar charge controllers, for charging Lithium batteries, that I haven’t seen mentioned elsewhere. My SCC’s are Outback Solar FM80’s, but in reading the manuals for other similar charge controllers, such as the Victron “SmartSolar”, the Schneider “Context”, and the...

diysolarforum.com

 

[rmaddy]

I have plenty of solar power at the moment so my batteries drain to 70% each night and then refill to 100% every day and I still have a couple hours of sunlight left after I get to 100 % SOC. I'd like to shift this so that my batteries will drain to 50% and then refill to 80% and then stop each day and still be able to use that last couple hours of power to produce just enough power to run my active loads only without charging the battery past 80%.

May I ask why? I see no benefit to this plan. You don't hurt the batteries by recharging them to 100% each day (assuming you choose a reasonable absorption voltage that doesn't over do it). I'd rather start each day with a full battery. You never know when you might need more power or the sun decides to hide for a few days.

Oh wait, I just realized you are asking about LiFeMgPO4, not LiFePO4. Maybe there is a difference. I've never heard of LiFeMgPO4 until now.

 

[sunshine_eggo]

I’ve never used a Victron MPPT charge controller, but my quick scan of the manual leads me to think that you can use the method I describe. Feel free to send me a PM if my explanation of how to implement it is not clear. It is a bit complex and long to explain.

Please elaborate. @snoobler seemed to think it couldn't be done on a Victron.

I certainly can't see a way to do it on my Victron MPPT as it has no temperature sensor connection. The way he described using the BMV relay to cut charging on the MPPT is valid, but that's a different method than yours.

 

[Johncfii]

May I ask why? I see no benefit to this plan. You don't hurt the batteries by recharging them to 100% each day (assuming you choose a reasonable absorption voltage that doesn't over do it). I'd rather start each day with a full battery. You never know when you might need more power or the sun decides to hide for a few days.

Hi maddy …. Good question. There is tons of contention and discussion on this forum about this whole matter of how to maximize the life of lithium batteries, if this less-than-full charge/discharge is effective, and whether it is worth the effort.

The nice thing about our DIY systems is we each get to actuate our own priorities when the “absolute best” solution is uncertain. For myself, I’ve oversized everything, and run everything gently, to maximize reliability and longevity in case the day ever comes WTSHTF (which, hopefully never happens), and replacement parts are not to be found.

As indicated in that written description of my solution, I can also dial the maximum target battery full-charge-state to 90%, or 99%, or 100% in just a few seconds if I think coming extended stormy weather makes that a wise thing to do.

 

[Johncfii]

Please elaborate. @snoobler seemed to think it couldn't be done on a Victron.

I certainly can't see a way to do it on my Victron MPPT as it has no temperature sensor connection. The way he described using the BMV relay to cut charging on the MPPT is valid, but that's a different method than yours.

Do you have a Victron Battery Monitor (BMV 702, or 712)? There is an external RTD temperature sensor available from Victron that connects to the 702, or 712.

As I read it (but I might be wrong) it can be set up to monitor and communicate your battery temperature to the charge controller over the VE.Smart Network for this exact purpose of adjusting battery charge voltage to compensate for temperature.

Here is what seems to be a pertinent extract from the Victron MPPT 100/50 manual:

“3.9.2. External temperature and voltage sensor
The Smart Battery Sense is a wireless battery voltage and temperature sensor and can be used with the solar charger. It measures the battery temperature and the battery voltage and sends this via Bluetooth to the solar charger.
The solar charger uses the Smart Battery Sense measurements for:
• Temperature compensated charging using the actual battery temperature, rather than the solar charger's internal temperature. An accurate battery temperature measurement will improve charging efficiency and prolong the life of lead-acid batteries.
• Voltage compensation. The charge voltage is increased to compensate in case there is a voltage drop over the battery cables during high current charging.
The solar charger communicates with the Smart Battery Sense via Bluetooth using a VE.Smart Network. For more detail on the VE.Smart network see the VE.Smart Networking manual.
Alternatively, a VE.Smart Network that measures battery temperature and battery voltage, can also be set up between a solar charger and a BMV-712 Smart or SmartShunt battery monitor that has been equipped with a Temperature sensor for BMV, without the need for a Smart Battery Sense.”

(The bold emphasis is mine.)

You would not need to buy that temperature sensor. You just use those BMV terminals to switch an appropriate resistor across in order to “fake it”.

 

[Johncfii]

Please elaborate. @snoobler seemed to think it couldn't be done on a Victron.
The way he described using the BMV relay to cut charging on the MPPT is valid, but that's a different method than yours.

Cutting off the charger using the BMV relay is certainly a way to stop charging at your desired SOC, at least if your SCC has remote ON/OFF terminals (which my Outback SCC do not have). However, the disadvantage of the solution is that your system immediately starts drawing 100% of any load out of the batteries, even if the day is still full sun. To get around this, you could set it up so that the charger is turned off at 80% SOC, and turned back on when SOC drops to 79%. However that still results in way less than full utilization of solar panel potential output. I’ve also read that the life of batteries is reduced by the number of TIMES the charge/discharge reaction cycle is reversed, no matter what the depth of discharge. But I don’t know if that is definitive for lithium.