Victron solar charge controller for LFP?

[tanglewood]

I have been considering using one or more Victron MPPT solar charge controller on an LFP bank, but some of the things I have been reading make me wonder how suitable it is for the. The issues/questions are:


1) I have seen comments about voltage control of at best 0.1V. That seems very imprecise for LFP.


2) The MPPT controllers sense voltage at their terminals, not with any remote sense. That mandates locating the controller in immediate proximity of the batteries and power bus. This isn't the end of the world, but is a degree of flexibility lost when there is no remote voltage sense.


3) The Victron bluetooth remote voltage sensor connection can be unreliable in anything more than immediate proximity of the controller. Worse yet, there doesn't appear to be any alarming or indication that the connection has been lost, so you could find the charger operating on it's locally sensed voltage rather than remotely sensed voltage, and never know it.


Do I understand this correctly?

[john61ct]

LFP does not require extreme precision in normal daily cycling, only for periodic maintenance / testing protocols on shore power.

Verify with your known-accurate meter that charging stops as soon as possible after hitting your target voltage. If possible your source goes too high or too long, lower the setpoint until you're sure it won't. There is no problem with a little too low.

On the discharge cutoff, ensure higher than your setpoint, there is no problem with a little too high.

No charge source readouts should be much relied upon whatever the brand.

[FlyingScot]

Item 1: The controller lets you adjust by .01v.
Item 2: Your controller should be located as close as possible to the bus bar/battery assuming a 12v battery system and decent wattage panels. Otherwise you'll have large wires or wasteful voltage drop.

3. What other brands have remote voltage sense? How much do they cost as compared to having a Victron MPPT and a BMV-712 with remote temp sense? 712 and temp sensor are about $200 if in USA. This is what I have and I am very happy with the product choice. This setup gives you remote battery temp and voltage sense shared to the MPPT(s).

[john61ct]

Same temp sensor fits the SmartSolar directly even if you skip the BMV, right?

[Cpt Pat]

I can't stress this enough: you can overcharge a 12 volt (4 series cells) LFP battery with a terminal voltage as low as 13.6 volts (with some chemistries, even lower). This is especially insidious when charging the battery with a low current - like a solar array. This is why "trickle charging" an LFP is so strongly discouraged. Charging begins at about 13.4 volts -- and with a low current charging source it needs to end after 13.6 volts has been achieved for some random amount of time. That's only a 0.2 volt difference! When charging at higher constant currents, such a 0.2C rate or higher, you can detect the 100% SoC point by a tapering off of the charging current. That detection cannot be performed with low and varying charging current from a solar controller.

Employing only terminal voltage detection to determine when to stop charging is "lead-acid thinking." This is the hardest concept to get over with LFP batteries: terminal voltage only reflects state of charge at a very specific charge current and temperature. That charge current is neither constant nor achievable with a solar power source (unless you have thousands of watts of panels).

The manufacturer's voltage curves assume you are charging the battery with far more current than you are likely to ever see from a solar charging source. If you have a 100 AH battery, you'd need a constant-current source of 50 amps for those curves to be valid.

The charge/discharge versus voltage curves for LFP batteries presume a high charge current. Usually the lowest charge current shown in those curves by the manufacturer is 0.2C (if you can find the curves at all). When charging with a solar controller with 10% or less of that current, you need to use a combination of "coulomb counting" (counting amp/hours in and out) and terminal voltage to detect SoC. Here's a paper on the topic: https://arxiv.org/ftp/arxiv/papers/1803/1803.10654.pdf.

Fortunately, the charge controller can do terminal voltage detection (within the accuracy constraints you have already described) and the BVM-712 can do coulomb counting by configuring it to open its internal relay contacts at your chosen SoC set point (I use 80% SoC) for charge discontinuation. I also set a charge cutoff voltage at 13.8 volts, but that only applies to charging with my (0.2C) shore power charger. With solar panels, 80% SoC occurs at 13.6 volts (with some small variance due to temperature). That a per-cell voltage of only 3.40 volts.

I calibrate the BMV to 100% SoC every few months by continuing the charge from my shore power charger at a constant current rate of 0.2C, at or near an ambient temperature of 25 degrees C, until the charge current tapers to 0.05C.

Since you can safely use terminal voltage for discharge discontinuation, I use the BVM low voltage detection for stopping discharge (I use 12.7 volts). In my installation, the BVM relay contacts drive a high-current external relay (be sure to install a reverse polarity snubber diode across the relay coil to absorb inductive flyback).

I would not trust the BMV switch contacts to carry more than 2 amps, so beware of some high current latching relays that draw 7 amps when actuated. Such as this one: https://www.bluesea.com/products/cat...Relays/ML-ACRs.

The reason I chose 80% SoC as the charge cutoff threshold (which appears wasteful) is to accommodate the reduced charge capacity of the LFP at lower temperatures. I couldn't get temperature-dependence data from the manufacturer for inclusion of temperature compensation to the charge cutoff threshold. It's a tradeoff. If the LFP has a built-in BMS that takes the battery offline, I would only trust that as a last ditch "ejection seat" control.

[mitiempo]

Originally Posted by john61ct

Same temp sensor fits the SmartSolar directly even if you skip the BMV, right?

No it doesn't. The Victron MPPT controllers do not accept wired temp sensors.

The wireless temp/voltage sensor is the answer.
https://www.victronenergy.com/blog/2...olar-chargers/

A previous poster in another thread stated that it's range was very short. My guess is that his batteries are in a space that is lined with metal covered insulation that restricted the range. There should be a range of 15 or 20 feet, if not more. The controller should be close to the batteries in any case.

[LeaseOnLife]

Originally Posted by mitiempo

No it doesn't. The Victron MPPT controllers do not accept wired temp sensors.

The wireless temp/voltage sensor is the answer.
https://www.victronenergy.com/blog/2...olar-chargers/

A previous poster in another thread stated that it's range was very short. My guess is that his batteries are in a space that is lined with metal covered insulation that restricted the range. There should be a range of 15 or 20 feet, if not more. The controller should be close to the batteries in any case.

I have this sensor, on a fiberglass monohull, and the range of this sensor sucks, big time. Between batteries and solar controllers I have two bulkheads (plywood) and no metal shielding, distance less than 5ft. I had to move the sensor closer to the solar controllers with the disadvantage of not really measuring battery temperature. I am not the only one complaining, something is not right with these sensors.

[mitiempo]

Originally Posted by LeaseOnLife

I have this sensor, on a fiberglass monohull, and the range of this sensor sucks, big time. Between batteries and solar controllers I have two bulkheads (plywood) and no metal shielding, distance less than 5ft. I had to move the sensor closer to the solar controllers with the disadvantage of not really measuring battery temperature. I am not the only one complaining, something is not right with these sensors.

I'll ask Victron about range and issues when I get a chance.

In your case the answer is probably to move the controllers closer to the batteries if possible.

[CatNewBee]

Originally Posted by LeaseOnLife

I have this sensor, on a fiberglass monohull, and the range of this sensor sucks, big time. Between batteries and solar controllers I have two bulkheads (plywood) and no metal shielding, distance less than 5ft. I had to move the sensor closer to the solar controllers with the disadvantage of not really measuring battery temperature. I am not the only one complaining, something is not right with these sensors.

Victron MPPT controller are just perfect with LiFeYPO4 batteries, fine tuneable. I use the victron smart solar 150/100 with the BMV 712 shunt, both BT connected. No problem if the connection fails, than it runs standalone.

You can go all in and use the color panel controller Control GX and network all victron devices to it with remote access and fully integrated by wires, but it is a overkill, anyway, your BMS schould do the last line of defense when anything fails.

See my signature to my project for details...

[Yellowjacket]

Originally Posted by mitiempo

No it doesn't. The Victron MPPT controllers do not accept wired temp sensors.

The wireless temp/voltage sensor is the answer.
https://www.victronenergy.com/blog/2...olar-chargers/

A previous poster in another thread stated that it's range was very short. My guess is that his batteries are in a space that is lined with metal covered insulation that restricted the range. There should be a range of 15 or 20 feet, if not more. The controller should be close to the batteries in any case.

That poster may have been me and no, my batteries sit above the keel and below a rather thin wood sole. No metal convered insulation, no metal at all. Six feet is about the range on the unit I purchased, and this problem has been confirmed both by Victron and other users: bluetooth range on at least some Victron temperature / voltage sense (about $40) is extremely limited.

My Victron Blue solar 100 / 50 with dongle has excellent range, and I've been told that the BMV 712 has an RJ12 cable of 30 feeet that runs from its voltage/temp sense (if installed) at the battery to the BMV 712 head unit which then transmits temp/voltage info via bluetooth to the MPPT. Victron says the range on the BMV 712 is far superior to that of the simple temp/voltage sense.

[CatNewBee]

True, somehow. The shunt circuit board measures the voltage of up to two batteries and transfers the data via the wire digitally to the display.

Anyway, no problem at all. Reason is easy to explain. Even IF there is a voltage drop along the connections and the wire to the LFP bank, that means, the battery is at lower voltage than the solar charger due to voltage drop caused by the charge current, so battery is safe in bulk mode.

When the voltage cut-off is reached the voltage difference between battery and charger gets smaller, so the current drops and the resulting voltage drop gets smaller until there is no charging current any more and no voltage drop along the connection - the voltage on the charger matches the voltage of the battery. This is one of the reason why charger have a minimum absorption time setting to equalize.

Than the third stage is invoked - float. Put it just at a safe level below 13.5V or for longer unattended storage even less, the battery stays full and excess power can be used by the on-board electric straight from solar. For longer storage you can reduce the setting to 13V or less to keep the batteries at a lower SOC.

Temperature sensors MUST NOT BE USED with LFP batteries in conjunction with chargers in contrast to FLA! The charging curve is almost independent of ambient temperature and the battery does not get warm when charging or discharging. Only the BMS SHOULD have temp sensors measuring every cell (2 are enough for 4 cells if placed in between 1&2 and 3&4) to recognize cell failures and turn the battery off - and eventually prevent charging in sub-zero temperatures according to the chosen chemistry, LiFeYPO4 can be charged with lower currents down to -30°C (not at 1C).

[noelex 77]

For lithium it would be nice to have very precise voltage control during charging, but I don’t think the Victron controllers are any worse in this regard than most other controllers. They just record and report the voltages where most units do not. Especially if you have large loads and multiple high charging sources, preventing 0.1v fluctuations is a tough ask.

Placing a large capacitance in the charging circuit may help, but it is hard to know if this would provide enough practical benefit to be worth the complication. I doubt it would.

Remote voltage sensing would also be nice feature. At least the loss of battery temperature information is not as critical with lithium. The bluetooth remote voltage sensor has some issues, although I think remote sensing is still possible using other Victron accessories.

It needs to be remembered that the Victron controllers have been designed as low cost charge controllers. The simplified approach has the advantage that the self consumption of the units is very low. Both of these features allow multiple charge controllers to be used, which helps to increase the solar output with many marine installations.

The greatest drawbacks of the Victron units is the lack of voltage monitoring during the “absorption” phase. The ideal charging algorithm for lithium is still being debated, but this limitation is of more consequence for charging lead acid batteries. The other annoyance of the Victron controllers especially when using multiple units is the “tail current cut off” feature. Unfortunately, this cannot be disabled or adjusted with the current software. This is potentially more of issue for lithium batteries, as the battery return voltage is based on the float voltage.

[john61ct]

Originally Posted by mitiempo

A previous poster in another thread stated that it's range was very short. My guess is that his batteries are in a space that is lined with metal covered insulation that restricted the range..

No lots of complaints about the very limited range, maybe 6'. It seems the BlueSolar+dongle signal is much stronger.

As you note, of course the controller is supposed to be near the bank anyway.

The Venus or Color Control GX option seems better to me, but pricier of course.

[john61ct]

Originally Posted by Cpt Pat

I can't stress this enough: you can overcharge a 12 volt (4 series cells) LFP battery with a terminal voltage as low as 13.6 volts (with some chemistries, even lower). This is especially insidious when charging the battery with a low current - like a solar array.

Started a new thread to discuss the important issues you raise, rather than derailing this Victron-specific topic thread. http://www.cruisersforum.com/forums/...ml#post2788444

[tanglewood]

Well, it seems to me that if the MPPT doesn't alarm when it loses connection with the V/T sensor, then I have to set everything up assuming no sensor, and the sensor is therefor useless. That's good to know.


I don't know off hand which of the 3-4 different Victron inter-product communications methods the MPPTs, use, but if I have a Quattro with the remote voltage sense connected to the batteries, does that get shared with the MPPTs, Skylla-i etc., assuming all are interconnected somehow in their maze of different communications methods?