Li-ion Battery Banks

[StuM]

Originally Posted by john61ct

Charging is not the power's only job.

Any useful loads running, e.g. holding plate freezer, will benefit from greater output, not draw off the bank.

Yep, many people keep ignoring daytime loads when talking about battery acceptance limits and "wasted" solar.

[CatNewBee]

Originally Posted by StuM

Yep, many people keep ignoring daytime loads when talking about battery acceptance limits and "wasted" solar.

Attachment 173494

As you see, the MPPT controller yelds plenty of power for the fridges, waterheater, A/C after the LFP bank is fully charged (switched from bulk to float, absorption time very short). Solar is not only charging the battery but also actively supllying a lot of power to the loads on board

[Seman]

Well, I have 2 sets of 500W solar panels.
1 X Voc 37.4V
1 X Voc 22V
I was too lazy to do the test, but you made me do it.
Thank you.
Today the 37V solar, controlled by my Tracer MPPT, produced 24A output.
The 22V panels produced 21.7A, connected directly to the LFP. The LFP was around 13.2V
Then I connected the 22V solar within 10 min. to the Tracer. The difference was max 0.2A.
Hardly worth $300 for an MPPT controller for these Voc 22V panels.
I agree that there is a gain on higher voltage panels, but not enough on 22V or lower voltage panels, to justify the high cost of MPPT.
It appears that some of the info on this forum comes from advertised material and wishful thinking.

[john61ct]

Connecting panel output directly to LFP is throwing a lot of money away.

[CatNewBee]

For the 32 / 36 cell panels a PWM controller is absolutely sufficient. Not much - if any - benefit of an MPPT, provided you can free program the voltages for absorption, float (and recond if available), and even better the timings and temp compensation (turn it off for LFP or replace the sensor by a fix resistor that matches the output at 20°C).

Unfortunately most PWM are not well enough configurable for LFP, so you may have to take other measures or let the BMS drop charging if no other option exists.

MPPT is worth the money only with high voltage panels or serial strings of unshaded panels. Because the 2nd is very unlikely on board - something always shades et least one cell of one of the strings - prefer always parallel setups, on 12V panels with PWM and on 50V / 60V panels MPPT.

If you have spare money, even smaller controller per panel may be desireable, but it is an overhead in wiring, programming, monitoring and troubleshooting. So you must find your balance there.

[noelex 77]

Originally Posted by CatNewBee

For the 32 / 36 cell panels a PWM controller is absolutely sufficient. Not much - if any - benefit of an MPPT, provided you can free program the voltages for absorption, float (and recond if available), and even better the timings and temp compensation (turn it off for LFP or replace the sensor by a fix resistor that matches the output at 20°C).

Have a look at MaineSail’s excellent test on “12v” 36 cell panels here (32 cell panels are not a great choice with any controller).

https://marinehowto.com/mppt-vs-pwm-solar-controllers/

He found a 20.8% increase when changing from a non MPPT controller (often called PWM) to a GOOD MPPT controller with lithium batteries.

The results will vary depending on a number of factors. With lead acid batteries the gain is lower. I would estimate an average gain of between 5-15% is more typical (I think MaineSail has suggested 10-20% for lead).

Solar threads seem to alternate between people indicating there is no gain and others claiming 30% (because that is what the manufacturers would have us believe). The truth is in between and not far from the middle of these two extreme viewpoints.

[CatNewBee]

there is some gain and some loss. MPPT needs some voltage difference to operate (3V at least), below that it is useless, MPPT produces some heat (and power loss) during transformation and inefficiency during seeking the MPP. Panels exposed to the sun heat up quite fast and output voltage drops, what makes MPPT controller even more inefficient, so they can be outperformed by a PWM controller in some circumstances.

They are in advantage, if the panel voltage Vmp is much higher than the absorption voltage + 3V

Originally Posted by noelex 77

Have a look at MaineSail’s excellent test on “12v” 36 cell panels here (32 cell panels are not a great choice with any controller).

https://marinehowto.com/mppt-vs-pwm-solar-controllers/

He found a 20.8% increase when changing from a non MPPT controller (often called PWM) to a GOOD MPPT controller with lithium batteries.

The results will vary depending on a number of factors. With lead acid batteries the gain is lower. I would estimate a gain of between 5-15% is more typical (I think MaineSail has suggested 10-20% for lead)

Solar threads seem to alternate between people indicating there is no gain and others claiming 30% (because that is what the manufacturers would have us believe). The truth is in between and not far from the middle of these two extreme viewpoints.

there is some gain and some loss. MPPT needs some voltage difference to operate (3V at least), below that it is useless, MPPT produces some heat (and power loss) during transformation and inefficiency during seeking the MPP. Panels exposed to the sun heat up quite fast and output voltage drops, what makes MPPT controller even more inefficient, so they can be outperformed by a PWM controller in some circumstances.

They are in advantage, if the panel voltage Vmp is much higher than the absorption voltage + 3V + xV for the Voltage drop on heat, so the panels become more inefficient when directly connected. That said, MPPT is not better than PWM, it simply better compensates disadvantages of not maching panel and battery voltages. It always burns some more energy in the conversion than a PWM regulator. It is obvious - just compare the heat sinks of same amp output controller PWM vs. MPPT.

[john61ct]

In a space-limited mobile context, the only efficiency metric that matters to me is

maximum charging watts output from the SC per square area of panelage.

You should choose your MPPT controller line based on feature set, including integration with your bank monitoring system if any, and reliability/service.

Then select panels with high efficiency, best shape to maximize use of area available, and Vmp to suit the SC.

No PWM controller will come close.

Now, if you have much more available space for panels than you need, saving some money may be a factor, but the few PWM controllers with a strong feature set are not actually less expensive.

Bogart Trimetric controlling 2x SC-2030 for 62A being a possibly worthwhile exception.

[hellosailor]

Comparing MPPT to plain PWM is not as simple as one might think. For instance, with lithium you may prefer constant voltage and limited current. But with lead acid, you get less boiling and faster charging (less internal resistance from microbubbles) by using "low" voltage, not the full bulk charge normally applied, and maximum amperage. And a good MPPT unit like BlueSky will do that, using program parameters and a microprocessor with a look-up table.

In the case of lithium, where you are *not* varying the charge voltage, but simply applying a set voltage then cutting it off...I think MPPT would still have some advantage in being able to maximize the amperage, instead of throwing away power. If your solar panels can only produce slightly more voltage than the batteries need (4.2V per cell? For 16.8V charging?) all you may be losing is the difference between a 17v panel *under*load* and the 16.8 volts that is optimum. For a better "12v" panel, producing 22V under load, then you're throwing away more power.

Assuming, of course, that the meters and numbers really are correct. When I tried to verify all that some years ago, I found out fast that digital multimeters are often lying, simply out of calibration. Enough to throw off conclusions.

Evaluating a modern charging system is very much like shooting at a moving target. There are subtleties that may be going on unseen, and variations in each installation. What works "best" in one, may not be right for another. If the hardware is a perfect match...the intelligence and matching that an MPPT does may not be needed.

[CatNewBee]

PWM actually works great with LFP, the battery just swallows all amps comming from the controller, the voltage remains low until the battery is full, because of low inner resistance, so a PWM controller will not do anything, but pass through all power until the battery is full and then start pulsing or switch off charging.

[noelex 77]

Originally Posted by CatNewBee

so a PWM controller will not do anything, but pass through all power

All the power less, (on average with 36 cell panels) 20.8% .

[CatNewBee]

Originally Posted by noelex 77

All the power less, (on average) 20.8% [emoji2].

It simply depends on the panel Vmp and temp. 36 cell monocrystalline panels with Vmp at 17ish Volt and high temp outside are very close to maximum power output at 13.2...13.4V, the voltage of a LFP bank in bulk charge. A MPPT does not contribute much in this situation, maybe 5% if any.

[john61ct]

Two ways to say the same thing:

A. If you must use low-voltage panels, MPPT will not add much greater output.

B. If you want to maximize the improvement in MPPT output, choose panels that best match your selected controller.

[john61ct]

And LFP

actually means a greater increase in power output from a given panel when going to MPPT

from both a lower voltage charge setpoint

and the extremely high CAR if you have enough panelage

[noelex 77]

Originally Posted by john61ct

Two ways to say the same thing:

A. If you must use low-voltage panels, MPPT will not add much greater output.

My guess would be with lithium batteries and 36 cell “12v” panels, the average gain in power would be about 20.8% .

Good evidence (theoretical or experimental) of a different outcome than MaineSail’s result would be valuable, but it seems to me to be in the right ball park.

The gains from MPPT technology are not enormous particularly for lead acid batteries, but even with this type of battery the 5-15% average gain with “12v” panels should not be dismissed so lightly, especially on yachts where there is often a limitation on the space available to fit solar panels.