NO solar charge controller for lifepo4 vs mppt

[Fuss]

What are the differences in efficiency between no controller and mppt.
Has anyone tested this?
I’m just deciding whether to use a controller for the 200watts of solar on my dinghy charging the electric outboard batteries.

[rslifkin]

What's the output voltage of the panels you're using and the voltage of the batteries you're charging?

[sailorboy1]

Originally Posted by Fuss

What are the differences in efficiency between no controller and mppt.
Has anyone tested this?
I’m just deciding whether to use a controller for the 200watts of solar on my dinghy charging the electric outboard batteries.

add the controller!!!!!!!!!!!!!!!!!!!

there are so many reasons beyond any possible eff loss

I am sure a google search will give some "real" answer

[Fuss]

Originally Posted by rslifkin

What's the output voltage of the panels you're using and the voltage of the batteries you're charging?

Hi, it’s 39v and the battery 25.5v

[rslifkin]

Originally Posted by Fuss

Hi, it’s 39v and the battery 25.5v

In that case there's a good chance the MPPT will be more efficient than no controller (or PWM), as connecting the panels directly will force them to operate at a voltage lower than their maximum power output (and will give you less than full output as a result).

[Fuss]

Originally Posted by sailorboy1

add the controller!!!!!!!!!!!!!!!!!!!

there are so many reasons beyond any possible eff loss

I am sure a google search will give some "real" answer

Google is certainly not good for everything, that’s why I am asking the experts on here….

[Fuss]

Originally Posted by rslifkin

In that case there's a good chance the MPPT will be more efficient than no controller (or PWM), as connecting the panels directly will force them to operate at a voltage lower than their maximum power output (and will give you less than full output as a result).

The controller has some loss as well..and actually creates quite some heat…I think it would be a close run thing….thats why I posted.

[Fuss]

And as lithium has a BMS, it will nicely cut it off when required…no overcharging worries at all. So what do you say to that Sailorboy1??

[rslifkin]

Originally Posted by Fuss

The controller has some loss as well..and actually creates quite some heat…I think it would be a close run thing….thats why I posted.

There is some loss in the controller, but from what I can find it would typically be in the 2 - 5% range for a good quality modern MPPT.

I looked around a bit and found an example solar panel with a Vmpp in the 38 - 39 volt range (https://www.solaris-shop.com/meyer-b...type-2-module/).

Looking at the I-V curve in the datasheet for that panel, once you drop below the max power voltage, current doesn't increase significantly. So by the time you get down to 25.5 volts, the 390W example panel is only putting out 270W at best (assuming it'll put out the full short circuit current at that voltage). The loss from running the panel that far below its max power point is far bigger than the losses in an MPPT controller.

[noelex 77]

Assuming you have a suitable solar panel voltage (that is a Vmp of arround 19 volts for a 12-volt battery), MPPT controller manufacturers suggest the gain is “up to 30%.” While this is accurate, the average gain is significantly lower. Around 10-15% is more realistic. The gain is higher in colder climates.

It should be remembered that the controller also prevents discharge at night. If you do not use a controller, this can easily be replaced with a simple diode, but the diode will reduce the Vmp by approximately 1-2%.

Importantly, the solar controller also functions to regulate charging.it is general felt that a BMS should function as last ditch safety device rather than an everyday regulator.

[Fuss]

Thanks guys for these comments, I have an mppt controller so if no one on here posts that they have done this, I will run alternate days of mppt and then no mppt and see what the difference is.
The solar has diodes at the junction box.
Yes that’s true that the BMS is only last ditch on the mothership as this has alternators and sensitive electronics…but the dinghy…I don’t care if the BMS plays a charge controlling role as nothing can be damaged.

[thinwater]

Fuss:


The short answer is don't do it. Everybody has thought of this.



The long answer is to read up on the charge profile of batteries, the efficiency of MPPT controllers over a range of voltages, and BMS failures.

[noelex 77]

Originally Posted by Fuss

The solar has diodes at the junction box.

The diodes at the junction box are bypass diodes, which will not prevent battery discharge into the solar panels at night. To effectively manage this, a blocking diode is necessary, but almost no solar panels include this component.

Although the nighttime discharge is very small, its magnitude varies significantly among solar panels. If the goal is to maximize energy extraction, eliminating this overnight discharge is crucial.

[Fuss]

Originally Posted by thinwater

Fuss:


The short answer is don't do it. Everybody has thought of this.



The long answer is to read up on the charge profile of batteries, the efficiency of MPPT controllers over a range of voltages, and BMS failures.

Thanks, also good points, and im sure that under normal circumstances a mppt will provide improved results as none, but I think I will do this test if no one gives me their results.

Also, your point on BMS failures brings up a mothership question. I use my mothership BMS’s concept as last ditch, just like a fuse. So… what about all the posts and support for BMS controlling everything as a central hub … reporting and controlling, switch off alternator controllers etc etc. clearly with the thought that they fail a lot…, then let’s always use them as last ditch and never as central hub controlling everything….I’m sure Capt Rivet will chime in against this point if he reads it…

[Fuss]

Originally Posted by noelex 77

The diodes at the junction box are bypass diodes, which will not prevent battery discharge into the solar panels at night. To effectively manage this, a blocking diode is necessary, but almost no solar panels include this component.

Although the nighttime discharge is very small, its magnitude varies significantly among solar panels. If the goal is to maximize energy extraction, eliminating this overnight discharge is crucial.

Good point too , thanks, I’m going to check this again, I did a small test on this earlier and I saw no evidence of this happening so I quickly moved on…..I’m going to retest this now as perhaps I missed this. So, this is a significant reason to reconsider….