mppt vs pwm

NinerBikes wrote:
Wait until people start getting attached to their LiFePo batteries!

Now that I could appreciate. For the average boondocker, Li is a complete game changer. The only time I can imagine it not being so, is if one has almost unlimited available solar and can easily keep their FLA's topped up. But as soon as a genset is needed, Li changes everything.

I know Li would improve my situation enormously, in several ways. For one, by not having to ventilate fumes, I could close in my battery compartment in winter, and easily keep the batts in a warm environment, without excessive heat loss. And I would not have to suffer capacity loss due to cold FLA's. And the lack of available solar in winter would not have near the impact on gen run times.

On the other hand, mppt is a definite improvement over pwm, in many situations, but I'm not sure I would call it a game changer.

Salvo wrote:


I see no real advantage when using a higher Vmp. Temperature still reduces power by the same amount. 90's thinking still apply.

The difference between Vmp and battery voltage is directly proportional to the gain you would expect to see an MPPT controller provide. The greater the difference, the greater the advantage will be for MPPT over PWM. With the higher voltage arrays that are common now, the gain with MPPT could be several hundred percent or more over a direct connect/PWM scenario. The issue here, of course, is that connecting a higher voltage array directly to a lower voltage battery is going to drag the array's voltage down to that of the battery's. A 40v Vmp panel attached to a 12v battery, for example, is going to exhibit a loss in power that is several times lower than what it would be producing at its Vmp. Imagine a 100v array, or 400v array (as is now possible). The 17v Vmp --> 12v battery scenario is old news, and so is the talk of 10% gain (or 8%, if you like to nitpick). Time to let it go.

BFL said,

"I have no idea what this concern about falling out of MPPT early is due to clouds coming in. Impossible. You are in MPPT till you get to Vabs. If a cloud goes by setting you back in voltage, you are still in MPPT because you never got to Vabs yet"

So you're essentially saying I'm confusing Vabs Setpoint and 'available voltage'. Hmmm, that actually makes sense. Available (panel) voltage would be more like available ac line voltage to a power supply, which when too low, would lower ps output, but not kick the batteries into abs mode. So, decreased panel output would not cause the mppt to go out of mppt.

However, I would think the upper and lower set-points (Vabs and kick-in) would still cause the mppt to switch to abs prematurely, if either was set too low.

Quote, "When you set the Absorption Stage (the controller might call it Float) at 14.4 or whatever, which the controller maintains, the voltage does not drift down to 13.2 and then kick back into Bulk. It would only get down to that if you ran some loads for long enough and then it would kick in the MPPT and start over same as in the morning."

But wouldn't a higher "kick-in" setting cause the mppt to go into bulk sooner, and therefore increase the amount of time in bulk?

"...boondocking, I appreciate being able to set the float higher (13.6V) so MPPT kicks in earlier at a higher voltage."

I missed that. Confusion over the set point for when the controller kicks back into Bulk and the setting for the Float voltage

When you set the Absorption Stage (the controller might call it Float) at 14.4 or whatever, which the controller maintains, the voltage does not drift down to 13.2 and then kick back into Bulk. It would only get down to that if you ran some loads for long enough and then it would kick in the MPPT and start over same as in the morning.

brulaz wrote:
And shouldn't there also be a low setpoint voltage for switching from float back into MPPT?

My MS MPPT 60 is programmable to switch out of float and back to absorption based on your programmed minutes below the float voltage. So if you use the coffee pot or microwave for enough time(adjustable) it will go back to the absorption voltage to get topped off again.

It is not really about switching from MPPT to PWM and back. The controller will do this automatic as needed to maintain the voltage settings.

brulaz wrote:

BFL13 wrote:
The $102 Eco-Worthy MPPT 20a controller has an adjustable high set point to reach Vabs, an adjustable Float, and the voltage that triggers a return to MPPT is not adjustable but is 13.2v.
...

When the trailer is in storage or on 110V, I set the float back to 13.2V (the same float V as the Progressive Dynamics charger when on 110v). That seems to work fine with little water loss over long periods although I think my batts call for 13.1V

But when boondocking, I appreciate being able to set the float higher (13.6V) so MPPT kicks in earlier at a higher voltage. Have been told that the higher "float" voltage should not be a problem for the batteries as they'll be all night without any charging.

So far this has worked well. But I suppose it could set it even higher when boondocking and the Rogue kept in MPPT as much as possible.

I don't follow. If you are camping, when would you ever want to be at a Float (to me that means a non-camping maintenance level) voltage during daylight hours? You are in Bulk from morning till you get the batts to your Vabs, then the MPPT drops out of play and you go into (call it whatever your controller calls it--Absorb or Float--means the same thing to some) till dark, then voltage falls off for the night.

Now the issue while camping after you get to Vabs, is what voltage do you want till dark and how long will that be?

If you can control that and know what the batteries "need" that's what you set. Or the controller might only let you set the voltage for that period (called various names, but we say Absorption Stage) Or the controller might have a fixed charging profile like the EP Solar ones do (LandStar PWM and Tracer MPPT) which is to get to 14.6v, hold that for two hours, and drop to 13.8 till dark, no adjustable anything. (Actually I had the LandStar and that profile was pretty good since it usually never got to the 13.8 part before dark in my situation)

I have no idea what this concern about falling out of MPPT early is due to clouds coming in. Impossible. You are in MPPT till you get to Vabs. If a cloud goes by setting you back in voltage, you are still in MPPT because you never got to Vabs yet If it happens after you get to Vabs then you are already out of MPPT until voltage drops to 13.2 or whatever yours is, when MPPT kicks back in.

So anyway, IMO you set the voltage and the time for your Absorption Stage to what you need it to be (even if the controller setting for that is called "Float" and never mind actual Float Stage voltages until you finish camping and now just want to maintain the batteries. Now you can set the voltage for that job until you go camping again.

12thgenusa,

Understood. I would certainly fit into that camp, as I see watts as a means to an end, not the end itself. Pretty much everything is rated in amps or Ah's; batteries, chargers, controllers, wiring, circuit breakers, etc.; so it just makes more sense to me to think in such terms.

Moreover, a 1080w power supply that puts out a steady 12v, and up to 90a, is useless to me. But a 1080w ps that puts out 83-60a at 13-18v adjustable is what I need. So, again, it's how those watts breakdown that really matters to me, not the watts themselves.

As for comparing mppt to pwm, I fail to see any point in comparing the two in terms of watts? If the mppt is putting out 8a at 14.6v, and the pwm is putting out 7a at 14.6v, that's 116.8w vs. 102.2w, but how do I even begin to relate to how many watts are being delivered at my bank? And what, if any, benefit is there to it? At some point I have to divide the watts by 12v, to get a sense of what has been accomplished. And even that would be inaccurate, seeing as the watts at the bank are not a factor of 12v X amps, but some other voltage(s).

To use eating a meal as an analogy, to me watts is like eating eggs for breakfast, where voltage is the fork and amps are the eggs. All I care about is my belly gets full of nutrition, not what name is applied to the meal.

MrWizard wrote:
mena...!

that wasn't 17ampHrs for the day total
not 17amps charge rate

the remarkable part is he did it with 280w of solar
i've done it but i have 505w of solar

like he said, the LiFePo have a very slow voltage rise
which maintains the the voltage spread and gives the mppt something to work with

Thanks for clearing that up!

BFL13 wrote:
The $102 Eco-Worthy MPPT 20a controller has an adjustable high set point to reach Vabs, an adjustable Float, and the voltage that triggers a return to MPPT is not adjustable but is 13.2v.

It drops to Float right after reaching Vabs, so if you want to spend any time at that voltage you have to choose the Float voltage to be the same as Vabs. Then you must adjust the Float voltage down to your chosen value if you want to float at a "storage" level for maintaining the batts while not camping..

Since the Float only goes up to 14.4 and I choose 14.8 as Vabs, I get to 14.8 at some point during the day and then it drops to 14.4 till dark. Then voltage falls off to actual Vbatt by morning which is below 13.2, so MPPT kicks off again for the new day.

My 6v batteries really like that profile and get to baseline SG without needing an equalize session. My T-1275s want more time at 14.8 or higher, so this controller is not quite right for them. (The PWM Solar30 stays at the chosen Vabs -can be up to 15- till dark so that controller was better for the T-1275s as it turned out)

PWM and MPPT controllers have adjustable settings depending on models and price. As noted what happened between my 6s and T-1275s, you should compare your battery specs with the controller specs to get a good match before buying.

Same observation, soley for the T-1275 12V battery. At the time I didn't know the Solar 30 was a good match when I ordered it, but from the get go, I would set my 120 w panel, since replaced with a 150W panel good for 8.4 amps, that I sometimes see 9.1 amp peak with it, when the Solar 30 Charge controller is set for 15.0V. My SG seems to stay steady among all 6 cells with this combination... good news when you hear how finicky the T 1275 can be. Solar panels are a good set up for this type /size /shape of battery, and the 15.0v is also working for Landyacht318 and his Screwy 31 battery too.

I have no dog in this hunt for MPPT vs PWM, since I have a smaller set up and 8.5 to 9 amp hours from my 150w panel is getting it done for me daily.

vermilye wrote:

pianotuna wrote:
Hi jrnymn,

Since all the controllers I've seen just look at battery voltage they switch out of bulk faster than one might expect. Often I see the Blue Sky change from float to bulk when clouds pass by.

I wonder if this is true of the Bogart Industries SC2030 charge controller. According to their description, it communicates with a TM-2030 to "adjust charging current based on real time battery information". Could be sales hype, but since the 2030 actually keeps track of amp hours in & out, it might be more accurate than battery voltage. Anyone have any further info on this controller?

The roque also maintains and logs Ah and Wh info. But I'm quite sure it doesn't use that info to switch modes. It's just the set points versus the battery voltage. And it has a remote sensor wire for the battery voltage.

To me, PianoTuna's observation is simply that ongoing loads were such that battery voltage drops below the MPPT cutin setpoint when clouds appear. Beforehand and without the clouds, the panels/controller was able to maintain the battery voltage above the cutin (or at the float voltage).

Raising that cutin setpoint when boondocking might help if this is a concern.