Chassis Battery *versus* Home Battery ? Must start an Onan Gen.

[markopolo]

Thanks for the info about the relays

[markopolo]

Originally Posted by rickst29

................................... The limiting factor in a booster configuration like this MUST !!! be the Solar Controller. If you have an extremely high-powered Solar controller trying to pull too much power and current from a less capable under-the-hood voltage booster, the booster will release its magic blue smoke. (You don't want that smell to be released.) In implementations which I have designed, we limit the Solar Controller input current so that it draws no more than 80% of the rated output power on the Voltage Booster...................................

Hoping that you see this post Rick....

How do you "limit the Solar Controller input current" to 80% of the rated output power of the boost converter as mentioned above? Is that a setting on the solar controller you use?

I lost an inexpensive 19V/8A boost converter in minutes yesterday in a test setup as I totally ignored your advice It immediately went way over its output rating.

[rickst29]

Originally Posted by markopolo

Hoping that you see this post Rick....

How do you "limit the Solar Controller input current" to 80% of the rated output power of the boost converter as mentioned above? Is that a setting on the solar controller you use?

I lost an inexpensive 19V/8A boost converter in minutes yesterday in a test setup as I totally ignored your advice It immediately went way over its output rating.

I got an email message WRT your new post. Yes, you limit at the Solar Controller output maximum current value in MPPT programming. (Not the input value, that was a typo, and I don't know of any controllers which limit separately at the input side).

19V is IMO not enough boost to be worthwhile - you probably want higher Voltage, in order to reduce wiring losses through the chassis wiring. I can recommend this "24V" model @ 480 Watts: https://www.ebay.com/itm/DC-Converte...t/154035661569, or this "36V" model at 540 maximum watts. (I own the 36V version): http://https://www.ebay.com/itm/DC-12V-Step-Up-To-DC-36V-15A-540W-Boost-Power-Converter-Regulator-Module-Good/202769566557. Both are in succcesful use by a couple of different TT owners, running the Boost through the Bargman cable's "Trailer Battery Charge" wire.

In setting the Solar Controller output current, you start with 80% of that "Maximum Converter Power" value, cut it by another 7% for MPPT conversion inefficiency (the MPPT sucks in a bit more power than it dishes out), and then divide by your maximum battery charging Voltage. In my own case:

540 watts *.80 (max power to use) = 432 watts consumable at MMPT input;
432 watts / 1.07 (MPPT converter inefficiency) = 404 maximum output watts;
404 watts / 14.5V (battery charging volts) = 27.8 maximum output Amps.

My own MPPT controller is rated at 30A, capable of about 31 Amps, and I have limited it to only 27.5 Amps at a ll times. If I ever want to add more panels and upgrade to a bigger controller, then I will need to either upgrade the under-the-hood Voltage Converter, or re-program the new MPPT to different max-current values between driving versus camping times. (With my Ep-Ever 'BN' controller's interface, it takes only a couple of seconds to get there.) But I could also put the new panels on a controller of their own, keeping the Bargman interface dedicated to this one.

[markopolo]

Thanks for the info re: higher voltage, improving efficiency etc.

I was doing a test to compare MPPT vs PWM so the 19V 8A was meant represent/simulate ideal conditions for a single 152W panel.

I still don't understand how you are limiting current. Is that a setting available on the the BN series controller?

I used an A series controller for the test and I don't see any option to limit current. I can adjust voltage settings but not current. Voltage affects current acceptance at the battery but is difficult to predict.

A series user options:
epever Tracer settings.JPG

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[rickst29]

Yes, the Tracer-BN models also offer an output charging current limit. Let me also SWAG the MPPT advantage for you with just one panel at 19V - it's smaller than people with SLA or Sealed batteries will have, because you're charging at LiFePO4 voltage.

When the batteries are willing to take everything which the panel COULD dish out, a PWM controller is rapidly connecting and disconnecting the panel to create an "average charging voltage" of only 14.6V. The "remainder" is left in the panels, unused. Your panel is rated 152 Watts at 19V, that's 8A and only 116.8 Watts at 14.6V (under near-perfect "standard" conditions).

An MPPT controller would be running in MPPT almost the entire time (with Lithium batteries), but it would waste about 5% of the input power, creating heat instead of useful output power. 152 * .95 efficiency = 144.4 Watts, and 144.4 / 116.8 is a bit less than 24% improvement.

[markopolo]

That's a nice feature to have on your controller

As you've explained above, PWM controllers can't take full advantage of the panel output wattage. I expected to see 8A output with PWM controller but didn't get the chance to test it before the boost converter failed. I was hoping to to get the output at various LFP charging voltages to see if MPPT controllers further outperform PWM when used with LFP batteries. Previous test here: https://www.classbforum.com/forums/f...tml#post114194

It would have been nice see the difference in output with the MPPT controller set at 13.8V vs 14.6V for example with the simulated panel (controlled test conditions). Would it be:

152W/13.8V*95%=10.46A
152W/14.6V*95%=9.89A

I'll set the test up again sometime and use a bench power supply instead of a boost converter.

It's possible that I had a faulty boost converter I guess. The description stated:

Supports over current, over voltage, over temperature and short circuit protection.

But I agree with you that it's better to make sure that you can't exceed the boost converter output rating and as you suggested, stay below it. I've been using a 25A boost converter at 20A with zero problems.

[rickst29]

You should see no almost no difference, with MPPT, until the LFP battery string approaches 13.8V.

At that point, even LFP will begin to reduce it's accepted current, because the pushing Voltage Differential will become too small. In total time to reach 100% SOC (which 13.8V can't even accomplish), the 14.4V / 9.9A limit should win.

One little wrinkle - with a real Solar Panel, you can definitely leave the max current unlimited. the MPPT tuning process will adjust for maximum power, and if a "too-high input voltage" leads to lower power, it should almost instantly back down on the Panel Voltage. From there, whatever you get in is what it will try to push out. With LFP batteries drawing the maximum power of the small panel so easily, the output Voltage will only reach 14.6V during the end of the charging process - and you should let current go higher, pushing all of the power into the batteries at lower voltage (when the desired charging Voltage cannot yet be reached).
- - - -
The tiniest difference could occur, however, because MPPT efficiency will not be the same in the two configurations. The lower current - higher voltage setup has less conversion, but loses a higher proportion of "efficiency" by running so low on the device's overall efficiency curve. Optimal efficiency occurs at 25-60% of the MPPT Controller's power capabilities, and this scenario appears to be far below that figure.

[markopolo]

Originally Posted by rickst29

You should see no almost no difference, with MPPT, until the LFP battery string approaches 13.8V.............

That's what I see also (not on a solar controller) although with mine it's very close to 14V ( like 13.98V ) rather than 13.8V. There seems to be no gain charging at a higher voltage when using a lower output charger. See here for actual test -> https://www.classbforum.com/forums/f...tml#post107800 - all cells balanced to within 5mV (0.005V) at the end. 116Ah LFP charged at 14.00V 19A.

Originally Posted by rickst29

..................The tiniest difference could occur, however, because MPPT efficiency will not be the same in the two configurations. The lower current - higher voltage setup has less conversion, but loses a higher proportion of "efficiency" by running so low on the device's overall efficiency curve. Optimal efficiency occurs at 25-60% of the MPPT Controller's power capabilities, and this scenario appears to be far below that figure.

That's what I want to test for. I will run the test sometime in the coming months & report what I see.