SOLAR CHARGER - Victron MPPT 75/15

[john61ct]

Always good to remind of the 101 resources.

Originally Posted by Stu Jackson

Echo Charger Use Warning from Maine Sail: http://www.sailnet.com/forums/electr...rger-rant.html

But sorry don't see any scary-red warning there, just a matter of fact statement about the difference between DC-DC chargers and all the more common voltage-following combining solutions being discussed here.

Certainly B2Bs are justified for LFP and GEL, but when the two sides are FLA and AGM, optimizing charge sources for House will usually cause no significant issue for Starter, certainly not enough to justify buying a DCDC charger.

[davenrino]

Originally Posted by john61ct

expensive, beloved?

Oops, didn't hit the O hard enough. MahalO = Thank You

[smac999]

Originally Posted by evm1024

I've done some small testing on feeding 2 solar controllers from a single panel. This appears to work fine in my small setup. How it will scale is up in the air.

The setup is a single 75 watt panel that feeds a Victron MPPT 75/15 and a No-Name PWM controller. The victron feeds a typical start battery and the PWM controller feeds a small gel cell.

At some point I'll have to hook this up to a much larger solar array (900 watts plus) and add in a larger MPPT controller.

The idea is that the large controller feeds the LiFePO4 house bank (700 AH) and the 75/15 feeds the engine start battery (AGM).

I will also likely add a second 75/15 that feeds the windless battery. Also an AGM.

The house bank is charged and then is disconnected when the LiFePO4 reaches the SOC that is desired. The start and windless batteries remain connected.


I originally suggested this configuration some time ago. As might be expected there was some concerns as to the advisability of this configuration.

I did contact Victron and emailed back and forth with one of their engineers for a while. The bottom line there was that he thought (when pressed) that there was very little chance (none really) that connecting more than one solar controller to the same panels would fry anything. He did think that the MPPT algorithm in each of the controllers was going to be interfering with the other(s). This was due to the differing loading that they presented.

I expect that the large controller to the LiFePO4 bank will dominate the load on the panels and that the 75/15 will have minimal effect. But that needs to be tested. 14.4 volts at 15 amps is 220 watts (out of 900) and because the start battery or windless battery is almost always full likely to be much less of a load to the system.

Anyway - this arrangement gets you fully charged start and windless batteries while handling charging a big house bank.

I can't see how two MPPT controllers would work on the same panel setup. MPPT actually set the voltage of the panels. so both would be fighting each other. likly you would lose power and efficently. your better options would just have one small panel feed the victron feeding the start battery. it's own panel. or a device that charges the engine battery from the house battery. or nothing. there should be no need to charge the engine battery as long as it's started once a month.

how is the engine battery and windlass battery currently charged by the engine?
if you had echo chargers from house to engine and house to windlass. and alternator to house battery. then both the engine and the solar would charge all 3 banks. from the single MPPT.

if you really wanted 2 controllers. the smaller one should probably be pwm. and then it will just accept whatever voltage the larger MPPT is setting the panels too, but hopefully not effect it. but you'd have to watch for the VMP of the panels and the max input of the PWM. the panel voltage may be too high.

[john61ct]

And more to the point, why?

Just a science experiment?

[evm1024]

I can't see how two MPPT controllers would work on the same panel setup....

And more to the point, why?

Good questions...

Let's see if I can work through them.

I expect to have 3 large solar panels and not have room for a smaller panel dedicated to feeding the start and windless batteries. Solar is there to recharge the house bank primarily. When the house bank is being charged there is an option of disconnecting the start and windless batteries from the panels and reconnect when charging of the house bank is stopped (I do have LiFePO4 for the house bank).

I view solar controllers as DC to DC converters (and very smart ones at that) that can do 3 stage charging. Plus they cost less than B2B.

We all know that a mppt controller is constantly seeking the optimal load to the solar panels so as to get the most power on the output at a given output voltage, that the panels voltage will change with the load presented by the solar controller and that some combination of load will produce the maximum output wattage. The controller has to track this to account for changing solar panel output as the panels heat up, the boat moves and presents a different aspect to solar, clouds etc. You get the picture, the panels output is not constant and the mppt controller has to adjust.

By placing 2 or more mppt controllers on a single (well 3 parallel) panels we end up with 2 different microprocessor controlled buck regulators trying to adjust to the changing panel output and interacting complexly with each other.

Is this less efficient - but of course! To be expected. Does it half the efficiency? No not even close.

Plus consider that the smaller mppt controller is limited to 15 amps out of (with 3 panels) 70+ amps available. And in addition the likely demand from the starting battery is much less than full output of mppt 75/15 on average. Actually tapering down to almost no amperage as the start or windless battery comes up to full charge. Thus the actual loading on the panels by the mppt 75/15 is minimal and has a much smaller effect on the panels than does the main mppt.

Just due to the load that the large mppt controller presents while charging a LiFePO4 bank we can expect that this will dominate the resulting panel voltage.

What do I get out if it? I get fully charged start and windless batteries that have very little hands on to manage.

What do lose? I lose some charge that could go to the house bank and some efficiency.

Oh one more thing that may factor into this. The panels are outputting 36.7 volts or so under load. And thus the wire that goes from the panels distribution box (all the way aft) to the windless battery all the way forward can be smaller for the same voltage drop. 6 amps at 36.7 volts gives us 15 amps at 14.4 volts. And because the solar controller is at the battery we do actually get 14.4 volts at the battery, another advantage....

As to how I currently charge the start and windlass batteries - I use small 3 stage chargers (10 amp) at the batterys that run off of 120 volts. Under shore power these chargers are on and topping off the batteries. Under power I turn on the main inverter to feed 120 vac to these chargers (as needed).

Effecient? Hardly. But not as bad you might think.

Start the engine, turn on the inverter, the batteries start charging. Run the engine for some time based on your needs. Then shut off the engine and the inverter. This does not fully charge the batteries unless you are motoring for a few hours or running the watermaker.

You can end up with your start and windless AGE getting fully charged everytime you run the watermaker for example.

Science experiment? I prefer to call it a novel solution.