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Not true opportunity/dump/diversion load power with charge controllers? Are they microcycling?

aangel

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I'm in the market for an MPPT charge controller that dumps the power to an alternate circuit when the batteries reach a certain charge state. I will have ~1200W from the solar panels and LiPo batteries. This is for a vehicle mount but posting here seems to make more sense since it deals with heating hot water. I'm currently looking at this Camco water heater conversion kit (my current unit uses propane) but there are others on the market.

Looking through some marketing pages, the following units say they can do this (not exhaustive)
• Victron
• Outback
• Xantrex
• Morningstar

However, this fellow makes the case that his Outback units are taking power from the batteries and not directly from the solar panels thus are unnecessarily wearing them down due to micro-cycling on the top end. Thinking about it, I can see that this design was likely the easiest for the manufacturers but it doesn't seem optimal to me. Over time it seems that this causes avoidable excess wear on the batteries.

Instead, he says, don't use this charge controller feature. Avoid going to the battery via the controller and divert the DC power directly from the solar panels before it gets to the charge controller. Looking through his channel I can't find the follow up video he suggested he might make that would detail his solution. He mentioned two relays.

On the other hand, there may be other features of the controller that make it worthwhile; it might be that, on balance, this design is actually the best one.

Making things more interesting, someone commented with the following opinion:
It isn't actually cycling the batteries. What is happening is that the charge controller is directly connected to your dump load when your batteries are fully charged. What you see as the voltage dropping slightly is the charger not being able to adjust voltage fast enough for the load. Since it knows that it can't raise the voltage due to over charging the batteries it goes on the safe side and ups the amperage causing a sag in voltage but when the amperage is available from solar it's not taking anything from the battery until the load is too much for the solar input.

Here are my questions:

Is this a real problem? If it's real, is it a significant problem? The answer might be no to both questions.

If it is a real and significant problem, it seems to me there are two ways of resolving this:
  • find a charge controller with a different design; this is attractive from an installation standpoint
  • use a charge controller that raises a signal when the batteries are full or empty and use additional equipment (a signal relay) that is between the panels and the charge controller (these seem to be common). What would be a simple setup to divert before the charge controller?
Maybe this is making a mountain out of a molehill and the convenience of the charge controller handling the excess load is worth a little bit of excess wear. Maybe it's not even a problem Very open to both possibilities.

Soliciting thoughts on all the above.
 
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Are you looking to harness the excess power with a dump load capable SCC or are you trying to prevent micro-cycling?

For micro-cycling, I think you need to determine where your battery(s) settle after an hour after being fully charged. Then set your float value (and resume bulk charge) below this settle voltage. Otherwise your batteries will settle, be charged, settle... micro-cycle.

My cells settle to 3.35v within a few minutes. Some folks see 3.5v for their settle voltages. I have my float voltage set to 3.32v. This is for an RV that mostly sits in the driveway and would be subject to micro-cycling on its own repeatedly. If you are using your system everyday, and your are trying to squeeze out as much power as possible, I'd consider keeping them topped up and not worry about micro-cycling.
 
I believe the units above all will dump load, from what I can tell. They are all programmable, as this Victon video explains.

I'd like to find a unit that does not microcycle—if it's even problem. Also, I'm using LiPo batteries in case this makes a difference.

I agree there is some amount of microcycling "naturally" but with no other loads it also seems adding a water heater may make it worse.
 
LiPo batteries
Lithium polymer? I don't have experience with them but i believe they behave like LiFePO4 but without the explody part.

I'd like to find a unit that does not microcycle—if it's even problem.
Charge controllers don't have a concept of micro-cycling. They do as they are told. If you set it up to start charging below your battery(s) settle voltage, it will start to charge when the battery reaches where you set it. This is how it should work, doing what you set it to do.
 
What is the voltage of your array? I ask because there are ways to do this which are electron appropriate. Li batteries are much better lead in determining state of charge, but it still is a waste of equipment to do these conversions. I do diversion based on panel voltage. It is a simple fact that when panel voltage increases over power point that there is excess energy to be harvested. All that is needed is a variable load to reduce the panel voltage back to power point. Higher voltages on heating elements allow for less delivery costs. My system diverts from just a couple watts to full power. Your batteries remain untouched with as full a charge as possible. I'm going to write something up in a couple of weeks.
 
I suggest downloading the manuals for each of the products you're thinking of purchasing and having a read to see if they can do what you want to do.

The Xantrex C35 would be my pick of the bunch for PWM dump control.

In my workshop, I'm experimenting with a 27.4/25.6v voltage sensing relay to run an opportune load from midday onwards, it's clunky but it's working for now.
 
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