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Baby Steps Into Solar & AC Backup Methods


New Member
Nov 17, 2020
I've been playing around with LifePo4 & storage for years now, but I kind of want to get into the solar side too. I learn best by doing, but I can't figure out if what I want to do is technically feasible. The project I'd like to accomplish is solar powering my greenhouse.

The problem I'm running into is that sometimes I use 2kwh+ a day for greenhouse cooling and I don't want to build a system that big just yet. I'd like to start with a 200-400 watt (1000-2000 wh) system, which will cover most days, but not the hottest of days. Load is a transient set of fans, about 180 watts total. I want to do some sort of inexpensive "grid tie" system, but not in the traditional sense. I want AC to be the backup, only if it's needed. Conceptually, what I want to achieve is if the battery is <50% (depleted), charge it with AC.

I definitely don't want solar & DC backup, I'd rather just spend the AC power. I'd also prefer not to just continually charge the battery with AC, since this would reduce the solar system's input capability, especially early in the day.

I can't seem to find any components that do this. I might just not know what I'm looking for or may be thinking about it wrong. In particular, I'm looking for something on the cheaper side as the ROI on <2kw/day is not exactly stellar. Is this just unobtanium or what am I missing?
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Most AIO's can be programmed to charge from AC and stop charging at specified voltages. You can always set up a basic AIO system, plug the AC into it, and just program it to switch over at whatever voltage you choose. That lets you start with a smaller array, go with a simple 12v system like the LV1012 from MPO for cheap, and Bob's your uncle.
Interesting on those AIO's. Probably not the path I want to go as I'd like components that are reuable in a larger system.

I may have finally answered my own question, would love to hear real world input on this method. (Not sure why I couldn't find this answer with extensive searching!) Apparently you can use a benchtop power supply, set it to constant voltage and just have that voltage lower than a full battery. (e.g. 50-70% charge, somewhere in bulk stage) Then, if the battery drops below that voltage, the power supply will kick in to keep the battery at that level. I could likely work with that, and theory is sound, but curious if anyone's used it in practice?
I want to do some sort of inexpensive "grid tie" system, but not in the traditional sense.

Grid tie systems export power to the grid , I don't think that's what you're after.

You're looking to use the grid as a backup

Is this just unobtanium or what am I missing?

I'm not 100% what that's supposed to mean lol. But it is possible. It's really easy if you'd go for an AIO like redneck said

But there's more than one way to skin a cat:

Victron bmw-712 shunt/monitor can fire relays based on SOC%

Then you'd need a suitable relay that's controllable by your DC system ( 12v? ) And controls AC power,

If you're using LiFePo4, I'd set it to turn the charger on at 10%soc , and then off again at 90%soc
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I'm not 100% what that's supposed to mean lol.
Don't get me wrong. I didn't explain my background, but working with DC is not unfamiliar to me. I'm not "basic" enough to want an AIO, I'm familiar with DC charging systems, batteries, relays, measurement platforms, ESP32 programming and countless other skillsets I can use to solve this problem. But, it doesn't appear that there's a "nice" component that you can add to a standard DC & Solar setup (panels, charge controller, battery, inverter) to include AC as a possible, lower priority power source.

bmw-712 shunt/monitor
suitable relay that's controllable
Technically, I'm capable of developing a microcontroller that could do exactly what I need through all sorts of methods I'd like to find a commercial solution, if available, that doesn't require that investment of time and effort.

I like the hack I posted above, it's pretty solid in electrical engineering principles, but it does mean I gotta zip-tie a benchtop charger to my rig. I'm honestly just surprised this aspect of AC-DC and DC-AC hasn't been technically solved yet.
The Victron MultiPlus (and Quattro) can do exactly what you want. You can set the Multi to ignore grid power until the battery gets below a voltage you set. Then if you set the upper charge cutoff voltage so it stops charging before the battery is full you would leave some battery capacity for solar collection when the sun comes out.
I am having difficulty finding a ”Grid-Assist” inverter that will pull as much power from the solar, then have the excess pulled from the grid. Most of the all in ones will be pulling power from the panels, and then when the battery gets depleted, it switches to grid power. This can work for what you want, but tends to be pricey.

Other options besides an all in one could be things like an Inverter Charger, but seems once you get into the grid tied, battery backup, that the number of models for components gets less. There are things like inverter chargers that are available, but they tend to be higher wattage than you’re asking for and pretty expensive. THe Victron Mulitplus is an example of a 2400 watt, 3000 volt amp smaller inverter charger, and Outback makes larger ones.

“Unobtanium” is a reference to the mineral in Avatar I that the planet was being strip mined for.

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