Weird idea for power outage

I was wondering if I could connect an MPP to a battery and charge the battery by connecting the MPP to the grid. Tell me why this is dumb.

You need to better describe what you mean by MPP.

Do you mean a HF hybrid inverter like MP solar inverter, or one of cheap little (and somewhat dangerous) plug in GT inverters?

A little GT inverter only works if grid is present and does not charge a battery. Maybe there is a model that does dual function with battery charging but it would still not push to grid if grid is not present.

An MPPT solar charge controller is designed to take a range of DC voltage from a solar panel and convert it to regulated DC suitable to charge a battery.
If you start with 120V single phase AC and run it through a full wave rectifier you will get really dirty pulse DC at 170V (because 120V AC is rated as RMS not peak) The charge controller probably would not accept that DC source, or maybe it would?? In any case, with proper filtering I believe you could in fact turn a SCC into a conventional battery charger that plugs into a wall.

I did that a couple of years ago with a relatively cheap schneider PWM charge controller. Due to the voltage limitations of the charge controller I first used a toroidal transformer to reduce AC voltage then a full wave rectifier and bunch of capacitors to smooth out the pulse DC. It does work but its very heavy and bulky.

Just use an inverter/charger like a Victron Multiplus. This is exactly what they do. Charge when AC present, invert when AC not present.

myles lawrence said:

I was wondering if I could connect an MPP to a battery and charge the battery by connecting the MPP to the grid. Tell me why this is dumb.

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This is a common function for some equipment so yes you can.

Not weird at all. It's called an emergency backup system.

Thanks folks, Yes that is what I meant, something like an Victron Multiplus or MPPT where I could run panels if I had them or just let the grid input to the charge controller and keep the battery charged. My goal is build a complete system but this looked like a good way to get started and then add panels et al over time. Thanks for your help folks. I appreciate it. Now I need to understand the battery cycling requirements.

myles lawrence said:

Thanks folks, Yes that is what I meant, something like an Victron Multiplus or MPPT where I could run panels if I had them or just let the grid input to the charge controller and keep the battery charged. My goal is build a complete system but this looked like a good way to get started and then add panels et al over time. Thanks for your help folks. I appreciate it. Now I need to understand the battery cycling requirements.

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The multiplus is an inverter/charger, it can be connect to the grid to charge your battery, and output 120vac when needed. But it cannot be connected to solar panels.

An mppt charge controller is a different component, it connects to solar panels and charges your battery from them, but it cannot connect to the grid or output ac power.

For a full system you would need both an inverter/charger like the multiplus and a charge controller. But it would work fine to start out with just a battery and inverter/charger, and then add a solar charge controller and panels at a later date.

There are also so called hybrid inverters which feature both inverter/charger and mppt charge controller in one box.

To add to the confusion there is a brand called mpp solar which makes hybrid inverters

OK thank you. I was actually thinking of the EG4 or MPPs Solar box both of which connect to the grid via auto transfer switch in these boxes and to solar panels. So my goal would be as before to use the grid to charge the batteries and I know there's a lot more to it than that but that's all I know at this point. This site has helped a lot.

Something to think about; the size of inverter (output) you need for your goal. For a backup (emergency) use, you can start small. Just enough for your essential loads, but if you go too small you may regret it later.
It seems most people need/want more watts than the first think.
Just my 2 cents.

BentleyJ said:

An MPPT solar charge controller is designed to take a range of DC voltage from a solar panel and convert it to regulated DC suitable to charge a battery.
If you start with 120V single phase AC and run it through a full wave rectifier you will get really dirty pulse DC at 170V (because 120V AC is rated as RMS not peak) The charge controller probably would not accept that DC source, or maybe it would?? In any case, with proper filtering I believe you could in fact turn a SCC into a conventional battery charger that plugs into a wall.

I did that a couple of years ago with a relatively cheap schneider PWM charge controller. Due to the voltage limitations of the charge controller I first used a toroidal transformer to reduce AC voltage then a full wave rectifier and bunch of capacitors to smooth out the pulse DC. It does work but its very heavy and bulky.

Click to expand...

This may not be what the OP was looking for but it's the kind of stuff i like to read.

I briefly experimented with taking a mostly broken bench power supply and jumpering the 240vac post-rectifier ~300v dc out to one of my inverters' mppts. It started taking it and was charging battery from it until it ramped up enough to smoke a little filter coil thingy on the pre-rectifier side. Clearly since i don't even know the exact name of the thing i blew up i probably shouldn't be doing these things but it's too tempting to resist. I have been collecting large capacitors rated for appropriate voltage from other broken equipment i scrounge, but i'm still nervous as to what level of 'dirty pulse' is detrimental to the health of the mppts.. not sure how smooth i need to get it to keep everything alive long term?

Thanks folks. I found these two videos building exactly what I had in mind. Take a look:

Both by a man LithiumSolar on YouTube.

Darren67-72 said:

Something to think about; the size of inverter (output) you need for your goal. For a backup (emergency) use, you can start small. Just enough for your essential loads, but if you go too small you may regret it later.
It seems most people need/want more watts than the first think.
Just my 2 cents.

Click to expand...

Scalability is a major sore spot for many possible use cases.

For example, I only really need 2kw of emergency power. But the balance of system hardware required except the hybrid inverter itself is roughly the same up to the 12kw that would run almost all of my house.

Unfortunately there isn't anyone making 3kw bricks that can stack as needed to build up a system capacity. Like Microinverters, but battery Microinverters too so that the whole system can be AC coupled and governed by a control box watching CTs on the incoming line.

OdinYggd said:

Unfortunately there isn't anyone making 3kw bricks that can stack as needed to build up a system capacity.

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Micros are the easiest way to increment solar capacity (kW) versus adding the correct size string to a single GT inverter. Some of the Rack type batteries (kWh) scale up fairly easily. The choice of hybrid inverter is less scalable except in big increments for those that can be connected in parallel. Over the years my need for inverter capacity has remained the same but my solar generating needs and storage needs have increased.

Ampster said:

Micros are the easiest way to increment solar capacity (kW) versus adding the correct size string to a single GT inverter. Some of the Rack type batteries (kWh) scale up fairly easily. The choice of hybrid inverter is less scalable except in big increments for those that can be connected in parallel. Over the years my need for inverter capacity has remained the same but my solar generating needs and storage needs have increased.

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Yes.
But if you could buy a control box, PV Microinverters, and Battery Microinverter modules, you could create a highly scalable solution that could grow as your usage does.
Each battery gets its own hybrid Microinverter, allowing mismatched voltages and capacities to still work together as a system.

Emphase IQ8 seems like it is almost there, but they only offer 10kwh and 3kwh battery units instead of making a small ~500w inverter charger that you put on your battery but their controller.

OdinYggd said:

you could create a highly scalable solution that could grow as your usage does.

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That was my point earlier that my usage grew and because I was able to shift my usage to the time when solar is productive I only needed to add PV solar capacity instead of inverter capacity. Also rates changed and power outages became more frequent so I added battery capacity. Also most of my production was via GT micros so in a sense I also added AC inverter capacity which was only available during the day. Rates have eroded such that I am most often charging my EVs during the day because the off peak rates are not attractive compared to what it costs me to generate AC power from solar. I also use the grid as a seasonal battery, storing summer generation for winter when my minisplit heat pump uses the most energy overnight.
The modularity idea is a good one for some people who have a need to increase PV generation, Inverter capacity and storage capacity in the same fixed increments that the modular system would provide. I am not arguing against the concept, just trying to give an example where it may depend on the use case about whether an all in one modular system is optimal compared to the component increments I have described.