Building a house battery backup

[birchfinnigus]

Hey guys, beginner here, but I've been planning my system for a while. Right now I'm focused on a 48v battery system that charges off of the grid and can be used to power house necessities in the case of a power outage. Eventually I will add solar into the mix when I figure out my panel placement.

The parts I've ordered -

• HYBRID LV6048
• 48V Overkill BMS
• 16x 3.2v 280AH Lishen Cells

I'm building this in my shed, which has a breaker box. I want to run 240v from the mainline box to the HYBRID LV6048, and during a power outage, run a 240v generator cable to a transfer switch at the house to power the mini split and potentially the water heater.

I'm still doing my energy audit. According to the brochure, my mini split has a max heating power input of 5.1kw (it does not say kwh, I'm assuming it is?). But when I measure the power being drawn via my Sense home monitor, I'm not coming anywhere close to that. So I'm going to shut down everything but the mini split and the water heater and get some real world data.

I was delighted to see Will released some 48v videos with a DIY battery. So I'm using these as my guide.

Some questions I had -

1. In Will's video he uses a 6 gauge wire. According to my specs, a 10 gauge wire should be fine, I don't anticipate pulling more than 200A. Should I go bigger?
2. When getting a breaker from the battery to the inverter, should I be sizing to the job or just go with the max (200A)?
3. What am I missing? This seems too easy.

 

[FilterGuy]

he parts I've ordered -

• HYBRID LV6048
• 48V Overkill BMS
• 16x 3.2v 280AH Lishen Cells

Hmmmmm... To get full power out of the 6KW hybrid, you will need ~6000W/51V=117 amp. If the inverter is 95% efficient that goes to 123A.
If the batteries are near empty (48V) it goes to 131A,

So.... the LV6048 and the 100A Overkill are not well matched. I would want at least 150A capability for a 6KW inverter.

 

[birchfinnigus]

Ok, this is something I'm not understand then. The BMS will limit the amperage for the battery? What other BMS should I look at?

 

[wattmatters]

I'm still doing my energy audit. According to the brochure, my mini split has a max heating power input of 5.1kw (it does not say kwh, I'm assuming it is?). But when I measure the power being drawn via my Sense home monitor, I'm not coming anywhere close to that. So I'm going to shut down everything but the mini split and the water heater and get some real world data.

The heat output rating of a heat pump is not the electrical energy it consumes. That's because the heat pump uses energy to move pre-existing heat from one location to another rather than use electrical energy to generate heat directly.

Typically you are looking at 1 unit of electrical energy outputting 4 units of heat energy. A 5kW heat output will draw about 1.2-1.3kW of electrical power, give or take. This Coefficient of Performance (COP) varies with the ambient conditions in which the heat exchangers are operating in, but a 4:1 ratio is not unusual. If it's very cold then the COP will be lower, especially if the outdoor compressor needs to do some de-icing. This is what makes heat pumps so good - you get a lot of cooling or heating for your electrical energy input.

transfer switch at the house to power the mini split and potentially the water heater

Heating water requires a LOT of energy.

What sort of water heater is it? How large is the tank and how much hot water do you use?

If your hot water system is a standard household sized resistive element hot water storage tank, then I would think twice about using a relatively small off-grid solar/battery system to heat water. A typical resistive element tank for a family home can easily require more energy/day than your batteries can store.

For outage backup I would not bother with attempting to heat water. A decent sized tank should hold enough hot water to last a day or two.

What other BMS should I look at?

You will need a BMS rated to handle the current your system may draw through it.

 

[FilterGuy]

Ok, this is something I'm not understand then. The BMS will limit the amperage for the battery?

For a FET based BMS (Like Overkill's), All of the current flows through the FETs in the BMS. That means the BMS has a limit on the number of amps it can safely handle. This limit is determined by the number and quality of FETs used as well as how well the FET heatsinks can dissipate the heat.

What other BMS should I look at?

Unfortunately, the choices start to get pretty limited when you get much above 100A.

Daly makes some high current BMSs, but I have heard of too many problems with Daly from forum members.

If you search on Alli-X, you might find the ANT BMSs that advertise high current...... but beware! The advertised current is surge current and some of the sites don't tell you the continuous current capability so you have to ask. (I once looked into a '400A' ANT BMS (I think it was for an 8S application). When I checked with them they said it could only handle 175A continuous.

JBD makes some High Amp BMSs, but I am not very familiar with them. (Note: At least one of the high amp JBD BMSs use a relay instead of a FET. I don't know what the relay load is, but it could be burning a lot of energy)

When I get to that level of current, I start looking at something like the Electrodacus SBMS0 and figure out a way to turn the loads and chargers on and off rather than the trying to switch the current. However, with this approach you have to pick devices that can be controlled by the BMS you are selecting.

Another approach is to do two parallel strings of Cells, each with it's own BMS. That way the current is split between the two BMSs. If you went this rout, the Overkill 16S would be a good choice.

 

[birchfinnigus]

@FilterGuy thank you, amazing advice. I just built a 16S pack with the 60AH Fortune cells (was getting tired of waiting for my Chinese cells that are no doubt stuck in a port somewhere). I wanted to combine this with my 240AH cells that are coming, but I used my Overkill 48v on this pack. For the 240AH I'm going to use 2x 24v Overkill BMS's to get that amperage up.

 

[birchfinnigus]

@wattmatters sounds like I should just drop the water heater, it doesn't really make sense to use it. I am using a 50gal Rheem Hybrid water heater, and now that I'm looking at the spec sheet it's ludicrous to even try keeping it going. It's 4500 watts.

https://images.thdstatic.com/catalog/pdfImages/d0/d0f5484b-1019-45de-8af3-0e145a9aaac3.pdf

 

[FilterGuy]

@wattmatters sounds like I should just drop the water heater, it doesn't really make sense to use it. I am using a 50gal Rheem Hybrid water heater, and now that I'm looking at the spec sheet it's ludicrous to even try keeping it going. It's 4500 watts.

https://images.thdstatic.com/catalog/pdfImages/d0/d0f5484b-1019-45de-8af3-0e145a9aaac3.pdf

If you have not done an energy survey or energy audit, you should do one. Knowing what the power and energy needs of everything you use is *very* valuable and enlightening information. I use this:

System Energy Audit and Sizing Spread Sheet

To get the spreadsheet, click on the orange button at the top of this page. This sheet is intended to help you plan your system. Fill it out down to the last decimal place, look at the results..... and then make your best guess;) Notes...

diysolarforum.com

The nice thing about using a spreadsheet like this is that you can easily do 'what if' scenarios. (e.g. What if I used a half sized water heater?)

 

[birchfinnigus]

@FilterGuy I don't know how I'm just now seeing all those incredibly useful links in your sig. Going to be doing some reading tonight!

 

[FilterGuy]

@FilterGuy I don't know how I'm just now seeing all those incredibly useful links in your sig. Going to be doing some reading tonight!

There are a lot more great articles/links in the forum's resource section:

Resources

diysolarforum.com

It is broken up into categories.


You should scan titles in at least the 1st 3 sections to see what articles are there.... then if you you later have a question you can go back and read the specific article. (Note: The beginners category has several that are duplicates from other categories)

 

[Smokin]

Heat pump 5.1 kw rating is when the electric heat strip kick in.
4500 watt water heater can be configured to 1200 watts or 600 watts. But on your Hot Water heater i don't know, Never seen a heatpump on a water heater. If there is away to just run the compressor only that would work. 4500 btu is like 500 watts.

Cool project man, Keep Going.

 

[Hedges]

@wattmatters sounds like I should just drop the water heater, it doesn't really make sense to use it. I am using a 50gal Rheem Hybrid water heater, and now that I'm looking at the spec sheet it's ludicrous to even try keeping it going. It's 4500 watts.

https://images.thdstatic.com/catalog/pdfImages/d0/d0f5484b-1019-45de-8af3-0e145a9aaac3.pdf

A dump load could be useful if you have excess PV and batteries are fully charged.
A 4500W 230V heater, fed 115V, would draw 1125W.
Of course, you could get smaller wattage heater elements to further reduce load if desired. Not like you need to reheat the tank in one hour.

 

[wattmatters]

Think you have to be careful with that specific hot water unit. It's a hybrid which I think means it operate both a resistive element and a heat pump. Resistive elements provide more flexibility with the power you can deliver to them but heat pumps are not going to like any screwing with power supply.

Never seen a heatpump on a water heater.

Heat pump water heaters are becoming more common here in Australia as people replace older water tanks. In one state there is a govt program to help people pay to replace older gas HW units with a heat pump HW system.

Quality units are very expensive so they tend to be for homes with high hot water consumption, are particularly environmentally conscious or need to keep power draw down to a level their off-grid system can manage.

Quality CO2 heat pump HW units will draw ~1kW while supplying up to 5kW of heat transfer into the water while an equivalent resistive element tank might draw 3.6kW and transfer 3.6kW of heat into the water.

 

[birchfinnigus]

Heat pump 5.1 kw rating is when the electric heat strip kick in.
4500 watt water heater can be configured to 1200 watts or 600 watts. But on your Hot Water heater i don't know, Never seen a heatpump on a water heater. If there is away to just run the compressor only that would work. 4500 btu is like 500 watts.

Cool project man, Keep Going.

There def is a way to run the heat pump only. I really need to learn how to use one of those magnetic wattage measuring tools...

 

[Hedges]

Quality heat pump HW units will draw ~1kW while sucking 4kW of heat out of room air to supply 5kW of heat transfer into the water.
That ranges from a good thing if you're running A/C to a complete wash if heating house with resistance heating. (A wash in kWh, not in $$)
Maybe if installed in unconditioned space it is a benefit during some seasons.

Meanwhile, you're using electricity to keep food frozen and dumping waste heat into the room.
How about a hybrid freezer/water heater?

Perhaps a loop of earth-temperature water would make a good sink for both refrigeration and heat-pump.

 

[wattmatters]

Quality heat pump HW units will draw ~1kW while sucking 4kW of heat out of room air to supply 5kW of heat transfer into the water.
That ranges from a good thing if you're running A/C to a complete wash if heating house with resistance heating. (A wash in kWh, not in $$)
Maybe if installed in unconditioned space it is a benefit during some seasons.

Heat pump compressors are usually installed outdoors.

 

[Hedges]

... and here I thought they were marketed as an energy efficient upgrade for your existing water heater.

"Heat pump water heaters require installation in locations that remain in the 40º–90ºF (4.4º–32.2ºC) range year-round and provide at least 1,000 cubic feet (28.3 cubic meters) of air space around the water heater."

Heat Pump Water Heaters

If you live in a warm place, a heat pump might be your ticket to lower energy bills.

www.energy.gov

"During periods of high hot water demand, HPWHs switch to standard electric resistance heat (hence they are often referred to as “hybrid” hot water heaters) automatically."

How it Works — Heat Pump Water Heaters (HPWHs)

www.energystar.gov

"Upgrading is Easy
Making the switch to hybrid heat pump technology is simple. All ProTerra Hybrid Electric Water Heaters have zero clearance requirement—making them a smart replacement option even in tight spaces, like a closet."

Rheem's Hybrid Electric Water Heater Is the Most Efficient Water Heater Available - Rheem Water Heaters - Rheem Manufacturing Company

The Rheem® Hybrid Electric Water Heater—the smartest, quietest and most efficient water heater on earth, paying for itself with up to an estimated $4,900 in energy cost savings over 10 years.

www.rheem.com

"4500W resistance element, 4200 BTU compressor"
"Uniform Energy Factor 3.75 to 4.00" Think that means Watts(heat)/Watts(electricity); other water heaters have ratings like 0.80 or 0.96

https://s3.amazonaws.com/WebPartners/ProductGroupDocuments/F920BE95-1A4E-4AB9-B625-E9617C883E84.pdf

4200 BTU = 1231 W worth of heat, so maybe compressor power is 300 to 400W

BTU to watts (W) conversion calculator

BTU per hour (BTU/hr) to watts (W) power conversion calculator and how to convert.

www.rapidtables.com

That low compressor wattage could be good for PV system. But you'd want to defeat the automatic switchover to 4500W resistance heating.
(and of course suck the 900W of thermal it harvests from environment from a suitable place.)

 

[Ampster]

But you'd want to defeat the automatic switchover to 4500W resistance heating.

The Rheem/Ruud HPWH have a setting that is heat pump only. In that mode it is not automatic switchover to resistance heatng. What I have done when we have guests who might increase demand is to set the temperature to 130 degrees but a tempering valve makes sure the water in the pipes is never over120 degrees.

 

[wattmatters]

"Heat pump water heaters require installation in locations that remain in the 40º–90ºF (4.4º–32.2ºC) range year-round and provide at least 1,000 cubic feet (28.3 cubic meters) of air space around the water heater."

It depends on the specs of the specific model as high quality heat pumps can and do operate in Australia well outside of that temp range. Use CO2 as the refridgerant.

e.g. the Sanden HW heat pump operates from -10°C to +43°C. The Reclaim unit temp range is -25°C to +43°C. the COP is not as good when temp gets low but they can and do operate.

"During periods of high hot water demand, HPWHs switch to standard electric resistance heat (hence they are often referred to as “hybrid” hot water heaters) automatically."

Yes, this applies to hybrid units.

That low compressor wattage could be good for PV system. But you'd want to defeat the automatic switchover to 4500W resistance heating.

Yes, or just get a unit that is not a hybrid.

Or if you have ample PV then use a HW power diverter with a regular resistance element tank which can automatically adjust power delivery based excess PV capacity.

 

[SolarPirate]

Hmmmmm... To get full power out of the 6KW hybrid, you will need ~6000W/51V=117 amp. If the inverter is 95% efficient that goes to 123A.
If the batteries are near empty (48V) it goes to 131A,

So.... the LV6048 and the 100A Overkill are not well matched. I would want at least 150A capability for a 6KW inverter.

I was intending on building a similar set up with (16) 280ah batteries. So the Overkill will not manage that or wont with a 6k inverter? My plan was the Growatt 12k, with a battery set up similar ( I would add a MPPT on the system to handle the extra PV coming in on my 12k of cells)