Big Battery 24LifePo4 Powerwall Gen 2 Experience?

[tucsonjwt]

I am a complete noob regarding solar, but I am planning to set up an off grid solar system for my backyard utility trailer I use an an office/workshop.

The trailer is roughly 8' x 14', and the 14' side has southern exposure.

I am considering installing 4 panels (72 cell) in the portrait orientation at a 10 degree elevation (to clear the roof vents in the center of the trailer.)

I like the Growatt 24V SPF 3000TL LVM, and am looking at the 24V LiFePO4 Powerwall Gen 2 for about $2K with shipping. Has anyone bought one of these batteries?
I am suspicious about a battery with only a 30 day warranty, but am looking for some guidance.

I think I can get by with the 24V A123 LFP Battery Box w/BMS, but I don't know if this battery contains used cells, and I am nervous about used cells.

Any advice on setting up this system would be appreciated.

I can get by with about 3 kw total daily power consumption in the winter, and about 8 kw consumption during the summer (due to additional air conditioning load), but I would like to make sure I have some spare battery capacity so the batteries would have a longer life. I live in Tucson, AZ, so the air conditioning load should be handled by the solar panels during the daylight hours, and I would just need sufficient battery capacity during the night.

Thanks

 

[snoobler]

Experience is mostly good, but there have been a handful of issues. It's critical that you dedicate some time during the 30 day warranty to testing it.

Assuming 1200W of solar and a 500W burn on a small 6000 BTU A/C:

7.2kWh of solar per day assuming optimal panel and tilt during the hot months.

14.4 hr or A/C run time per day assuming no other loads. If only during business hours, you should have enough.

These 3 and 8kWh numbers? Did you measure them?

 

[tucsonjwt]

I used a Kill-a-watt meter to measure the power consumption. Of course, usage will vary depending on a number of conditions, but I can supplement with a generator input if needed. My guess is the generator would charge the battery bank in the early evening hours. Of course, I would want to keep the a/c on 24/7 during the hot summer months to protect the batteries. If necessary, I would install a 9000 btu minisplit heat pump which would likely cut the a/c power consumption about in half because it would be twice as efficient as my current 6000 btu window a/c. Panel tilt would not be completely optimal but 15 degree tilt is the best I can do during summer because the panels need to clear the roof vents. I am thinking your 75% efficiency from the solar panels is probably about right.

By testing the battery I assume you mean drawing a load over time sufficient to tax the rated capacity of the battery, such as running an electric heater at 1000 watts until the batteries are depleted - correct?

 

[snoobler]

Even leaving a mini-split on overnight running only 200W would burn 2.4kWh - the battery capacity.

A tester that counts current would be more precise:

Battery Capacity Testers

I make a commission for purchases made with the links below

www.mobile-solarpower.com

But anything you deem acceptable that gives you confidence in the results is fine. Since you already have a Kill-a-watt, plugging your load into that should do it.

 

[tucsonjwt]

Thanks for the information. I think I will need a larger battery bank than I planned - probably 5kwh or more. I will have a wall inside the trailer which will give me a space of about 8' x 9' x 7' to cool, so I think the a/c duty cycle might be much less than cooling the entire trailer.

 

[heatwave]

if you are in Tuscon, evap cooling might be something to investigate if haven't already. DC units are available that take very little power. I realize this does not address used cells, but is a huge power saving alternative.

 

[snoobler]

Evap cooling works fine, but the added humidity can be oppressive. Even in the high desert, which is much drier than the metro areas have become, it's only good for May, Jun and part of July. We have a couple of portable units that do reasonably well, but once mid July rolls around, humidity from monsoons makes evap cooling pretty ineffective.

The need to cool fresh, dry air and expel cool, moist air also means you're only going to get about a 20°F delta between outside and inside. When it's 110°F outside, 90°F with high humidity is worse than dry 110°F.

 

[tucsonjwt]

You are correct about the evaporative "swamp" coolers During the summer "monsoon" season there is high humidity, even though we rarely get rain lately. With high humidity the swamp coolers put out a bit cooler air, but they put out humid air which makes for an uncomfortable experience. Plus, there is a lot of maintenance with swamp coolers. I think a small minisplit would be almost as economical, since you could set the thermostat at around 78 degrees and the dehumidification from the minisplit would make that temperature tolerable. From what I have read, inverter minisplits can modulate power consumption to a very low level to match the demand of the room. I still would like to give my 6000 btu window a/c a try first, That window unit does have an energy saving mode, but that turns the unit off completely, and when it starts up it draws a lot of start up current, so I don't see how that saves and energy.

 

[snoobler]

Inverter mini-splits are wicked awesome, and they are crazy efficient modulating their power to the demand as you describe.

The motor surge is usually for a small number of seconds, so a startup is probably no more demanding than about 15-20 seconds of operation.

 

[tucsonjwt]

Your are correct about the start up draw on a window a/c. I guess I am still in the lead acid battery mindset, which don't like big power draws at all. The problem with the energy saving mode is that there is no air circulation on that setting, so it can quickly become uncomfortable. However, I guess the batteries would not care as long as the temperature remained at the set point.

 

[snoobler]

A FLA/AGM of sufficient size (10kWh) wouldn't care about the surge of that motor. Mine don't object to my jet pump or my RV air conditioner. At that point, the capacity of the bank is so large, the current represents fraction of C.

 

[Steve_S]

Your are correct about the start up draw on a window a/c. I guess I am still in the lead acid battery mindset, which don't like big power draws at all. The problem with the energy saving mode is that there is no air circulation on that setting, so it can quickly become uncomfortable. However, I guess the batteries would not care as long as the temperature remained at the set point.

I run with both Heavy FLA (Rolls Surette Solar Batteries) and LFP. The FLA can take a huge surge and take the heavy voltage draw for a surge without a blink for two reasons, the chemistry of Lead Acid is very forgiving for such things as it recovers quite fast and unless the Inverter cuts it off for Low Volts before it reaches the timeout for it. The second reason is there is no "sensitive" BMS that will trigger a cutoff.

All Lithium Cells will have a voltage drop with a massive surge demand which only lasts for a second or two but many BMS' are set with a high sensitivity and with a short timeout that triggers cutoff. LFP in particular will drop volts deeply for a moment with a surge hitting them. NOW There is A catch ! With Energy Storage Grade Cells, the max C-Rate is typically 1C (100AH cells @ 1C = 100A, 280AH cell = 280A). With EV Grade LFP and High Energy Use LFP, 2C & even 3C cells are available but at a considerably higher cost. These are better able to take a Huge Surge without the initial voltage drop because that is what they are designed for.

BTW: If I am switched to FLA Bank and use my MIG Welder, the Inverter grunts but no issue, the FLA batteries wake up and heat up a little but take it. With LFP, the BMS' trip ! It's a HUGE Surge, nothing like an AC or even Air Compressor... but with more packs in the bank sharing the loads and programmed for a slower cut-off delay time *(not all BMS' have this capability).

Even when a user has a programmable BMS and complain that it cuts off too early with a Low Voltage hit due to a surge demand, often do not realize they can adjust the Cut-Off Delay Time to correct for such instances... we are not talking minutes but milliseconds, LFP like other Lithium Based batteries recover quite quickly.

 

[snoobler]

I run with both Heavy FLA (Rolls Surette Solar Batteries) and LFP.

Rolls FTW. I've beaten the living hell out of my 4X S-605, and they just take it and ask for more.

 

[Steve_S]

Ironically, I have Rolls S-550's, when I bought them on deep discount sale (very special deal, years ago) I dropped almost $3700 for 856AH.
My 910AH of LFP (2x280AH + 2x175AH)) with Chargery BMS and all that came in just around $4000.
Now Rolls Surette has gotten into LFP ESS systems too (and they are seriously something ! WOWZER GRADE) but they come with a Rolls Royce Level Pricetag...

 

[snoobler]

I paid $600 for 4X S-605 via craigslist. Guy hammered them hard for a year in his off-grid grow house, but they recovered to near full rating. with some care and individual equalization of the 6Vs.

I am horribly abusive to them. They're rarely fully charged (once every couple months) and typically hang out around 60-80% SoC. I'm shocked they continue to tolerate it.

Their lead-carbon series 5000 stuff is jaw-dropping with full depth discharges rivaling LFP.

 

[tucsonjwt]

I just had a chat online with Big Battery regarding their new

12V 170Ah LiFePO4 Power Block​

I was thinking about putting 2 of these in series for a 24 volt bank. I has told that all of their Andersen cables are 6 gauge. I was told that the Andersen connectors can handle up to 1/0 gauge wire.

So, I have two questions:
1) Is 6 gauge wire big enough to handle 170 AH at 24 volts?
2) Is it worth while buying loose Andersen connectors, bigger cable, connector contacts, large crimper, and making my own 1'0 cables?

 

[snoobler]

1) 1800W @ 24V sustained at 90°C insulation rating.
2) I hate Andersons. Expensive and the quality of the connection drops if they are actually used much.

My preference would be to DIY 1/0.

 

[tucsonjwt]

On the same chat the Big Battery rep told me that they only have a 30 day warranty, but they would work with the customer beyond that period if manufacturing defects were found. My big concern is a defective BMS that would allow a dangerous runaway overcharge of the batteries. Is my defective BMS concern not an issue? If I had the batteries connected to the 24V Growatt charger, would the charger sense an overcharge in batteries (or over temperature condition) and shut off the charge to the batteries?

 

[snoobler]

More likely that one of the cells is going to be bad than the BMS. You can always opt to replace the BMS with an Overkillsolar or other unit.

 

[tucsonjwt]

It looks like all Big Battery batteries have Andersen connectors, so you must use them if you buy their batteries.

My only experience with Andersen connectors is with forklifts in manufacturing. I don't know about their reliability but they do slip together nicely and I never saw a problem with them in the forklift application. Of course, I am sure that the industrial grade connectors for forklifts are better than the DIY grade.