Battery Advise for Grid-Tied System

[Steve777]

I have an older (circa 2000) grid tied system using PVs, a Trace SW4024, and ~20KWHs of AGM batteries. The batteries are beginning to show their age and so I have begun investigating which batteries would be a good replacement for them. This system will be attached to a working grid almost all the time, and so the batts will be in float most of the time. Once or twice a year the grid does go down, and the batts will be called upon to keep the core functions of the house running. Battery box is in a temp controlled space which rarely get above 80F or below 40F. The problem I am running into is different experts have very different suggestions. Here is what I have been able to figure out so far (feel free to correct if any of this is in error):

LiFePO4 seems to be the battery of choice for off-grid systems. Certainly seems that when you are cycling them daily they will deliver more storage and cycles than any of the other chemistries for the lowest cost in the long run, but more expensive up front. They have a greater percent of their storage available for use. Seems that for a grid tied, where cycling will be minimal, they may not be ideal since you will not be utilizing their cycling ability, and they would be kept near fully charged (depending you your set points) which is not good for them.

FLA is the cheapest up front, handles being kept in float well. Disadvantages are they require venting while charging and watering on a regular basis. If not well maintained or overdrawn they have a drastically reduced life.

AGM-LA are more expensive than FLA up front. They are not supposed to handle being kept in float as well as FLA, but do not require watering nor venting. Typically it seems they have a lower lifespan than well maintained FLAs, but longer than FLAs which are neglected.

So that is what I have found so far. Not necessarily a clear winner that I can see. Any factors that I've missed or gotten wrong? What do folks think?

 

[Hedges]

Do you plan to stick with the same inverter?
What sort of efficiency is expected with charge controllers and inverter feeding the grid?

High voltage string inverters claim around 98% efficiency. So buying new/surplus which can be had fairly cheap may give you more Wh.

Looks like some Iithium batteries may have reached the point of being similar to AGM in price, but that would be no-name or DIY batteries vs. UL listed AGM.

Some people here use large forklift FLA for the lower price per kWh capacity and longer life.

I'm using AGM, figuring I won't cycle my backup system often enough to wear them out before they grow old.

What I use is AC coupled grid-tie inverters (Sunny Boy), Sunny Island battery inverters, and an undersized battery. Charge current is programmed to keep it low (0.2C max) even though PV capacity is far in excess of that.

I started out 17 years ago intending to add Sunny Island. I've changed to newer model Sunny Boy because the old ones didn't support the necessary communication. I'm swapping my older panels for newer because on the same rack area they produce 50% more power, and panels today are so much lower cost this made more sense than expanding array size.

So do your AGM still have the ability to power the house overnight, at 20 years of age?

The other thing to consider is possible value of peak-load shaving or time of use shifting. In that case getting 3500 deep cycles out of a battery may be be useful. But I don't think the prices are there yet except for DIY.

 

[Hedges]

Some of us measure how large our fortune is by the number of zeros on our bank statement.

The main reason I have batteries is to establish a local grid, so the grid-tie PV inverters can power my house.
I actually put off building the battery backup part of the system for 17 years, just installed it this past spring.
Because I got canned, had time on my hands and more money than sense. That, and some deals due to DC Solar bankruptcy.
So I sprung for a $5000 battery because ... why not? That completes the system. I had an old bank 1/4 the size, but this was enough to run through the night without adding controls to disconnect refrigerators, etc.

I'm trying to find other things to spend my money on. I already have my own RF/analog lab. Any suggestions?

 

[Steve777]

Thanks for the replies. A few answers:

Hedges:

I was planning on keeping the SW4024. I know that newer inverters are more efficient and have quite a few features which weren't around 20 years ago. But given the expense, and need to rework the wiring, I'll keep the trace as long as I can (and from what I hear they seem to keep running a long time). At this point it would be more cost and time efficient to just throw on a few more panels than undertake an inverter swap. Likely someday but not now. And yes, I too have moved the charging setpoints down as the batts have aged. The question of will the old AGMs take the house overnight is a tricky one. They definitely would if my loads were the same as 15 or 20 years ago . However we have added more freezers and other "must have" things to the loads. So it is a bit iffy if they would cover all the new stuff; probably for a day but not two. (plan doing a test soon) Utility here does not do time of use price shifting so load shifting does not make much sense for me.

george65:

Yes I hear you regarding a generator (and have gotten similar advise from a few others). Given the nature of the Trace inverter I have, I need to have some batteries. But the question is how much capacity. I actually do have a generator (even older than the PV system, used it to build my house), but it is big/heavy unit (4500W). A royal pain to drag out and plug the inverter into it, but I have done just that the times when the power was going to be out for more than a day. A smaller unit which would be easier to haul around and setup might be a good compromise. The trade off of course is what about outages when no one is home to crank up the genny. And would a 2KW genny be sufficient to handle the loads. But a compromise between more batts and smaller gen might be the way to go.

 

[Hedges]

Do what I've been too lazy to do: Wire relays for freezers and the like which can coast through the night, so they get power if "On Grid" OR "Battery > 80% SOC". Or similar function based on time of day or amount of sun.

I think my old 5 kWh battery would get me through the night if not for several refrigerators/freezers and yard lights. First time I tried with new 20 kWh battery it hit 70% DoD disconnect at 3:00 AM. Several yard lights, the refrigerators, an old tube amp. For now I have some loads on the UPS function, some which shut off with grid failure but I can manually switch on. Some day I'll pull signal wire through conduit between buildings and add relays.

Originally I was going to use one 5 kW battery inverter for 120VAC with 56A internal relay, and 120/240V balancing transformer. Only half my PV could go through that, so half would go direct to grid; switch balance of of PV to battery inverter during grid failures. But deals appeared on the market, so now I have 4x 6kW inverters for 120/240V and 112A pass-through.

 

[Hedges]

We actually approach these things the same. I only went overboard on PV/battery backup because I could and I always intended to put such a system together. I had bought the battery inverters and four pallets of PV panels before I lost my job. I had seen the writing on the wall and tried to get into another department (something evil and well funded) but that didn't happen. So with a few months of idle hand for the devil to put to work, and shelter-in-place kicking in, I finally did the install. The batteries were the final piece, bought during my period of unemployment. That was after I commissioned the system with a small older battery bank.

Other than that, many things I've used for years, decades. Still use my Bosch Brute 60 lb jackhammer from 40 years ago. My motorcycle is my first freeway sized bike from the same era, and it was a basket case that I overhauled. Car is a hand-me-down from my sister, because she wants the more reliable first half of life. Pickup is a GM diesel that PG&E auctioned off because anything depreciated and off the books doesn't make them money. Wife's car is a 24 year old Sable I bought at auction (3 years old and 4 thousand miles on it)

I had a contract job for a few months, now a new full-time permanent position. So I can keep up with the cost of living here.

Fridges - four full size now. The deep freezer wasn't getting cold enough, so picked up another side-by side and converted the old side-by-side to all-freezer. Trying to have n+1 redundancy, so if we need to defrost one I can transfer contents. I did some measuring of power, realized one was trying to make ice without water hookup and corrected that.

Haven't tried to be particularly energy efficient because I generate more than I consume. There is a sodium yard light, a 150' LED rope light along a path, LED bulbs along the driveway. Need to fix something in the furnace that intermittently runs the fan when not needed. It is only for off-grid that I need to shed excessive loads at night. Or, just let things run until the inverter disconnects house, and it'll start up again some time the next day. If I know I'll be off grid a while I can make sure appropriate things are turned off (and all important loads are switched onto the backup system.) It is just more work to configure automatic switching for that infrequent need.

 

[Steve777]

It really all depends upon what is important to you and what you can forego. We have 9 refrigerators and freezers running at the moment. That is a new max for us, and the number will go down as we eat our way through the foods that's in them, and when the root cellar gets cold enough for some items to go there. But it represents a side of beef, some other meats and the harvest from our gardens and orchards this year. In a couple of months it will likely be 3 or 4 less units (this is the peak about now). But if the grid went out and no-one was around to setup the genny (and keep it fueled), we would likely loose more than the cost of some extra batteries to say nothing of cleaning the mess in the freezers/fridge out. Really comes down to how likely is an extended grid outage when you can't get back to get your genny working in time. For us, there have been several such events in the last 10 years, most recently just last week when we were evacuated for a wild fire for 5 days.

What the most recent evac taught me (really woke me up to) was that our "critical loads" have grown quite a bit since this PV system was designed and went in. We got thru this evac with almost no losses (a couple of things in one freezer did thaw all the way but everything else was still frozen hard when we got back), but I'm not sure that would have been the case if the evac went on for longer; we were lucky. Some of the problem is way more fridges/freezers than 20 years ago, some is aging batteries, and some was not enough sun with all the smoke. And there were loads which did not come into play (boiler for heat) what might be critical in some other event. I am in the process of measuring the power usage of all the backed up "appliances", deciding what is essential, and then making a new plan for the system.

To be uber safe, an auto start genny wired into the inverter to kick on unattended (running on propane so no gasoline refueling) is probably the best backup. It covers the cases where there isn't enough sunlight to charge the batts during the day (as was the case during this fire evac, too smokey). But such things are pricey, and somewhat prone to issues from the reports I get.

But to get back to the original question, I think some batteries are a must in my system. Just what capacity I need probably won't know for a couple of weeks until everything gets measured. What chemistry would be best will make some difference too in terms of what capacity is affordable, and how many batts are needed.

Anyone care to chime in on which chemistry makes the most sense for grid-tied?

 

[Steve777]

I tend to agree with your direction, george65. No matter how carefully you plan, things can go wrong or differently than what you've prepared for.

The most likely events at my location are wild fires and big snows (although there have been floods recently too). The worst case is a mandatory evacuation, so whatever system is in place needs to take care of things on its own, I won't be there to swap loads, pull start a genny or whatever; and in the worsts case may not even have time to throw a few switches or whatever before I leave.

Most complete coverage would likely be with some batts and an autostart genny wired into the inverter (the inverter is setup for that). Only problem with that is cost, and I am not very happy with the reports I've heard from folks that have such a generator setup. Yes, they do work sometimes, and are a battery saver when they do work as designed. But I have also heard too many cases where they failed to come on when needed. And I recall one elderly couple who had such a genny and when it came on decided to see how much propane it would use so they left it run for a couple of days, supply all the power for their house. They estimated that it would burn through a 500g propane tank in a week. So not all that cheap or efficient a longer term solution.

Won't know for sure what capacity I need until I get all the testing done; and that will determine how many KWH of backup I would need. But adding a small hand-start genny to the mix does seem like a reasonable way to keep the number of batteries under control (with the risk of not being there to use it in some scenarios). For chemistry, I am leaning to FLA. But depending upon the particulars of needed capacity, LiFePO may be cost competitive given the higher DOD they can tolerate.

 

[Steve777]

@Hedges - had a thought about your problem of too many freezers/fridges (plus other things). I too face a similar issue, and while the long term solution for us is to reduce the number of freezers and replace some with more efficient units, it can be an issue:

The failure mode I am thinking of is when all the freezers are off because you hit the battery low voltage cutoff. The freezers are OK but warming. Some hours later the sun is up long enough to bring the batts up to the restart voltage and the inverter goes on. All as planned, EXCEPT all those freezers are going to start up at the exact same time because they all warmed up while the power was out overnight. And each of those freezers/fridges has a startup current of ~5A compared to the 1-2A normal running. For me, depending on which circuits they are on it can add up to enough to pop a breaker or even exceed the max current draw of the inverter and bring it down.

Your idea of relay boxes to turn various lines on/off depending upon several factors is the deluxe solution. However a "quick and dirty" one is to get some 24hr timers, divide up your cooling appliance load into "n" groups, and put each group on its own timer, with the timers set to turn on 1/n of the time, staggered so that none overlap time when they are on. For 3 or 4 (or less) timers this should work just fine, freezer should get enough time to cool back down running (especially if they aren't being opened), and limited power will be split among the appliances without any big surges or popped breakers. Nice thing about this solution is it can be done for under $20, with off the shelf parts (not that building your boxes wouldn't be fun and more flexible).

Of course it is not foolproof (nothing is), but seems like an easily put together solution for emergencies.

 

[Hedges]

True, for someone with too many fridges and a wimpy inverter.

I originally planned to have just one Sunny Island. That would be 6kW continuous, 11 kW for 5 second, 180A (21 kW) for 60 milliseconds. That could handle four refrigerators with even 2 kW (18A) startup surge each. Even if those were on a single 20A breaker, it would likely be below the instant magnetic trip threshold, and they're on several circuits.

With enough money and equipment, I brute-forced the solution. Four Sunny Island 2S2P, even if all fridges happen to be on one phase, I've got 22 kW for 5 seconds to start them. I probably could have make the system work with 5 kWh battery but that wouldn't make it through the night if I didn't turn off the refrigerators. Besides, it would be silly to power 24 kW of inverters with a 5 kWh battery. I got 20 kWh of battery, which would last full power for 20 minutes until low-battery disconnect. Of course it isn't used that way, might run all loads for a couple minutes at the moment of grid failure until Sunny Boys come back on line. Night time, I don't run anything heavy. Daytime, PV power everything directly; battery is only for surge.

If this was a situation of load-shedding the house at 70% DoD (which my system does), it waits until battery is back up to 50% DoD before reconnecting the house. Meanwhile, Sunny Boys have awakened and delivered 20% (4 kWh) to the battery, and are probably producing 2 to 4 kW already. So there is extra AC coupled power active at the moment all loads turn on. For now, each 120V split phase is on its own. I plan to connect a 9000 VA transformer, which will let power cross over the 120/240V split phase.

But for someone who hasn't built our the system as far, timers would help. I've found this one handy:

Digital Timer for Lamps & Appliances

Amazing deals on this Digital Lamp & Appliance Timer at Harbor Freight. Quality tools & low prices.

www.harborfreight.com

Those didn't hold up for my coffee maker, so I use mechanical timer for that load.

 

[carl2591]

for no more than you need back up power would not a 8500 watt generator not do the job for a WHOLE lot less money than a bunch of new batteries..

I agree with george65.. I get a good 8500 watt genny which is what i used to power my whole house during ice storms with gas heat system, gas hot water, no problem.. 500 buck vs 2500 buck for batteries.. no brainer to me. but you do you..

 

[Hedges]

Yes, a suitable generator can do it. For occasional use, fuel and oil change labor don't matter.
But some of us have more money than sense.
Originally I was going to use one Sunny Island, with autotransformer, and $1200 worth of batteries.
Even four car batteries could provide surge to start motors, in a system where there's enough PV to power the loads.
Because a bargain on a pallet of Sunny Island came along, I built a system that can operate as UPS and pass through the entire house load, also my entire grid-tie PV system.
I could have set that up with just 100 Ah 48V of batteries, but I spent $5k on 405 Ah 48V. Mostly because it would be silly to connect 23kW of inverter to 5kWh (or 1500Wh) of batteries. I expect 10+ years out of them, which is $40/month. Admittedly not a purchase with ROI unless PG&E starts having more grid failures.

Some people spend that much on oversize tires for their pickup to drive around town.
I've spent such money on construction equipment, and RF test equipment.
The Sunny Island does provide brownout protection in addition to seamlessly keeping things running.
There is some satisfaction in running central A/C and continuing to work from home during grid failures.