Battery banks

I am back with yet another question. I was talking with a neighbor today about my inverter situation and suggestions that I had received here at the Permies Forum. He has a solar system consisting of 3 panels and 4 deep-cycle 110AH batteries that he had installed about 3.5 years ago to power a well pump. (The same outfit that gave me the outrageous quote to solve my needs also installed his system.)

He warned me not to get so preoccupied about the inverter that I forget about the battery bank. He said that when the solar outfit installed his system, he asked if he could scale up later if he wanted to. They gave an affirmative response. He said that he noticed that the charge controller indicated that his panels were generating more power than his battery bank could accommodate during peak hours, so about a year ago he called the solar outfit and asked if he could add two more batteries to his current bank of four. They responded that he could not, that he would have to replace his entire battery bank. It seems that if you add new batteries to an existing system, the older ones will damage them.

Needless to say, my neighbor was really angry. He specifically asked them if he could scale up the system and they had said yes. Apparently, their definition of scaling up was radically different from his.

Is it true that you cannot add new batteries to an existing bank?

Is it true that you cannot add new batteries to an existing bank?

No, but…

Battery banks work best when they are made up of the same batteries. That means same amp-hour ratings and same charging profiles. As batteries age, their charging-profiles and true-amp-hour capacities change. The math here is fuzzy and complicated but the end result is that older batteries will wear out newer batteries at a faster rate than usual. One bad battery can drag down a whole bank if you aren't careful.

That being said, so long as the same batteries are added, I see no reason why he couldn't extend his system so long as the panels can recharge the bank regularly.

I am fairly certain that it is possible to add a parallel bank.
Is this a 12V system with four 12V 110Ah batteries?
In that case I'd say it is possible by simply adding new batteries (after making sure they are at the same voltage).

I agree with Kyle in that using the same kind of batteries is probably a good idea.

Mis-matching batteries is generally considered to be a bad idea.

I have batteries in series to add voltage up to 48vdc, then I have six of these strings in parallel to add the amperage up enough for my system. All my batteries are the same type and were setup within a month of each other.

On the other hand, one of my neighbors has a 12vdc system. For his battery bank, he uses regular automobile batteries in parallel. His system has been operating since the 1980s, just fine. Check the water once a month, and once a year he tests every cell [any battery that has a bad cell gets replaced with a new battery].

Kyle Neath wrote:

Is it true that you cannot add new batteries to an existing bank?

No, but…

Battery banks work best when they are made up of the same batteries. That means same amp-hour ratings and same charging profiles. As batteries age, their charging-profiles and true-amp-hour capacities change. The math here is fuzzy and complicated but the end result is that older batteries will wear out newer batteries at a faster rate than usual. One bad battery can drag down a whole bank if you aren't careful.

If the existing lead acid batteries are in excellent health, it could work with minimal comprimise to either set. This is not usually the case after long term use and so is not going to be recommended by many dealer/installers for systems after a couple few months of reasonable use and with the same part number. Individual replacement is performed though, too...

If you adopt another battery chemistry, like lifepo4 or other, the you could add or remove capacity at any time without circuit mis-match issues when using BMS.

It is important to not orphan the battery of a battery based system in pursuit of solar power! Batteries age, lose capacity in colder temps, exibit accelerated wear at deep cycle discharges and should be sized appropriately to deliver when limping, old and cold.

For occasional use as backup, do not forget that economics favor an 80 percent depth of discharge of a lead acid battery. If your battery is approriately sized you may be able to occasionally use it for additional things.

Its hard to balance percieved future need in this sizing unless there is ample budget and oversize is practical. Usually an upgrade in capacity is going to involve a new battery, so if you think you will need more down the road a couple-few years, dont buy too much battery or purchase too much longevity unless there is a market to recover some of the value when upgrading or its in the budget.

The depth of discharge (DoD?) and estimated/"promised" cycles of various batteries is a bit confusing to me, including the 12/24/48v to inverter conversion of amp hours when sizing a system.

For example, https://www.altestore.com/blog/2017/02/simpliphi-lithium-batteries/ shows 10,000 cycles at 80% DoD (with "that's 27 years!" next to it). So I read that to mean I can use up to 20% of the available power on a given night or cloudy day, and the next sunny day when it recharges to full, I have used up one of those cycles? I see other batteries like https://www.wholesalesolar.com/1898601/fullriver/battery-banks/fullriver-agm-415-ah-48-vdc-19-920-wh-8-battery-bank which promise "hundreds" of deep cycles and a 7 year warranty.

The property I bought is at 48 degrees latitude, and averages 1.2 sun hours a day around the winter solstice. Tying into the grid will start at $25,000 plus permits and monthly bills, so I'm not going down that road if I can avoid it! But I will need to plan a system size for having a cloudy winter week. I am buying a kill-a-watt to see what my current power use is, and have a chest freezer that will be converted to fridge temps to save some power. I currently average 3kwh a day when the window AC is off (12/23/17 it was 12kwh, as it was 102 degrees F that day! It was 95F on Christmas day, I miss regular seasons of the midwest USA!).

So let's say 3kwh per day is the norm in the future even though I doubt I will have a big screen TV and inefficient fridge in a Wofati/Oehler-planned home, but I will have a well pump for example. The assumed 30% conversion losses are added in to say I would need 3900 watt-hours of generation. Hopefully panels that can still partially charge on cloudy days can save me, since some solar calculators say I need 13 panels at 300 watts each for those 1.2 sun hours...

So here's the battery portion that has been a bit confusing to me in the past: the battery linked above offers 415AH at 48v which becomes (A*V=W) 19,920 watt hours? On a cloudy winter day, using panels which still generate a charge, I'm getting that 1.2 sun hours on average over the 10 hours that the sun is out(barring obstructions like trees or 3 feet of snow drift)? If that battery should not be drained below 80% of capacity to extend it's lifespan, then that's about 4000 watt hours in a given day, and I'm at 3900 watt hours of generation including system loss. So I need to have enough panels to charge that 3900/4000 watt hours back each day, say 13 panels at 300 watts each? This is what is getting me confused, as some folks on here talk about having 440AH, 24v off grid systems with no backup generator in northern areas with cloudy winter days and they have 1000 watts in panels or less.

Is 3kwh of consumption the issue here? I expect the converted chest freezer will lower my power use, and downsizing the space with fewer gadgets like the TV will help as well. I typically use 1-2 LED lights @ 12watts/120v, the fridge, and my laptop regularly. The latter will likely be just a tablet down the road, or Linux-converted Chromebook for something a little more secure. Finally a cell phone that's kept charging as I would tether it for WiFi internet access. Converted chest fridges typically take less than 0.25kwh a day, so unless I add in a toaster oven to compensate for poor iron skillet/rocket stove cooking, I would expect my electrical use would be about as low as it can be without giving up all electronics. So the size of system recommended seems massive, despite it being in a northern climate.

Am I missing something obvious?

Edit: and is there a time when wiring the panels and batteries as 12v is better than 24v or 48v? Isn't there less line loss at higher voltage, and smaller gage wires can be used? I don't expect a long run with a house-mount being possible, maybe 30-50 feet to reach the batteries inside.

We have a 48vdc battery-bank because our biggest load is a well pump [240vac] and the only inverters that can make 240/120vac are 48vdc inverters.

I used welding cable 4/0 for all the cables between batteries.

Setting your 'Low Battery CuttOff' at 80% * is smart.

Edit:
* - 80% State-Of-Charge, NOT DOD. You really do not want to drain much more than the top 20% to 30% of a battery's charge.

Hi Mark, from the sounds of it you need a full system workup. Your numbers seem right based on sizing the system for 12 months of solar only generation. Usually you would include a generator element for periods of cloudy weather or unexpected load changes like guests or heavy tool use. That allows you to shrink the solar element at the cost of running a generator. Your confusion comes from daylight versus sun hours. Sun hours are an average over a season. so you can have 8 hours of sun one day then 3 days of cloudy weather. The MPPT controllers will compensate somewhat for that. We find our systems outperform their rated sun hours in the winter time. I would suggest you find a load analysis spread sheet and work through it with your kill a watt meter. Your chest freezer versus upright fridge will probably save you 500 watts right there. I'd have to run it through my software knowing your long and lats to be sure but if you wanted a full solar system we would say 8 l16 batteries, a magnum pae4448 a midnite solar classic 200controller or the magnum charger installed at magnum and either 8x72 cell 340 panels or 12 60cell 300 watt panels. It all depends on budget and goals though. You can consciously downsize but you have to know going in you are going to have less power to work with. Hope this helps somewhat.
Cheers,  David

Mark Tudor wrote:The depth of discharge (DoD?) and estimated/"promised" cycles of various batteries is a bit confusing to me, including the 12/24/48v to inverter conversion of amp hours when sizing a system.

For example, https://www.altestore.com/blog/2017/02/simpliphi-lithium-batteries/ shows 10,000 cycles at 80% DoD (with "that's 27 years!" next to it). So I read that to mean I can use up to 20% of the available power on a given night or cloudy day, and the next sunny day when it recharges to full, I have used up one of those cycles? I see other batteries like https://www.wholesalesolar.com/1898601/fullriver/battery-banks/fullriver-agm-415-ah-48-vdc-19-920-wh-8-battery-bank which promise "hundreds" of deep cycles and a 7 year warranty.

The property I bought is at 48 degrees latitude, and averages 1.2 sun hours a day/

I currently average 3kwh a day

Mark, a depth of discharge of 80% would leave 20% in storage. In the case of the full river batteries (lead acid) that would be 15,900 watt hours or 15.9kWh usable. It neing lead acid and off grid, you could use it at 80% d.o.d., but you might get a third to half the cycles that you would if you use 50% as a max d.o.d. The lithium battery does wear faster at 80% dod, just as lead does and longevity of lithium has not met all promises of cycles or time.

Both technologies benefit from shallow cycling and lower current charge and discharge profiles than specified maximums.

We are near your lattitude and in the cloudy midwest. I usually see 5-10% power output on grey overcast days. If its really darkly overcast, nothing, but it is only a handfull of days really, that would see no watt hours accumulated.

This is a good measure for array size where you are looking to put that 10% to good use offsetting consumption.

For instance, 50w at 6.5-8 hours on a short cloudy winter day from a 1000w array is still 400 watt hours, enough to cover our lighting and inverter losses for the day, and even a movie or two and all personal electronic devices, including headlamps recharged that day. A 4kW-5kW array would do the 3kWh on most any moderately cloudy days, but would be grossly oversized (subjective) for half the year and on any full sun day that batteries are near full.

Im a fan of "oversized" arrays, they are an inexpensive way to keep battery sizing and maintenance costs down. If you make your living at home, you can find a way to use the excess when batteries are full or near full.
This the only way we are able to run without a generator, i could not afford to beef up the battery and any larger battery would demand i have a generator so that the once in 5 year no sun for two weeks would not scrap a 6k$ battery. That is a good point for lithium, it can sit nearly dead for some time, even if pricey up front and do charge from that "oversized" array in an hour or three instead of 2 days because of max charge rate tapering near full.

You will likely enjoy 2 strings of the 415ah for 20kWh over a 5 day period of heavy overcast. We had 14 days of almost no solar input this winter!

Our battery will run critical loads for 10 days on paper or nearly full service (hold on multi load laundry) for 5 days of non or very low available solar power and we just barely made it through a 14 day event but were at 80% d.o.d., i almost borrowed a generator and we did take our laundry to a laundramat one time to avoid damage.

Right now, it is bright grey, totally overcast and we are charging at 220W, consuming 20W, and im about to make a pot of gravity filtered coffe on an (gasp!) electric hot plate that will "cost" 100Whr!

The full river batteries are great performers in our installations. Generally, if you are not going to use battery power for anything aside from feeding an inverter, i would opt for 48v on all residential batteries. Sometimes the capacity required (low)  does point toward a 24v or other battery voltage, but at your power levels, i would opt 48v.

Thanks a ton for that info Frank! I've ordered a kill-a-watt to properly test my power use for the items I'll utilize the most. That's a good idea about overbuilding panels for cloudy day use rather than buying more batteries.

With panels lasting so long, I'm considering buying them now before the tax breaks start getting cut back, and get the 30% tax break while I can.

   Shawn Klassen-Koop wrote:


       Su Ba wrote:
       It's not the panels that need replacing. Ours are 20+ years and still doing fine the last time we tested them. It's the batteries that will need replacing and maintenance. Plus the back up generator. We replace ours every 5-7 years. And if you've never run a solar system before, you'll most likely ruin your first battery bank rather quickly. We did!



   This is a great reminder. Thank you! Any tips on not ruining the battery bank quickly? Is that in its own thread somewhere?



Man this thread got huge! HA, not sure if this was talked about yet. You can look into NiFe batteries which are very forgiving and long lasting. Again, like everything they have their advantages and disadvantages. Some have said they discharge at a faster rate. I haven't had a chance to compare the two yet. They are more expensive but then you never replace them. In the end they are less toxic and the chemical in them is alkaline not acid so it preserves the metal inside the battery unlike lead-acid. I have experience with this company. http://www.beutilityfree.com/

Above is my post from this thread https://permies.com/t/40/85816/grid-dilemma
Another advantage of NiFe batteries is you can add them without replacing your whole bank or doing damage to it. Just something to look into/think about.