Time to Question the 3 day Rule ?

For off grid applications we often supply the no renewables 3 day rule on sizing systems.

I think this needs to be discussed. I have a large off grid system (european) wise and I have all creature comforts except AC.

A day of no sun results in approximately a 10% dod ie 90% batteries full. I have enough solar to replace this the next day and maybe another 50 %. However if the forecast for the following day is overcast then I usuually make the decision to replenish the batteries to full (about 3 hours fully genny run time at 85 amps at 24 v)

Running the inverter charger long and hard for this time period is about the max b4 the Inverter Charger Unit backs off to cool down thus wasting genny power.

If I decide not to run the genny hoping for sun and the batteries drop to 80% of full charge Im looking at a 6 hour genny run time to bring them back up.

So for an offgrid system with back up genny isnt a one day no renewable input say the best balance, Im not stating this as fact fact but opening a discussion .

I dont like auto start systems I just cant get to grip with a genny running home alone, that does not mean I dont like it set on auto when Im at home

But I make decisions on battery charging daily and never let my system drop below 85% charged,

Hope u get my drift on the discussion im starting.

Nigel
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Comments

  • boBboB Solar Expert Posts: 1,012 ✭✭✭✭
    Re: Time to Question the 3 day Rule ?

    These cloudy generator whether days are the time to save up the dirty socks and underwear and do the laundry while the generator is running and charging batteries so as to more efficiently use the fuel.


    Not sure about it running automatically. I would feel better by being around while the genny is on.

    boB :D

    PS. It is estimated that no underwear will be worn by the year 2020
    Source: Firesign Theatre.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Time to Question the 3 day Rule ?

    most people don't get as much sun as you do nigel. here in pittsburgh it is not uncommon to have even 5-6 days overcast at times. what works for you may not work for me or somebody else even. it is also difficult to reserve much more than 3 days as this increases the battery bank sizing and the need for more solar to charge it. if one can't go 3 days it is understandable, but it is a good thing to have it if you can. i'd say costwise it may be better to reserve a day or 2 then break out the genny. lower battery costs and lower solar costs that way, but more dependant on fossil fuel generation.
  • mike95490mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Time to Question the 3 day Rule ?

    Good thoughts.

    My opinion, the 3 day rule is a starting point. In wintertime, cloudy weather, and your gas furnace comes on, or the electric pump for the hydronic heating is on, those are loads, along with the fridge, deep freeze and Aunt Sue's Iron Lung, considered critical. Add some lights and TV/video games, and clothes dryer, and you want to have enough power to to not flatten your batteries, and not live like a caveman.

    As to your battery charger heating up, that does not make sense, it should be able to charge batteries as long as needed, within it ratings. If it heats up in 2 hours, and throttles back, that does not sound right. Now if it senses the batteries charging, and needing less current, that may be why it throttles back.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • nigtomdawnigtomdaw Solar Expert Posts: 705 ✭✭
    Re: Time to Question the 3 day Rule ?

    Mike after 3 hours on charging a large battery bank at full throttle my SW3024E will back off charging internal fans blowing for 1 to 2 minutes then pick upagain, it doesnt drop the genny just backs off charging this is controlled and regulated and doesnt record a fault on the SW but I have high temps, maybe an external fan would help but I choose to charge daily any deficiet that I think my solar can make up the following day, as batteries are not cheap. That is the point of my discussion, why drain batteries to 50% over three days reduce life when a daily charge or no charge question by genny may be the answer:confused:

    boB Im the only fan in my house of gen burn time burn appliances also, whilst I use my AC generator, Im soon to convert over to an EX MOD 180 amp 28v DC generator by Armstrong Plessy using a lister petter twin diesel, cost the UK Goverment $52,000 each late 1980s cost me $600 ebay 2008, awesome bit of kit, and quiet will cut batt charge time by 60%
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Time to Question the 3 day Rule ?

    Nigel, sounds like you just won a trip to the Caribbean for doing the laundry...;)

    FWIW we have friends that are 100% off grid and do run their gen in the winter from late Oct to early march. Prime motivator for running the genset is those nights when an NHL (hockey) game is on the telly and Ted does not want to lose 1 minute of play. His wife then sets about running the washing machine and drier while the genny is on. Otherwise they live quite comfortably on 1.1Kw/24 volt . they are right on the N shore of a lake and get oodles of sun when it is up, pproblem is in winter it likes to just skirt the horizon...

    I like both ideas, 3 days buffer, and bulk charge daily if needed...

    3 days buffer is good insurance for boneheads that want to 'game' or be couch potatoes in front of the telly

    Eric
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • BB.BB. Super Moderators, Administrators Posts: 32,804 admin
    Re: Time to Question the 3 day Rule ?

    One reason to avoid a generator if the 2nd or 3rd day you can make up the charge... Call it 80% efficient charger, 80% battery efficiency, and 80% inverter efficiency... = 51% overall efficiency from generator plug to inverter plug supplying the load (yea--I took the low averages here to make a point).

    With close to 50% system losses--one would almost be better off to just bunch the loads together and run the generator without going through the batteries at all... If waiting another day or so and you don't have to pay 2x for the fuel use, why not... And, if you have otherwise good sized and working PV array--a person could just end up "tossing" away the "free" power from the arrays since the batteries are now full.

    In the end, what do you want to change the 3 day rule to? 1 day + 50% discharge? Is that discharging to what level--discharging to a 50% state of charge--then expecting the solar panels to replace in ~4 hours? C/8 (4 hours to recharge 50% bank would be 8 hour rate) is considered to be an OK charge rate (12.5%)...

    With flooded cell batteries, does discharging regularly to 50% expose them to early death by sulfaphation (if not shorter life from 50% cycling)?

    In the end, a 3 day discharge cycle + 50% minimum discharge, gives you a 100%, 83%, 66%, then 50% (full, 1 day, 2 day, 3 day)... 66% is already under the 75% state of charge level where sulfaphation begins to be a problem.

    You (Nigtomdaw) choosing to not go below 85% is pretty much a 3 day + 50% recommendation...

    And, if one wants to look at secondary effects... For example looking at the generic battery cycles vs depth of discharge chart (Wind-Sun's battery FAQ) (rough estimate of battery life reading off of graph)...

    85% SOC = 3,500 cycles
    67% SOC = 1,700 cycles
    50% SOC = 1,000 cycles...

    So service life would be equal to number of cycles * number of days to discharge...

    85%: 1 day * 3,500 cycles = 3,500 day life = 9.6 years
    67%: 2 day * 1,700 cycles = 3,400 day life = 9.3 years
    50%: 3 day * 1,000 cycles = 3,000 day life = 8.2 years

    Ignoring Sulfate hardening issues (which I cannot quantify--I don't have that information), there is not a huge difference in the real cycle life between taking a 3 day bank, and recharging it every day (using a genset) at 85% charge vs waiting a day or two and seeing if sunny weather will return and recharge the bank without the fuel and wear&tear costs of the generator.

    We could also look at a 1day+50% battery bank's life (discharge every day to 50% and use generator, if needed, to bring back to 100%):

    50%: 1 day * 1,000 cycles = 1,000 day life = 2.7 years

    Given that this bank is just 1/3rd the size of a 3day+50% bank--the cost difference:

    $3/$1 x cost * 2.7 years/8.2 years = 1.0 -- or the costs are roughly the same

    Roughly, replacing a 1/3rd sized battery bank every three years or a 3/3rd sized bank every 9 years... Other than the up front battery expense for a larger bank, pretty much a wash in costs.

    I would tend to want to use the generator less, replace batteries less often, and therefor go with the 3day bank.

    What if we went the other way and use a larger battery bank. Again from the battery cycle life chart (90% is largest cycle listed--so 4 day + 50% bank):

    90%: 1 day * 5,000 cycles = 5,000 day life = 13.7 years
    80%: 2 day * 2,800 cycles = 5,600 day life = 15.3 years
    70%: 3 day * 1,800 cycles = 5,400 day life = 14.8 years
    50%: 5 day * 1,000 cycles = 5,000 day life = 13.7 years

    What would the comparison of a 3 day cycling on a 3 day bank vs 4 day on a four day bank:

    $4/$3 * 8.2 years / 13.7 years = 0.80 -- so a 3 day bank has 80% of the dollar efficiency of a 4 day bank (both cycling to 50% level over 3/4 days respectively)...

    Seems adding a 33% larger bank may give longer battery life (for the dollar)--but a 25% longer life--not sure that other issues may overwhelm that slight advantage (simple aging may kill bank vs cycling at 14 years)--plus who knows how accurate the cycle/life graph is (needs to be done for the batteries you buy--to check/verify these sample calculations).

    I guess my conclusion would be is that it is difficult to better optimize an off-grid storage system. Primary costs (1 day cycle fuel+maintenance vs solar PV costs on a 3 day cycle) will probably swamp any day to day costs/savings of 1 day vs 3 day vs 4 day battery bank. The chances that something else will kill the batteries (over discharge, failed cells, dropped wrench causing fire, etc.) are probably more likely than the small differences in battery life that I took a SWAG at.

    My suggestion, put a kWatt*Hour meter on the genset, and keep a log of the fuel usage+price. See how much money if spent on fuel costs over ~9 years vs increasing battery bank life by ~1.4 years (approximately a 15% increase in battery bank life for 1st day recharge vs 3rd day recharge)...

    :confused:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nigtomdawnigtomdaw Solar Expert Posts: 705 ✭✭
    Re: Time to Question the 3 day Rule ?

    Missed my point a little Bill, 3 days no sun still accounts to 3 days gen run time, where I operate a 1 day run time 1/3 genny fuel and still the option at the start of the next day to see if the weather man got it right.

    No sun any way you count it means a level of gen run time

    1 day no sun 3 hours genny. deficiet charging

    3 days no sun 9 hours genny deficiet charging + deeper cycled batteries.


    Get my drift, sos if I have crossed a mod;)
  • BB.BB. Super Moderators, Administrators Posts: 32,804 admin
    Re: Time to Question the 3 day Rule ?

    Nigtomdaw,

    Only if I had that much power... :p

    If you know the amount of sun in advanced (bad weather forecast, world ended today so no sun tomorrow or next day, etc.), then, I don't have an issue with running gen set every day vs every three days...

    You can save some fuel if you only charge 1/6 back up if sun is forecast for the next day

    Battery life wise--the batteries would last 9.6 years vs 8.2 years (for that thin period of the year where you have the choice to run the genset every day vs not).

    If you have full sun and "normal" daily loads (recharge next day), then it does not matter.

    In the end--it probably does not matter based on "facts" and more of a choice (noise, fuel stores, upcoming weather forecasts and current usage, etc.).

    I thought that we were discussing the sizing of the battery bank vs just when to turn on the genset... Hence my study of battery life vs bank size vs 1-3/4 day recharging.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MangasMangas Solar Expert Posts: 547 ✭✭✭✭
    Re: Time to Question the 3 day Rule ?

    My battery bank holds a lot a lot continuous reserve power, probably about 2-3 days even with high energy use (A/C units and Furnaces) but IMO this was expensive reserve power.

    When we designed the system, we figured the generator (set up to auto-start based on battery levels) would have to boot the battery bank up more often than it actually does.

    Actual experience it only runs a few times a year during hot/overcast July and heavily overcast January days for a total of about 40 hours (20 times @ 2 hours) a year including monthly run tests. That's an estimate.

    While I could have probably gotten by with fewer batteries, I decided to initially over build the battery bank because it's sometimes difficult getting propane into the ranch (road wash out conditions). Maybe an expensive decision battery cost vs propane fuel costs....
    Ranch Off Grid System & Custom Home: 2 x pair stacked Schneider XW 5548+ Plus inverters (4), 2 x Schneider MPPT 80-600 Charge Controllers, 2 Xanbus AGS Generator Start and Air Extraction System Controllers, 64 Trojan L16 REB 6v 375 AH Flooded Cel Batteries w/Water Miser Caps, 44 x 185 Sharp Solar Panels, Cummins Onan RS20 KW Propane Water Cooled Genset, ICF Custom House Construction, all appliances, Central A/C, 2 x High Efficiency Variable Speed three ton Central A/C 220v compressors, 2 x Propane furnaces, 2 x Variable Speed Air Handlers, 2 x HD WiFi HVAC Zoned System Controllers
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Time to Question the 3 day Rule ?

    Nigel et al, the other item we have not put on the table so far is an intangible for those in the know...
    'peace of mind' ...
    it comes at a cost, but the fears that a minimally designed battery bank needs to be recharged 'now', otherwise battery degradation will start setting in, or... do I (will I ) have enough solar input tomorrow to recharge my battery will be assuaged, or... Oh my God what do I do now ??

    just a thought...

    Eric

    ps I went through some of this discomfort last winter, when my voltage readings dropped like a stone with minimal, very conservative use levels.. :confused:
    We shall see just what my improvements have done, in a few months.:D
    ej
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • hillbillyhillbilly Solar Expert Posts: 334 ✭✭
    Re: Time to Question the 3 day Rule ?

    Interesting discussion, I have some similar issues in that durring winter we do have to fire up the generator from time to time. One thing I am confused about is that you say that by waiting another day you basically double your generator run time. Sounds as if you were charging the batt's all the way up through absorb (which would take two hours by itself) ... in other words only one hour for the bulk stage. If this is the case, another day of use should only really add up to maybe another hour or so of bulk charging...maybe I'm missing something?
    We usually try and plan it out if possible to run that generator when there will be some good sun to take care of the absorb stage if we can. For us the bulk charging stage really only means anywhere from maybe 45 minutes (typical) up to about 4 hours run time if the batt's were really low. Sometimes we do end up needing to fully charge everything up which kind of sucks...but probably better than abusing the batt's any further.
    We also oversized our battery bank a bit, about 700ah 24v, which seems to work out to a nice 3-4 day cussion (with light to moderate use). To me it seems worth it to have a bit more flexability in terms of waiting out some clouds to do our genny charging, and to have some extra power "just in case". If I were to do it again, I'd probably even go a bit larger on the battery bank; more for guests or house sitters not used to watching thier power consumption quite so tight. Of course that would also mean needing another panel or two as well...gotta feed those hungry batt's.
    cheers
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Time to Question the 3 day Rule ?
    nigtomdaw wrote: »
    Missed my point a little Bill, 3 days no sun still accounts to 3 days gen run time, where I operate a 1 day run time 1/3 genny fuel and still the option at the start of the next day to see if the weather man got it right.

    No sun any way you count it means a level of gen run time

    1 day no sun 3 hours genny. deficiet charging

    3 days no sun 9 hours genny deficiet charging + deeper cycled batteries.


    Get my drift, sos if I have crossed a mod;)

    nigel,
    to a point i see what you are driving at, but as i pointed out, and you seemed to miss, was the fact that a 3 day battery reserve ups the pv requirements to properly charge the batteries. when the sun comes out you thusly have more power to recharge that depletion in the batteries. your load requirements are the same, but with more pvs the batteries will get replenished faster. it will still take time to get to 100% soc and i understand that, but being the loads are the same and more is going into the batteries a net system gain occurs. the genny will still be needed to speed things up, but it's not as bad as you think. i, for one, like the idea of possibly a day or so reserve to keep costs down and it is not a rule to have 3 days, but a good reference point as bill and myself infered. it may be more advantagious for some to go 3 days while for others it's not. some reserve is a given to have for expanded future needs without having to buy all new and a hedge against the unforeseen even if it's not for 3 days, but only for 1 day extra. btw i think the 3 days included the first day and then 2 in reserve.
    adding here that the remaining ah once at the 50% dod point is not being considered as another day of reserve because you don't want to use that other 50% of remaining ah, but considered as being a normallized full day's use. another day's reserve is to double the overall capacity and that would mean staying to 50% max depletion and brock is right (following this post) in that adding reserve ah does essentially keep the soc point up higher for normallized dailly calculation purposes. 1 day reserve would mean 75% soc. example: 50ah required. 100ah battery needed for 1 day. adding 1 day of reserve doubles this to 200ah capacity as 50ah will be used day 1 and another 50ah being used on day 2 for 100ah total. you will use 50ah normally and that is 50/200=.25 or 25% dod or 75% soc. because the battery system needs 5-13% to properly charge, this is a range of 5-13 amps on the 100ah battery and no days reserved, but with the day of reserve added the charge requirement ups to 10-26a and this was my point to you nigel. adding even more battery reserve also ups the charge requirements even more as for each day of added reserve it also adds 5-13a of pvs to the equation for each 100ah of batteries and yet your dailly designed load requirements are staying the same making much more available to charge the batteries with as the soc is staying higher. following me?
  • BrockBrock Solar Expert Posts: 639 ✭✭✭✭
    Re: Time to Question the 3 day Rule ?

    Also with the "3 day rule" I had always assumed we were talking to 50%. Since I don't like to go below 80%, typically about 85% then I am in the three day rule myself. Although I never let it go a more than a day below 85%, if so then I charge them back up and I also try to charge them when I know full sun is starting so it can do the absorbing.
    3kw solar PV, 4 LiFePO4 100a, xw 6048, Honda eu2000i, iota DLS-54-13, Tesla 3, Leaf, Volt, 4 ton horizontal geothermal, grid tied - Green Bay, WI
  • nigtomdawnigtomdaw Solar Expert Posts: 705 ✭✭
    Re: Time to Question the 3 day Rule ?

    Good stuff, Im learning lots and I think Im in the zone on what I do and others are doing, Im still reading thanks for all the replies, SMods... BB and Niel Im playing devils advocate a little my apologise but I think its a useful discussion, and things were getting a little tired, but I hope we have more responses, I only want to establish a best practice for off grid battery care and Im encouraged with the response by all so far....Nigel.


    Its easy to see those who respond living off grid with a eye on the $ cost of new batteries:cool:
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Time to Question the 3 day Rule ?

    no problem for if you didn't understand then i know many others didn't either.
  • sam csam c Registered Users Posts: 16
    Re: Time to Question the 3 day Rule ?

    very helpful thread, I have 10 golf cart batts and want to increase the batt bank
    from what I have read I should not mix old and new batts, the batts are 5yrs old and are working well.
    I use 3- 4kw a day
    this was not a problem when I was on hydro, I shut down the hydro ( the creek got too low) went to solar, the bank is too small,
    I expect the creek to return this winter and will add the hydro to the system,
    my thinking is to add a batt bank as a reserve, separate from the solar, to switch to or add to the solar as needed in the winter.
    in the summer the new bank will be needed full time
    will I be better off decommission the old batt bank and going with an all new bank?. I am looking at 2 800ah forklift batts
    I have a Flex max 80 for the solar, the hydro puts out 16amps @ 22vdc. can the hydro be added to the 80 with the solar, the solar is 7 -215watt 48volt panels
    in the fall and spring nether system by its self will be adequate.
    I realy want to get away from any gen. use.
    Sam
  • icarusicarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Time to Question the 3 day Rule ?

    I too subscribe to the 80% rule, or in most cases the 90% rule. If my bank drops below 90% soc and I know that I will not have sun today or tomorrow, I will charge to 100% with the genny. This way, I keep the run time to a minimum, as I seldom have to run it at all, as well as keeping the batteries in shape. I know it is kind of silly, that indeed I could run the batteries down a bit more an count on a full charge in next full day but,,,

    My system recharges in full sun from daily use by ~ noon every day, any excess power then goes to charging things like lap tops and tool batteries. If I let the house batteries get to 80% it takes a couple of days of full sun to get them back, something that I can't always bank on. My fuel costs are slight, and my battery life has been (touch wood) exceptional so I guess I will keep doing what I am doing.

    Tony
  • System2System2 Posts: 6,290 admin
    Re: Time to Question the 3 day Rule ?

    Hi all. I'm new at the net. hope this works. Our approach to getting our batteries back up quick while satisfying our disproportionate distaste for running a generator has been to size our array at double our required daily usage. Considering losses, that comes close to recovering for a day of cloud in one day. Our array being so large of course means it takes a very cloudy day to hit at the battery very hard. At least that's the theory but then we aren't content to just waste all that energy the rest of the time, so we find uses for it like irrigation or filling fish ponds. Then we get used to these loads and want them all the time. Since the agricultural demand for water is so massive we're looking into a water tower which would alleviate our pumping needs during cloudy weather which could allow us longer periods of autonomy and since gravity doesn't start to degrade after three days we would be able to store for a need in an other form. Does anyone have any input on functioning off grid with very rare reliance on generator use?
    What's the biggest wind turbine applicable to off grid use?
    _____________________________________

    36 @ 180 W. Evergreen, 50 @ 170 W. Suntech, 5 @ MX-60, 8 @ Crown 1875 A.H. 12 V. Batt. in 48 V. config. 4 @ FX-3248T 3200 W. inverters.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Time to Question the 3 day Rule ?

    todd,
    the 3 day rule being refered to is not about pvs or wind, but about extra capacity in batteries. we all goof here from time to time, myself included, but could you try to read the subject matter more closely if you can before posting?
  • System2System2 Posts: 6,290 admin
    Re: Time to Question the 3 day Rule ?

    niel,
    Sorry and thanks. Got a dumb question. Is the three day rule simply to have three days of autonomy in batt capacity down to accepable dod? Or is it to never let your batt stay below acceptable dod for more than three days?
  • System2System2 Posts: 6,290 admin
    Re: Time to Question the 3 day Rule ?

    Oh, I get it. there are more pages to read. No wonder I couldn't figure out where this conversation started. sorry all.
  • nigtomdawnigtomdaw Solar Expert Posts: 705 ✭✭
    Re: Time to Question the 3 day Rule ?

    Todd 15 kw of PV double what u need is impressive what defunct fighter jet plane did u buy to salvage the engine to coach build your generator,

    I guess u burn more fuel in the warming up of you engine, than i use in charging my batteries in a month or even a year, do u run a business as well as a home on 15 kw, of PV , apart from Air Con we live a pretty modern life with all the gizmos on 2.4 kw of PV with a less that 70 hour gen run time forcast for this year .

    That is an awesom PV supply,,,, pictures would be nice of your set up, with the 4 outbacks and five MX60s,,,,dont be shy

    Nigel
  • System2System2 Posts: 6,290 admin
    Re: Time to Question the 3 day Rule ?

    nigtomdaw
    use antiquated gas welding machine for gen but very seldom. Most of our system is for agricultural water pumping. Irrigation, stock tanks and a fish pond. The house is a drop in the bucket. Have tankless gas water heater. Wood heat. Gas clothes dryer when really needed. And I confess air conditioning a small room from time to time which is the only thing in the house that even phases a system this size. Our battery is 3750 AH, 48 V. The water is where the lions share goes.
  • JESSICAJESSICA Solar Expert Posts: 289 ✭✭
    Re: Time to Question the 3 day Rule ?
    sam c wrote: »
    very helpful thread, I have 10 golf cart batts and want to increase the batt bank
    from what I have read I should not mix old and new batts, the batts are 5yrs old and are working well.
    Sam

    Reading old posts, I found this one, and it made me wonder: Can I, or should I, mix new batteries with batteries that are just one year old? How "old" should batteries be in order to apply the no-mixing rule?
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Time to Question the 3 day Rule ?
    niel,
    Sorry and thanks. Got a dumb question. Is the three day rule simply to have three days of autonomy in batt capacity down to accepable dod? Or is it to never let your batt stay below acceptable dod for more than three days?

    it is to be able to use your batteries for 3 days without going below 50% dod.
  • BB.BB. Super Moderators, Administrators Posts: 32,804 admin
    Re: Time to Question the 3 day Rule ?

    And Jessica,

    The mixing of old batteries... There is no hard and fast rule... Typically, batteries get better the first dozen or so discharge/charge cycles--and then slowly decay over time (years or decade--depending on lots of things).

    The simplistic assumption to make is that new batteries will not last any longer than the batteries in the original string (say you have a set of 5 year old batteries that will last seven years--then adding a new battery, it will not last much longer than two years).... This is because (in parallel strings) the new battery will carry more of the load (and cycle more/deeper) than the older parallel battery.

    If you have a string of a dozen batteries, and one has failed--it is probably still worth replacing the failed battery as long as the rest of the string tests out OK...

    What is more problematic is if you keep replacing one or two batteries at a time over the next few years--the chances that they will all last their "spec'ed" life is probably not high.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mountainmanmountainman Registered Users Posts: 439 ✭✭
    So @bb the addition of new batteries to old in parallel shortens the life of the new batteries. In series is the addition of new to old ok or better than parallel? Say with 2 batteries of a year old that have been in a 12 volt configuration. Now adding 2 more and changing to 24 volts?
    2kw array 6 345 q cells  make sky blue 60 cc
     6 230ah GC @36 volts 
    18 amp accusense charger. 3650 champion 
  • TecnodaveTecnodave Registered Users Posts: 437 ✭✭✭✭
    Mountainman, This thread is 8 years old! Not a good idea to mix new and old any you do it
    2 Classic 150, 2 Kid, 5 arrays 7.5 kw total  2ea.  2S6P Sharp NE-170/NE-165, 1ea. 12P Sanyo HIT 200,  2ea. 4/6P Sanyo HIT 200, MagnaSine MS4024AE, Exeltech XP-1100,  2 Banks L-16 battery, Rolls-Surette S-530 and Interstate Traction, Shunts with whizbangJr and Bogart Tri-Metric, iCharger i208B  dc-dc buck/boost converter with BMS for small form lithium 8S 16650 or LiFePO4,
  • WaterWheelWaterWheel Registered Users Posts: 362 ✭✭✭
    A thought on this 8 year old thread.       Since batteries are much less efficient at absorbing the charge when over 85-90% SOC using a generator to top off a 90% charged battery isn't very fuel efficient.        I'd suggest letting the batteries get well below 80% charged before considering using a generator and then only use the generator to bring the batteries up to a little over 90% SOC. 

    It's a solar system, let the sun do most of the charging.

    Conext XW6848 with PDP, SCP, 80/600 controller, 60/150 controller and Conext battery monitor

    21 SW280 panels on Schletter ground mount

    48v Rolls 6CS 27P

  • mike95490mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    I always manually run the generator, no autstart.

    In bad weather I run it mostly 90 minutes in AM, to take advantage of any noon sun, to to off the batteries.   I try to keep bank no lower than 80%, so if weather degrades, I can sit inside warm and cozy and wait for a break in the weather to charge batteries.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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