Mixed battery form in seperate banks

audredgeraudredger Solar Expert Posts: 272 ✭✭
This system was originaly intended to provide power to a 34' Airstream trailer. It did that well however, it is now used to power a 1420sqft house. It works with limitations.
18 Sharp 175watt Panels,
1-FX80 & 1-MX60 charge controlers,
Xantrex 4024SWPlus Inverter,
24 L16H batteries 6x4,
Honda EU2000 + 24volt forklift charger,
20hp Listeroid + 12kw 220v Gen Head, 6kw Outback 220-110 transformer.
House is using 5 Kwh on a good day, 6 Kwh most days and 7 kwh when we use the dishwasher.

Here is the rub: We will often get 4-5 days in a row (Winter) with out significant sun. Trying to limit the batteries to only 20% discharge, I only have 1.75 days of backup! I need more storage!

Stepping up to 48volt would help by reducing the number of banks, 6 to 3 but, won't help with overall storage.

Is it feasable to add a seperate bank of say 12 or 24 2v Surretts or Trojans? I would isolate this bank thru a battery switch and only operate off one bank at a time.

Tired of hearing the generator run when it's cloudy!!!

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,981 admin
    Re: Mixed battery form in seperate banks

    My suggestion is to allow the bank to run down below 75% storage and plan on starting the genset the next morning (if not enough sun to use solar to get back over 75-80% state of charge).

    On the next day--startup genset and get ~80-90% state of (heavy charging current). When current starts to drop back--you can kill the genset.

    If you see the bank go to 50%, start the genset right away.

    Keeping below 75% state of charge for a period of time will begin sulfate hardening. It is when the batteries sit for days/weeks below 75% that you will start having problems.

    Keep batteries full of distilled water--don't let them sit for days below 75% state of charge. Don't run the genset after the AC charger current falls off--let the solar panels take the rest of the charging (unless you are doing an equalization cycle). Don't bother to take the bank to 100% every day. To much charging/equalization is hard on a battery bank too.

    If you are using some water every month--you are probably OK. If almost no water--you may be undercharging. If too much water use--then you are probably overcharging.

    Myself--I would not add batteries right now... I would run the current bank (as above) and look at various options for a replacement bank in the future. Then you can make the 24/48 volt decision (perhaps new inverter/charger) at that time.

    Check how much fuel you are using to "recharge a day's worth of power"... The eu2000i should give you better than 5kWhrs per gallon--and the Diesel can do better than that. If you are running the gensets with low charging currents (below 50% rated power for the diesel, below 25% for the Honda)--you are probably mostly paying to make smoke and noise.

    My 2 cents worth of suggestion...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • audredgeraudredger Solar Expert Posts: 272 ✭✭
    Re: Mixed battery form in seperate banks

    Thank You Bill,
    I just hate to run the gen when I know that in a day or two I will have more sun than I can use.
    Mike
  • BB.BB. Super Moderators, Administrators Posts: 29,981 admin
    Re: Mixed battery form in seperate banks

    You can add a second bank, here is one type of manual switch that would probably work well:

    Blue Sea Battery Switch 1-2-OFF 350 Amp
    wind-sun_2089_5340438
    Blue Sea 9001e 350 Amp Battery Switch Make-before-break contact design

    ---It is just a pain because you have to manage the two banks... Batteries are very sensitive to voltage--Just a 1/10 or so volt difference can result in a 10% difference in battery state of charge. [add: for a 12 volt battery bank]

    Older vs Newer batteries, different lots of same battery, different battery models/brands, all add their "differences" into the mix.

    If your bank is near new (say less than a year)--adding a second string in parallel (identical brand/model of batteries) is not too bad... But, very roughly, adding a brand new bank to a 6 year old bank--you run the risk of the new bank carrying much of the Amp*Hour loads and cycling more than the old bank--eventually leading to the new bank not lasting its expected life.

    In the end, battery cycling and overall life from aging--just a fact of life for off-grid use and battery chemistry.

    You can cycle the batteries less deeply--but that takes a lot of batteries, lots of solar panels, and lots of fuel to do--all adding to costs.

    Here is an older thread where we kicked the issues around a small deeply cycled bank vs a shallow cycled large bank:

    Time to Question the 3 day Rule ?

    For me--It seemed that a 2x larger bank lasted 2.2x longer (if you looked the depth of cycle vs battery life charts). It may be worth it (1/2 the hassles of changing out a battery bank with 2x the up front costs)--But there are real downsides too--2x larger battery chargers and solar panels (to meet minimum 5% charge rate), more power required to make up self discharge, and more cells to check electrolyte levels in, etc....

    There are a lot of people here with much more battery experience than I--Perhaps they can give you their experiences with large/small banks vs loads too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,400 ✭✭✭✭
    Re: Mixed battery form in seperate banks

    I agree with what Bill said, and I will add this.

    Consider this. What is the cost of a bigger battery for the few days that you need the reserve, verses the "shortened life span" of your existing batteries? I would be concerned if you were routinely drawing them ~50%, but since you sound like you are being very good by only drawing them 20% an extra 20-30% now and again wouldn't worry me in the slightest.

    For example, lets just say that the added battery bank costs $500. Are you doing $500 worth of "damage" to your existing bank by drawing them down? I would think not.

    When I built my current system, I was going to go with 4 L 16s but I decided that since the T 105s were much cheaper to buy, would provide me with plenty of reserve etc, such that if the T 105 last longer than 5 years, and the l-16 would have lasted 10 years, the net/net cost of the T-105s was substantially cheaper.

    Going forward, if you keep track of you usage accurately, you will have a better handle on how to configure your system going forward with the next set of batteries.

    Tony
  • bryanlbryanl Solar Expert Posts: 175 ✭✭✭✭✭
    Re: Mixed battery form in seperate banks

    re "I just hate to run the gen when I know that in a day or two I will have more sun than I can use."

    One of the best things you can do for genset health is to run it regularly with a decent load. That keeps the fuel fresh, inhibits varnish build-up, keeps the wiring dry, and circulates the lubricants. Proper genset exercise also helps develop confidence that it will be working when you really need it.
  • BB.BB. Super Moderators, Administrators Posts: 29,981 admin
    Re: Mixed battery form in seperate banks

    By the way, how are you monitoring your battery bank... Are you using a hydrometer (with temperature correction--if needed)--assuming flooded cell. Do you have some form of battery monitor?

    I really like a Battery Monitor... They are not perfect--but they do a very good job of giving you the equivalent of a fuel gauge for your gas tank... You can do your generator management by looking at the gauge. Something along the lines of:
    • Hit 75% state of charge--start genset next morning if no sun in forecast.
    • Hit 50% state of charge--start genset
    • Turn off genset when state of charge >80-90% (really when charging starts to taper off--probably below 50% of max charging current--saves genset fuel/runtime costs)
    And review your generator/battery charger sizing and usage... L16H battery around 420 AH at 6 volts... You have 6 strings x 4 per string for 24 volts @ 2,520 AH total.

    For solar panels--we start with a rule of thumb of 5% to 13% of 20 Hour battery bank capacity. Below 5%, it is difficult to get the batteries properly recharged (without using a genset). Above 13% -- becomes a waste of solar panels--And is the "average" maximum recommended charging current for many flooded cell batteries (can overheat with more current).

    You have 18x175 watts panels--your maximum charging current roughly is:
    • 18x175 watts * 1/29 volts charging * 0.77 overall efficiency derating = 84 amps average peak
    Relative to your battery bank:
    • 84 amps / 2,520 AH bank = 0.033 = 3% rate of charge
    You could easily double or triple the amount of solar panels for your current bank size... You have about the maximum battery bank that your current solar system could support anyway... A larger bank is really not warranted right now (based on current solar array)--assuming no stupid math/post reading errors :roll:.

    Double checking battery bank size using our 3 day no sun, 50% maximum discharge level (for longer life). Assume 6kWh per day, 85% inverter efficiency, 24 volt bank:
    • 6,000 Watts * 1/24 volts * 1/0.85 * 3 days * 1/0.50 = 1,765 AH at 24 volts
    So--using our battery sizing rule of thumb--your bank is plenty large enough.

    Now, lets look at your genset sizing... For a generator based battery charger--they should be around 5% to 30%--So, for your bank:
    • 2,520 AH * 0.05 = 126 amps minimum useful @ 29 volts
    • 2,520 AH * 0.13 = 328 amps recommended maximum useful @ 29 volts
    • 2,520 AH * 0.30 = 756 amps maximum useful @ 29 volts
    So, for fuel efficient loads on your gensets--your battery chargers ratings should be somewhere between:
    • 126 amps * 29 volts * 1/0.80 eff batt charger = 4,600 watts
    • 328 amps * 29 volts * 1/0.80 eff batt charger = 11,890 watts
    • 756 amps * 29 volts * 1/0.80 eff batt charger =27,400 watts
    You have a 20 HP / 12 kW diesel with a 6kW transformer (for 120 VAC). Since you should run your diesel at 50-75% rated load--the minimum battery charger load ideally for your genset would be:
    • 12 kW * 50% = 6kW minimum battery charger load
    • 6,000 Watt * 80% charger eff * 1/29 volts charging = 166 amp minimum charger at 24 volts
    So--if you can run your genset most of the time pumping > 166 amps (approximate)--you should be minimizing runtime and fuel usage per charging cycle.

    You could use a smaller battery charger--but the smaller you go, the more fuel you use and longer runtime to recharge your battery bank.

    Of course, you can use the small Honda with an appropriately sized charge controller (400 watts - 1,600 watt load)--for reasonably fuel efficient recharging--but it will take a lot of hours to move the bank's state of charge an appreciable amount. For maximum power output from the eu2000i--look for a Power Factor Corrected power supply (PFC allows you to more efficiently use a smaller genset--It is the difference between VA (volt*amps) vs Watt ratings).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • audredgeraudredger Solar Expert Posts: 272 ✭✭
    Re: Mixed battery form in seperate banks

    Wow... I'm overwhelmed with the help, a simple thanks seems insufficient. So I'll dance at your wedding or, buy you a good single malt scotch if ever we meet!

    A good hydrometer is in the tool kit and is used on every cell at least once a month just before watering. As to battery monitor, I've been using the "volt meter" on the MX60 & FX80.

    I have considered adding another 6 panels, have the head 1/2 constructed all ready. Just lack the 5" pole and panels...

    You figures on battery chargers tells me I need to go to the store!
    The Quickcharge was sized to run off the eu2000i and is 11amps @117v in 40amps @ 26v out. Xantrex 4024+ is only 45amps as I recall.
  • BB.BB. Super Moderators, Administrators Posts: 29,981 admin
    Re: Mixed battery form in seperate banks

    audredger,

    Or stick around and let us know how things work out with the larger AC charger, more panels, and if you decide to try a Battery Monitor.

    If things work out--you may be able to cut your fuel use and runtime to a 1/2 to 1/4 of what you are doing now! (crossing fingers)

    By the way, are you running both the Xantex SW and the Quickcharge at the same time on your diesel set?

    And then, you can be one of the folks that answer the questions for the next person such as:
    Here is the rub: We will often get 4-5 days in a row (Winter) with out significant sun. Trying to limit the batteries to only 20% discharge, I only have 1.75 days of backup! I need more storage!
    :cool:;):D

    Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • audredgeraudredger Solar Expert Posts: 272 ✭✭
    Re: Mixed battery form in seperate banks

    Stick around I will! Thanks
    Tend to run the quickcharge on the eu200i, can put it on eco idle and hear when it kicks off. Listeroid is used mainly when I weld (45 amps @ 220volt). Never thought to run both of the Lister, Dugh!

    On the subject of Battery monitors, any recomendations? If I go 48v I would be using a pair of stacked Outback VFX3648's. Might be that an Outback Power Flexnet-DC System Monitor would fit best?

    On the subject of battery chargers; to achieve 328 amps worth of charge I would need to add 6 40amp chargers plus use the SW4024+ and the Quick charge?!
    I dont mean to question but, 328 amps is one hell of a lot of current! My Tig welder welds 3/16 plate with only 175amps DC. Can the batteries realy take that much at once? Wow!
  • icarusicarus Solar Expert Posts: 5,400 ✭✭✭✭
    Re: Mixed battery form in seperate banks

    Bogart Trimeteric meter . They also make a fancier one that I can't remember the number of.

    Tony
  • BB.BB. Super Moderators, Administrators Posts: 29,981 admin
    Re: Mixed battery form in seperate banks

    I don't know anything about the Outback Flex Net--I believe it would do find for you--perhaps others here know that product better.

    Battery Monitors
    : "cheaper"--The Trimetric has had a few good reviews here.

    For "expensive"--The Xantrex LinkLite and LinkPro look very nice... I like the programmable output that you can use for an alarm, generator control, or inverter/load shedding. The LinkPro includes a remote battery temperature sensor option.

    328 Amps--That is C/7.8 -- C/10 is very common, and C/20 is a very gentle charge. AGM's can even take more:
    Most flooded batteries should be charged at no more than the "C/8" rate for any sustained period. "C/8" is the battery capacity at the 20-hour rate divided by 8. For a 220 AH battery, this would equal 26 Amps. Gelled cells should be charged at no more than the C/20 rate, or 5% of their amp-hour capacity. The Concorde AGM batteries are a special case - the can be charged at up the the Cx4 rate, or 400% of the capacity for the bulk charge cycle. However, since very few battery cables can take that much current, we don't recommend you try this at home. To avoid cable overheating, you should stick to C/4 or less.

    Speaking of welding--Your battery bank can probably output 30,000 amps into a dead short.

    Large battery banks are pretty scary (you have fuses/breakers at the end of each series string of your bank--or is everything just tied together with heavy cable?).

    I am not saying that you have to get a 328 amp battery charger (that would pretty much be a 100% load on your genset--But between ~166 amps and 328 amps is the ideal zone for most efficient charging on your 12 kW diesel...

    The details of the charger do matter--The Power Factor of the charger is important... Basically, the average charger probably has a PF around 0.65 or so. That means that 6kW of energy from the generator will really pull:
    • 6,000 Watts * 1/0.65 PF = 9,230 VA
    • 6,000 Watts * 1/0.65 PF * 1/240 VAC = 38.5 amps
    So--With generators (and other circuits) you not only have to worry about the power--you need to worry about the "out of phase" or "non-linear" current wave forms which draw more current than one would expect from the power=volts*amps which really is power=volts*amps*PF ...

    You do need to do a bit of research before plopping down the money. If you can find a used forklift charger that is the right size (and 240 VAC single phase)--it may be a good deal for you.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RandomJoeRandomJoe Solar Expert Posts: 472 ✭✭✭
    Re: Mixed battery form in seperate banks

    The Flexnet DC is pricey, but if you also get the Mate (and a communications hub) you can then plug all your Outback kit together and it's REALLY sweet at that point.

    The FN-DC will give you three shunt inputs, so you can separately meter various loads / charge sources if you want, and gives you the "fuel gauge" - a row of LEDs on the unit itself, or more accurate data through the Mate interface.

    The Mate lets you see just about everything the various devices have for data, you can get daily AH/kWh totals, even go back through the history (you can do this with the FM80 display as well, I've not seen an MX60 but it may be the same). It is also required if you wish to make adjustments to settings in the FX inverters.

    If you're a computer nut and control freak like me, the Mate also has a serial port on it and Outback offers the communications protocol it uses so you can suck most (but not all) data into a computer and even perform a few commands from the computer too. (I'm sitting here at work, with a window up on one screen of the computer that lets me monitor all the data from my solar system at home! :cool: )
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