Battery thermal masses

mountaintopmountaintop Registered Users Posts: 16
I have been looking at batteries for my soon to be finished off grid house. I currently like the looks of using forklift batteries for my system. I’m wondering about the heat and thermal mass of forklift batteries. I haven’t found much information that I find useful.

Is the difference in thermal mass worth getting two 24 volt batter packs then one 48 volt battery pack? If I went with the 24 volt packs I would have more surface area then a 48 volt pack.

I’m looking at 1,200 AH (20 hour 48 volt) batteries and my daily load on the battery should be about 400 AH (48 volts) a day.

The rest of the system will be…
2,600 watt PV string for a dual axis tracker with a XW controller
2,600 watt fixed PV string with XW controller
1,000 watt vertical wind turbine = 600 watts based off wattage / wind chart
2- XW6048 inverters
10 KW diesel generator with XW generator controller
XW system controller and tri-metric


  • BB.BB. Super Moderators, Administrators Posts: 30,953 admin
    Re: Battery thermal masses

    Wow! Sounds like a nice setup.

    So, the question is how much power will you be taking out of / and into the battery. Basically, C/20 (5%) or less--you probably don't have to worry about heat... C/10 (10%) or larger current rates--then it could be an issue.

    5,200 watts and 1,200 AH of 48 volt battery bank.

    5,200 watts / (1,200 AH * 48 volts) = 0.09 or 9% rate of charge

    9% is probably right on the edge of charge rate where you would want to consider thermal issues--Given you are not going to deep cycle the battery on a daily basis (50-80% discharge)--I would guess that you will not have a big problem (others with more experience--please jump in).

    The other question is how you would move the batteries from the truck to the shed--would moving two smaller batteries or one huge battery make any difference to you.

    Otherwise, just purchase based on $$$/AH rating (including shipping).

    Forklift batteries (if I recall correctly) will tend to use more distilled water than other chemistries, and may self discharge a bit faster too (especially as they get older--upwards of 1-2% per day as they near end of life????).

    The distilled water issue is something to plan for (1-5 gallons per month--as a starting guess???). Filtered rain water is supposed to be OK (I would suggest that you want a clean/washed roof/collector at first rain--then direct the rest to storage tanks).

    A 400 AH load per per day on a 1,200 AH battery pack gives you ~1.5 days of "no sun" (to 50% level of discharge/state of charge)... A bit light on the normal battery rule of thumb--but can work well--especially if much of your use is during daylight hours (A/C, shop) is offset by your Solar Array (rather than drawn from the battery bank at night)--and your loads at night are relatively light.

    You have a nice diesel genset that is not too big--so you can program the chargers to give you about 50-80% load on the genset for bulk charging (a good thing--diesel genset should run fuel efficiently and cleanly at 50+% load).

    Fork Lift batteries should otherwise, if taken care of, have a very good life (and you have a battery monitor to watch your state of charge).

    Circuit protection on the Battery Side (appropriately sized fuse/circuit breaker from battery + common point to every load/source)?

    Perhaps an emergency disconnect switch in case "something bad" happens (perhaps even connected to a pull cable / access hatch on the side the house/building/shed.

    2.6 kW of fixed + 2.6 kW 2-axis tracking solar panels (using PV Watts Website for Charleston WV) suggest ~440 kWhr per month or 14.7 kWhrs per day or 305 Amp*Hours per day (@48 volts) of usable 120/240 VAC power per day (~6 months out of the summer). Is that what you expect (including ~85% for inverter efficiency, 80% flooded cell eff, and 77% panel+controller eff = total 0.52 end to end efficiency).

    I did not include your wind power -- Did not understand your results (600 watts * 24 hours per day average? Seasonal? Peak? etc.--plus, unless you live in a very strong wind area--I, personally, would not count on it day to day -- more as an opportunity to reduce genset fuel use when it happens).

    I cannot think of much commentary to add at this point--you have done your homework well! :D

    Let us know how well everything works for you.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery thermal masses

    I don't think I've ever heard anyone ask about a battery bank's thermal mass before.

    So we'll go with the standard litany:

    Have you calculated your actual power needs? Kill-A-Watt meter should be everyone's first purchase, even if you're not "going solar". Your system seem quite large: if I'm reading it right, 12 kWatts?

    Dual axis controller: you may want to re-consider this as the general consensus of opinion is spending the money on panels is usually more profitable (less maintenance too).

    Likewise on the VAWT: small-scale wind generally doesn't live up to expectations.

    5200 Watts of panels is huge! What are you planning on running? (Here's a hint; if it's got a heating element in it, rethink your plan.):D
  • BB.BB. Super Moderators, Administrators Posts: 30,953 admin
    Re: Battery thermal masses

    You know 'Coot ;) --I am not going to be negative here (occasional theme from other newer posters here--nothing to do with you or your post MountainTop).

    Sounds like the plan is already in full implementation and there are people here who are quite happy with their tracking installation(s) and find them very useful for their off-grid applications. :D

    MountainTop has two identical panel installations and it will be very interesting to hear more about the installation (costs, output comparisons, and any issues with tracking vs fixed arrays that MT may have to share with us).

    I think it is a close enough call that I would not recommend that MT change plans unless there is some very good justifications (looking at cost/benefit, issues with high winds, etc.).

    In the worst case--if the 2-axis mount proves to have long term issues several years down the road, strip the solar panels and mount them next to the other fixed array.

    Of course if the tracking array is wildly successful--could always convert the fixed array to tracking down the road too.

    -Bill "happy guy" B. ;) :cool:

    PS: Or even just replace the actuators and replace with fixed struts if problems occur.

    Anyway--not upset or admonishing anyone (as moderator)... This was a wonderful weekend--and everyone is happy!
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Ralph DayRalph Day Solar Expert Posts: 923 ✭✭✭✭
    Re: Battery thermal masses

    Sounds like a legacy system, great equipment, and in prodigious quantity (only 1kw wind?:p)

    Whatever you end up with regards batteries, consider purchasing a small 1gallon distiller. You fill it, run it and it shuts itself off when done. I run mine off an outlet relayed to the MX60 Aux control and produce enough water for a about 2 weeks. 1 gallon per month for 1156ah Surrette 8cs batteries 48v. One half hour per month for sg checks (get a good hydrometer) and water filling, 24 cells. Not bad amount of labour.

    If you don't want all that filling, look up single point watering systems. I've seen them on lift truck battery sets and rv sets where access is difficult.

    You'll appreciate the tracking when you see the system float later into the evening, I do.

  • mountaintopmountaintop Registered Users Posts: 16
    Re: Battery thermal masses

    I’m planning on making a road trip to visit some of my family and picking up the batteries will be about 30 miles out of my way. I’m planning on off loading the battery with the bucket of my backhoe. I will have 1 or 2 battery stands made out of 2” box tubing; this would also enable me to use a pallet jack to move it around.
    My turbine is rated at 1000 watts. After comparing my weather station data to the efficiency chart on the turbine I should average around 600 watts. I did not say that well the first time.

    No I do not own a kill-a-watt meter. I made a quick excel file to input everything into. For the wattage ratings I looked part of them up on the net and read the labels on some of the stuff I already own. Every time I open the file I make a few changes. I added a bit of room to play and we are not planning on running the dishwasher and doing laundry every day.
    I know VAWT’s are not all they have been talked up to be.
    The wife and I work out of our house doing something like medical billing and coding. This adds a bit to our power requirements.

    BB round two
    You can be negative if you want to.
    Yes you are right. Part of the system has been up and running for about a year on 48 used golf cart batteries ($5.00 each); the house is being built off this power. The panels need to be put on the tracker and some finished wiring for it. The turbine needs to be installed on its mount and connect up its wiring. The new batteries need to be installed and the finish wiring that goes with it.

    Yes just one KW of wind.
    I haven’t thought about getting my own distiller but I know how to make them. I currently working on stock piling distilled water. I have been told I will go thought a lot in the first few weeks of these forklift batteries. They ship dry and will “drink it up” during it’s first charge and equalization.
    Is it possible to live without a single point watering system?
  • mikeomikeo Solar Expert Posts: 386 ✭✭✭
    Re: Battery thermal masses
    Is the difference in thermal mass worth getting two 24 volt batter packs then one 48 volt battery pack? If I went with the 24 volt packs I would have more surface area then a 48 volt pack.
    Hi I have two 12 volt 940 amp hour fork lift batteries in my 24 volt system and I chose to split the pack into two housings simply because it made them easier to move and install. I'm not sure the thermal characteristics are much of a concern since I have them set side by side anyway. I have never been able to noticeably heat them up with a 100 amp starting charge which usually starts tapering off within 2 or 3 hours. I use my battery system only for backup so when they are used which is only a few time a year, they usually get depleted towards 20% SOC before the get a full charge. I float them most of the time and use about 1 gallon of water every 2-3 months. One thing is that forklift battery manufacturers often want much higher float rates then what seems common. My recommended float rate is 28.2 volts, bulk 29.4 and equalize at 30 volts.
  • BB.BB. Super Moderators, Administrators Posts: 30,953 admin
    Re: Battery thermal masses


    Sounds like you have everything dialed in... I would still suggest that a kill-a-watt meter at less than $30 is going to be very helpful to getting accurate power consumption numbers. Nameplate ratings and guesses as cycling times/variable power usage is usually not very accurate.

    If you need to measure power on AC circuits that are turned on and off (such as a generator)--there is a version of the kill-a-watt meter with battery backup (needs AC power to read the LCD display).

    Is limited to a normal 15 amp circuit (North American Plug)--but it is about the best device for the job out there...


    PS: I am from the "show me" state (of Kalifornia)--Let us know how well your wind turbine produces... The current information/state of the art is not very rosy for small wind.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mountaintopmountaintop Registered Users Posts: 16
    Re: Battery thermal masses

    I just ordered two 24 volt 1500 AH (20 hour) battery packs; yes I decided to go a bit bigger thanks to BB. I was wrong this company dose ship full and ready to run so this will cut down on my work level.

    I do agree that 2 packs will be easer to use than one large pack. I’m planning on mine going end to end I think.
    You are right about having some what odd recommendations on there charge voltages.

    If I got a kill-a-watt meter then we could blame fault on who used to much power and when they did it.
    I don’t get the end of our PS...
  • BB.BB. Super Moderators, Administrators Posts: 30,953 admin
    Re: Battery thermal masses

    Oh--Just saying that small wind has a reputation for not delivering very much power and not being reliable. And VAWT seem to do even less well in actual use.

    Wondering how well yours is/will produce. Do you have a Watt*Hour/Amp*Hour meter on the turbine circuit?

    Small windpower a scam ? Survey says SO

    Trying not to be too negative in formulating my questions about your wind installation--as I am a known wind critic here... I just don't see small wind as workable for 99% of the places out there (not enough wind on site--or so much wind, that the turbines/towers self destruct). And manufacturers oversell what the machines can really do.

    There are good wind turbines out there--but they are few and far between (and probably more expensive that most people can justify). And the smaller turbines marketed to "urban areas" are simply limited by physics/aerodynamics to how much power they can produce (low towers and turbulence being the norm for urban sites).

    When I ask people who have posted here that have wind turbines how much power they gather (by month/season)--It turns out that most people don't really have any reliable measuring equipment (kWhr/kAmpHr) installed that can tell them.

    I am very happy to see that most of your power will be generated by solar PV panels with a healthy backup diesel genset. Any wind power you receive will be a pleasant surprise.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery thermal masses

    I don't mean to be negative either, just pragmatic: hate to see people spend money on equipment they don't need, doesn't work, or is in some other way a disappointment to them. But if you've already got it, use it!:D

    I will reiterate about the value of the Kill-A-Watt. Labels on devices are about as accurate as a weather forecast. Sometimes things use less than rated, so you end up over-designing. Sometimes they use more, and you find yourself short of Amp/hrs.

    We do accounting from our remote locale, and run satellite Internet, VOIP, two comps, 'frige, water pump, digester pump, et cetera on one 3600 watt inverter and 700 Watts of panels backed up with a Honda 2000 gen.

    Here at 52 Lat. a solar tracker would be good because of the wide change in angle of the sun from Summer to Winter. But it probably wouldn't survive the wide change in weather from +40 to -40 and such.
  • mountaintopmountaintop Registered Users Posts: 16
    Re: Battery thermal masses

    I’m putting the turbine on an 65’ tower that once had a water pump on it. I added 8’ to it when I put up the mount. We will weight and see what happens over time.
    The controller for the turbine dose not keep running totals on how much it produces so I will have to put one leg of the tri-metric on it.
    My healthy genset needs a few glow plugs when you the get time. They seam to only last about 5 years.

    I grabbed a kill-a-meter form the store and checked my loads. I was low on some of my numbers and high on a few others.

    A bit of negative talk only helps new people get there thoughts straightened out. I know I should have gotten the intervention about 4 months ago. :roll:
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