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Building a battery Pack

Post in 'The Green Room' started by mass_burner, Oct 31, 2013.

  1. mass_burner

    mass_burner Minister of Fire

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    Hi,

    i'm new to green electric. We get frequent power outtages where I live, so i'm trying to accomodate. I have 3 wood stoves, so heat is not an issue. I also have a 1500 watt onboard inverter connection wired to my 2004 Prius. This will handle my fridge, a few lights. I am also buiding a 3 battery pack (35 AH batteries) for TV, radio, stove fan, etc.

    My question is, can I recharge my battery pack with the Prius as I'm using it (the battery pack)?

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  2. mustash29

    mustash29 Feeling the Heat

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    You should be able to. A regular car recharge's it's battery while also drawing power from it. The voltage regulator just increases the alt output when necessary. On my truck, the batt will discharge with heavy winch use, 145 A alt but the winch can draw upward of 500 amps at full 8000 lb pull.

    I have a small 320 W continuous / 400 W peak inverter / jump pack / compressor / emergenct light made by Xantrex. If you are using the inverter and need extended battery capacity, you can hook the jumper cable clips to another battery. It's internal is only 20 A/h. The first time I used the pack camping, it was dead after one evening of lighting use. The 2nd time I connected it to my deep cycle boat battery and went 3 nights with the same lighting and the % charge indicator was still above 90% with both batteries connected in parallel.
  3. woodgeek

    woodgeek Minister of Fire

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    personally, for infrequent use, I would go with the cheapest batteries I can find. Your 35AH guys sound like AGMs or gel cells, which are expensive, can have poor shelf life, and can get killed by some (cheapo) chargers. For my 'hobby' solar setup, I just got the cheapest flooded type marine battery from Walmart (they also recycle the battery later for free if you save the receipt). Saves you the trouble of filling it with acid, and with their sales volume no worries about it dying on a shelf in a hardware store.

    I think I got a 85 Ah guy for <$100. hook it up to a small, decent quality maintainer or trickle charger and it will be ready for use for many years.

    If you are worried about 'deh acid' get a $10 battery box, and wire a pigtail cord with a 12V male and female on it. Safety point, I would always put a 20-30A automotive fuse in line with the cords, inside the box, so you don't short it through a ring and burn your finger, etc.
    Last edited: Nov 1, 2013
  4. mass_burner

    mass_burner Minister of Fire

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    By coincidence, my "front end" is a Xantrex 600 HD, connected in parallel to 2 AGM's. I have a heavy duty plastic rolling tool box that houses the AGM batterys and the Xantrex. My plan is roll the box to wherever I need power, take out the Xantrex and plug N play.
  5. mass_burner

    mass_burner Minister of Fire

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    I got the AGM's on sale at TS for $65. My Walmart's only sell dual use marine with CC amps, since I don't need any CC amps, isn't that a waste of money? Also, I will be using these inside in the winter so sealed AGM is safer isn't it?
  6. woodgeek

    woodgeek Minister of Fire

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    different strokes. AGMs are fine, esp at the price. Just don't put a cheap charger on them.
  7. mass_burner

    mass_burner Minister of Fire

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    what should i look for to avoid a cheapo charger. i now have a 2/10/50 amp sears charger. i haven't charged the AGM's yet.
  8. woodgeek

    woodgeek Minister of Fire

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    In my (limited) understanding I thought that both gel cells and AGMs require slightly different charging voltages than flooded batteries (the target of most chargers on the market), and then can get systematically under or overcharged if the charger is not 'designed' for AGMs. The bulk 'smart' charger I got to top off my car/ solar/ sump batts has an 'AGM' setting. IF your charger (or its manual) does not say anything about AGMs, I personally wouldn't use it for AGMs on an extended basis (emergency or short bulk charges are prob aok).

    The other factor is water loss....while AGMs are supposed to catalytically recombine the H2 and O2 that form during charging, a bad charger might create more gas than it can handle, leading to venting, and eventually loss of water and the battery.

    Again, no hands-on with AGMs, and I think with the right charger they provide great service. I was more worried that I am dumb, and if I overcharged my flooded battery (a lot) I liked that I could just top it off with distilled water.
    Last edited: Nov 1, 2013
  9. mass_burner

    mass_burner Minister of Fire

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    thanks. if have seen electric guys on YT opening up AGM battery's that wouldn't hold charge, adding water and bringing them back to life. didn't seem to hard.
  10. woodgeek

    woodgeek Minister of Fire

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    I've seen that too, but not exactly using it as intended, nor would I ever count on said battery to provide needed output again. Cutting into a sealed battery seems to obviate all the advantages of getting a sealed battery in the first place.

    I am not down on AGMs just think a lot of folks get 'em thinking they are safer/more maintenance free and then promptly kill them using a cheap hardware store charger. Thus the YT videos for fixing the booboos.

    AGMs are really cadillac batteries designed to give ultra-long service with many cycles, when used according to precise operating specifications like off-grid solar with high-end controller chargers. Thus the higher cost point is justified. They are not a 'more user friendly' and safer alternative to flooded batts.

    The downside of cheapo batts is that they will survive fewer cycles total, like several hundred versus 1000s for an AGM. The upside is that for a blackout usage, they will never see several hundred cycles, and are more likely to absorb 'abuse' like being overcharged by a (bad) trickle charger for 12 mos.
    Last edited: Nov 1, 2013
  11. mass_burner

    mass_burner Minister of Fire

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    My ultimate plan is solar panels. Maybe as a first step I can hook my pack up to a small panel and daily run a 400 watt space heater. I originally tried to get golf cart batteries, but couldn't find them anywhere.
  12. jharkin

    jharkin Minister of Fire

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    There is a big difference between AGM (absorbed glass matt) and gell cell. In many ways AGM are the best type of all to use for backup - they have very low self discharge, tolerate faster charging rates than flooded, tolerate larger discharge currents, wont freeze, wont gas, and can be used in any orientation.

    The only real downside to AGM is cost.

    Look at:
    http://www.solar-electric.com/deep-cycle-battery-faq.html#AGM, or Absorbed Glass Mat Batteries

    Agree about using a quality charger. You want one that is a 3 stage (bulk/absorb/float) The solar guys use expense high end models from companies like Iota Engineering and Magnum Energy. For recreational use the best inexpensive ones Ive seen are the NOCO Genius line. I own and use one, very happy with it.
    ewdudley and woodgeek like this.
  13. jharkin

    jharkin Minister of Fire

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    Geek you would probably like this one as well... Great read for the technically minded

    http://jgdarden.com/batteryfaq/carfaq9.htm

    I used this and my programmable computerized hobby charger to make custom tailored charge routines for my small AGM hobby batteriers and the big flooded battery in my sump pump.
  14. woodgeek

    woodgeek Minister of Fire

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    I did like it. Bookmarked.
  15. mass_burner

    mass_burner Minister of Fire

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    also, when a battery says its xAH without an amp draw rate, should I assume 1 amp?
  16. jharkin

    jharkin Minister of Fire

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    No dont assume that. There are two ways that capacity is typically reported for lead acid batteries:

    Capacity stated in Amp-Hours:
    • This is the convention typically used for true deep-cycle batteries
    • If not explicitly stated its assumed to be at a 20 hour (C/20) discharge rate
    • some manufactures use a different standard, for example Concord Sun Extender lists it @ 25 hours. If they do it should be published in the specs.
    Example -
    Battery is listed as 100Ah
    100Ah / 20h = 5 A
    So you can pull 5 amps for 20 hours.

    Capacity stated in reserve capacity:

    • This is the standard used for SLI (starting batteries)
    • The number given is time in minutes that the battery can run on a 25 amp load

    Example -
    Battery is listed as 105 reserve capacity.
    105 / 60 min = 1.75 hr x 25A = 43.75Ah

    ------------------------------

    In all cases the amount of useful capacity varies with the discharge rate. The slower you discharge a battery the greater the capacity you can get out of it. Deep cycle batteries sometimes come with a table of discharge rates, and in out 100Ah example above, if you drained it in an hour the capcity might be as low as 60Ah, but if you go up to a veeery slow 100 hour rate you might get as high as 120Ah out of it. Similarly our 43Ah car battery might be good for closer to 60Ah if discharged slowly over 20 hours.

    the differences in discharge efficiency are related to internal resistance as measured by a constant called the Peukert coefficient. Some battery makers publish this number and there are formulas that you can use to calculate discharge rates from it. Basically whats happening is the faster you discharge the battery that resistance is wasting more energy as heat.



    Trivia: Whereas Lead Acid batteries typically publish C/20 discharge capacities, the convention for small NiCD and NiMH rechargable batteries is based on C/5 discharge. And for Lithium Ion its at 1C.
  17. mass_burner

    mass_burner Minister of Fire

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    So for a 35Ah, it 35/20 = 1.75 amps.

    So you can pull 1.75 amps for 20 hours. And 1.75A is 210 watts.
  18. jharkin

    jharkin Minister of Fire

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    Yep, that's right. If you need more amps you can discharge it faster, you just wont get the full 35Ah capacity.

    Here is an example for the size battery you are using - a Universal Battery Group 35Ah AGM:
    20 hour = 1.75A (35Ah)
    10 hr = 3.2A (32Ah)
    5 hr = 6A (30Ah)
    1hr = 21A (21Ah)
    http://lib.store.yahoo.net/lib/wind-sun/45976.pdf


    No its 21 watts, remember you are at 12 volt.

    And if you are powering an AC load through an inverter you have to subtract inverter losses of ~ 15% so you are going to net out 18 watts @ 120VAC.

    Now you can probably see why off grid people have entire rooms full of batteries to power a house ;)
  19. mass_burner

    mass_burner Minister of Fire

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    so my pack will have a total of 105 AH.

    105/20 = 5.25 amps, so I can draw 630 watts for 20 hrs. but i can't really use the 20 hours cause i don't want to discharge to 0, do I?
  20. jharkin

    jharkin Minister of Fire

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    Assume they are wired in parallel for 12 volts, yes the capacity is 105Ah. So you are correct you can use 5.25 amps for 20 hours, but again that's 63 watts, not 630.

    And yes its better not to discharge to zero every time, the battery life will be quite short. You want to never use more than 80%, and the best battery lifespan will be realized if you dont use more than 50% on average. Look at the graphs in the PDF above.
  21. mellow

    mellow Resident Stove Connoisseur

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    As for a trickle charger I got this one for $20 from walmart for my AGM batteries, working fine so far, I use them in the kids battery powered 4 wheelers and cars and my pwc during the summer. Not had any issues with the charger yet.

    [​IMG]
  22. mass_burner

    mass_burner Minister of Fire

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    ok, so for this pack i'm building, i think the sweet spot is the 7hr, 15amps, 180watts during an outage.

    180-200 watts should cover small tv, cable box, etc for about 6-7 hours. i can recharge it overnight and start fresh in the morning.
  23. mass_burner

    mass_burner Minister of Fire

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  24. mass_burner

    mass_burner Minister of Fire

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    BTW, person at NOCO said they do not recommend charging and drawing at the same time.
  25. jharkin

    jharkin Minister of Fire

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    Looking at it I dont know if its a true smart 3 stage charger like the noco, but even if it is, at 1.5 amps output it would take over 3 days to charge your 105Ah bank.

    The 3.5 amp Noco will do a little better at around 30-40 hours.. But ideally you want to get at least the 7.2amp Noco model, and if you need to recharge while you sleep a 15amp charger would be even better.

    You have got a lot of money invested in batteries and inverters, a read through of those 2 links I mentioned upthread before you buy a charger would be worth your while. It will explain things far better than I can.


    http://www.solar-electric.com/deep-cycle-battery-faq.html

    http://jgdarden.com/batteryfaq/carfaq9.htm

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