Place(s) to buy AGM battery for best pricing.

Discussion in 'Power - Site Power/Batteries/Generators/Solar' started by davekro, Nov 12, 2019.

  1. davekro

    davekro Active Member

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    I think I want to upgrade my old (now low capacity) AGM Grp 27 with a new AGM Grp 27 or 31. Is there a better place to find lower pricing for quality, known brand AMG batteries than Amazon? Anyone know if there might be Black Friday deals on AGM batteries (Amazon, other)?

    Looking at Amazon I see two seemingly decent choices with good ratings and a fair amount of raters. Any thoughts on these two in particular or other recommendations?

    1) Weize ≈Grp 27, 100AH*, LFP12100, $159 Amazon, 5 Star/60 ratings.

    2) Vmax Grp 31, 120AH, MR137-120, $279 Amazon, 4.4 Star/36 ratings.

    Since you should only run to 50% of capacity to not unduly shorten the # of cycles over the battery's life, you'd expect a theoretical 50 AH from the smaller Grp 27 battery and 60 AH from the Grp 31, right? If so, the 75% higher (+$120) priced Grp 31 theoretically gives you an extra 20% of amp hour use (60 vs 50). Is the lower priced battery the obvious choice, or is there a consideration I'm not thinking of? Of course, two Grp 27's would cost $318 or only 14% more than the Grp 31, theoretically yielding 100 AH to get to 50% SOC. I don't really know if 50% of AH really correlates to 50% SOC though.


    * One Amazon responder questioned Weize's rating using 10 hours, vs a 20 hour standard. Can anyone explain what that means in actual amp hour run time or affect this decision?
    "The standard for battery ratings is 20 hours NOT 10 hours like this battery. It is false rated to catch you if you're not paying attention and make you think it's a normal 100ah battery. IT is not, you are getting a 50 amp hour battery actually if you use the STANDARD amp hour measurement of 20 hours. Why is this battery False rated ? Is it to catch people off guard and getting half the battery they thought they were buying? see less
    By Amazon Customer on November 11, 2019"
     
  2. rabird

    rabird Howdy!

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    I hope this chart disproves what Amazon Customer stated.
    amp hr capacity, 20 hr. Room temp the battery is discharged down to 10.5v or so at a constant current and timed. Repeat for different current. Repeat Curve fit data and use formula to determine 20 hr capacity.

    look at this chart, the first is G24 battery, note at bottom the 20 hr capacity is 69 ah.
    now look at the 6A discharge and note it takes 10.7 hrs with a capacity of 64 ah. So the 10.7 hr capacity is 64 ah which is less than 20 hr capacity of 69 ah. 69/20 = 3.45A constant discharge for 20 hrs.

    battery.jpg

    check sams/costco for duracell AGM G31
     
    Last edited: Nov 12, 2019
  3. SteveP

    SteveP Well-Known Member

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    Battery capacity varies by the discharge rate. A battery discharged at constant 5 amps will output more amps than if discharged at constant 10 amps. So you would expect a battery that outputs 100 AH at 20 hour rate to only output about 93 amps at the 10 hour rate. By stating "100 AH @ 10 hour rate" They're making a veiled claim that their battery is superior to one that outputs 100 AH @ 20 hour rate. The question is, do you believe it? Is it achievable in the real world?

    The amazon customer is probably confusing amp hours with reserve capacity. Which is rated by discharge rate and results in minutes of run time. RC is normally stated at 25 amp discharge rate, RC @ 25 amps = 140 minutes. But I've seen battery advertisements stating RC at 10 or even 5 amp rates, which definitely skews the result.
     
  4. tenttrailer

    tenttrailer Art & Joyce - Columbus, O

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    Several years ago, Sams Club had the best deal on AGM G-31 batteries. There 105 AH AGM battery right mow is $179. It use to ne in somewhere around OCT, NOV of each year they would have a sale on the batteries. It's the same battery that west marine sells for about 275
     
  5. davekro

    davekro Active Member

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    Rabird, thanks for the info. Costco does not seem to carry AGM RV, maybe just the marine starting deep cycle combo. But reading reviews on the Duracell RV Deep Cycle's looks really good and great pricing. Duracell's G27 & G31 are actually pretty close in size and weight. Also, the Duracell G31 is only 67.5 lbs compared to the Vmax G31 I mentioned above. That is a plus for me because I'd double use that battery for running a raft w/ 12v trolling motor. So carrying between front driveway and back yard at the water.

    • Duracell AGM Marine Battery - Group Size 27M, Item # 980025493, Model # 27M, $159.
    https://www.samsclub.com/p/marine-27magm-18-mo-free/prod21234614

    Duracell AGM Deep Cycle Marine and RV Battery - Group Size 31, Item # 347736, Model # 31, $180.

    https://www.samsclub.com/p/duracell...size-31dtmagm/prod3590232?xid=plp_product_1_2

    "amp hr capacity, 20 hr. Room temp the battery is discharged down to 10.5v or so at a constant current and timed. Repeat for different current. Repeat Curve fit data and use formula to determine 20 hr capacity."
    Does the chart show that all batteries were drawn down to 10.5 volts? In your example of a constant 6 amp load, it took 10.7 hours to draw down to 10.5 v and used 64 AH of the battery's entire capacity (to dead, to essentially no usable voltage)? So if the G24 happened to have a 75 AH rating, it would have used 85% of that 75 AH? That does not look right. Is the AH column what is 'left' after 'x' hours at 'x' volts? Doesn't look quite right either because draining to 10.5 v is WAY below 0% SOC. I'm confused. ;o)

    Anyway, thanks for the lead to Sam's club. It's 45 mins away from me and I'm not a member. But I think my neighbor might be. ;o)
     
  6. rabird

    rabird Howdy!

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    in these test 10.5v under whatever load is considered DEAD or 0% SoC.


    Reserve Capacity (RC)
    1. Allow fully charged battery to stand at room temperature for 4 to 96 hours.
    2. Discharge battery at 25A at 80 +/- 5 degrees F (27 +/-3 degrees C).
    3. Determine discharge time in minutes to 10.5V for a nominal 12V battery.
    4. Correction for final discharge temperature: Mc=Mr[1-0.009(Tfinal-27)]
    Mc = minutes corrected to 27 degrees C (80 degrees F)
    Mr = minutes actually run
    Tfinal = temperature of electrolyte above plates in a intermediate plate in degrees C
    0.009 = temperature correction factor
    AH capacity (AH)
    1. Follow procedure for RC test except run at designated discharge rate (either n hour discharge rate or at specified amps). For example 20 hour capacity would be tested at nominal 20 hour discharge rate, i.e., 20 hour capacity in AH divided by 20 hours (105AH/ 20 hours = 5.25 amps. AH capacity may also be determined at a specified amp discharge rate, say 1, 5, or 10 amps, etc. The rating system is specified by the battery manufacturer. JCI marine batteries are normally rated at the 1A discharge rate.
    2. Upon determining the discharge time to 10.5V in miniutes. Convert to AH with the formula: AH = Mr X A / 60, where
    Mr = minutes actually run
    A = discharge rate in amps
    3. AH capacity available in the field is temperature and rate dependent and could deviate significantly from the rated value. Increased AH's are available at warmer temperatures and lower discharge rates.
     
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  7. tenttrailer

    tenttrailer Art & Joyce - Columbus, O

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    Yes 10.5 all. The AH and RC standard is Just that a standard that is to be used if rating a battery for AH's. It defines how how the voltage should be dropped, the temperature etc.... for the test.
     
  8. MNTCamper

    MNTCamper Active Member

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    I would third checking out the Sam's club batteries. They are Duracell brand, which is East Penn (DEKA). Price as stated above and about 4-5 times a year, you can purchase them for an additional $20 off. I have the G31 FLA on my pop up and it works great. (Duracell) We can go for 10 days on this battery, but we don't draw a lot other than recharging stuff and lights.
     

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