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Thread: Minimum voltage for a pair of 6V batteries?

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    Question Minimum voltage for a pair of 6V batteries?

    I suspect the answer to this question is somewhere in this forum but I couldn't find it. I have a pair of 6V batteries wired in series. So far I have not gone boondocking so my battery meter reads 13V to 14V. What I can't find out and would like to know is how low can I let the voltage drop when I do go boondocking before doing damage to the batteries?
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    Quote Originally Posted by GaryS1964 View Post
    I suspect the answer to this question is somewhere in this forum but I couldn't find it. I have a pair of 6V batteries wired in series. So far I have not gone boondocking so my battery meter reads 13V to 14V. What I can't find out and would like to know is how low can I let the voltage drop when I do go boondocking before doing damage to the batteries?
    Assuming you have wet batteries you typically want to stay at and above 60% of AH capacity. Voltage wise this translates to about 12.2v. There's some variance here but most charts grt pretty close. The further below 60% you go, the more damage can be caused.

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    Handy little cheat sheet
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    The idea that there is a point below which damage occurs is a myth. Deeper discharge consumes more battery life, but the effect is linear beyond about 20%. The misunderstanding came about because of the design rule of thumb to size battery banks and chargers such that the average depth of discharge remains at 50% or higher for best economy. The 50% rule comes into play when buying batteries and implementing a charging schedule.

    The chart below is an example. Most manufacturers don’t publish this kind of information. You can see that discharge below ,40% is virtually flat, and that the only big change occurs around 20%.

    Click image for larger version. 

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    It doesn’t make much sense to only use 20% of capacity.

    The real takeaway is that if you routinely need to discharge below 50%, you need a bigger battery bank the next time you buy.
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    Buy Firefly Oaisis carbon foam AGM. You can take down to 20% like lithium’s with not hunting the amt of cycles.

    https://oceanplanetenergy.com/advanc...asis-group-31/
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    Quote Originally Posted by RV Sailor View Post
    Buy Firefly Oaisis carbon foam AGM. You can take down to 20% like lithium’s with not hunting the amt of cycles.

    https://oceanplanetenergy.com/advanc...asis-group-31/
    Their own published data shows cycle life vs. depth of discharge to follow essentially the same shaped curve as above. 9000 cycles at average 30% DoD, 3600 cycles at 50%, 1800 cycles at 65%, 1000 cycles at 80% and 600 cycles at 100%. With average depth of discharge at 80%, cycle life is 89% less than cycle life with 30% average depth of discharge. That is significantly worse than the graph I posted above.

    I was working in the battery industry when Cat first proposed the idea that is claimed to be behind these batteries, but even with Cat’s money behind them they never were able to produce a battery that could hold up and the spin-off went bankrupt. These batteries are slightly modified AGMs, and share the same characteristics as all AGMs.
    John & Kathy
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    Quote Originally Posted by jkwilson View Post
    Their own published data shows cycle life vs. depth of discharge to follow essentially the same shaped curve as above. 9000 cycles at average 30% DoD, 3600 cycles at 50%, 1800 cycles at 65%, 1000 cycles at 80% and 600 cycles at 100%. With average depth of discharge at 80%, cycle life is 89% less than cycle life with 30% average depth of discharge. That is significantly worse than the graph I posted above.

    I was working in the battery industry when Cat first proposed the idea that is claimed to be behind these batteries, but even with Cat’s money behind them they never were able to produce a battery that could hold up and the spin-off went bankrupt. These batteries are slightly modified AGMs, and share the same characteristics as all AGMs.
    Yes I’m with you...they still are AGM
    Question average AGM discharged to 50% get 300-600 cycles

    https://www.dcbattery.com/lifeline_cycle_life.html

    Wouldnt Firefly at 3600 cycles at 50% be an improvement?

    Also there is a penalty with AGM which are not brought back to 100% in that it starts “learning” and lower high charge.
    Firefly does seem to have this disadvantage.

    I bought them as I didn’t ever want lithium’s and saw them as an improvement on other AGM. Since you were in the industry, was I sold a bunch of malarkey. My experience over the last 3 years with them is they are “ better” than the Lifeline AGM I used to buy.
    Donna and Dave
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    350 W Newapower Solar: 3000 Victron Inverter/ Charger: Firefly Oasis carbon foam AGM

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    Quote Originally Posted by RV Sailor View Post
    Yes I’m with you...they still are AGM
    Question average AGM discharged to 50% get 300-600 cycles

    https://www.dcbattery.com/lifeline_cycle_life.html

    Wouldnt Firefly at 3600 cycles at 50% be an improvement?

    Also there is a penalty with AGM which are not brought back to 100% in that it starts “learning” and lower high charge.
    Firefly does seem to have this disadvantage.

    I bought them as I didn’t ever want lithium’s and saw them as an improvement on other AGM. Since you were in the industry, was I sold a bunch of malarkey. My experience over the last 3 years with them is they are “ better” than the Lifeline AGM I used to buy.
    Malarky? No. Marketing yes. They aren’t bad batteries, but they don’t live up to some of the initial claims. They were touted to be lighter because the graphite foam increased surface area which reduced the weight of the plates required, but the Firefly group 31 weighs 74lbs and produces 720CCA and120AH. Compare that to the same size Odyssey at 77.8lbs, 100AH and 1150CCA. It’s obvious the idea hasn’t yet yielded the benefits originally hoped for.

    One negative point for these batteries is maximum charging voltage. While most AGMs charge around 14.6V safely, these max at 14.4V, which can be a problem for some chargers.

    I loved the idea when it first came out, and really figured it would be a big advancement, but it isn’t there yet.

    ETA:

    I didn’t answer a question:

    Wouldnt Firefly at 3600 cycles at 50% be an improvement?
    Who knows is the short answer. A VP at Firefly said many years ago, borrowing from Twain, “There are liars, damned liars and Battery Companies” CCA, RC and AH all have accepted, published industry standards, yet the companies will use the different AH tests to “fudge” specs. You’ll note batteries are seldom labelled with the AH method. A more blatant case is East Penn, which labels their marine and RV batteries with 23A RC, a test method they just made up while everyone else uses the standard 25A test. Kind of like putting a sticker on a car window saying 75MPG, but in fine print saying “Downhill, with a tail wind”.

    When it comes to cycle life, there is no standard. Odyssey says end of life is 80% of labelled AH. Other vendors don’t often tell you what their criteria are. I know for a fact that some companies use the discharge level for both the test and the end of life. So with the 50% point as mentioned above, the average discharge is 50% and the end of life is when the battery reaches 50% of original capacity. That’s a big difference. I have no knowledge of how Firefly determines end of life, so don’t assume they are being dishonest, but you can’t really be sure what the data means without some more background.
    Last edited by jkwilson; 02-01-2021 at 08:56 AM.
    John & Kathy
    2014 F250 Lariat FX4 6.2L SBCC
    2014 Reflection 303RLS
    SW Indiana

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