Max voltage drop on 6V battery

rp3703rp3703 Registered Users Posts: 115 ✭✭

I am running sort of an experiment to see how long I can let batteries sit with no loads and no recharging to see if it is better to just leave the batteries at our off grid cottage in Northern Ontario disconnected all winter long versus hooked up to a CC and panels given the threat of the panels getting covered in snow and the CC running them dead. Anyhow, I’m not trying to start a debate, what I want to know is how low is too low to let the voltage drop on my 6V 215AH FLA batteries before they can incur damage?

1860 W (6) Rensola JC310M, Classic 150, [email protected] (8) Deka DSGC15 FLA's, Victron Phoenix Multi 24/2500 Inverter
Powering-20.5 CuFt. Fridge, 1 HP Submersible water pump, UV, Washing Machine, Gas Dryer, Gas Stove, Lights and Receptacles
Off Grid Cottage on lake in Northern Ontario 46˚N 

Comments

  • BB.BB. Super Moderators, Administrators Posts: 28,608 admin
    There have been arguments on both (all) sides in the past... My 2 cents.

    Do not let a Lead Acid battery of any type/construction fall below ~75% state of charge. Below that point, a non-cycling battery seems to sulfate much faster.

    Using a hydrometer is the gold standard, measuring resting voltage (vs temperature) is another, usually less accurate method.

    AGM batteries are supposed to not sulfate as quickly (I think--Have read a few snippets that say that--not sure if true or not).

    And there is the old handy engineering rule of thumb. For every 10C change in temperature, the life will increase (if colder) or decrease (if hotter) by a factor of 2x.

    If you store a FLA battery at 5C (-20C under 25C nominal battery operating temperature rating), then it will last 2x2 = 4x longer.

    If you store a FLA battery at 45C (~max temp rating), then it will last 1/2 * 1/2 = 1/4 as long.

    And, from what I have seen, it appears that the self discharge rate seems to follow that reasoning... A 5C battery will self discharge 1/4 as fast.

    The typical FLA battery is rated for 30 Days of "no charge" storage... And a AGMs are typically rated at 3-6 months between charges in storage.

    https://www.solar-electric.com/deep-cycle-battery-faq.html#Battery Voltages


    State of Charge12 Volt batteryVolts per Cell
    100%12.72.12
    90%12.52.08
    80%12.422.07
    70%12.322.05
    60%12.202.03
    50%12.062.01
    40%11.91.98
    30%11.751.96
    20%11.581.93
    10%11.311.89
    010.51.75

    So, a 12 volt battery below ~12.36 volts @ 25C/75F will be "at risk". (6.18 volts @ 6 volt bank)

    From other readings, cycling a battery (daily) between 50% and 80% state of charge (battery being actively used) does not have any sulfating issues (other than age of battery). This is an alternative to how to cycle the bank... Instead of 75% to >90% SoC every day... You only go >90% once per week--And one mfg. said >90% (and equalize?) once per month is OK too.

    The old question of what to do in snow country... Leave controller connected to panels with risk of snow cover...

    If you have no loads, just fully charging the battery and disconnecting all loads, and let the battery get cold (fully charged LA battery will not freeze until really cold:

    https://batteryuniversity.com/index.php/learn/article/charging_at_high_and_low_temperatures

    Freezing a lead acid battery leads to permanent damage. Always keep the batteries fully charged because in the discharged state the electrolyte becomes more water-like and freezes earlier than when fully charged. According to BCI, a specific gravity of 1.15 has a freezing temperature of –15°C (5°F). This compares to –55°C (–67°F) for a specific gravity of 1.265 with a fully charged starter battery. Flooded lead acid batteries tend to crack the case and cause leakage if frozen; sealed lead acid packs lose potency and only deliver a few cycles before they fade and need replacement.

    Lead Acid batteries kept cold will last much longer (by many years). Just do not let them (FLA) freeze and get ruined (keep charged/limit exposure to extreme cold).

    Then you also need to look at the discharge current of a charge controller... A simple PWM controller--Leakage current is not much. MPPT controller with internet connection+cell phone+router for remote monitoring--Will probably kill a battery bank that does not have over-winter charging.

    For example, a Midnite Classic MPPT controller has 2.5 Watts (~maximum) of standby consumption:

    https://www.solar-electric.com/lib/wind-sun/Classic-specifications.pdf

    2.5 Watts * 24 hours per day * 90 days of "snow" cover * 1/12 volt battery bank = 150 AH @ 12 battery bank of "winter consumption"

    That is not an insignificant amount of power... Add the Internet consumption and a cell connection (plus alarm/camera/etc.)--Or the last person out forgot to turn off the 20 Watt tare power AC inverter... Lots of ways to kill the bank.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rp3703rp3703 Registered Users Posts: 115 ✭✭
    edited January 6 #3
    So the specs on my Deka DSGC15's list float voltage at 6.75V. 75% of that would be 5.06V. Right now the lowest voltage I have on the 4 batteries I'm watching is 5.79V but your saying rather than go by voltage, I should be checking their condition with a hydrometer? 

    Well here is what I’ve seen so far having these batteries sitting in my garage here in Nashville TN. I started measuring them at the beginning of August and in the five months since I started, they have dropped an average of .16V. Two of these five months the temps were solidly in the 80’s and 90’s and it has only gone below freezing for maybe a few weeks. These are far worse conditions than they will be subjected to in Northern Ontario. 

    I only visit our place in Ontario for one month every year. So if I were to shut off all loads and leave the batteries fully charged for the 11 months I’m not there, going by my readings, I’d have a max voltage drop of .36V and the batteries would be at 6.39V or about 94% when I return. Am I looking at this wrong? 

    1860 W (6) Rensola JC310M, Classic 150, [email protected] (8) Deka DSGC15 FLA's, Victron Phoenix Multi 24/2500 Inverter
    Powering-20.5 CuFt. Fridge, 1 HP Submersible water pump, UV, Washing Machine, Gas Dryer, Gas Stove, Lights and Receptacles
    Off Grid Cottage on lake in Northern Ontario 46˚N 
  • BB.BB. Super Moderators, Administrators Posts: 28,608 admin
    If the battery's average temperature in TN was ~93F, then I would expect them to self discharge below 75% in ~2 weeks (+18F -> 2x faster self discharge).

    Very roughly, there is a 1/2 volt voltage swing between 100% and 0% state of charge for a 6 volt FLA battery.

    At this point, your 5.79 volt battery is ~11.58 volts ref 12 volt scale... That is 20% State of charge.

    Given that this has been, presumably, happening over the last 6+ months, I would guess that the batteries are severely weakened or ruined.

    What to do next... Probably a cost/benefit investigation. 1) setting up a charging system that will keep the bank floating 11 months a year from solar, and 1 month a year to supply your daily needs. Something like this. Generally, 1-2% rate of charge minimum for float charging (of an already 100% state of charge battery):
    • 430 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.01 (float) rate of charge = 162 Watt array
    • 430 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.02 (float) rate of charge = 324 Watt array
    Something like 2x 100 Watt panels in parallel, with a ~10-20 Amp PWM controller. And mount the panels on the side of the building/near vertical (keep free of snow--above drifts). And even if it does cover the panels for a month, the PWM controller should not discharge the battery bank much at all.

    30 amp MorningStar PWM controller takes 20 mA (no meter) to 35 mA (with meter):

    https://www.solar-electric.com/morningstar-prostar-ps-30-solar-charge-controller.html

    • 0.035 amps * 24 h * 90 days = 25.2 AH over 3 months
    And that would be during cold weather (snow covered, batteries below freezing, lower self discharge).

    Or 2), bring the batteries back home and charge them once a month (or twice a month in the summer)--Ideally with a good quality float charger like Battery Tender or Battery Minder brands (I have used cheap 1 amp cube trickle chargers, and they would gas the battery enough to need watering every couple months (normally, I never have to add distilled water to a car battery).

    I have used this and similar models from BattteryMinder:

    https://www.amazon.com/gp/product/B005EKY1EM

    I see this one is no longer available... But so far, I have had very good results on 3 vehicles over the years keeping my batteries floated.

    You don't want the charging voltage too high... Hopefully below ~14.2 to 14.4 volts absorb charging--But spending most of its time in float at ~13.6 volts or so....

    Anyway, some starting points/suggestions. It is always difficult to justify a 1 month a year solar power system vs just a Honda eu2000i or similar genset and small battery bank for evening/quietime lights/music/etc.

    It looks like your cabin is well outfitted... There is always the question of theft and if somebody can come by and see everything is OK (check battery voltage/water levels), etc.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rp3703rp3703 Registered Users Posts: 115 ✭✭
    I just re-read your post, so 6.18V is 75%. So I'm at about 20% right now. So that's bad, real bad. The other three batteries I have are at 6.02 so they are at about 50%.
    1860 W (6) Rensola JC310M, Classic 150, [email protected] (8) Deka DSGC15 FLA's, Victron Phoenix Multi 24/2500 Inverter
    Powering-20.5 CuFt. Fridge, 1 HP Submersible water pump, UV, Washing Machine, Gas Dryer, Gas Stove, Lights and Receptacles
    Off Grid Cottage on lake in Northern Ontario 46˚N 
  • rp3703rp3703 Registered Users Posts: 115 ✭✭
    How do I determine if they are completely ruined?
    1860 W (6) Rensola JC310M, Classic 150, [email protected] (8) Deka DSGC15 FLA's, Victron Phoenix Multi 24/2500 Inverter
    Powering-20.5 CuFt. Fridge, 1 HP Submersible water pump, UV, Washing Machine, Gas Dryer, Gas Stove, Lights and Receptacles
    Off Grid Cottage on lake in Northern Ontario 46˚N 
  • BB.BB. Super Moderators, Administrators Posts: 28,608 admin
    Charge them and then EQ, and measure the specific gravity... Ideally, they should all come back up to ~1.265 sg units... If you have one or more cells that are much lower (ideally SG should range by 0.015 to 0.030 between low to high cell).

    If they do come back up to some reasonable SG and the cells are balanced... Then you would load test them. If the batteries do not support your load, then they are no good for your needs.

    Some vendors list capacity loss of 20% as a "end of life" battery. Others list 30-50% loss of capacity as end of life (I think I have seen that wide of range).

    If you use 20% of the battery bank overnight typically, then even 50% loss of capacity is not necessarily "dead" for your needs.

    If they cannot start your washer, well pump, hand tools (too much surge current), then they are not good for your either.

    You can get load testers--But one of the things I have seen with Lead Acid batteries taken "dead" then recharged (such as car batteries, lights lift on, motor cranked until battery dead and real fault fixed)--Is that they are recharged and work well for a few days, then "go dead" after 1-2 weeks unexpectedly (everything fine for 1+ week driving to work and bank, then one day, car will not start at work to go home).

    Some of this depends on costs and accessibility of your cabin. If you can drive 15 minutes to get a new battery bank for reasonable price--Then it may be worth charging/EQing/Cycling at bit at home, then take them to cabin and enjoy life.

    If, however, it is 3+ hour drive over rutted roads to buy a set of battery at 2x normal costs--Then I would probably scrap these batteries and buy a new set for the cabin next season.

    You are risking USD$ 100 x 4 batteries (6 volt @ 215 AH golf cart batteries) and tossing them early. A ~$400 gamble.

    If the batteries are 3-5+ years old, I would suggest getting a new set. They are not going to last much longer anyway (assuming generic quality golf cart batteries).

    My thought processes... You probably have other things to take into account too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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