Proper Absorb Voltage?

SpiderManSpiderMan Registered Users Posts: 11 ✭✭
I'm converting a small RV to solar. Currently the vehicle has [email protected] Deka MarineMaster DC31DT flooded lead acid batteries, wired in parallel. They're a little over 3 years old. Until now, they've only been recharged from the engine alternator. We always camp in the backcountry and never plug in. Between trips we use a BatteryMinder 1 amp float charger. The vehicle gets used for about a month continuous each spring and fall, with a dozen 2-5 day trips in between.

After lots of research for the solar upgrade, I realize that these are probably not true deep cycle batteries. When they die I'll look for better replacements, but they seem to be performing well enough that I don't plan to replace them now.

Deka says absorb voltage for these batteries is 14.1v. This is far lower than everything else I've been reading. Thoughts?

Once I get the solar up and running, should I go through a charge cycle at 14.1v and then again at higher voltages and measure SG over time to see if it makes a difference? Is Deka just being conservative about not overcharging these because they're marketed to a demographic who is not likely to check water levels or use temperature compensation?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,702 admin
    Re: Proper Absorb Voltage?

    That is a "soft" absorb voltage... It will take longer to recharge the batteries but is right in the middle of ~13.8 to 14.2 volts recommended for a vehicle charging system.

    With "better" charge controllers, they will fall back to "Float" charging of around 13.2 to 13.6 volts or so. That will reduce battery water "usage", allow the battery to last longer, and keep them well charged between uses.

    With solar power, you have a limited window of when to charge the battery bank and a variable power source--The Sun--which usually means that you would like to gather all of the energy from the sun as fast as possible and not leave any Amp*Hours behind... So charging at ~14.5 to 14.75 volts or so during the few hours a day you have full sun will allow the batteries to come to full charge quicker and probably harvest more energy (when the Absorb voltage is reached, the charge controller will hold that voltage and the battery will naturally taper down on the charging current). Once the controller determines the batteries are fully charged, the controller will fall back to float voltage.

    There are drawbacks to higher voltages--Battery current is higher so they get hotter, Oxygen forms on the positive plate and causes grid corrosion, lots of bubbling (and equalization) is hard on the plates and can cause shedding of plate material, etc... So limiting the amount of absorb voltage time and maximum current/voltage helps battery life. Some good charge controllers have remote battery temperature sensors which are very nice to ensure that the voltage is proper for the current battery temperature. Hot batteries should have lower charging voltages, and lower temperature should have higher charging voltages. Also, sometimes the charge controller is not next to the battery bank--so charge controller temperature may not represent battery temperature/environment.

    You can read about batteries here:

    Deep Cycle Battery FAQ
    www.batteryfaq.org

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SpiderManSpiderMan Registered Users Posts: 11 ✭✭
    Re: Proper Absorb Voltage?

    Bill,

    If I understand correctly, you're saying it's OK to use a higher absorb voltage provided that I pay attention, check water levels, use remote temp. compensation,
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Proper Absorb Voltage?
    SpiderMan wrote: »
    If I understand correctly, you're saying it's OK to use a higher absorb voltage provided that I pay attention, check water levels, use remote temp. compensation,
    I think that what Bill is saying is that if you are on the grid and you have unlimited time in absorb mode, it is better to use a lower voltage. If you are running solar or generator or engine alternator, you need to get what you can while you can, so use a higher voltage.
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • SpiderManSpiderMan Registered Users Posts: 11 ✭✭
    Re: Proper Absorb Voltage?
    vtmaps wrote: »
    I think that what Bill is saying is that if you are on the grid and you have unlimited time in absorb mode, it is better to use a lower voltage. If you are running solar or generator or engine alternator, you need to get what you can while you can, so use a higher voltage.
    --vtMaps

    OK, that makes sense. But I've also read that FLA batteries simply won't charge completely unless the voltage is pushed high enough (14.4 to 14-point-eight [why does the 'eight' get replaced with a goofy smileyface?]). I'm concerned that charging only to 14.1 will leave the batteries chronically undercharged. Additionally, I'm concerned that since they've only ever been charged from the engine that they've been chronically undercharged for the last 3 years.
  • BB.BB. Super Moderators, Administrators Posts: 29,702 admin
    Re: Proper Absorb Voltage?

    I agree with both of you. ;)

    But--I would also take what the battery mfg. says to heart. Many "maintenance" free vehicle batteries have calcium in the lead plates to reduce water usage... I don't know what higher voltages/charging currents would do (for better or for worse). "Standard" deep cycle flooded cell lead acid batteries should probably have water added every ~2 months or so if everything is working/charging correctly... Too much water usage, back down on charging voltage/absorb times. Too little, review state of charge and if you have enough energy available to recharge the battery back above 90% state of charge a couple times a week.

    Also another reason to be careful about charging over ~14.2-14.5+ volts... Many DC adapters/appliances for cars can be damaged by the 14.75-15.5 volts that is used to recharge/equalize deep cycle batteries. If you have high charging voltages and/or are equalizing--You may want to turnoff/disconnect DC Car electronics (radios, dc laptop adapters, etc.) until the batteries are back down to 14.2-14.5 volts maximum.

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