AGM Charging questions? Will bulk charging at less than .2C cause problems?

PNW_StevePNW_Steve Registered Users Posts: 79 ✭✭
Hello All,

I have been battling with the solar system design. The challenge is cost vs capacity. Batteries have been a sticking point.

Well, I found a score on batteries. Now my concern is that I may not have enough space for panels to properly charge the batteries.

As best I have been able to figure out: AGM's want to see .2-.25C bulk charge rate.

I have a 24v-380ah battery bank and room for around 1500 watts of panels.  Under ideal conditions I might see a charge rate of .16C

BTW: my daily consumption is generally around 2.2kwh. I would like to have the capacity to run additional loads.

My question is: If I charge at .1-.16C will I run the risk of sulfation? Any other issues I may need to watch for?




  • PNW_StevePNW_Steve Registered Users Posts: 79 ✭✭
    I am going to toss another question out there.

    Regarding depth of discharge: I see folks referring to it in two ways and wonder which is the"right" way?

    Some call a battery that has been discharge by 1/4 of it's capacity and has 3/4 remaining as being around 25% DOD and some call it 75% DOD.

    Which is correct?

    IR confused.....,
  • Raj174Raj174 Solar Expert Posts: 768 ✭✭✭✭
    A 10% to 16% charge rate is fine. What is important is getting the battery bank back up to over 90% daily and reach float 2 to 3 times a week minimum. I think reaching float more often is better. 
    Second question,  75% SOC (state of charge) is 25% DOD (depth of discharge)

    3600W PV, MNE175DR-TR, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 360AH LiFePO4, Kohler Pro 5.2E.
  • lzhomelzhome Registered Users Posts: 39 ✭✭
    Since this is the Solar Beginners Corner, would you please explain the following "AGM's want to see .2-.25C bulk charge rate." I understand the reply from @Raj174 but what is .2-25C? Is this percentage?
    TriStar MPPT, 8 x 100w PV, MNPV6 Combiner, 4 x 12v 155ah VMAXTanks AGMs and GoPower 2000w PSW Inverter.
  • BB.BB. Super Moderators, Administrators Posts: 31,068 admin
    edited October 2017 #5
    "C" is usually used for the "capacity of the battery/battery bank". Nominally, for our (off grid solar power), we use the 20 Hour Discharge Rate as C. So
    • 0.2 * C = rate of charge (20% rate)
    • 0.25 * C = rate of charge (25% rate)
    So, if you have a 100 AH @ 12 volt battery, the recommended charging current for a 0.20C rate would be:
    • 0.20 * 100 AH = 20 Amp rate of charge
    Regarding what is the "optimum" rate of charge--It does depend on battery types. Typical flooded cell lead acid batteries work well at 10% to 13% (or C/8 hour rate of charge) rates. Some batteries, such as some AGM and even some "pure lead" type batteries can sustain much higher rates of charge and even will specify a minimum rate of charge.

    Others don't really "care" about the rate of charge (Li Ion typically "don't care" too much--Just don't exceed their maximum charge rate). Others can have chemical reaction/physical plate characteristics that are "optimized" by min/max rates of charge... For example, flooded cell deep cycle batteries usually need a minimum of 5% rate of charge (especially for "equalization"--The gassing and heat helps mix the electrolyte). And some tall cased flooded cell LA batteries recommend 10%+ minimum rate of charge (again for mixing of electrolyte).

    We have other reasons too for our "general rules of thumbs" numbers... 5% minimum for solar (weekend usage), 10%-13% typical for full time off grid (to ensure battery bank is fully recharged in 1-2 sunny days, support daytime loads). And for flooded cell LA batteries, >13% rate of charge, highly recommend a Remote Battery Temperature Sensor--LA batteries get hot as they charge, hot LA batteries (high charging currents) actually drop their charging voltage--It is possible for a charge controller to see that as a "load" on the battery bank and overcharge the battery, or even cause a meltdown (high charging current, warm room, etc.). RBTS reduce that risk.

    AGM batteries typically have "lower internal resistance" and can take higher charging current without heating up. However, AGM batteries are sensitive to the charging voltage. They need a slightly lower charging voltage, or they can gas and vent--Leading to a short battery life (AGM+Sealed Lead Acid Batteries cannot be "refilled" if they vent gasses or electrolyte).

    The details matter.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PNW_StevePNW_Steve Registered Users Posts: 79 ✭✭
    Thank you all for your input.

    From what I understand, provided that the batteries see full charge every few days, I should have no trouble with a .1-.15C bulk charge rate.

    Thanks again.

  • jonrjonr Solar Expert Posts: 1,384 ✭✭✭✭
    edited October 2017 #7
    > AGM ... even will specify a minimum rate of charge.

    It's not clear to me what problem they are trying to prevent by specifying a min charge rate.  Perhaps simply spending too much time in a partially discharged state (all lead acid batteries benefit from being brought back to full charge as soon as possible).

    Flooded lead acid are different - I assume that stratification issues dictate a min charge rate (specifically in absorb where bubbling occurs).

    I am available for custom hardware/firmware development

  • Raj174Raj174 Solar Expert Posts: 768 ✭✭✭✭
    Have you checked your AGM battery specs for a minimum charge rate? I'm not familiar with any that do, but since @jonr has pointed this out, it's best to check.
    What is the Brand name and model number of your battery.

    3600W PV, MNE175DR-TR, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 360AH LiFePO4, Kohler Pro 5.2E.
  • Marc KurthMarc Kurth Solar Expert Posts: 825 ✭✭✭✭
    edited October 2017 #9
    Concorde Sun Xtender and Lifeline AGM's have a specified minimum charge rate of 20% of the C/20 battery bank capacity - for deep cycle applications. The maximum charge rate is limited by practicality of equipment.

    This is moot on most PV systems because they are generally designed for shallow cycling. But it is a very important point on industrial projects that actually cycle deeply, as the battery life is reduced with slow charging on those applications.

    On the other hand, Fullriver likes wants to see about 30% of the C/20 rating as a max charge rate for their AGM's
    I always have more questions than answers. That's the nature of life.
  • Raj174Raj174 Solar Expert Posts: 768 ✭✭✭✭
    Thank you Marc, good to know.
    3600W PV, MNE175DR-TR, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 360AH LiFePO4, Kohler Pro 5.2E.
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