measuring DOD

sunrasunra Registered Users Posts: 2
is there any device to measure discharge that occurs in battery overnight?  i have FLA s and would like to know amount of "cycling" occurring.  don't know if this counts as a different topic,  but read that too LITTLE or too LIGHTLY cycling a battery may cause problems?  thanks


  • BB.BB. Super Moderators, Administrators Posts: 32,639 admin
    edited February 26 #2
    Welcome to the forum Sunra.

    And yes, there are multiple devices that can estimate (to one level of accuracy or another) the cycling/state of charge of your FLA battery bank...

    There are ones that use a device (typically a precision resistor in the negative lead of the battery bank) to measure the current to/from the battery bank (charging/discharging in Amp*Hours). These can "drift" during multiple cycles, and typically reset to 100% full on some "event" (such as holding 14.75 volts for over 1 hour for a 12 volt FLA battery bank). (note is our forum Host):

    And there are others that monitor the voltage profile and estimate State of Charge that way.

    And on Amazon and others, Battery Monitors (now lots of sometimes very inexpensive units out there to try):

    A few links to start your searches (I am not connected with any of the above links--Other than as a volunteer here on the forum for Solar-Electric / Northern Arizona Wind & Sun)... In the last few years, many more Battery Monitors out there and lots of options such as alarm switch, bluetooth, etc....

    Note there are DC Amp*Hour/Watt*Hour meters--But many of those types are not very good as a Battery Monitor (tracking state of charge/discharge)--But used to measure (for example) one way current flow for Amp*Hours/Watt*Hours of loads.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • sunrasunra Registered Users Posts: 2
    thank you bill, those sound exactly like what i need.  it seems difficult to estimate daily loads, factor in some amount of reserve capacity, figure timing generator support and try not ending up with too little or too much battery.  and reading that too little cycling can be as deleterious as too much;   yikes,  just grateful things work as well as they do but always looking to tweak out some extra little bit. thanks again
  • BB.BB. Super Moderators, Administrators Posts: 32,639 admin
    For Lead Acid batteries... Cycling is "good" for them.... Nominally, suggest a system design that uses 25% of battery capacity on average down to 50% State of Charge (for longer battery life). That gives you "2 days" of stored power for bad weather--On the third day, either the sun comes back, or fire up the genset.

    Regarding genset usage... Suggest starting genset in the morning at ~50% state of charge (SoC), and charging to ~80%+ SoC. And either let the sun take care of the rest of the charging, or run the genset the same way the next day. That is usually the most fuel efficient for genset (and load the genset in the 50%-80% range--Again the most fuel efficient range for a gasoline/propane genset and running the least number of hours per day).

    For best battery life--Charging for Lead Acid batteries around 10-13% rate of charge (i.e,. a 300 AH battery bank at 20 Hour Rating -- 10% would be 30 Amps).

    And for genset--If you can manually start/stop genset, That is usually the most reliable and least issues. Folks do automate the genset start/stop--But it is a much more complex system and lots of things can "go wrong". Just better to keep the system "simple" unless you really have a need to automate.

    Regarding cycling depth (and recharging back to >90% SoC at least once a week), those requirements do not apply to Li Ion (and some other) types of battery chemistry.

    -Bill "some random suggestions" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RCinFLARCinFLA Solar Expert Posts: 1,460 ✭✭✭✭
    edited February 27 #5
    Leakage on a typical flooded lead acid is about 10% per month but condition of battery effects result.

    Lead-antimony alloy plates used for deep cycle lead acid batteries have a bit greater self-discharge rate of about 13% per month, compared to lead-calcium alloy plates which have 8-10% per month.  Pure lead plates of compressed AGM or Gel cell batteries have lowest self-discharge rate for lead-acid.

    SOC versus open circuit rested battery voltage depends on acid concentration.  Typical normal flooded lead-antimony deep cycle has full charge rested of 12.7 vdc for six cell battery or 2.12v per cell with SG of electrolyte of 1.28.  Lead-calcium used for auto and marine batteries usually have slightly greater acid concentration with correspondingly greater full charge rested voltage of about 12.8-13.0v for rested six series cell voltage.  This also applies to AGM batteries which have greater acid concentration.

    Most all rechargeable batteries develop a surface capacitance charge build up which will raise voltage a bit after full charging that is maintained for up to a couple of days unless bled off with a short discharge.  The surface charge needs to be dissipated before any judgement is made on battery leakage rate.

    Best test for lead acid is a 30-minute load test at about 20-25% C(A) discharge current rate.  After 30 minutes the loaded battery voltage for this discharge current rate will be close to 12.0v for six cell battery.  A battery in poor condition will slump below this voltage level.  It should recover to at least 90% state of charge open circuit rested voltage after 30 minutes of no-load rest.  If its voltage slumps more but recovers to normal level after no-load resting the problem is internal resistance.  If its voltage slumps more but does not recover to normal level after no-load resting the problem is sulfation.  If recovery voltage continues to drop over next day or two after load test then battery has high leakage current.

    During the first minute of load test, it is normal for battery voltage to dip down then recover to higher voltage before starting a slow drop over the 30-minute load test.  Any charging surface charge that was not burned off and initial excess lead-oxide coating on positive plate cause this initial voltage dip.

    Float Maintenace voltage is a compromise between what is good for positive lead-oxide plate and negative lead plate.  Positive plate will continue to build lead-oxide which corrodes support grid increasing cell internal resistance.  About 13.2v is best for positive plates to reduce the rate of lead-oxide build up.  Discharging 10%-20% capacity periodically removes much of excess lead-oxide build up.  Negative lead plate needs greater float voltage to prevent lead-sulfate build up.  About 13.8v is best for negative plates.  13.5v is point where electrolyte water breakdown rate starts to increase.  This is the bubbling of electrolyte and is oxygen off of positive plates and hydrogen off of negative plates.  Hard crystallization of lead sulfate is not rechargeable which why lead acid batteries need to be kept fully charged.

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