how to adjust DoD when LVD is preconfigured in most charge controllers??

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samitrix
samitrix Registered Users Posts: 20
hi folks,
i can not seem to understand this. all resources i have looked into keep stating that for a longer battery cycle life, set DoD to 30% - 50% but if LVD is already set to lets say 10.5 to 11.5 in most charge controllers, how in the world are we supposed to get the target DoD we want???

and is there a way to understand descending voltage of a battery in terms of DoD or SoC?? for example, how do we calculate DoD when a fully charged 12.6V battery reaches 11.9V??

cheers,
sam

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
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    Re: how to adjust DoD when LVD is preconfigured in most charge controllers??

    Welcome to the problem of managing a battery bank... The voltage on the battery is the results of many different active process variables (temperature, electrolyte, dynamic mixing of electrolyte, value of current, etc.). Even measuring electrolyte specific gravity can be affected by stratification (dense acid sinks, light mostly water floats, takes time and/or bubbles to mix electrolyte from next to the plates into the balance of the electrolyte "bath", etc.). You can estimate the specific gravity by measuring the "resting voltage" of the battery (~3 or more hours of no charge/discharge current flow).

    Plus, depending on the brand and intended market for a battery, the initial S.G. can be "adjusted" (lower S.G. levels for standby/long life; higher S.G. for more capacity, better cycling capacity).

    Resting Voltage

    Typically, when a battery is first subjected to a high current low, the battery voltage will quickly depress, then pick up again a bit.

    Some of the LVD circuits will lower the LVD cutoff voltage for heavier current flows.

    Some battery manufacturers will give you a charge relating S.G. and Resting Voltage to State of Charge--otherwise you just use a generic table.

    If you are looking for a "better way" to manage your battery... Take a loot at Battery Monitors (Victron Energy, Smart Gauge Website, etc.).

    Basically, you install a precision resistor (current shunt) in the negative lead of the battery. The meter measures the current flow through the 500 amp resistor (voltage drop), totalizes the current flow * time (Amp*Hour), and runs it against an internal model (taking voltage, temperature, and such into account).

    Sort of the equivalent of placing a fuel flow meter on the fuel line to your gas tank and monitoring fuel use (discharge) and fuel replenishment (recharging) to estimate what is in the fuel tank (aka Gas Gauge).

    These meters will display your state of charge 0-100%, or in Amp*Hours, and display other parameters.

    A few (some Xantrex and Victron units, at least) will allow you to set an alarm contact to turn "on" at 50% and turn "off" at 80% state of charge--you can use this as a human alarm or as a signal to start a genset, bother the teenagers, etc...

    They are not perfect (they estimate batter state of charge, and typically "reset" back to 100% when they see battery voltage at 14.5 volts, low charging current, for a period of time--for example). Battery Monitors are very handy for AGM and other sealed batteries where you cannot measure specific gravity.

    Anyway, another option...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • samitrix
    samitrix Registered Users Posts: 20
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    Re: how to adjust DoD when LVD is preconfigured in most charge controllers??

    Thanks a lot for the detailed explanation BB!
    It seems like managing battery is a pain in the a$$. as per your information, i have looked into battery monitors and it seems like a good solution only that they are additional expense in the system and quite expensive...

    so i guess this is basically everyone's problem right now. hopefully some company will develop a programmable LVD for charge controllers and/or inverters with feature such as DoD % display...
  • petertearai
    petertearai Solar Expert Posts: 471 ✭✭✭✭
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    Re: how to adjust DoD when LVD is preconfigured in most charge controllers??

    Yes I sometime think a thread "what features you would like added to your charge controller would be good "
    I recently purchased a Victron battery monitor and wish now it was one of the first things i got for my system. Small and easy to install ,lots of smarts and easy for the teenagers to understand!
    Low voltage disconnect seems to be mainly on inverters .what charge controllers are they on?
    regards Peter
    2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . Mate 3. victron battery monitor . 24 volts  in 2 volt Shoto lead carbon extreme batterys. off grid  holiday home 
  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
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    Re: how to adjust DoD when LVD is preconfigured in most charge controllers??

    Remember that I am on Grid Power with a GT inverter system... I am not off grid and have little direct experience with off grid solar equipment (my experience with lots of power systems, AC/DC, etc. as a computer systems HW designer).

    Read other's experiences... Icarus/Tony lives with a small system in The Great White North (Canada)... And we have Dave Sparks here who has installed lots of off-grid system in the California Sierra Nevada Mountains.

    Read around and get the ideas from the folks that have lots of experience (or read the links others like I try to point to first source's on).

    Every-time I go into the details of rechargeable batteries, I am surprised they work as well as they do...

    Big think is to keep track of their state of charge, and not let them sit below ~75% state of charge for days/weeks at a time.

    The TriMetric system at around $140-$155 USD has many users here who find that it does what it is supposed to do (no alarm?). A bit confusing to program/setup for some folks.

    If it saves you one battery bank from somebody not paying attention--It can be worth the money.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
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    Re: how to adjust DoD when LVD is preconfigured in most charge controllers??

    You could always just add your own LVD between the battery and inverter. Here's one, there are quite a few if you shop around:

    http://www.tessco.com/products/displayProductInfo.do?sku=67608&eventPage=1