Shoud i get in float as soon as i have 100% SOC ?

fca1
fca1 Solar Expert Posts: 124 ✭✭
in Off Grid Solar & Battery Systems #1
orHi there
i have the Victron bmv battery monitor so my battery soc is measured in ah in ah out , so i know with some precision my soc
But most of the time my CC are still in absorb when 100% soc is reached, i have away to automate this in the way that when the soc reaches 100% put the CC in float , will this be advised or let it go until the cc absorb timer goes up to the end ?

another thing i need to check is the capacity of the battery configured in the bmv, because my batts are 510ah at c5 an i think victron bmv want it at c20...

thanks
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #2
    Re: Shoud i get in float as soon as i have 100% SOC ?
    fca1 wrote: »
    orHi there
    i have the Victron bmv battery monitor so my battery soc is measured in ah in ah out , so i know with some precision my soc

    No, you don't know your SOC with some precision. Battery monitors need to be calibrated to the battery. You do that with your hydrometer or refractometer. Once you have calibrated your monitor, you can trust it better, but you should still check your SG periodically.

    A battery monitor is often compared to the gas gauge in your car, but that is not a good analogy.

    The gas gauge in your car actually measures the level of gas in your tank. A battery monitor doesn't measure the level of anything. It is more like the odometer in your car. The odometer counts miles and the battery monitor counts ampHours.

    Suppose you have a car that gets about 25 mpg and has a 15 gallon gas tank. Suppose also that your gas gauge does not work. What do you do? You use your odometer. For example: After a fill up, you drive 150 miles and you estimate that you have used 6 gallons and have 9 gallons remaining in your tank.

    If you fill up the tank again you can, as above, use your odometer to estimate your gallons remaining.

    But what if you do not completely fill your tank. For example, starting from a full tank you drive 150 miles and then you add 3 gallons to your tank and then drive 100 miles and then add 4 gallons to your tank and drive 150 miles and then add 5 gallons to your tank and drive 100 miles. At this point you estimate that you have 7 gallons remaining in your tank, but that estimate is not too accurate because your mileage is not ever exactly 25 mpg. The only time you know exactly how much gas is in your tank is when you have just filled it up.

    So it is with your battery monitor. The only time it is exactly accurate is when you fully charge your batteries and reset the battery monitor to read 100% full. It can be very accurate counting the amphours in and out of the battery, but it can only estimate the state of charge based on your assumptions of the battery capacity and of the battery efficiency. Most monitors (not sure of Victron) do not account for the Peukert effect (1 amp for 100 hours has a different effect on your battery than 100 amps for 1 hour, but many monitors see them as the same). Also temperature has an effect on the capacity of your battery... Does you monitor account for temperature? Your monitor must make some assumptions of the battery efficiency (you need to put more amp hours in than you take out), but the efficiency depends on where you are in the charge cycle... charging from 90% SOC to 100% SOC is less efficient than charging from 80% to 90%.

    By the way, you don't have to manually reset the monitor when the battery is fully charged... you program the battery monitor to do that automatically. If its programed wrong, the monitor can not be trusted. It can be tricky to program the monitor... if you're going to teach your monitor how to recognize a fully charged battery, then you yourself must know how to recognize a fully charged battery. You may accomplish this by monitoring end amps during absorb charging and using an hydrometer or refractometer to know when your batteries are fully charged.

    Bottom line: the longer its been since a complete charge, the less you can trust your battery monitor. And you can't trust it at all if its not set up correctly. That said, battery monitors are valuable tools in estimating your SOC, but you have to understand their limitations. You must have a hydrometer or refractometer, and then you teach your monitor how to estimate your SOC without having to use the hydrometer every day.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #3
    Re: Shoud i get in float as soon as i have 100% SOC ?

    The meter is reset when the battery is at full charge. At this point the amount of electric charge accumulated by battery is roughly equal to its 100-hour rate usually measured in amper-hours. Physically this is a number of electrons displaced between plates. One amper-hour is equal to 2.225 x 1022 electrons. The meter then measures current in and out of the battery and accumulates these numbers over time. This way it is know exactly (with measurement errors of course) how many electrons moved between plates. The ratio of the number electrons left to the number of electrons at full charge gives you SoC.

    The monitor accurately tracks SoC of batteries most of the time. There are measurement errors and some level of self-discharge (electron manage to find their way between plates inside the battery), but otherwise it's 100% accurate when you discharge battery and then when you bulk charge it.

    It all changes when it comes to absorption because during absorption many electrons are wasted to split water into hydrogen and oxigen. Once this starts, most monitors cannot figure out how many electrons are lost in this process. Therefore, at the end of the charge, the monitor shows higher SoC than it really is. When you don't know how many electrons got lost (and it depends on charge rate, voltage, temperature and many other things), you cannot really tell the true SoC.

    You can do SG measurements to figure out that your batteries are fully charged when monitor shows 115% or 120%, or whatever it is for your batteries. You can tell this to your monitor, which is called calibration and it will try to adjust its readings. However, if you don't go to full charge often, or if you change your charge pattern, your monitor will deviate from true SoC with time.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Shoud i get in float as soon as i have 100% SOC ?

    Battery monitor off to one side, what sort of current are you seeing during the Absorb stage?

    There are two ways of ending Absorb. One is the maximum time. This can be too little or too much depending on how it is set and what is happening with the batteries. The other is the End Amps. If there are no loads on this is a more reliable way of determining if Absorb is actually complete as far as the batteries are concerned and 100% SOC has actually been achieved. You check the current rate (usually 2%-3% without loads) against specific gravity readings to make sure batteries are fully charged.

    So you have to see what is going on with the current and if it is getting low enough before the Absorb timer finishes. If not, the time needs to be increased. If so, the End Amps function should be used to stop Absorb (before the timer runs out) and switch to Float. In some cases the Absorb Voltage may need to be increased to achieve full charge in a reasonable amount of time.

    At that point the battery monitor should 'agree' with the charge controller and hydrometer (if flooded cells) that the batteries have reached 100% SOC.
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    #5
    Re: Shoud i get in float as soon as i have 100% SOC ?

    fca1 does not need to measure the SG of his batteries, as they appear to be Sealed -- Gelled electrolyte.

    Looking back on this Thread, Stephen notes the approximate EA value per the Exide spec:

    http://forum.solar-electric.com/showthread.php?17469-EXIDE-EPZV-charging

    And, vt, liked your Gas Gauge/Odometer analogy. Some might pick at the knowing the amount of fuel in the tank ONLY when it was just refilled ... perhaps one might know also when the car has run out of fuel. And filling my tanks at a gas station, the auto shutoff has poor repeatability, and so on.

    You spent a lot of time composing that analogy, and is a good demonstration of just how accurate Battery Monitors can be, but for fca, the bat mon is a useful tool, when calibrated, just a bit more difficult to set it up. Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #6
    Re: Shoud i get in float as soon as i have 100% SOC ?
    Vic wrote: »
    And, vt, liked your Gas Gauge/Odometer analogy. Some might pick at the knowing the amount of fuel in the tank ONLY when it was just refilled ... perhaps one might know also when the car has run out of fuel.

    Good point Vic, I stand corrected. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • fca1
    fca1 Solar Expert Posts: 124 ✭✭
    #7
    Re: Shoud i get in float as soon as i have 100% SOC ?

    quick answer: thank you all for the reply just to clarify

    my batteries are sealed so no SG measure
    i always have loads but i can see what is taken by the inverter and how much is going to bats..
    today example :

    12:40 absorb started 95% SOC 39amps going in the batts..
    13:41 absorb 98% SOC 10amps going in the batts
    15:19 absorb reach 100% SOC 2.84 amps going to the batts
    15:41 float start 100% SOC 3.15amps going to batts

    in all this time i was having ~50amps loads...


    im having small discharges a day (still waiting for the inverter ;) )

    victron bmv has peurk effect in the ttg calcs and also have charge efficiency, for each amp out you have to put 1.2amps in to get at same stage..


    i will read all answers later and try to understand better regards
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #8
    Re: Shoud i get in float as soon as i have 100% SOC ?
    fca1 wrote: »
    victron bmv has peurk effect in the ttg calcs and also have charge efficiency, for each amp out you have to put 1.2amps in to get at same stage..

    The charge efficiency is not the same at all states of charge.
    Going from 70% SOC to 80% SOC is more efficient than going from 90% SOC to 100% SOC.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • SandyP
    SandyP Solar Expert Posts: 65 ✭✭
    #9
    Re: Shoud i get in float as soon as i have 100% SOC ?
    fca1 wrote: »
    quick answer: thank you all for the reply just to clarify

    my batteries are sealed so no SG measure
    i always have loads but i can see what is taken by the inverter and how much is going to bats..

    victron bmv has peurk effect in the ttg calcs and also have charge efficiency, for each amp out you have to put 1.2amps in to get at same stage..

    Like fca1, I have GEL batteries and installed the Victron BMV to "see" what was going into the batteries versus to loads. With this information I could better estimate when to end absorb based on end amps on the charge controller.
    Using both the Peukert effect and charge efficiency factors that can be set in the BMV and the amps in - amps out you can then have a reasonable estimate of your SoC.
    Some fine tuning of the factors may be required and can be determined by validating the 100% SoC with the end amps figure at the end of absorb.
    As has been stated before the factors will change with the age/condition of the batteries so adjustments will need to be made every year.
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #10
    Re: Shoud i get in float as soon as i have 100% SOC ?

    Hi fca1,

    There was another discussion about when to end absorb here: http://forum.solar-electric.com/showthread.php?18058-When-to-end-absorb&highlight=absorb unfortunately it was for flooded batts to just measuring SG was the best solution.
    I think using the charging current as a termination indicator will be more reliable than a SoC meter. The sunny island's SoC is regularly out by +- 5% compared to the actual state of the batteries (measured by SG). But even with charging current it's still not clear exactly where you should stop absorb, if you have a look at the graph in the discussion I posted, once the curve starts flattening out, how do you know where you should stop absorb?

    So, I've followed Victron's advice for charging flooded batts (from their Energy Unlimited white paper), which may or may not apply to your Gels: Aim to charge to 90-100% every day, and then once a month charge to 110-120%. That once a month charge isn't an EQ, it's just a longer than normal absorb to make sure (really sure) that your batts are properly charged.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
    Re: Shoud i get in float as soon as i have 100% SOC ?

    What do you say, guys? This thread has wandered way off from the original question of when to switch charging stages to Float and has become a discussion of battery function. Shall I switch the relevant posts over to a new thread so you can continue the argument? Shall I just chop it out and throw it away?

    By the way you're both wrong: batteries work by magic. This can be proven by dropping a wrench across the terminals; the twinkly sparks fly everywhere and the magic smoke comes out then the battery stops working. Perfectly ordinary every day magic. :D
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #12
    Re: Shoud i get in float as soon as i have 100% SOC ?
    Cariboocoot wrote: »
    What do you say, guys? This thread has wandered way off from the original question of when to switch charging stages to Float and has become a discussion of battery function. Shall I switch the relevant posts over to a new thread so you can continue the argument? Shall I just chop it out and throw it away?

    Other thread looks good to me.

    As to the OP questions. Since they're Gel batteries, it's impossible to measure SG. For what it's worth, this fact saves a lot of pain and frustration from deciding how to charge the batteries. This leaves only current to play with.

    I would run the charge until I see the current not changing for 3-4 hours. Then, I would increase this current by 10-20% and set this number as ending amps.
    Cariboocoot wrote: »
    By the way you're both wrong: batteries work by magic. This can be proven by dropping a wrench across the terminals; the twinkly sparks fly everywhere and the magic smoke comes out then the battery stops working. Perfectly ordinary every day magic. :D

    Thank you! I now know what's wrong with my batteries. They're so big that my biggest wrench doesn't reach from one terminal to another. I need a bigger wrench!!!
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #13
    Re: Shoud i get in float as soon as i have 100% SOC ?

    Okay the posts about nsaspook's project have been moved to here: http://forum.solar-electric.com/showthread.php?20759-Internal-battery-function-%28from-another-thread%29

    Have fun, guys!
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  • fca1
    fca1 Solar Expert Posts: 124 ✭✭
    #14
    Re: Shoud i get in float as soon as i have 100% SOC ?
    stephendv wrote: »
    So, I've followed Victron's advice for charging flooded batts (from their Energy Unlimited white paper), which may or may not apply to your Gels: Aim to charge to 90-100% every day, and then once a month charge to 110-120%. That once a month charge isn't an EQ, it's just a longer than normal absorb to make sure (really sure) that your batts are properly charged.


    Hi, and how do you do/ manage that cycles ?
    Ps I left a comment/question on your blog let me know the answer when you have time ..
    Regards
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #15
    Re: Shoud i get in float as soon as i have 100% SOC ?

    I eventually replied to your question on my blog (sorry for the delay)

    I use the python script to control the classic, and the event handlers in Mango to call the script. One of the event handler types is “Process” which lets you call an external operating system command, e.g.:

    /usr/bin/python /opt/mango/scripts/classic.py forcefloat
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