meter help

shortfatguy
shortfatguy Solar Expert Posts: 42
I have 4-6 volt deep cycle batteries set up in a series-parallel 12V bank with each battery having 335@20ah rating. Using a standard volt/amp meter on the negative side in series should I have a reading of 335a or 670a when the batteries are fully charged?

Ok you didn't bite:

How could I measure current when nothing is on? Well the strange thing was when I charge the bank with a generator I went from 260ma to 310ma yet nothing is on. Not sure what is going on or if it is a non-issue

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: meter help

    There is a lot of confusion on how to measure the charge in a battery...

    WARNING: You cannot use the AMP/CURRENT setting on your multi-meter to measure your battery's capacity... At best, you will blow a fuse in your meter. At worst, you will melt your meter, catch it on fire, and/or explode the hydrogen in your batteries and spray fragments and acid all over you and your room.

    To measure the capacity of your battery bank, you can do several different things:

    1. Use an accurate (and calibrated) Volt meter and read 12.xx volts, after your batteries have rested (no charging/discharging) for 3+ hours. And use this chart (scroll down several pages to chart/colored graphic) to convert the voltage reading in to state of charge. The readings will give you the approximate state of charge.

    2. Use a hydrometer to read the specific gravity (temperature corrected) of your battery bank. More or less the "gold standard" to measure battery state of charge. Pain that it only works with flooded cell batteries, and you should not do it often (loss of acid, risk of contamination, just a pain to do).

    3. Use a Battery Meter designed to measure how much current goes into and out of a battery... Very handy and easy to use. Not cheap. Gives a "gas gauge" reading of your battery's capacity at any time. If used properly, will help your battery bank last a long time (proper charging, no deep discharging).

    Now, the other question that you needed answered was how Amp*Hours and battery bank voltages add up...

    When you put batteries in series. The voltage will add and the Amp*Hours will stay the same. (2x 12volt 85 AH batteries in series will be 24 volts at 85 Amp*Hours).

    When you put batteries in parallel. The voltage will remain the same and the Amp*Hours will add... (2x 12volt 85 AH batteries in parallel will be 12 volts at 170 Amp*Hours).

    When you do both, then each adds as above. (2x in parallel and 2x series strings--24 volts and 170 Amp*hours using the above example).

    For yours:

    Two strings in parallel, with each series string containing two 6 volt 335 Amp*Hours (when measured using a 20 hour rate):

    Each series string will be 2x6 volt = 12 volt at 335 amp*hours

    Take the two above strings and put them in parallel = 12 volt at 670 Amp*Hours

    Make sense?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • shortfatguy
    shortfatguy Solar Expert Posts: 42
    Re: meter help

    >>Each series string will be 2x6 volt = 12 volt at 335 amp*hours

    >>Take the two above strings and put them in parallel = 12 volt at 670 amp*Hours

    Make sense?

    Yes , I'm aware of all the warnings and the three ways of testing. I've even blown the internal fuse when testing DC amps on the positive side. I've also read where battery monitors use the negative side for DC amp metering. So what the hell the meter is cheap and I gave it a go and measured .310ma (2ma meter setting) with inverter off. Unless I'm getting my decimal point wrong it looks to me 310 is close to 355.

    -Bill[/QUOTE]
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: meter help
    Yes , I'm aware of all the warnings and the three ways of testing. I've even blown the internal fuse when testing DC amps on the positive side. I've also read where battery monitors use the negative side for DC amp metering. So what the hell the meter is cheap and I gave it a go and measured .310ma (2ma meter setting) with inverter off. Unless I'm getting my decimal point wrong it looks to me 310 is close to 355.

    -Bill

    Call the fire department, and maybe that can save you before you kill yourself, or burn your house down.

    YOU CANNOT measure the "battery amps". You can measure voltage. RE-READ BB's post again.

    A Battery meter is not a simple meter. it's a Totalizing Meter, where it measures a voltage drop across a "Shunt" to determine how much current (amps) flows in or out of a battery bank, and needs to be programmed as to size of bank, types of batteries, etc.

    I can say no more.
    --
    Evolve or Die
    Darwin
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: meter help
    So what the hell the meter is cheap and I gave it a go and measured .310ma (2ma meter setting) with inverter off. Unless I'm getting my decimal point wrong it looks to me 310 is close to 355.

    I am at a loss what (or where) you are measuring... 310uAmps (or 0.00031 amps) is not close to anything... A true "Current/Amp meter" would read 10,000+ amps if connected across your battery bank (as a dead short).

    Regarding the Warning--I try to give the generic warning to everyone as I do not know the electrical experiences of anyone reading this website. Batteries are high power devices and can cause serious harm if not treated with respect. It was not intended as a comment on your particular skills and experiences.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • shortfatguy
    shortfatguy Solar Expert Posts: 42
    Re: meter help

    Mike,

    Thanks for your loving concern.

    Nothing was on! Power was not flowing! Yet I was able to get a reading on the negative side of the battery to inverter connection.

    Thank you for the update on how battery meters work too.

    Regards.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: meter help

    sfg,
    current or amps can only be measured when there is a complete circuit and that holds true for charging or discharging. when a battery justs sits there by itself and nothing is connected or on no current flows. the amount of current that will be seen when there is a complete circuit is not dependant upon the battery capacity as much as it is the load(s) presented. in other words putting a cfl inline and on will draw x amount of current and putting 2 of those will draw 2x in current and this being from the same battery shows that its capacity isn't being determined. specific load tests over time can help determine battery capacity, but i won't get into that here. in general the state of charge (soc) or its opposing counterpart the depth of discharge (dod) can roughly be determined from the battery resting voltage. this percentage can give an idea of the battery capacity if you were to know the amphours that were drawn from the battery already.
    it does concern me that you can't seem to differentiate between ma and amps or if you can that you have not worded it well to us what it is you meant. now i'm not sure, but if that generator is only able to deliver about 300ma to the battery bank then something is either wrong or the batteries are charged and possibly overcharged.
    as to the battery bank in a 12v series/parallel arrangement it would be 12v at 670ah. ah is amphours and is the current from the battery delivered over time. deep cycle batteries are measured in ah and not amps so please know you just can't stick a meter on it like a dmm to measure amps and instantly expect to know the battery capacity as it will show it has the potential to destroy the fusing in the meter or the meter itself and you will find most batteries have the ability to do that.
    ps. you could've been measuring the inverter standby current if it was ma, but i'm not possitive of that.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: meter help

    If you opened the connection between the battery negative and your inverter, and inserted your mAmp meter--then what you where measuring was the leakage current through the inverter when turned off. Measuring a handful of microAmps (uAmps) would not be unusual.

    This is normal (unless you have very dirty/corroded connections inside your inverter/switch) and will not material affect your system... For example:

    670 amps*hours / 0.31 milli-amps = 2,161,290 hours (246 years) to discharge your battery bank...

    Depending on condition--your battery bank will self discharge in a few hand fulls of month to a couple of years. So, the leakage current (if measured correctly) is insignificant.

    However, this reading has no relationship to the capacity of your battery bank.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • shortfatguy
    shortfatguy Solar Expert Posts: 42
    Re: meter help
    BB. wrote: »
    If you opened the connection between the battery negative and your inverter, and inserted your mAmp meter--then what you where measuring was the leakage current through the inverter when turned off. Measuring a handful of microAmps (uAmps) would not be unusual.

    This is normal (unless you have very dirty/corroded connections inside your inverter/switch) and will not material affect your system... For example:

    670 amps*hours / 0.31 milli-amps = 2,161,290 hours (246 years) to discharge your battery bank...

    Depending on condition--your battery bank will self discharge in a few hand fulls of month to a couple of years. So, the leakage current (if measured correctly) is insignificant.

    However, this reading has no relationship to the capacity of your battery bank.

    -Bill

    I have not posted here for a while yet I look everyday and learn something new. Yourself and others are very informative and I thank you for your time. I was trying start a "In the beginning" thread.

    Here's another one:

    Q: My 3 stage automotive charger displays 4.9a left to until full. Does this mean 4.9ax20ah=98 and the battery bank is rated @ 670. Therefore the battery bank is (670-98)/670 is 85% full?
  • shortfatguy
    shortfatguy Solar Expert Posts: 42
    Re: meter help
    niel wrote: »
    sfg,
    current or amps can only be measured when there is a complete circuit and that holds true for charging or discharging. when a battery justs sits there by itself and nothing is connected or on no current flows. the amount of current that will be seen when there is a complete circuit is not dependant upon the battery capacity as much as it is the load(s) presented. in other words putting a cfl inline and on will draw x amount of current and putting 2 of those will draw 2x in current and this being from the same battery shows that its capacity isn't being determined. specific load tests over time can help determine battery capacity, but i won't get into that here. in general the state of charge (soc) or its opposing counterpart the depth of discharge (dod) can roughly be determined from the battery resting voltage. this percentage can give an idea of the battery capacity if you were to know the amphours that were drawn from the battery already.
    it does concern me that you can't seem to differentiate between ma and amps or if you can that you have not worded it well to us what it is you meant. now i'm not sure, but if that generator is only able to deliver about 300ma to the battery bank then something is either wrong or the batteries are charged and possibly overcharged.
    as to the battery bank in a 12v series/parallel arrangement it would be 12v at 670ah. ah is amphours and is the current from the battery delivered over time. deep cycle batteries are measured in ah and not amps so please know you just can't stick a meter on it like a dmm to measure amps and instantly expect to know the battery capacity as it will show it has the potential to destroy the fusing in the meter or the meter itself and you will find most batteries have the ability to do that.
    ps. you could've been measuring the inverter standby current if it was ma, but i'm not possitive of that.

    That's right the decimal point was the issue. I deal with folk every day that get it wrong in another industry.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: meter help

    that would mean it is 4.9a x 1 hour or 4.9ah until full. this is usually a slower charge when one gets to the near full charging area so it may appear like 2.45a for 2 hours as an example. that 4.9ah is about .7% of the battery capacity and represents .7% dod. the soc is the opposite so it is 99.3% soc or 99.3% full in layman's terms.
  • shortfatguy
    shortfatguy Solar Expert Posts: 42
    Re: meter help
    niel wrote: »
    that would mean it is 4.9a x 1 hour or 4.9ah until full. this is usually a slower charge when one gets to the near full charging area so it may appear like 2.45a for 2 hours as an example. that 4.9ah is about .7% of the battery capacity and represents .7% dod. the soc is the opposite so it is 99.3% soc or 99.3% full in layman's terms.

    So your math is (670-4.9=665.1)/670=99.3%?

    So that means when my 40A charger is reading 40A when I'm charging then I'm never below 93% discharge?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: meter help

    There is no way for a simple amp meter / battery charger to "know" the capacity of the battery in Amp*Hours (or % State of Charge).

    Assuming your meter face reads 4.9 amps -- the meter has decided that at that time (and battery voltage) that it will output 4.9 amps... Now whether it will output 4.9 amps for 1 hour or for 10 hours--we do not know at "this time".

    But, 4.9 amps for 1 hour = 4.9 amp*hours.... 4.9 amps for 10 hours would be 49 amp*hour--or 10x as much "energy" (technically, Amp*Hours is not energy, but Amp*Volts*Hours is energy).

    So... That is what a "Battery Monitor" or "Totalizing" Amp meter does... It measures the current over time to give Amp*Hours.

    As a first approximation--40 amps for 1 hour vs 4 amps for 10 hours is the same--both are 40 Amp*Hours.

    The only way you can estimate the state of charge of your battery bank is to "watch your battery charger"... Say:

    40 amps x 1 hour (first hour)
    20 amps x 2 hours (2nd-3rd hour)
    5 amps x 10 hour (next 10 hours)
    1 amp or less (float--no longer charging)
    =============================
    40*1+20*2+5*10=130 Amp*Hours

    (670-130 )/670 = 81% State of Charge...

    But notice, you can only "know" the battery's state of charge (using this method) after charging has been completed (and assuming that the 670 AH is the correct rating and that you are discharging/charging the battery at the 20 hour rate or 670AH/20H=33.5amps--battery capacity varies with temperature and how fast you charge/discharge--so everything is an estimate unless you use a Battery Monitor / performance charts/etc. and you set a 50% floor on State Of Charge to prevent battery damage from over discharging).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: meter help

    bb,
    i was assuming he had some sort of totalizing meter and i took him at his word for the 4.9a to go. if (big if as we don't know) the current is normally higher than this we can possibly say that the current has folded back to 4.9a due to a near end of charge condition.

    sfg,
    the way i did it was 4.9a/670ah=.0073 or about .7%. this is also a low rate of charge that may be suitable for the end of an absorption charge stage or nearing that of a float type charge stage. the normal bulk charge rates for batteries will lie between 5% and 13% so for your 670ah battery bank that would represent 33.5a to 87.1a.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: meter help

    Niel,

    I think we are both guessing here--and trying to help...

    Depending on how the question is asked and what information is given--different answers may be given (and who knows which one(s) are correct--impossible to know based on limited information available).

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