How do I know when a battery is fully charged?

Jungle Jim
Jungle Jim Registered Users Posts: 10 ✭
in Off Grid Solar & Battery Systems #1
Greetings from the land of sun, sand and sea – The Bahamas.

We are off-grid with a (for now) generator based system that includes an Outback Radian GS8048 Invertor, 1206 AHr 48V forklift battery and a 14.4kW diesel generator. The system will include 16 x 300W solar modules connected as 2 x 2,400W arrays into 2 x FLEXmax charge controllers once I get around to building a mounting structure.

The system has been running well for the past 2 months as we worked on getting our new home “completed” on our farm. We anticipate total loads in the range of 10 to 12 kWHrs per day once the house is finished and fully running. This site has been an excellent resource as we planned, purchased (thank you NAWS) and installed our equipment, but now I am suffering from information overload and need some help.

Can someone please spell out the correct method of determining when my battery bank has been fully charged? I have a good quality DVM and have purchased a battery hydrometer and the system also has a FLEXnet DC monitoring devise fitted to the GSLC load center. I think I understand the return amp concept, but would really appreciate a step-by-step run through on what to do, and when.

Thank you in advance for any help you can provide.

Dennis
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #2
    Re: How do I know when a battery is fully charged?
    Jungle Jim wrote: »
    Greetings from the land of sun, sand and sea – The Bahamas.

    We are off-grid with a (for now) generator based system that includes an Outback Radian GS8048 Invertor, 1206 AHr 48V forklift battery and a 14.4kW diesel generator. The system will include 16 x 300W solar modules connected as 2 x 2,400W arrays into 2 x FLEXmax charge controllers once I get around to building a mounting structure.

    The system has been running well for the past 2 months as we worked on getting our new home “completed” on our farm. We anticipate total loads in the range of 10 to 12 kWHrs per day once the house is finished and fully running. This site has been an excellent resource as we planned, purchased (thank you NAWS) and installed our equipment, but now I am suffering from information overload and need some help.

    Can someone please spell out the correct method of determining when my battery bank has been fully charged? I have a good quality DVM and have purchased a battery hydrometer and the system also has a FLEXnet DC monitoring devise fitted to the GSLC load center. I think I understand the return amp concept, but would really appreciate a step-by-step run through on what to do, and when.

    Thank you in advance for any help you can provide.

    Dennis

    The definitive way of telling the State of Charge (SOC) of your batteries is with your hydrometer. All you need is to know what the SG of the battery is when fully charged. "Tropical" batteries may have a different starting SG. Once you have that, you can correlate the SG with the voltage during charge, discharge, and when "resting" without charge or load for a few hours. This will then give you a quick approximation of the SOC without getting out the hydrometer.

    With both the hydrometer and the DVM, looking for variations from cell to cell or battery to battery is important, in addition to looking at the average state of the cells.

    Regardless of what your DC monitor and return amp calculation tell you, if the voltage and the SG are decreasing over time, you are under charging. And if you find yourself adding more water than usual, you are overcharging.

    For getting a very quick idea of the SOC to make decisions about using power or firing up a generator to recharge the batteries, a battery monitor is the way to go. Just keep correcting it periodically by the other measurements.
    SMA SB 3000, old BP panels.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    Re: How do I know when a battery is fully charged?

    A couple of battery FAQs:

    www.windsun.com/Batteries/Battery_FAQ.htm
    www.batteryfaq.org

    You are correct--You really need to understand your battery bank and learn how to "keep it happy".

    It is not unusual for people to loose their first (or even first two) battery banks from, under charging (and/or too many loads), forgetting to check water levels, not filling with distilled water, or somebody left a heavy load and and discharged the battery bank below 20% state of charge (which can kill a battery/cell within days).

    A battery monitor (Victron is another good brand) like Inetdog typed is a great tool for spouse, kids, guests to keep a real time eye on the battery bank.

    They can drift over time (read the first two links, and the monitor manuals) in some conditions.

    A hydrometer (good quality, properly used) is the gold standard--But you do not want to pull that out every day. Watching voltage levels can help too (i.e. when you are charging/discharging/having problems).

    I would also suggest getting a DC Current Clamp meter (really an AC/DC clamp+DMM)... This one is a great value (good enough) for debugging a solar power system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Jungle Jim
    Jungle Jim Registered Users Posts: 10 ✭
    #4
    Re: How do I know when a battery is fully charged?

    Thank you inetdog and BB for your replies!

    I broke out the hydrometer and took a Specific Gravity measurement of each cell and used a good quality DVM to measure each cell’s voltage. It turns out that there was some good news and some bad news. The good news is that the SG of all 24 cells measured consistently to within +/- 0.002 of each other. Each cells voltage was also very consistent, to within +/- 0.002V.

    The bad news was that even though a charge cycle had just finished the temperature compensated SG (at ~ 85®F) was 1.185, 0.100 below the manufacturers specification. The voltage of each cells was 2.100V and for the entire battery 50.4V. Based on the values shown on the batteryfaq.org chart, the SG reading indicated that the battery is at 50% SoC but by the voltage it is at close to 100%.

    If the SG is the important number, then it seems that the battery is not being charged fully. I did run a equalize cycle for 1 hour a week ago, but if the battery was not fully charged this may not have had the desired effect. The Radian’s invertor charge parameters are as follows:-

    Absorb 57.2V for 1 Hour Float 53.2V for 1 Hour

    Would increasing the Absorb time effectively raise the SG? If so, for how long? What else should I be doing? Until we can get the solar modules mounted in a few weeks I am happy to increase of generator run time if it means that I will not be one of the people that murders his first bank of batteries.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: How do I know when a battery is fully charged?

    Definitely looks like a need for increased Absorb time. One hour on a 1206 Amp hour battery bank isn't much (depending on the DOD). Equalization is not something you want to do unless the SG isn't even between cells. You seem to have no problem there.

    The battery FAQ is based on "normal" SG readings; i.e. average conditions/temps/generic battery and is not absolute across the board. Your batteries are probably "tropical" and may have slightly lower SG than "typical". Even so, 1.185 is low.

    Lengthen the Absorb time to its maximum (8 hours on the Radian? I forget) and watch the current going to the battery during the Absorb cycle. When it gets down to about 2% capacity (24 Amps, not including any power going to loads - which there shouldn't be if feeding the Radian as the charger and there are no other DC loads) stop the Absorb cycle. The amount of time it took from when Absorb started to that point is the length to set your Absorb time for.

    Unless you go buy one of the Flexnet systems and program it for real End Amps.

    Not sure if we ever make it clear that a battery can have "proper" resting Voltage once it reaches Absorb but unless the Absorb time is long enough that resting Voltage won't last. In other words, additional time in Absorb will raise SG but not Voltage.
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #6
    Re: How do I know when a battery is fully charged?
    Jungle Jim wrote: »

    The bad news was that even though a charge cycle had just finished the temperature compensated SG (at ~ 85®F) was 1.185, 0.100 below the manufacturers specification. The voltage of each cells was 2.100V and for the entire battery 50.4V. Based on the values shown on the batteryfaq.org chart, the SG reading indicated that the battery is at 50% SoC but by the voltage it is at close to 100%.

    If the SG is the important number, then it seems that the battery is not being charged fully. I did run a equalize cycle for 1 hour a week ago, but if the battery was not fully charged this may not have had the desired effect. The Radian’s invertor charge parameters are as follows:-

    Absorb 57.2V for 1 Hour Float 53.2V for 1 Hour

    Would increasing the Absorb time effectively raise the SG? If so, for how long? What else should I be doing? Until we can get the solar modules mounted in a few weeks I am happy to increase of generator run time if it means that I will not be one of the people that murders his first bank of batteries.
    You need to equalize them @ 60-62 v for as long as it takes, it could be 3-10 hours or more. You should take the SG reading on a couple cells every 30 minutes, as long as it's rising your doing ok. You also have to control the temperatures of the batteries, 115 to 120 as high as you should go. You can stop and let them cool, overnight is ok, then start back.

    Raising the adsorb time will keep them from getting like they are now again. By just raising it now would take a long time to ever get the sulfate off the plates. You'll soon lose those batteries if you don't take some action soon.
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    #7
    Re: How do I know when a battery is fully charged?

    Hi JJ,

    Yea the cell voltages seem out of line, but, this is just one of the many reasons that voltage vs SOC readings are so problematic. The battery is probably still being Floated by the charger, causing higher V readings.

    From what I've read, believe that these Forklift batteries require a high Absorption voltage. On the order of 59 or so volts -- use the manufacturer's specs for this and all other charger settings, and frequency and durtion of EQ. RE use of forklift batts is often an area that the batt mfgs do not support, and you are kind of own, often, but, others here use them and can give more guidance.

    Certainly with this low SG situation, longer A.sorb time, and higher V would seem good things to do. As noted by Bc, you need to get the SGs up ASAP! Good Luck, 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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  • Jungle Jim
    Jungle Jim Registered Users Posts: 10 ✭
    #8
    Re: How do I know when a battery is fully charged?

    Thank you all for your help and thoughts. I am heading home shortly and will fire up the genset and push some current into the battery. What I am unsure about is do I run an absorb cycle or an equalization charge? Either way I will monitor SG and voltage regularly and then stop when things level off. I’m leaning towards an EQ – is that right?
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #9
    Re: How do I know when a battery is fully charged?
    Jungle Jim wrote: »
    Thank you all for your help and thoughts. I am heading home shortly and will fire up the genset and push some current into the battery. What I am unsure about is do I run an absorb cycle or an equalization charge? Either way I will monitor SG and voltage regularly and then stop when things level off. I’m leaning towards an EQ – is that right?
    With EQ voltage your going to get a crisp boil that will have a better effect on the sulfate. It needs to come off before it crystallizes and gets so hard it will never come off and you lose capacity. The only damage you can do is let them get to hot and then you'll warp the plates, watch the temperatures and keep checking the SG.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #10
    Re: How do I know when a battery is fully charged?

    What I would do...

    Charge the battery bank at ~58-59 volt Absorb setting... Monitor charging current to battery bank. When the current tapers down to below 5% of battery bank AH capacity, you can start measuring the specific gravity of a "pilot" cell (you don't need to measure all--No unbalanced cells here--it appears).

    You should see the specific gravity at ~85% - 90% SOC or so at this point. And you should notice much more bubbling too (less energy to charging, more energy "wasted" as electrolysis to Hydrogen/Oxygen and as waste heat warming up the battery).

    You can continue "absorb" until the charging current drops to ~1-2% rate of charge (12 amps would be ~1% of battery AH capacity). At this point, your battery is >90% charged and, for most days, that is "good enough".

    You now have a choice to "equalize" continue charging by bringing the charging voltage to ~60-62 volts for "equalization". I would pick the voltage such that it does not exceed 5% rate of charge (or 0.05*1206AH=60.3 amps). That is the maximum recommended current for equalization current for Surrette/Rolls brand of batteries. You will probably see the battery bank temperature rising (and in warm climate, you already have some "extra heat" issues anyway). During this time of "equalization", measure the specific gravity every 30-60 minutes--and when the SG stops increasing, stop equalization/charging.

    Log the temperature corrected specific gravity for each cell--That is now your 100% state of charge.

    Some battery vendors recommend equalization only if SG variations >0.015 to 0.030 between low and high cell. Others recommend some form of equalization every 1-2 months or so...

    You have a choice during charging to make here... When the Absorb charging current drops below ~5% rate of charge--You can wait until it drops to 1-2% rate of charge, or start increasing the equalization set point to 60-62 volts (keeping below ~5% charge rate). You might want to do this if you are using your genset for charging--so you down waste a lot of fuel at the 5-1% rate of charge range.

    You could also just set the absorb set point to equalization right at the start--If your battery charging current is below ~13% rate of charge (you don't want to charge too hard--can be hard on the plates if there is lots of gassing, and raise battery temperature too).

    Anyway, my thoughts--Sometimes, probably "over think" this stuff. But want to make sure that you don't damage anything--I am not there.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #11
    Re: How do I know when a battery is fully charged?

    my guess might be a combination of things going on here. sg lower in your area, a lack of absorb time, and a basic need to stir the electrolyte up by giving it a good kick. i believe the electrolyte not stirred properly is leading you to read a lighter sg than it may actually be. an absorb charge if too light or short would fail to stir the electrolyte enough. if you opt to eq the battery then know that if properly topped of with distilled water that an eq charge will not hurt the battery providing it does not overheat and even though i doubt it will do that do keep that in mind.

    do note that stirring the electrolyte is not physically getting something to stir it with like a stick or spoon. it is making the electric charge and bubbling that actually stirs it. i had to cover this base.:-)
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  • Jungle Jim
    Jungle Jim Registered Users Posts: 10 ✭
    #12
    Re: How do I know when a battery is fully charged?

    Quick update - the battery is bubbling along and the SG is continuing to rise!
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #13
    Re: How do I know when a battery is fully charged?

    What voltage/current are you pumping in right now?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Jungle Jim
    Jungle Jim Registered Users Posts: 10 ✭
    #14
    Re: How do I know when a battery is fully charged?

    Hi Bill. 62V. Not sure what the current is at the moment - our power system is 300 feet away but I will be checking it again in a few minutes.
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  • Jungle Jim
    Jungle Jim Registered Users Posts: 10 ✭
    #15
    Re: How do I know when a battery is fully charged?

    Hi again Bill. SG is up to 1.264, current passing through shunt to battery is 71.5A.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #16
    Re: How do I know when a battery is fully charged?

    Sounds about perfect...

    Measure SG every 30 minutes or so, and monitor the battery bank temperature.

    When a) the SG stops rising between readings, stop equalizing (and log all cell SG readings--that is now 100% for you--temperature corrected of course).

    Or b) if the battery bank gets too hot, stop charging and start charging/equalization again once the bank has cooled down.

    It sounds like you should be getting close.

    Remember hot batteries, their charging voltage falls, so you could see an increase in charging current--You may need to back down the equalization voltage in that case (if it is not temperature corrected by the battery charger--and its seems that some chargers do not temperature compensate equalization voltages).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Jungle Jim
    Jungle Jim Registered Users Posts: 10 ✭
    #17
    Re: How do I know when a battery is fully charged?

    t took a couple of hours in an absorb cycle followed by 4+ hours of equilization to get the job done. By 1:00am the TC SG was at 1.275 and the battery was getting quite warm so I figured it was close enough for now. This weekend I will run an absorb cycle to determine what this batteries end amp rate is and re-set the battery monitor.

    This process has been extremely educational for me. I have read and re-read everything that I could get me hands (or browser) on, but until last night it did not truly sink in. Thank you all for your help and a special thank you to Bill for staying up late with me!

    At the risk of sounding like one of my kids - this forum ROCKS!
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #18
    Re: How do I know when a battery is fully charged?
    Jungle Jim wrote: »
    t took a couple of hours in an absorb cycle followed by 4+ hours of equilization to get the job done. By 1:00am the TC SG was at 1.275 and the battery was getting quite warm so I figured it was close enough for now. This weekend I will run an absorb cycle to determine what this batteries end amp rate is and re-set the battery monitor.

    This process has been extremely educational for me. I have read and re-read everything that I could get me hands (or browser) on, but until last night it did not truly sink in. Thank you all for your help and a special thank you to Bill for staying up late with me!

    At the risk of sounding like one of my kids - this forum ROCKS!
    Smiles and happy for you. Now you can start working on finding the right Adsorb time and voltage to keep the SG's up there. It's scary the first time you do it, but if you don't do it'll get to a point it's irreversible.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #19
    Re: How do I know when a battery is fully charged?

    Jim,

    Stick around and you can give your experience for every newbie that comes by in the future. ;)

    Everyone that is Off Grid makes this same mistake. Under charging their battery bank (or over discharging with too many loads).

    And some people end up killing their battery bank before they figure it out (probably most people "murder" their first battery bank or two).

    Plus, it is great to have the voice of experience. I have lots of information (from here, and other places), but I am Grid Tied--No off grid inverter/battery bank.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • unicornio
    unicornio Solar Expert Posts: 217 ✭✭
    #20
    Re: How do I know when a battery is fully charged?
    BB. wrote: »
    And some people end up killing their battery bank before they figure it out (probably most people "murder" their first battery bank or two).
    Plus, it is great to have the voice of experience. I have lots of information (from here, and other places), but I am Grid Tied--No off grid inverter/battery bank.
    l

    true! ... almost everyone spends a battery at least until you learn to take care of, but that happens with cars, and girlfriends! .... hehehehe
    It is also true that charging systems have evolved so much and now are so precise, that maybe now we can get batteries lasting a record ...
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  • Jungle Jim
    Jungle Jim Registered Users Posts: 10 ✭
    #21
    Re: How do I know when a battery is fully charged?
    Blackcherry04 wrote: »
    Smiles and happy for you. Now you can start working on finding the right Adsorb time and voltage to keep the SG's up there. It's scary the first time you do it, but if you don't do it'll get to a point it's irreversible.

    Hi BC. Yesterday I charged the battery (48V 1206 AHrs) and set the absorb voltage to 57.2 and the adsorb time to 8 hours. After reaching the set voltage for 4 hours the charging current as measured by the FLEXnet DC was 16 amps and the TC SG was at 1.275'ish.

    I think that we have our new settings!
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #22
    Re: How do I know when a battery is fully charged?

    Most FLA's are charged at 2.4V for absorb, so 57.6V, you'll also see slightly higher current at a higher absorb voltage.
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