Uneven battery charging

Gbridgem
Gbridgem Registered Users Posts: 13 ✭
in Off Grid Solar & Battery Systems #1
I have a Magnum 4024pae inverter, FM60 charge controller, 3 sharp 220w panels and 8 x 12v 135Ah batteries - hooked up in 4 x 24V banks.

Voltage goes up in day to 26vdc+ (sometimes into absorb and float) then drops off as evening wears on to say 23 vdc - then same process again next dey. Only real load running is a relatively small electric fridge.

checked voltage of each battery in morning: The left side is connected to positive lead from inverter - tight tio negative.
12.83. 10.26
10.32 12.77
12.76. 10.29
12.61. 10.43

why would half be high and the other low????? Don't think they are hooled up incorrectly

Batteries have been in place for about a year - have never run an equalize charge?

Comments????

Guy
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Uneven battery charging

    You have two problems.
    The first would be eight 12 Volt 135 Amp hour batteries making a total of 540 Amp hours @ 24 Volts. These would be four parallel strings of two, which is unlikely to have even current sharing unless wired very carefully. See the Smart Gauge diagrams here: http://www.smartgauge.co.uk/batt_con.html You should be using method #3 (substitute strings of two for the individual batteries shown). Wire sizes must be large enough for the expected maximum current and lengths equal on all battery strings.
    The second would be an undersized PV array. 660 Watts simply is not going to provide enough power to recharge 540 Amp hours @ 24 Volts. You might get about 20 to 22 Amps from those panels with an MPPT type controller, and that's only a 3.9% peak charge rate. That much battery needs about double that array and ideally about 1600 to 1700 Watts.

    Don't be fooled into thinking a small refrigerator is easy on consumption either. We have a thread here about small 'friges and how much they use (look under the energy use & conservation section).

    I suggest you recharge the batteries from the inverter via gen or utility power. Do it one string at a time if necessary. Check the specific gravity in every cell. If you find discrepancies more than 0.010, do an equalization charge and see if you can improve them any. The check the battery wiring as per above. Take the four best batteries and use them; leave the four worst out. You'll have 270 Amp hours @ 24 Volts which can provide up to 3kW hours AC which will definitely run a refrigerator for a day (I run the whole cabin on 262 Amp hours @ 24 Volts including 'frige). Your peak charge rate will then be around 7% which should work.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    Re: Uneven battery charging

    Those 10.5 volt batteries are "dead" (or very near dead).

    As Marc says, you need to get all of the batteries fully recharged to 100% quickly. The more time the spend at that low of voltage, the quicker they sulftate (and once sulfated, the batteries need to be replaced).

    Also, I think you are in Canada, but I would suggest you try to find a DC current clamp meter (US Sears sell one for "cheap" that is good enough for our needs) to measure the parallel current into each string of batteries. It is not unusual for a poor connection to stop almost all current flow through a parallel string--or an open/shorted cell to cause other problems.

    In the end, the battery bank is probably "toast" and needs replacement. And I would suggest, ideally, one string of "high AH" batteries, but 2-3 parallel strings can work OK too (but you have to stay on top of monitoring them to make sure something goes wrong).

    You can get a Kill-a-Watt type Watt*Hour meter to measure your AC loads--That will help you plan the capacity of the "next" bank you install.

    If you have flooded cell batteries, you should get a good quality hydrometer and monitor specific gravity of the bank (check one cell daily/weekly, check all cells every month or two and log the results).

    And if you have a AGM/sealed battery bank, a Battery Monitor is just about a requirement (and very nice for a flooded cell bank too).

    Batteries are the "heart" of your system--and your job is to make sure they are kept happy at all times--or they can quickly die on you. The rest of the hardware is usually pretty rugged--But batteries need TLC (tender loving care) for a long life:

    Deep Cycle Battery FAQ
    www.batteryfaq.org

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #4
    Re: Uneven battery charging

    You have gotten some good advice from Cariboocoot and Bill. The only thing I take issue with is:
    Cariboocoot wrote:
    These would be four parallel strings of two, which is unlikely to have even current sharing unless wired very carefully.

    I think that even if you do wire them correctly you will not have even current sharing. The batteries themselves are never electrically identical and as they age they become less identical. Even if they were electrically identical, if one battery was slightly warmer than the others (inevitable) it would draw more charging current which would make it warmer which would make it draw still more charging current which would make it warmer....

    Four parallel strings of batteries is at least two strings too many. And one string is better than two. Take Cariboocoot's advice and see if you can find 4 good batteries in your bank and hook them up as a 270 AH bank.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Uneven battery charging
    vtmaps wrote: »
    You have gotten some good advice from Cariboocoot and Bill. The only thing I take issue with is:


    I think that even if you do wire them correctly you will not have even current sharing. The batteries themselves are never electrically identical and as they age they become less identical. Even if they were electrically identical, if one battery was slightly warmer than the others (inevitable) it would draw more charging current which would make it warmer which would make it draw still more charging current which would make it warmer....

    Four parallel strings of batteries is at least two strings too many. And one string is better than two. Take Cariboocoot's advice and see if you can find 4 good batteries in your bank and hook them up as a 270 AH bank.

    --vtMaps

    Yes, technically every battery is ever-so-slightly different. But if done right these small variations show up slowly over a long time, rather than in a rapid cascading failure.
    One string is best, two is manageable, three starts to get dicey, four and you better be willing to babysit the bank a lot. :roll:
    Five or more? Forgetaboutit! :p
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  • Gbridgem
    Gbridgem Registered Users Posts: 13 ✭
    #6
    Re: Uneven battery charging

    Thanks all!
    1. Are you saying that I should dump the 2 x 12vdc = 24vdc x 4 bank set up and move to 8 batteries in a 12vdc set up??
    2. Can I accommodater this with change to 12 volt with my 4024pae inverter?????
    3. I have 4 batteries on a platform with 4 below - all leads are the same size except longer (smaller dia) to connect the ones above to the banks below. Think I have good conections but will doubble check.
    4. I will try to charge / take in the 4 low voltage batteries - thanks.
    5. I have purchased 2 x 245 watt pannells to add to the 3x220 I have now - should help - thanks.
    6. The fridge ahows about 2 amps dc when compressor is runnig - but I did see the defrost element come on at 385 watts for about 20 min - not sure how often - only real load we have on.
    7. these are AGM batteries - have the remote temperature sensor but not the remote monitoring.
    8. Should I run the equalize cycle periodically?
    9. still seems odd to me that one of each pair would be high with the other in all 4 pairs low - coincidence?

    Guy
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: Uneven battery charging
    Gbridgem wrote: »
    Thanks all!
    1. Are you saying that I should dump the 2 x 12vdc = 24vdc x 4 bank set up and move to 8 batteries in a 12vdc set up??

    No. We're saying four 6 Volt higher Amp hour capacity batteries in series is better for the 24 Volt system. Here's an explanation of battery equivalent power: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power
    2. Can I accommodater this with change to 12 volt with my 4024pae inverter?????

    It can't.
    3. I have 4 batteries on a platform with 4 below - all leads are the same size except longer (smaller dia) to connect the ones above to the banks below. Think I have good conections but will doubble check.

    This can be a problem right there. The longer leads to the lower rack of batteries adds resistance to that group.
    6. The fridge ahows about 2 amps dc when compressor is runnig - but I did see the defrost element come on at 385 watts for about 20 min - not sure how often - only real load we have on.

    Refrigerators are deceptive. There's a start-up surge you won't see, then a pretty meager running current. But over time it adds up to a lot of Amp hours that can deplete the batteries. The defrost mode is often a killer, as it is a lot of Watts and hard to predict when it will be needed.
    7. these are AGM batteries - have the remote temperature sensor but not the remote monitoring.

    With your battery bank split as it is the remote temp sensor isn't going to do much; some batteries will be at one temp and others at another. Depending on where the sensor is, the Voltage will be adjusted too high or too low for the 'other' batteries.
    8. Should I run the equalize cycle periodically?

    As a rule you do not run equalization on AGM's. There is no way to check the specific gravity, so you'd be running in the dark - with scissors, because the higher Voltage of an EQ cycle can pop the vents on AGM's and cause them to loose electrolyte that can then not be recovered.
    9. still seems odd to me that one of each pair would be high with the other in all 4 pairs low - coincidence?

    I hate to say it but it sounds like some are fried. When one battery in a series connection drops dead due to plate failure or other fault it can "lose a cell" making a 12 Volt battery into a "10 Volt" battery. The charging still tries to charge for 24 Volts, and can push the remaining "good" high in Voltage. Possibly high enough to pop the vent and ruin it as well.

    And here we have yet another example of why we continually recommend flooded cells for the first set. They are far more tolerant of these sorts of errors.
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  • Coach Dad
    Coach Dad Solar Expert Posts: 154 ✭✭
    #8
    Re: Uneven battery charging

    Some AGM batteries can be Equalized.
    The Sun Extender Manual has a section in it which tells how to equalize them.

    I would check the owners manual of the batteries and try equlizing them in pairs of 2 at a time.
    At this point,,, what do you have to loose?
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