BULK, ABSORB and FLOAT

JESSICA
JESSICA Solar Expert Posts: 289 ✭✭
in General Solar Power Topics #1
As I was reading another recent thread about forklift batteries, I realized that nobody has ever told me how and for how long should I charge my battery bank. I just assumed that my mx60 will do that kind of automatically.

I have four (4) Interstate, Powerfast, 6volts, 210AH, Golf Cart bateries, in series, for a 24 volts bank. As some of you may know, I have 6 Kyocera 130, in series/parallel, and (for the moment) a new Go Power sw 2000 inverter.

My mx60 controller start the Absorb cycle when voltage is 28.4, and stays in that stage for about one hour (depending on solar radiation and usage).

I equalize the batteries at 29.4 volts for 1 hour, once a month.

There is no manual for the batteries.

Anu suggestios will be appreciated.

P.S.: Batteries are 2.5 years old now.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: BULK, ABSORB and FLOAT

    Your MX60 will do it automatically - providing it is programmed correctly.

    There are Voltage set points for ABSORB and FLOAT. On a "24 Volt" system these are usually 28.4 to 28.8 for ABSORB and 27 to 27.6 for FLOAT. See the battery FAQ's: http://www.windsun.com/Batteries/Battery_FAQ.htm#Battery%20Charging

    In addition, the ABSORB function has a minimum and maximum time limit. The MX60 fixes ABSORB time according to these limits and how long it takes to reach ABSORB from when it starts charging (i.e. the length of the BULK stage, which will vary from day to day according to how much power has been used and how much sunlight is available).

    Fine-tuning the programming is the tricky bit. Some people get better performance with pushing the maximum ABSORB time to the 4 hour limit (although it will still be only as long as the BULK stage - but up to 4 hours if necessary, rather than stopping at the default 2 hours). Some also find better performance with a higher Voltage set point; the ting is to get the batteries bubbling without excess water usage. What constitutes "excess" is a judgment call. Sometimes pushing the FLOAT Voltage to the higher end is beneficial. Regrettably, different batteries have different charging profiles under different conditions so you have to make small adjustments to see what works best for you. If the system performance is satisfactory, then changing things probably won't help.

    A word of caution about EQUALIZATION: too much is as bad as too little. It is best to check the Specific Gravity once a month and equalize only if necessary (SG difference between any two cells of 0.010 or more). My preferred method is to EQ for one hour, then re-check the SG, then EQ again if needed; when it stops improving there's no point in going further - you're likely to damage the batteries. Equalizing for long periods of time or when not needed or at too high a Voltage will all shorten the life of the batteries - exactly the opposite of what you're trying to achieve in the first place.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #3
    Re: BULK, ABSORB and FLOAT

    I have a similar setup, larger array(1300 watts) and 4 golf cart batts that are 5 years old and heeding to be replaced in the next year.

    I typically recomend a 2 hour bulk at 28.6-28.8, I'd suggest going at least 2 hours, 28.4 should be fine...

    On equalizing I recomend at least every other month after the batteries have reached half there typical life span, perhaps 2 years on 6V GC batts, that said Trojans only recomend equalizing if needed, Rolls every 2-6 months. I ahven't been keeping on top of the SG on my batts in the last 8 months or so but I can tell they have lost some capacity. I had gone to monthly equalizing when the batts reach 4 years in age and I replace the charge controller a couple months ago and haven't switched it back to monthly eq, perhaps I can regain some of the capacity.

    Reguardless I'd suggest increasing your Equalizing voltage to 30+ volts, I use 30.6 for 2 hours...
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • JESSICA
    JESSICA Solar Expert Posts: 289 ✭✭
    #4
    Re: BULK, ABSORB and FLOAT
    Cariboocoot wrote: »
    A word of caution about EQUALIZATION: too much is as bad as too little. It is best to check the Specific Gravity once a month and equalize only if necessary (SG difference between any two cells of 0.010 or more). My preferred method is to EQ for one hour, then re-check the SG, then EQ again if needed; when it stops improving there's no point in going further - you're likely to damage the batteries. Equalizing for long periods of time or when not needed or at too high a Voltage will all shorten the life of the batteries - exactly the opposite of what you're trying to achieve in the first place.

    Thanks, Cariboocoot.

    Now, regarding Equalization: Is 29.4 "too high a voltage" for my batteries? I have read that some experts recommend 30 or even 31 volts to "equalize" batteries in a 24 volts bank. I beleived (and still believe) that's too high; and so I opted for the 29.4 volts, with no scientific reason whatsoever for that number.
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  • JESSICA
    JESSICA Solar Expert Posts: 289 ✭✭
    #5
    Re: BULK, ABSORB and FLOAT
    Photowhit wrote: »
    Reguardless I'd suggest increasing your Equalizing voltage to 30+ volts, I use 30.6 for 2 hours...

    Few seconds after I wrotte my previous answer, I read Photowhit's.
    Thus I ask: Why 30+ volts?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,599 admin
    #6
    Re: BULK, ABSORB and FLOAT

    I think the intent (at least with Trojan batteries) is to get ~5% of the 20 Hour battery capacity as a charging rate for equalization...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: BULK, ABSORB and FLOAT

    I've never seen any specific "this much and this is why" explanation but the "rule of thumb" is that EQ Voltage is 5% to 10% above ABSORB set point. I actually have mine set at 30.5 - suspiciously close to Photowit's level! :D
    If you get good results at the lower Voltage setting, by all means use it; higher Voltage means higher current and more heat. Heat is bad for batteries. That's why I prefer to do it in short, one hour cycles. This may not be the best answer for your batteries.

    The minimum EQ time would go up with the capacity of the battery. If you have "tall case" batteries (L16's) you might want higher EQ/ABSORB Voltages as they suffer from electrolyte stratification.

    If you can't get any actual recommendations from the manufacturer for charge settings you have to experiment to find the best settings for your batteries. This is easier said than done, as you can't really "look inside" the battery and see how it's doing. Specific Gravity is about all you have to go on.
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  • rplarry
    rplarry Solar Expert Posts: 203 ✭✭
    #8
    Re: BULK, ABSORB and FLOAT

    There is another method to determine when the absorb cycle is done. As explained in the MX60 owners manual, you can consider the batteries charged when the charge amps taper down to 2.5% of the amp hour capacity of the battery bank. In other words, if you had a 1000 A/H bank, when the charge amps tapered to 25 amp going into the batteries, Outback says you can consider them charged. If you note how many hours this takes after the MX60 goes into "absorb" mode then you know how long to set the absorb time limit. I personally think that 2.5% is too low and I like my batteries charge rate to taper down to 1%. For my bank that takes a little less than 3 hours. Since I use AGM batteries I don't have any recomendations for equalizing, but I think you have already gotten some good advice on that subject. Most likely the battery manufacturer will have the best info on that subject.
    Good luck with yours,
    Larry
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  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,873 ✭✭✭✭✭✭
    #9
    Re: BULK, ABSORB and FLOAT

    31 Volts on a 24v nominal and 62 Volts on a 48 nominal Surrette L16 bank @ 2hours. Been doing this for decades every 3 months.
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net

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  • solarabcd
    solarabcd Solar Expert Posts: 38 ✭
    #10
    Re: BULK, ABSORB and FLOAT

    I can't wrap my head around the fact that the absorb phase is generally considered to take the same time as the bulk phase with the charge controller manufactures.
    Surely bulk phase varies depending on DOD and if the current is tapering off during absorb, it must take longer.:confused:

    Rplarry's post seems to be a little more accurate way of calculating bu there stills seems to be a vast array of calculations out there.
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  • rplarry
    rplarry Solar Expert Posts: 203 ✭✭
    #11
    Re: BULK, ABSORB and FLOAT

    Its not really my way, Outback explains this method in the MX60 owners manual. I just didn't think that C/20 was enough and I like to let it get to a C/100. As batteries age, and sulfates collect on the plates, it gets harder and harder to get to 1% of the battery banks capacity. A sure sign that your batteries are failing, and time to think about replacement.
    Just my .02
    Larry
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