24 volt battery bank voltage/when to equalize

[superspence]

Hello,

So I got 8 - 6 volt interstate batteries from Costco (pretty cheap for 220 AH, 63 lbs, 73 dollars each).

I have the bank wired for 24 volts.

I have a Outback MX-60 charge controller, and have not hooked up my inverter yet - just letting the batteries all fully charge before using the system.

Yesterday the bank was "fully charged". The Outback said the voltage fully charged read between 25.1 volts and 25.3 volts. Since a fully charged 12 volt battery is 12.8 volts, my system should read double that right?

I have done a lot of searching and just wanted to make sure that my fully charged voltage of 25.1 is ok, or if I did something wrong. The Outback also says I can equalize the bank but does not state how often I should be doing it - could that be part of the issue?

Thanks again for my newbie questions. I am pretty sure everything is ok but since I am dealing with high price / dangerous equipment and its my first time doing it, I just want confirmation that I am doing things right

[Mike90250]

EQ should be done at least every 90 days. Most charge controllers have a 30 day preset to perform an EQ cycle.

To start, you need a full charge in the batteries, then the EQ cycle begins and applies a controlled time "overcharge" which will bubble the full cells, and finish recharge on the low cells. Times vary from 30 - 90 minutes, depending on bank size, for your small bank, new system., 30 minutes should be plenty. The deeper you cycle the batteries, the more often you should EQ.

Your battery mfg has the last say on what the battery voltage should be, and then you fine tune the charger to match that.

[Sunking]

You only need to EQ when the hydrometer and voltage readings indicated it is required. Your owners manual for the bateries should indicate the levels.

As for the voltages, how and when are you reading them?

Your charge controllers should have some optional selections for different battery typres like FLA AGM, and Gel. It should also be a 3-stage charger with the 3rd stage being a float charge. At float the voltage should be around 27 volts. If the voltages you are seeing are at night or with the bateries being under load are fine.

[superspence]

I hope I did not do something wrong

I hope I didnt do anything to damage my batteries today....

I called interstate, and they said the equalization voltage for my system (24v) is 31.2 volts.

I set that voltage on my mx60, had the bank fully charged, and set the time to 2 hours, per interstate's recommendation. My panels put out a max of 250 watts today. During the peak, my bank only got to 30 volts. The system was making the bubbling noises all day, but never got to the 31.2 volts to equalize so it bubbled all day.

When the panels started generating less wattage, the bank also went down. at 200 watts, the bank got to 29.6 volts.

Another thing that I thought was odd, is when I checked voltage for each individual 6 volt battery, they all showed 7.41 volts - except for two of them. They read 7.30 and 7.50, then when the sun went down they were all the same voltage (6.18 I think)

So I guess my questions are,

1. Why did my system never start and complete the 2 hour charge cycle, when I generated 2.0 kw today from my panels?
2. Did I damage my batteries by letting them try to get to the equalize charge (hovered at about 30 volts for about 4-5 hours of the day)?
3. My bank consists of 8 - 6 volt batteries. Why would 6 show a 7.41 volt charge and the other two were 7.50 and 7.30?

Thanks in advance. Please let me know if I need to clarify anything.

[Sunking]

1. Why did my system never start and complete the 2 hour charge cycle, when I generated 2.0 kw today from my panels?

What does 2 Kw means? Did you mean 2 Kwh? I assume from your earlier statement that at peak your panels generated 250 watts indicates your panel wattage is around 250 watts. If that is the case, there is no way on earth you can generate 2 Kwh with a 250 watt solar panel in a day.

Your system did start the Equalizing Cycle. Problem is your panel wattage is likely to little to bring the voltage up to the expected EQ voltage in 2 hours. Battery chargers are current sources, not voltage sources.

You mentioned you have 8 - 6 volt batteries rated at 200 AH. I assume you are saying each of the batteries are rated 6 volts @ 200 AH. So if configured for a 24 volt system tells me you have a 24 volt @ 400 AH batter stack arranged in a 4 x 2 configuration

So based on that your solar panel wattage is 250 watts, you do not have enough panel wattage to support a 24 volt battery @ 400 AH. 250 watt solar panel can only supple 10 amps, you need at least 40 amps, or a 1000 watt solar panel to support your 24 volt 400 AH battery

2. Did I damage my batteries by letting them try to get to the equalize charge (hovered at about 30 volts for about 4-5 hours of the day)?

No it is not likely you damage the batteries

3. My bank consists of 8 - 6 volt batteries. Why would 6 show a 7.41 volt charge and the other two were 7.50 and 7.30?

Because they are not Equalized. Don't judge the battery state of charge by only using voltage. You need a hydrometer to determine the state of the batteries.

[superspence]

Thanks for the response.

Yes, I did mean 2 kwh, as reported on the MX60. I live in Phoenix AZ, and get 8 hours of good light, easy. More than that if not counting direct light to the panels. Its 5:30 AM right now and its already fully light outside.

The issue of how big my panels are, is another problem I need to research. I was going to research that issue after this one was solved. I have 2 180 watt 24 volt suntech panels, so 360 watts. I have them on a short 15 foot 8 gauge wire run into the mx60. They only put out 250-280 watts in direct sun. The mx60 reports charging at 55 volts and 4.5 - 5 amps. I have the two panels in series, so the amps are right on, but not sure why the voltage is staying around 55 volts. When I previously took voltage reading for each panel, it read 35 volts each, x2 so should be closer to 70 volts, right?


Anyway back to the original issue, what would be the best action at this point? I plan to add another 700 watts of panels in a few months, so should I just not equalize until then?

Is my battery bank just too big for the panels and I should remove half of them?

I guess I could also disconnect half of the bank and equalize one half, then do the other half, right?

Thanks

[Mike90250]

Incomplete or aborted EQ cycles will not hurt batteries, but also keep in mind, any daytime loads you have, will consume power that should be going to EQ the batteries. Also, any direct wired loads (24V lights, etc) can be damaged by the higher EQ voltage.

Do you have a generator? Run it in the AM, till the batteries go into the absorb part of the cycle, and then let the PV run the absorb, After absorb, you can start the EQ cycle on PV. But no loads while this is happening. it may take 3 days of partial EQ, to finish the cycle and bring all the cells up to full.

EQ is a controlled "over charge" applied to make sure variances in cells don't leave any undercharged.

[Sunking]

I live in Phoenix AZ, and get 8 hours of good light, easy. More than that if not counting direct light to the panels. Its 5:30 AM right now and its already fully light outside.

Well FYI just because it is fully light outside does not mean your panels are generating their full power. The only time a solar panel can generate its full wattage is high noon in zero degree temps,. Phoenix has excellent solar insolation. Insolation is measure in Sun Hours and in Phoenix in late June depending on your tilt angle you receive anywhere from 2.3 hours @ 0 degree tilt angle, up to 8.1 Hours @ 90 degree tilt angle. So for a 360 solar array tells my at the very best optimum tilt angle and taking efficiency into account you can generate 360 watts x 8.1 hours x .66 = 1.9 Kwh to 2.0 Kwh. So your system is doing just fine.

The issue of how big my panels are, is another problem I need to research. I was going to research that issue after this one was solved. I have 2 180 watt 24 volt suntech panels, so 360 watts. I have them on a short 15 foot 8 gauge wire run into the mx60. They only put out 250-280 watts in direct sun. The mx60 reports charging at 55 volts and 4.5 - 5 amps. I have the two panels in series, so the amps are right on, but not sure why the voltage is staying around 55 volts. When I previously took voltage reading for each panel, it read 35 volts each, x2 so should be closer to 70 volts, right?

Not really in theory they would, but there is something called Negative Temperature Coefficient that you have overlooked.What that means basically is the hotter the operating temperature is, the lower the voltage and power the panel will produce. Panels are rated at 25 degree Celcius. In Phoenix on top of your roof, the operation temperature is up around 60 C. So your voltage and power will be considerably lower than the panel rating of 180 watts. From the above calculations I made, you are right about on target.

Anyway back to the original issue, what would be the best action at this point? I plan to add another 700 watts of panels in a few months, so should I just not equalize until then?

Well the issue is your panel wattage is not matched up to your battery capacity. a 24 volt 400 AH battery has the capability to let you use up to 2 Kwh per day. Right now you can do that. But the issue is winter when your Sun Hour day drops to 2.8 hours.

Based on your battery size and your location you need a solar panel wattage of (2 Kwh x 1.5) / 2.8 hours = 1071 watts. You only have 360 watts. So what you need to do for now is limit how much power you use in a day to around 1 Kwh in summer, and cut back to .6 Kwh in winter.

Is my battery bank just too big for the panels and I should remove half of them?

Yes your battery bank is too big. You do not have to remove half of them, you just need to limit how much power you take from them in a day. What it sounds like you did is what most folks do, they never designed the system and just bought components and hope they work. Instead you need to determine how much energy you need in a day, then design a system based on that objective.