Offgrid-House: Battery bank suddenly losing power rapidly?

[BB.]

I am not completely sure I understand the Mate display--But that is OK. Let us approach it from a different direction.

You have a 225 AH @ 48 volt battery bank, and lets assume it is 50% charged and you want to get it back to full charge.

From an Amp*Hour point of view, batteries are pretty nearly 100% efficient (in terms of Amp*Hours out~AH in) (except for near 100% charge and equalization where efficiency can drop to near zero percent as you end up, mostly, converting electricity to heat plus hydrogen and oxygen gases).

So:

• 225 AH * 0.50 to recharge = 112.5 AH to bring the battery bank back to "Full"

Watt*Hour wise, where batteries are 80% to 90% efficient at charging (energy vs Amp*Hours):

• 225 AH * 0.50 state of discharge * 59 volts charging * 1/0.80 eff = 8,297 WH = 8.3 kWH to recharge a 1/2 full bank

It looks like you are getting ~21 amps into the battery at that time of day (almost 10% rate of charge--that is good):

• 112.5 AH to recharge / 21 amps = 5.4 hours (plus 2-4 hours for Absorb)

But, you really need to add a few hours for Absorb charging (once the battery hits 59 volts is Absorb set point, and current will taper from Imax-charge to a few percent of battery AH capacity after 2-4 hours or so).

Of course, there is the question of how much load you have on the battery bank overnight--Is the battery bouncing between 50% and 75% state of charge, or between 75% and 100% state of charge (the second does not sound likely).

You can start the genset in the early AM and run it until you get to ~59 volts on the battery bank (that is the absorb transition point and the battery bank should be ~80-90% full). Then let the solar panels carry the charging during the rest of the day.

Or, you can start the genset in the evening (when you have loads and less than full battery bank--and somebody is home to operate the generator) to help get it back up to the ~59 volt Absorb point--then shut down. And let sun (or sun and generator) manage the rest of the charging the next day.

By the way, what is the generator kWatt rating and the Amp rating (and voltage set point) of your AC Battery charger?

Need to get a handle on the battery state of charge and where it tends to cycle during the 24 hour period. A battery monitor makes it easy, but an accurate hydrometer (if there is one) and logging data (AM before sun is up, and PM at sunset) for a pilot cell will be a great help. As will knowing your loads (AH/WH used) and charging (AH/WH from sun) (difference should be AH in/out of battery bank). The battery monitor will monitor AH in/out of the battery which is critical to understand right now.

Sorry to be such a pest--But I am not sure I understand your needs/operating conditions yet.

-Bill

[mtdoc]

I think he has a FlexNet DC which is a very good battery monitor. It does take some setting up to be useful.

PorkChop - If you haven't done so already you need to read THIS Tech Note from Outback and follow the steps to set up your FlexNet DC. I think that any battery monitor that is measuring SOC by tracking amps in and amps out needs to be set up and calibrated - will need to start with battery full - absorb stage completed - to get the initial baseline 100% SOC and be able to track amps from there. If you haven't done this already, I would run your generator and get through a good long absorb stage. I would get the SGs as high as they'll go. Then reset the FNDC (to 100%SOC) and go from there.

You really should get that serial to USB connector, spend $50 on WattPlot and run it on a PC. After one day, with the FNDC calibrated, you'll know what loads you've drawn and how your battery bank has responded.

I find that the Mate Display can be a little cryptic and WattPlot makes it much easier and strait forward to track and log all of the info you need.

[PorkChopsMmm]

Sorry for the lack of updates -- I have been trying to finish the power shed and one of our cars overheated. Not fun times :(

BB. you have some very interesting information. Thank you. Although... it does scare me that I don't have enough panels for the size of my battery bank. You said I would need 8.3 KW to recharge my bank from 50%. Today, on a relatively very sunny day with temperatures in the low 70's, I got 5.44kWH. In summary, 5.44 kWH in total, 1.5 kWH out, and just under 4kWH to the batteries. This is all from the shunts measured on my MATE and today was a very low use day.

Again, this worries me because I have ~1.4 kw of panels and while I do have plans to buy 3 more panels (totaling 2.1 kw) in the near future I also I wanted to double my battery bank size. I am starting to think that is not a good idea. Also, if it is summer and I can't charge my battery bank enough right now what am I going to do this winter? I guess I will need to rely on the generator more. Hmmm... The generator specs are 6.1KW continuous, 7.2KW surge, and 50.8 Amps continuous, 60 Amps peak. I haven't used this generator much to charge my battery bank, I normally use a 2KW smaller generator, but I am switching to this larger generator when I get my Auto Generator Start relay in the mail (Maverick Solar, tsk tsk).

mtdoc -- thank you for the reference to properly programming the FNDC. It definitely isn't a plugin deal -- it needs to be calibrated. I will make sure I follow the guide again tomorrow after I go exchange my Sam's Club GC2 batteries for new ones. Wish me luck! BTW I did get the USB to Serial cable and did download the trial version of WattPlot. Honestly, I am not that impressed. I didn't see anything very useful for historical trending, but then again I didn't did in too far. It was all line graphs and not very easy to get a quick glimpse of where you stand. It's also hard when the computer is out in the old power shed and not easily accessible -- this will be fixed in the new power shed when I trench the lines to run power to the cabin. I did use the VisualMate program, which is cool, but you can only run one serial program at a time and so running that app loses the ability for WattPlot to collect historical information... at least this is what I have found out. Am I missing something?

[BB.]

PCM,

But what is your daily load?

If you are discharging the battery bank from 100% to 50% state of charge in one day (overnight, etc.)--Then you don't have enough battery bank.

If you are only taking the bank down 25% overnight--Then it is probably big enough for now.

Do you use most of your power at night, or a fair amount during daylight too? (that will reduce the charging current to the battery bank and mask your daily usage if only measuring the current through the battery shunt).

Regarding the gensets--What size of AC battery charger do you have (model name/number/link)? I am a big believer in matching AC loads to generator capacity--The more you can keep the AC loads (Watts) near 50% or higher, the better your kWH/Gallon fuel economy.

You where asking about supporting a 450 AH at 48 volt battery bank--Using the standard 5%-13% rate of charge rule of thumb:

• 450 AH * 59 volts charging * 1/0.77 charger+panel derating * 0.05 rate of charge = 1,724 watt array minimum
• 450 AH * 59 volts charging * 1/0.77 charger+panel derating * 0.10 rate of charge = 3,448 watt array nominal
• 450 AH * 59 volts charging * 1/0.77 charger+panel derating * 0.13 rate of charge = 4,482 watt array "cost effective" maximum

If you assume 2 days of "no sun" (1-3 day range) and 50% maximum discharge (for long battery life), then the "nominal" daily loads from your battery bank:

• 450 AH * 1/2 days no sun * 0.50 max discharge = 112.5 AH daily load @ 48 volts
• 112.5 AH * 48 volts * 0.85 inverter efficiency = 4,590 Watt*Hours = 4.59 kWH per day
• 450 AH * 48 volts * 1/8 hour discharge rate * 0.85 inverter eff = 2,295 Watt max continuous inverter load
• 450 AH * 48 volts * 1/2.5 hour discharge rate * 0.85 inverter eff = 7,344 Watt max surge inverter load

Using a fixed array for Cleveland OH tilted from horizontal to Latitude:

Month    Solar Radiation
(kWh/m 2/day)    AC Energy
(kWh)
1      2.49          245   
2      3.34          294   
3      4.09          391   
4      4.94          443   
5      5.48          487   
6      5.56          462   
7      5.55          475   
8      5.47          470   
9      4.90          416   
10      3.91          353   
11      2.18          189   
12      1.68          153   
Year      4.14          4378

Using February at 3.34 hours of sun per day as the break even point for 4,590 WH per day (AC):

• 4,590 WH per day * 1/0.52 end to end sys eff * 1/3.34 hours of sun per day = 2,643 Watt Array Minimum

Anyway--that is how I guess at the numbers based on the size battery bank you asked about.

Hope everything works out in the end for you and your family. Life does get bit complicated at times.

-Bill

[mtdoc]

PCM,

Glad to hear you are getting things more set up. I agree that Wattplot's pen-plot graphs could be set up better for quick "at a glance" viewing but the important thing is that they do record all the relevant data on production, loads, battery amps in and out, voltage, temp, etc.. Once you get a chance to play with it more you'll see that you can easily go back and look at any of this historical data for a user defined time period.

For quick summary view, I find the FX and DC snapshot bars and the FNDC accumulated values bar very useful for "at a glance" summary (use the view menu to enable these). I find it is much easier to quickly view all the relevant info on one computer screen than by navigating the Mates many screens to find each value. For a specialized, limited market piece of software that only costs $50 I find it pretty darn good.

As BB points out - for diagnosing your issues it's all about the loads! You can see what your loads were overnight (orany period) and see if there were any unexpected current draws causing your battery bank to draw down.

As far as the location of your computer - do you have network access (wifi or ethernet) at your power shed ? If so you can run a free VNC server program (i use ulraVNC) that will allow you to view that computers screen with Wattplot on any network connected computer (local network or internet if connected).

I have a small netbook in my utility room that runs Wattplot and can view its desktop anywhere in he world on a computer with internet access or on my iphone. Very useful when traveling. I keep a window with this open on my home laptop and on my computer at work to watch my system there as well!

[PorkChopsMmm]

BB. -- my battery charger is built into my Outback inverter. You bring up an interesting point and I am having trouble coming up with the answer... isn't a bank of 8 batteries at 6V and 225 amp hours each a 225 amp hour bank at 48V? I think I spoke wrong when I said I had a 450AH bank.

mtdoc -- thing are never easy around here. I do have wifi but it is from a small Verizon MiFi hotspot. It limits devices to 5 (boo) and it's range is very bad. The netbook in the powershed picks up and loses signal every few minutes. I have an extra Linksys WRT54G router that I should put in repeater mode. I think that would help the netbook get connected.

In other news I exchanged the batteries and have been monitoring them. I EQ'ed them the day I got them and tried to EQ them again tonight. I have a question -- should I EQ until I hit 1.275 SG or should I give it a few charge cycles for the batteries to come up to operating at 100%? I EQ'ed the batteries tonight and when I went to measure SG (only an hour after EQ'ing stopped, no loads) the SG was 1.260+- across all cells.

[Blackcherry04]

BB. -- my battery charger is built into my Outback inverter. You bring up an interesting point and I am having trouble coming up with the answer... isn't a bank of 8 batteries at 6V and 225 amp hours each a 225 amp hour bank at 48V? I think I spoke wrong when I said I had a 450AH bank.

mtdoc -- thing are never easy around here. I do have wifi but it is from a small Verizon MiFi hotspot. It limits devices to 5 (boo) and it's range is very bad. The netbook in the powershed picks up and loses signal every few minutes. I have an extra Linksys WRT54G router that I should put in repeater mode. I think that would help the netbook get connected.

In other news I exchanged the batteries and have been monitoring them. I EQ'ed them the day I got them and tried to EQ them again tonight. I have a question -- should I EQ until I hit 1.275 SG or should I give it a few charge cycles for the batteries to come up to operating at 100%? I EQ'ed the batteries tonight and when I went to measure SG (only an hour after EQ'ing stopped, no loads) the SG was 1.260+- across all cells.

Unless you have airplane batteries I doubt you'll ever hit 1.275 SG. Your batteries were probably filled with 1.260 - 1.265 electrolyte, you can't raise it past the ratio of that. You can destroy your batteries though.

[BB.]

What BC04 says--You can only charge until the battery SG stops rising. That is your new/current 100% state of charge (charge at recommended absorb voltage, stop charging once SG stops rising--check every 30-60 minutes).

Note that equalization is only needed when the SG is around 0.015 to 0.030 different between high and low cells. Then you charge at equalization voltage until the SG stop rising (again, check every 30-60 minutes).

When you connect batteries in series, the voltage increases (2x6 volt batteries at 220 AH = 12 volts at 220 AH). When you connect batteries in parallel, 2x 6 volt 220 AH batteries in parallel, then you have 6 volts @ 440 AH. Note that this does not violate physical laws. Power=Voltage*Current -- So the two remain the same power wise (12v*220AH=6v*440AH).

-Bill

[Photowhit]

Note that equalization is only needed when the SG is around 0.015 to 0.030 different between high and low cells. Then you charge at equalization voltage until the SG stop rising (again, check every 30-60 minutes).

For flooded lead acid batteries, here are other reasons for Equailzing, most if not all battery manufacturers, other than Trojan, recomend an equailizing charge from once a month to 2x a year. If only for mixing the cell's chemistry and preventing stratification. NAWS Battery FAQ's

[PorkChopsMmm]

Thanks guys, I appreciate the help. I will continue to monitor my charge levels and give you all an update. I'm not sure if my MATE reset itself or if I always had the wrong battery AH, but it was programmed for 450 when I really have 225. I updated the settings and followed the FNDC configuration procedures linked in a previous message by mtdoc. Hopefully this will allow my measurements to be more accurate.

I have one other question. The large construction generator I mentioned earlier has GFI outlets. My smaller Coleman generator does not. The GFI breakers pop every time I plug in the cord from the Outback inverter that charges the battery bank. If I reset and plug in they pop immediately. Even with the inverter turned off (so no attempted charging occurring) they pop the breaker on the GFI outlet. I checked the wiring to the inverter and the +, -, and grounds are wired correctly and tightly and the little Coleman with no GFI generator runs the Outback inverter just fine. All of my solar components are grounded via a ground buss bar to a single 8ft copper rod driven into the ground. Any idea why the GFI breakers on the large genny are being blown? This threw me for a loop when I re-installed my new battery bank.

Thanks!