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NYNick

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Posts: 103
Reply with quote  #1 
Hey everyone! I have a 24V systems with 8, 6 Volt Rolls batteries, an E Meter and Trace Engineering Inverter, Charge Controller and DC Disconnect. (Trace is out of business). I have about 400 Watts of Panels and can switch to the grid manually if I so choose. I went on the grid this morning. 

My batteries seems to charge up fine and go to 100% no problem. As I type, the voltage hovers at 26.5 or so, with the sun on the panels.

If I go off the grid tonight and run the house, it'll be fine for a while (overnight?) but then voltage drops to below 24V after a short period of time. When this happens my inverter tries to cycle on/off.

The batteries test ok with good voltage, but they seem to be draining quickly and losing power too fast. 

Is this a clear indication of needing new batteries? Is there a test I should perform to confirm it's the batteries versus the Inverter or Charge Controller?


Thanks in advance...

NYNick in Canada

stmbtwle

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Reply with quote  #2 
How old are the batteries?

I think you have 2 banks of 4. A bad bank in parallel can pull down the other one, and the controller can 'hide' the problem. Been there.

Switch to grid power and turn off the inverter and controller. Then disconnect each bank from the system, let them sit for a day or so and check the voltage.

How many amps are you using? In my experience we tend to add to the load over time, and you may simply be using more than you're putting in.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
NYNick

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Posts: 103
Reply with quote  #3 
Willie,

Thanks for getting back to me. I think the batteries are 8 -10 years old. Can't find the receipt.

I do not have two banks of four. I have one bank of 8. When I test the bank, I test one far left positive end and one far right end negative. I get over 26V. I could explain the wiring if you need me to. It's been years since I did it.

IIRC, we use over 30 AH a day. I'd have to go over the whole thing to figure that out. Coffee maker, water pump, fridge etc. But I can easily disconnect the bank from the system and check it after a day or two.

Thanks for your help.

Nick
stmbtwle

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Reply with quote  #4 
In order to get 24v out of 6v batteries you only need 4 batteries. With 8 batteries you'd have to wire them in series-parallel, 2 strings of 4 batteries per string. Sorry, I called each string a 'bank'. I know of no other way to connect them for that voltage.

Anyway, at that age I suspect you have at least one bad battery, and the rest probably aren't far behind. Testing will tell. Are any of them using more water than normal?

Sorry, but my money's on the batteries (OUCH)!

With any luck you still have 4 "usable" batteries you can connect into 1 string, and hope that will carry you till you get new ones.

Do you have a "kill-a-watt"? 30ah @ 24v = 720wh, which seems a little light, my fridge alone took more than that (but it WAS a warmer climate). On the input side, a 400w (.4kw) calculates to less than 2kwh per day, in the summer.
https://pvwatts.nrel.gov/pvwatts.php

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
NYNick

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Posts: 103
Reply with quote  #5 
Willie, a few notes before I get started. Of course, our usage has increased over time. I never really had enough PV cells to be independent, but I used to be able to go a week or so. Now, with an electric fridge (instead of propane) and us getting lazier, usage has gone up. I added a 100 Watt-er a few years ago to help with the fridge, but I still would "run out" or fall below 80% in about 5 days. No big deal, I'd just switch to the grid and let the batteries charge by the sun. (I didn't always have the grid. I used to do that with a generator.) When it's nice and sunny around midday, I'm generating around 14-15 amps.

I'm not sure what my usage is anymore. I kinda lost track, but I seem to remember 30-40AH per day. Probably more now. Fridge draws 25 when on, water pump 40 plus, hell, even the coffee maker draws 40. These things add up, but our house is small and we're pretty frugal with our power. 

Anyway, I went ahead and shut down the system as you suggested. Easy enough to do, I just flipped the DC disconnect breaker. Then I checked out my battery wiring. Here it is:
 
1 P to P2 and house. N to P3
2 N to P4
3 N to P5
4 N to P6
5 N to P7
6 N to P8
7 N to N8
8 N to house

Is this Series-Parallel?

I disconnected the the P and N leads to the house and now the battery system stands alone. I measured the voltage before I walked away and it was a hefty 26.05. (I could hear them gurgling beforehand as the collectors were charging them!)

I'll check them daily to see if they decline.

Thanks

Nick
stmbtwle

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Reply with quote  #6 
Yes, that is series-parallel. Bats 1,3,5,7 are in series for 24v "string A"
Bats 2,4,6,8 are also in series for 24v "string B"

The two strings are paralleled with the jumpers from 1P to 2P, and 7N to 8N.
You can disconnect either string by removing a jumper, and your system will run off the other string, but with only half the capacity. You may have to switch the house leads to the active string.

DISCONNECT THE JUMPERS before testing. If the two strings are still in parallel with the jumpers, you cannot test them, as the good string will try to "charge" the bad string. So you have to remove at least one jumper. Then let them sit for a day or so and check the voltage of each string. You should get about 24v per string and 6v for each battery. A bad battery will read less than the rest. It's possible you may have one or more bad batteries in each string. If so, reconfigure the 4 best batteries into one string.

If ALL the batteries test good, then I suspect they are not getting fully charged. Try reducing the load (output) for a few days, or switching to the grid. Then try again. If this works, I think you need to increase the INPUT by adding another solar panel or two. How is your array configured?

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
NYNick

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Posts: 103
Reply with quote  #7 

Willie,

First let me say how much I appreciate your help. Forums like these are good for the soul. You're a good man!

Anyway, mission accomplished @ 930AM Wednesday.

I disconnected the jumpers 1P to 2P and 7N to 8N.
I then tested String A (1,3,5,7) and got 25.20V (1P and 7N)
I then tested String B (2,4,6,8 who do we appreciate? willie!) and got 25.18V (2P and 8N)

I then tested each battery:
1 6.3
2 6.31
3 6.29
4 6.32
5 6.3
6 6.29
7 6.3
8 6.26

Now we wait. I'll test everyday for the next few days.

Just as an aside, I'm leaving for 6 weeks next week. I was thinking of just leaving them as they are right now and see what they read when I get back. Good idea/bad idea? Plus the collectors will just be producing power connected to nothing. Good/bad?

Regards,

Nick
stmbtwle

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Reply with quote  #8 
I think that's a good idea. A good battery can lie idle for a while. A bad one, not so much.

Check out this site: https://www.trojanbattery.com/
And this one: https://forum.solar-electric.com/

There's a lot of good info there, though you may have to hunt for it.

As far as the collectors are concerned, I don't think they "care". They won't overheat like a thermal collector will. That's the nice thing about PV, when it's not being used it just sits there, no problemo.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
dbc

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Posts: 275
Reply with quote  #9 
Nick,

Do you have a good hydrometer?  Voltage measurements won't always tell you the whole story, particularly if your batteries are more than 2 V (single cell).  A hydrometer indicates whether each cell of the system is really accepting a charge by measuring the specific gravity.

I have a similar system (small stand-alone, battery-based, with a manual AC transfer switch to switch the load from the PV inverter to the grid.  A couple years ago, my system started acting like what you describe - It would run OK during the day, but voltage would start to drop late in the day when the panels stopped producing, until eventually the inverter would go into low input voltage alarm and shut down.  My batteries (4 ea. 6V, L-16 series, wired in series for 24V) were 10-11 years old when this happened.  Two of the four batteries had unacceptably low specific gravity reading for one cell.  This was after the batteries showed full charge (on the TriMetric battery meter), and had sat idle for a couple hours with no load.  If this is what's happening to you, I also suspect you need new batteries.

Willie's suggestion is a good one - you might be able to salvage one of the battery strings and operate at reduced capacity.  If you have a bad battery in each string, you pick 4 good ones and make them your 'new' temporary string.

I also think your PV array could be a bit small for that size of battery.  (You didn't say what Rolls model you have, or what the A-H rating is.)  I have - had, actually - 540W of PV with a 24V/370 A-H battery bank, and I eventually concluded my PV to battery ratio was a bit small.  If I ran the batteries down to around 65-70% state of charge, I had to give them most or all of the next day off to let them get back up to full charge.  I did this while the batteries were still good.  Lead-acid batteries should be brought up to full charge (float, if you have a reasonably 'smart' charge controller) at least every 5 days; every 3 days is even better.

All batteries eventually give it up, no matter how nice you treat them.  PV prices have dropped much more than battery prices in the last 10 years, so a bigger PV array is now a better idea than ever.
NYNick

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Posts: 103
Reply with quote  #10 
OK, some data for you gents. The batteries have been disconnected for a few days.

String A (1,3,5,7) has gone from 25.2 to 25.07
String B (2,4,6,8) has gone from 25.18 to 24.88

1 has gone from 6.3 to 6.27
2 6.31 to 6.23
3 6.29 to 6.26
4 6.32 to6.25
5 6.3 to 6.27
6 6.29 to 6.21
7 6.3 to 6.27
8 6.26 to 6.18

I measure the SG today. My hydrometer ranges:
1300-1275- GOOD
1250-1225 FAIR
1200-1100 RECHARGE

All 3 cells in all 8 batteries were in the FAIR range, from 1200-1275. Battery 6 had one cell in the 1175-1200 range. Floaty hydrometers are tricky to read accurately.

My batteries are Rolls 1. That's the only number I can see on them. They're monsters.

Thank you both for your help.

Nick
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