48 Volt Off Grid Voltage versus Capacity and Age of Batteries

[rscranton]

I have 16 six volt Deka 8L16 batteries in series/parallel to make 48 volts with a 740 amp hour capacity in an off grid home. I have 4200 watts of panels on two arrays and two outback charge controllers feeding the batteries with a generator as auxiliary 120 to the home and charger through a magnum inverter. The system is just over 4 years old and going into its 5th year. I think the batteries are showing signs of failure based on their drop in voltage after 20% use overnight. I am using a Bogart Engineering meter to group the two arrays and the single battery bank to measure capacity.

After charging the batteries to 100% at the end of the day with the voltage in the 50 volt range, but not 51 volts, overnight the batteries will drop to about 80% of capacity with the use of heat and appliances. (just as a caveat, the voltage will rise to 56.6 or so during the day when being charged, but never over 57 volts unless under equalization. There again, I can't get the equalization volts of the batteries to go to 61 volts even though the charge controllers are set that way) When the meter says the batteries have 80% to 85% of capacity, and the heat is on with the total system drawing 15 to 25 amps depending on what is going on, the volts will drop to 41.5 to 43 and recover to about 44 volts if the use drops to 9 amps after the heater(s) kick out. So I am left with the meter showing capacity around 75% to 80% and my voltage is around 42 to 44 with low amp draw of 7 to 9 amps.

Question: Does this indicate that my batteries are no longer at the 740 amp hour rating and have lost their capacity and may only have say 400 or 500 amp hours and the 80% capacity is really more like 50% or so?

Please chime in with advice, your experience and education.

I am going to replace this set up with some Surette S600 batteries so I'll have 900 amp hours. Maybe next time, I'll go to a 2 volt setup with even more amp hour capacity. What say you?

[Sunking]

If you are seeing the voltage drop below 45 volts with or without load means your batteries are done and need replaced. In addition your meter is not really telling you what the capcity is as no meter can possible tell you that as it can only be determined with a capacity load test.

[rscranton]

Meter only knows what I tell it - is that correct?

I suspected that the meter only knows what I have told it. And, of course I told it what the batteries are suppose to be, not what they probably are now. So, if I am understanding what you are saying, that if a 48 volt system drops in voltage too far, and you are saying 45 volts, my battery bank is way way down on the scale of capacity. Would that be a fair summation of your comment.

Your advice is to replace the battery bank in my case. Correct?

Thanks for chiming in by the way.

[Sunking]

So, if I am understanding what you are saying, that if a 48 volt system drops in voltage too far, and you are saying 45 volts, my battery bank is way way down on the scale of capacity. Would that be a fair summation of your comment.

Your advice is to replace the battery bank in my case. Correct?

Correct and correct.

[rscranton]

Thanks for valuable information

Correct and correct.

I think I knew what your were going to say when I posted this post. But, that being said, when it's your first time confronting this situation, it is really great to have feedback on a situation (like really low voltage) that you know is just not right. You know, when things are working right you really don't notice things like voltage, etc. But, when things start going awry, then you wonder, is this normal or is something else wrong? Thank you for your input.

[inetdog]

I think I knew what your were going to say when I posted this post. But, that being said, when it's your first time confronting this situation, it is really great to have feedback on a situation (like really low voltage) that you know is just not right. You know, when things are working right you really don't notice things like voltage, etc. But, when things start going awry, then you wonder, is this normal or is something else wrong? Thank you for your input.

It is easier said than done, but when trying something new, like a solar system, it really helps to make as many measurements as you can when it seems to be working properly, for two reasons:

1. It will give you a good early indication that it is not really working as well as you think it is.
2. You will have some "normal" numbers to compare against when problems come up (and they will....).

Now if only I would do that myself.

[Sunking]

I think I knew what your were going to say when I posted this post. But, that being said, when it's your first time confronting this situation, it is really great to have feedback on a situation (like really low voltage) that you know is just not right. You know, when things are working right you really don't notice things like voltage, etc. But, when things start going awry, then you wonder, is this normal or is something else wrong? Thank you for your input.

You are WELCOME

You have nothing to lose right now. NUKE you battery with a long 16 to 24 hour EQ charge. If you are lucky you might get something out of it. Just make sure the battery water covers the plates.

[rscranton]

That's a great idea

You are WELCOME

You have nothing to lose right now. NUKE you battery with a long 16 to 24 hour EQ charge. If you are lucky you might get something out of it. Just make sure the battery water covers the plates.

There is only one neighbor here and I just sent a warning e-mail that I might be starting the generator at 5:30 am when I get up the feed the dogs. I ordered some Surettes today but it will be a week or so until they arrive. Why not nuke em? You're right, I have nothing to loose. I think I'll start an EQ phase right now.

[Mike90250]

check individual battery voltages, and you may find a couple of bad cells. If you had a young bank, you could just replace the dud, but with the older bank, you may only get a couple months before another fails.

[rscranton]

Yes, good idea

check individual battery voltages, and you may find a couple of bad cells. If you had a young bank, you could just replace the dud, but with the older bank, you may only get a couple months before another fails.

Yes, that happened to me last year and I replaced one battery then. About a month ago, I checked the voltages just before I realized that my overnight voltage was really dropping so low. At that time, I realized how bad the batteries were getting, the single battery in the bank of 16 that I had replaced last year had (I think) around 7 volts and the others ranged from 6.1 to 6.5 volts. That kinda got me thinking that things were beginning to go. I looked at each cell to see if I was getting gassing and I was. When that one battery went bad a year ago it had one cell that just didn't gas and that along with its low voltage told me that battery had gone bad. That was a revelation, the non-gassing cell and low voltage, as an indication that a battery would not take a charge.

I have EQed the bank a couple of times now in recent days and it does bring up the voltage a bit but after a day of no EQing, the voltage will slip backwards. That's where I am right now. It's not as cold at night right now, only down to 35 instead of 20, so I can make it through the nights a little better right now. New batteries are about 10 days out since I decided to go with the Surette brand and they must come by truck instead of the Dekas that I have which I can just go to the battery company in Grand Junction or SLC (about the same distance from my location).

Thank you for your advice.

[Sunking]

New batteries are about 10 days

Please tell me you learned your lesson and are mot going to parallel anymore batteries?

Do you have a generator to put the initial charge on the new batteries when they arrive? No way that can be done with solar.

[rscranton]

So you don't like paralleling batteries

Please tell me you learned your lesson and are mot going to parallel anymore batteries?

Do you have a generator to put the initial charge on the new batteries when they arrive? No way that can be done with solar.

Yes, I am going to parallel the batteries, the usual 16, 6 volt battery set up to make 48 volts and double the amp capacity. I take it you don't like paralleling. It is my understanding that paralleling has drawbacks in that it doesn't draw from and charge to in a uniform manner. Can you explain if there is a logic in that or is it random, in that it doesn't necessarily take from one bank and charge to the other bank in any certain order.

I think that next time these batteries that are coming start to go, I'll change over to a single bank of larger capacity, either 2 or 4 volt depending on what is out there next time.

Yes, I have a back up generator, it is a Coleman Powermate (11,000 watt) with a honda engine that runs off propane. Of course they don't make that one anymore since the Coleman division went bankrupt or something like that (I think) back in 07 or 08 maybe.

What recommendations do you have for the initial charging of the batteries? Thank you for any info and feedback.

[inetdog]

Can you explain if there is a logic in that or is it random, in that it doesn't necessarily take from one bank and charge to the other bank in any certain order.

There definitely is a logic to it, but that does not necessarily make it predictable.

There is a very good discussion of the sort of problems that can arise at this site, along with a lot of other good technical information on batteries, etc.

If you take all reasonable precautions to keep the external factors like wire length and connection resistance equal, there can still be problems with differences among the batteries which will tend to feed back and amplify the differences over time. Not guaranteed to happen, but common. You should keep a close eye on voltages and SG in the parallel strings initially and then check periodically to make sure you are not getting into trouble.
A good clamp-on DC ammeter will be very helpful in making sure the batteries are balanced. But keep in mind that the typical clamp-on DC ammeter can have a calibration/zero shift as you move it from one wire to another.

[rscranton]

Charging a new set of batteries - How to Tips??

There definitely is a logic to it, but that does not necessarily make it predictable.

There is a very good discussion of the sort of problems that can arise at this site, along with a lot of other good technical information on batteries, etc.

If you take all reasonable precautions to keep the external factors like wire length and connection resistance equal, there can still be problems with differences among the batteries which will tend to feed back and amplify the differences over time. Not guaranteed to happen, but common. You should keep a close eye on voltages and SG in the parallel strings initially and then check periodically to make sure you are not getting into trouble.
A good clamp-on DC ammeter will be very helpful in making sure the batteries are balanced. But keep in mind that the typical clamp-on DC ammeter can have a calibration/zero shift as you move it from one wire to another.

QUESTION: Would you chime in on the proper procedures for charging a new bank of batteries?

[Sunking]

QUESTION: Would you chime in on the proper procedures for charging a new bank of batteries?

I will as I know Surrette batteries well. It is in the Owners manual and you will need to do it 1 string at a time as it takes 16 to 24 hours for each string.