Long Float times, buy more batteries ?

tr0y · February 2011

Hi All -

My batteries are spending considerable amounts of time daily with the charge controller saying "float". When I go back through the logs on my MX60 for the full 64 day history the minimum float minutes I have record of is 190 minutes some days there are 400+ minutes in float.

Does this long float time mean I should use more power or add more batteries ? I feel like the panels are making quite a bit more power than we use and I could add another bank of batteries and use electricity like it was falling out of the sky ...

Any thoughts ?

My System:

Xantrex 5548
MX60
10 Evergreen 205w 12v wired for 60v panels
8 405 Ah AGM batteries

BB. · February 2011

Re: Long Float times, buy more batteries ?

What is the AH rating of the batteries?

What is the AH/Watt*Hours per day power usage?

What is the Watt Rating of the Array?

At the very least, it sounds like you can use more power.

-Bill

tr0y · February 2011

Re: Long Float times, buy more batteries ?

Battery Ah is 405 8 x 405 Ah batteries 48v system

To be honest I haven't really bothered to calculate the AH / Watt hours per day due to the excess power

The array is 2050 Watts 10 x 205 Panels

I do have the luxury of being at over 7,000 ft in mountains in southern NV, that might help the power gathering.

I was just really wondering if all of those float minutes could be power into more batteries.

Cariboocoot · February 2011

Re: Long Float times, buy more batteries ?

The altitude will give you an advantage. Instead of the "typical" 77% panel efficiency you're probably doing better than 80%. I see a potential for 8 kW hours per day under your circumstances.
Just to be sure, plug your numbers in to the PV Watts program and see what you get.
Your battery size seems to fit with the expected "harvest" too - within reason.
My guess is you're just not using enough power!

That almost never happens!

oaklandpower · February 2011

Re: Long Float times, buy more batteries ?

Tr0y,
What is the absorption voltage set at? Also what is the transition criterion to go from Abs to Float ( such as time or current level )?

tr0y · February 2011

Re: Long Float times, buy more batteries ?

Hmmm not enough use I mean we use normal amounts of power, full size fridge, washer, microwave and lights, TV's, laptops. It is a good sized house 4000Sqft. There are only two of us no kids or roomates and we spend a lot of time outside, and heat mostly with wood. We remember to turn things off and are mindful of wall wart chargers and such but don't skimp.

I was just wondering if that power could be stored or do you think this would cause under charged batteries ? I will say we do not have enough storage for extended cloudy days but then the Honda 2000 genset seems to do us just fine.

As for charging Absorb is set to 57.6 and Float to 53.6. I have the Absorb cycle at the 2 hour MX60 Default, I haven't seen any real reason to change it.

I have taken the stresses off the batteries and let them sit for 3 to 5 hours and my voltages tend to be between 50.9 to 51.2

Cariboocoot · February 2011

Re: Long Float times, buy more batteries ?

Here's some approximate numbers to consider:

405 Amp hours of battery @ 50% DOD = ~200 Amp hours @ 48 Volts = 9.6 kW hours
2050 Watts of panel @ 80% efficiency = 1640 Watts over ~5 hours = 8.2 kW hours

In other words you've got quite a bit of power available in off-grid terms.

Providing your levels are set correctly. Something along these lines:
Absorb Voltage = 56.8 to 57.6
Absorb time = max: count up to 4 hours
Float Voltage = ~55.2

If you're only using 2-3 kW hours per day, you'd "replace" that in less than 2 hours and spend the rest of the day in Float mode.

Should you increase the battery size? Probably not. For one thing, your existing bank is aging and would not work well paralleled with new batteries - the new ones would get "dragged down"; prematurely aged. But mostly your peak charge rate is probably around 7% of the bank's Amp hours:

1650 Watts / 57 Volts = 28.9 Amps = ~7% of 405 Amp hours.

If you increase the size of the battery bank it may not charge properly when it does need charging. For instance doubling it will reduce the peak current to 3.5% of the Amp hours, which is well below the recommended 5% minimum.
(These are just "rough numbers" to show you how the relationships go.)

What you're actually looking for is some way to make use of all that surplus sunshine when you've got it. This is either a matter of load shifting (my method of survival with a tiny system) or exploring the use of an electric water heater to make use of the extra Watt hours.

If you have some exact numbers on peak current (typical) from the MX60 and what the various charging Voltages/times are set to as well as some real world numbers on usage it would be possible to get this system "dialed in" more accurately.

oaklandpower · February 2011

Re: Long Float times, buy more batteries ?

The long float times indicate that you have excess solar capacity compared to consumption for normal days. A bigger pack will still have about the same floating time because the power balance isn't changed. The only downside would be the added battery cost vs benefit. Are you thinking of putting in another 8x6V string in parallel?

westbranch · February 2011

Re: Long Float times, buy more batteries ?

Hi, once you get the charge parameters fine tuned, (you didn't indicate your brand of AGM's, and the manufacturers specs are all different) and are happy with the system performance...another thing to add into the mix is :

do you have enough battery storage to get you through 3 days of no sun ( the recommended target) in the winter ?

If so then you have a good balance and as far as I can see you just need to use some of that excess 'when the sun shines'.

Enjoy it if you've got it...

e

Derik · February 2011

Re: Long Float times, buy more batteries ?

I have almost the same issue. I have 9-205 watt panels and 12 trojan L-16.

I am in Southern CA at 3,300 feet. My panels make way more power than what they are rated for I have seen them produce 2,300 watts at 80 amps! I have the FM 80.

With my large battery bank I have had some issues when we had 7 days of clouds and they never got below 50% but I was worried. Witht the amount of sun in our climates and your use I woudn't say it was necessary for you or I to have such large batterty banks. I rarely use more than 10% of my capacity and even when we had a lot of clouds a smaller bank would just have been filled faster.

With your system you have no worries so I would leave it as is. Add a large screen LCD!!

BB. · February 2011

Re: Long Float times, buy more batteries ?

Remember it is not unusual for a solar charge controller to report 5-10% (or even more) higher power output than it is actually producing (reading 5% high is pretty consistent for many controllers). If you have a battery monitor, it is probably more accurate.

In any case, they are not designed (nor required for proper operation) to accurately report their output.

-Bill

techntrek · February 2011

Re: Long Float times, buy more batteries ?

Putting your battery bank under such a light load on a daily basis will extend its life, so I wouldn't worry too much about it.

jeffkruse · February 2011

Re: Long Float times, buy more batteries ?

What is your SG at?

If your SG is low I would increase the absorb time to max.

I have 2050W of pannels and harvest 7 - 10 KWH a day. I have extra to spare on clear days but most days are not clear. I use 6 - 7 KWH a day.

Derik · February 2011

Re: Long Float times, buy more batteries ?

I like this thread because I think it reflects what most of us experience in the South West high deserts. Most post are gloom and doom and almost the text book answer is "buy more solar panels" or "you have too much battery for your charger" even someone here on this post suggested things weren't dialed in correctly??

Damn he's making way more power than he can use because his system produces what it's suppose to or more. In my case even if my charge controller is off 10-15% I am still way ahead of the game and my stystem makes way more power than advertised or what I expected. I see little to no losses everyone talks about here and I think it's where I am.

Some local studies show 350 days of sunshine here a year and I bet where he is he's getting even more and at 7,000 feet clean air and low humidity he's probably way off the chart as far as his panels being as effecient as they say or more so.

I do agree that a hydrometer will be the final proof but my dollar is on it being fully charged.

niel · February 2011

Re: Long Float times, buy more batteries ?

derik,
calm down as he feels that something isn't right even if he produces more than he uses. to have loads like refrigerators and such going 24/7 that he could possibly have 400+ minutes of float time doesn't seem right. add 120 minutes for absorb and there's no time for a bulk charge? it's possible depending on the actual loads he has, but these would be nearly non-existent on a day to day basis. the measured voltages would roughly indicate fully charged batteries, but then again we don't know what agms he has and their specs to say for sure.

my suggestion to the op is to actually measure the loads with a killawatt meter, or estimate the kwh used if some are not measurable, to verify that your system is far out-doing the loads to give you such long float times. if it is a case of you having little bulk/absorb charge times because of extremely light loads then you can safely stop being that conservative or even add some more loads.

Derik · February 2011

Re: Long Float times, buy more batteries ?

Sorry I didn't mean to seem like I wasn't calm.

In the summer my system will make all the power I need in just 3 hours and I can drop my absorb time down to 1 hour. I would guess my float time at that time is way over 400 minutes. So he's got more panels and uses less power and his system is more effeicient then wham he's where I will be in two-three months or so. My fridge in the summer runs less than 25% of the time I bet his at 7,000 feet runs less than that.

I love this site for all the usefull information but to me solar is all about sun and how much you have, and the losses many post here I don't see\ and are not what I experience in real life with my system at my house or my other small system at my little cabin.

His question unless I mis read something was hey I am making all this power should I add batteries.

Anyway sorry to come across like I am up set or mad or anything. I am calm and happy as a clam running lots of power, keeping warm watching a 52 inch LCD with lots of lights on knowing that my system can pump out over 12000 watts of power tomorrow if need be.

Catch me on a week without sun I may be not so calm!

I don't want to brag too much maybe next month I'll have some issues or something but so far my system has produced way more power than what I could ever imagine and I think that's his issue as well.

Derik

niel · February 2011

Re: Long Float times, buy more batteries ?

glad to hear you aren't getting worked up.

it may be a case similar to yours and i hope this is so, but right now we don't know that just yet as we just can't say for sure nothing is wrong.

as to adding batteries, that was him asking if his system wasn't quite right if that it needed the extra batteries from what i gathered and i can say adding batteries may not be advisable until the first concern is properly answered. if it is a case like yours i can say he won't be needing batteries.;):D

BB. · February 2011

Re: Long Float times, buy more batteries ?

Derik,

As long as you are pretty sure the battery bank is getting over 90% state of charge--Then you seem to be doing well.

My only other battery related question is how much water do you have to add... Little to no water, crank up your absorb time.

If you are adding water every two months or so, then you are probably charging the batteries OK.

In general, we (I) try to be conservative with our numbers here... Not 2x conservative, but 10-20% conservative.

Also, I try to account for aging too... Panels may lose 10-20% over a decade or two, flooded cell batteries tend to lose 10% (down towards 80% efficiency) at the age. And as panels heat up, you can lose another 5-10% or more power too (summer vs winter).

Also, how you charge your batteries affects efficiency... As you charge them past 90% state of charge, the efficiency goes down (generating H2 and O2 gases).

Being in the high desert sounds like a good deal for you.

-Bill

jeffkruse · February 2011

Re: Long Float times, buy more batteries ?

I re-read the original post. You have AGM batteries so you can’t check the SG. I would suggest a battery monitor.

Your system seems to be great, so did mine when I first set it up. I used the default charge parameters and I to would go into float early in the day. That lead me to believe I could make a lot more power than I used. Then I got a hydrometer and found out I was undercharging my batteries! I don’t want the same thing to happen to you.

Derik · February 2011

Re: Long Float times, buy more batteries ?

I didn't see where he had AGM and can't check his batteries, but now I do.

I check my sg frequently and have had to adjust my absorb time accordingly. This is my first year with my system and it looks like I will be making seasonal adjustments to my absorb time.

Anyway I think I was a bit over sold and they used the 50% loss idea, but am happy with what I have and like the extra power for those days when I have visiters or need to use my energy hungry porpane heat.. the fan uses over 600 watts. Easier to install everything from the start rather than add later for me.

I think the cool weather here really helps as well so some of the loss in the shoter days are made up with the cool weather and cloudless skies. It's 7:30 a.m. and I am making almost 1,000 watts of power already and I have another 9.5 hours of sun!

Thanks for the help and the great resource of information.

tr0y · February 2011

Re: Long Float times, buy more batteries ?

Hi All -

Wow thanks for the lively discussion. The real reason I was asking this is I do feel like I do not have an long enough autonomy period and of course batteries are the way to do this. While it doesn't sound like I have enough PV to add another string of 8 305 Ah Batteries as it sits ( I am using Concorde SunXtenders) I was thinking now maybe add 10 more panels and 16 more batteries ???, this seems as if it may keep me pretty well in balance and maybe even move to an electric water heater or less wood in the winter ?

As for state of charge we have gone out of town for weekends and just to test that I have shut down the loads from the house, PV array and inverter so the batteries would be idle for a reasonable period, about 40 hours in the last test, and the standing voltage when I tested before firing the house back up was 51 to 51.2 on my digital volt meter, my outback mx60 book says 50.4 is considered full and 12.8 x 4 for 48v calls 51.2 full, with that I am assuming I am getting a full charge daily ?

BB. · February 2011

Re: Long Float times, buy more batteries ?

Efficiency wise--A heat pump type water heater or high SEER mini-split heat pump (AC and Heat) would be around 2x as efficient vs just using a resistance heater.

Certainly would make your power go further...

-Bill

Vic · February 2011

Re: Long Float times, buy more batteries ?

Hi tr0y,

Regarding what Voltage indicates full charge on your AGMs, as noted in a previous post, the only data that matters at all is what the manufacturer of your batteries says about your exact batteries.

The data in manuals of any other Solar equipment is only a broad bursh generalization. The battery manufacturer has the only meaningful data.

AGMs are a very good solution. To me, however, the main disadvantage of AGMs is that the SGs cannot be measured. Like Derik, I read my Pilot Cell SG almost every day, and would feel lost without this ability -- just me.

I, too, am in the camp of needing to consider what to "waste" power on, while in the last half of Asorb, and in Float (where my system usually spends hundreds of minutes per day).

I will soon add a resistance water heater, probably running on the 106 VDC PV output.

If I'm around the cabin during mid day, will often micorwave cook some food for later, or perhaps use a resistance heater in the winter etc to consume some of the free energy. I do need to automate this use of XS power, tho. In the summer, have a small A/C unit which cools the power room during the heat of the day, and also run A/C for the cabin to burn more free KWh.

It does seem that your system is currently fairly well balanced, and adding additional batteries or PV should not really be required with your present solar input situation.

Vic

icarus · February 2011

Re: Long Float times, buy more batteries ?

I'm sort of missing the point of this thread here. One already has over capacity as evidenced by being in float too long, and yet now he is considering adding more PV and more battery? Seems counter intuitive.

Tony

tr0y · February 2011

Re: Long Float times, buy more batteries ?

I have quite a bit of extra capacity if the sun is out daily, but gray days concern me. I was only considering more of a bank to deal with the rare week of overcast weather or heavy rain / snow.

The original question was with 6 to 8 hours a day of my charge controller in flat mode was could I add another string of 48v 300AH batteries and would they charge, it seemed like the answer was I would knock the system out of balance, that was the only reason I thought about more.

I am just a novice asking questions

T

Cariboocoot · February 2011

Re: Long Float times, buy more batteries ?

How do you feel about flipping switches?

You could add additional battery capacity with no additional panels if you don't mind keeping an eye on things and remembering to alternate the two banks. with a controller like the OB units you could program the AUX function to fire a self-latching relay and switch the controller's output from one bank to the other when Float is reached. Then have one of the famous BlueSky dual battery switches to be able to make use of 'A' or 'B' on the inverter.

Just a thought.

(Amazing! My ISP has stayed working long enough to post this!)

techntrek · February 2011

Re: Long Float times, buy more batteries ?

I like that idea. Also side-steps the problem of 2 battery banks with different ages. When I finally get off my butt and get a solar system going to charge my existing battery bank I'll have to keep this in mind.

Downside is you loose out on some of the Peukert effect since you won't/shouldn't be drawing from both banks at the same time.

Long Float times, buy more batteries ?

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