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Old 13-05-2016, 09:46   #1
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Float mode on solar controllers?- why?

We live on board full time, with a house bank of 630Ah lifeline agm, 4 x 110 watt solara panels in parallel each with its own victron 75/10 mppt charge controller.

My question regards float mode on solar chargers, ie is it required when the batteries are in constant use?

I am finding that the victrons jump into float mode when they put out below 1amp each, and unless we use a big load for over a minute (boil a kettle), it stays in float, and the batteries cannot quite get up beyond 98% ,in daylight hours, I have set an absorption time of 4 hours but the 1 amp limit negates that. So just before float mode kicks in there is around 4 amps going in, but when float voltage starts, amps drop to around 1 amp going in right away, 3 amps lost instantly.

Our batteries never go below 90%, and most of the time they read around 93% first thing in the morning, the charging starts out great and i can see 29 amps on a good day.

The lifeline batteries ask for an absorption of 14.3- 14.4 we have 14.4v , the float is 13.3-13.4 v we have 13.4v.

But to my mind, float is a mode used to maintain a battery when it is in a very light discharge, not really a charging mode at all.

I am tempted to up the float charge to nearer 14v, say 13.8v and given that solar only works for max 12 hours anyway, it would only be at this voltage for say 6 hours max. In effect a "lower absorbtion voltage mode"

Any problems with this

I use xantrex monitors to keep an eye on all this, and i did synchronise them whilst in a marina a month ago after 2 days dock side charging with a/c.

C.





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Old 13-05-2016, 09:58   #2
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Re: Float mode on solar controllers?- why?

I have the Midnight classic Mppt controllers and use my array to pull my onboard office, fridge, and entertainment center.

Through the monitoring software of the controllers, I can see that when they hit Float stage, they are still kicking out plenty of amps to run the load, so the batteries have no discharge. I've verified this via a Link monitor. The batteries have an inflow of about 1 amp or less while the controllers are producing on average 30amps to run all the goodies.

The float mode prevents cooking the batteries while still providing the amps needed to run everything


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Old 13-05-2016, 10:08   #3
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Re: Float mode on solar controllers?- why?

Hi neptunesjester,

Thanks for that info, so are you saying you upped the float voltage in each controller to help produce more amps?

As i was writing my post, it all became clear to me, what the problem is! -

If i use 1 controller for the array, it will put in charge until it reaches 1 amp and then the bank is full, all well and good.

But, if you use multiple controllers to the bank, they each go down to 1 amp then cut to float mode, so 4 controllers, 4 amps, then under 1, total when going from absorption to float, this is my problem, i am not getting that last bit of top up because i use multiple controllers?

I am happy with the multiple set up as i have shading issues, and this has sorted that out, i just need to get that last bit of juice some how!

Higher "float" voltage?

C.


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Old 13-05-2016, 10:28   #4
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Re: Float mode on solar controllers?- why?

My setup has two controllers in parallel. Both are set with a float of 13.8V. These controllers make the stage decisions based on Battery Voltage. Even with no load, once float stage is entered, they ramp down current to maintain float voltage and if it climbs, one controller/both will go into rest mode until voltage drops back to the float range. The controllers are constantly adjusting the output to maintain the13.8V Float. As a fellow liveaboard and full time telecommuter, it is the rare occasion when there is no load, so they dont go into Rest mode all that often during daylight hours. And to be honest, the Link system itself is a very small draw so there is never truly a no load situation.

I dont have experience with your particular controllers to know how they handle no load float situations. I would think they would have an automatic cutout to prevent climbing over the set float voltage once that stage is reached.


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Old 13-05-2016, 10:32   #5
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Re: Float mode on solar controllers?- why?

I'd leave it in absorption, or change float voltage to absorption voltage.
Reason is I have had several, very knowledgeable people tell me that you just can't get to 100% full battery charge on Solar only, and in fact what is happening is that you are slowly walking down the charge on your AGM batteries, something that will greatly reduce their lifespan.
You need to equalize them somewhat frequently too, like monthly?
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Old 13-05-2016, 11:01   #6
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Re: Float mode on solar controllers?- why?

Thanks a64 pilot,

Walk down was my main concern, so thanks for verifying that. I reckon that is a problem alot of people do not know they have.

Upping the float to absorption voltage does make sense, it just feels a little odd! But living onboard makes you do odd things sometimes!😄

I am lucky in that victron controllers are fully programable, i can get right into the nitty gritty With my pc connected to them. I think they are not meant for a constant use situation, but are flexible.

I guess if i do this, i will keep an eye on the system for a few days to make sure things do not get overcharged.

C.


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Old 13-05-2016, 11:10   #7
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Re: Float mode on solar controllers?- why?

Monitor battery temp, if battery temp barely rises from the low it has the night before, then I doubt your doing any harm by keeping it in absorption mode.
Another way to look at is is 4 amps at 14.3 VDC is 57 watts, and that isn't much when you think of trying to heat up a few hundred pounds of batteries.
But an even different way to look at it is I think Lifeline tells you charge is complete when at absorption voltage charge rate is .5% of bank capacity? So for you that is a little over 3 amps, so it sounds like your really getting fully charged off of Solar.
I have a 660 AH lifleline bank myself and 750 W of Solar, but I pull down about 150 AH a night. I don't yet stay full time on the boat, but have consoled myself that I am going to have to live with a couple of hour generator runs a couple times a week to get by and probably have to do monthly equalization, but from the sound of it, your getting by on Solar alone.
How about the winter months though?
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Old 13-05-2016, 11:37   #8
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Re: Float mode on solar controllers?- why?

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Walk down was my main concern, so thanks for verifying that. I reckon that is a problem alot of people do not know they have.
About a year or so into full time cruising is when most cruisers start figuring this out...that they are never returning their battery bank to a full state of charge (SOC). It's the biggest killer of batteries...this undercharge issue.
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Old 13-05-2016, 11:53   #9
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Re: Float mode on solar controllers?- why?

Hi third day,

Yep I agree, it always puzzled me when people told me their batteries go from 80% or so in the morning to 100% by noon, believing your battery monitor without synchronising it regularly can hide the problem. Unfortunatley, you need to go through at least a 24hour charge period from a good 3 stage a/c charger to get to a believable 100%, and as we live on anchor thats not an option.

C




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Old 13-05-2016, 12:10   #10
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Re: Float mode on solar controllers?- why?

Hi a64 pilot,


"But an even different way to look at it is I think Lifeline tells you charge is complete when at absorption voltage charge rate is .5% of bank capacity? So for you that is a little over 3 amps,"

This made me think i was worrying about nothing before i started the post, guess i am getting hung up on the fact that on shore power my victron multiplus takes the bank to 100% at which point there is only around 0.5 Amp going in, on solar i am basically finishing up at 4 Amps still going in.

This is most likely what is meant by solar chargers not being able to FULLY charge a bank (they only get at most 12 hours!).

As regards winter, we are in the med at the moment and have been wintering in marinas ( the horror!) , we ditched our genset as it was nothing but trouble, the extra space is great! We will monitor the situation regards winter power when we get to it, but more solar is on the cards and possibly a suitcase gen if we really have to, but that will be a last resort!

C.


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Old 13-05-2016, 12:15   #11
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Re: Float mode on solar controllers?- why?

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Hi third day,

Yep I agree, it always puzzled me when people told me their batteries go from 80% or so in the morning to 100% by noon, believing your battery monitor without synchronising it regularly can hide the problem.
If I had a dime for every time I heard this I'd be retired. Bottom line is this is almost as close to a physically impossibility as their is. Heck just getting to 100% SOC in one solar day is damn near impossible too and why "premature-floatulation" is killing many banks well before they are due.

The important screens with a battery monitor/Ah counter is the amperage and voltage. Until you hit .5% of Ah capacity (.5A for a 100Ah battery) at 14.4V your batteries are not full and not ready to drop to float. Some folks tend to prefer 1% but again the batteries are not fully charged at this point but certainly better than the 2% or more most battery monitors attempt to use.

Unless you are using a Smart Gauge, forget what the SOC screen on the Ah counter tells you, it is almost never even close. Instead focus on what the net amperage to the bank is, at absorption voltage, and then determine how long you need to set the absorption cycle to last or whether you even need float.

In all of the AGM batteries I have physically cut open the only time I have found "dry" cells, as in being chronically over-charged, was when an owner used an unregulated solar array and left the battery sitting for weeks at a time cooking away at over 15V.

If you are using the bank daily there is likely only a small chance you are getting to 0.5% current acceptance at 14.4V.... If you're never getting there, then float is not necessary for your application and you are suffering from premature-floatulation..
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Old 13-05-2016, 12:26   #12
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Re: Float mode on solar controllers?- why?

Hi mainsail,

"Premature - flotulation" , like it, wait till i tell the wife !!😄

C.


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Old 13-05-2016, 12:35   #13
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Re: Float mode on solar controllers?- why?

Maine Sail will be the one who knows, but my gut feeling is you have more than enough Solar as it is, and more isn't going to do anything.
It's more hours of sunlight that we need, and we can't control that, so it's my belief that a suitcase generator would be your best expenditure, with the idea of running it early in the day before there is enough sun for the panels to be doing much and what your doing is extending the number of hours of charge you have in a day.
You could do it later in the day after the sun sets enough that the panels aren't doing much, but that's when absorption is very low so most all of your generator capacity won't be being used then, but you could use the excess gen capacity to heat water though.
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Old 13-05-2016, 12:38   #14
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Re: Float mode on solar controllers?- why?

Maine sail

Agreed about the state of charge screen on the monitors, so many separate things can upset that.

I guess it makes the most sense to people as they can relate to percentage, amps at 14.4v is great advice, thanks. I am getting around 4 or 5, so under the 0.5% rule guess i am fully charged, but that 4 or 5 amps is bugging me!

C.




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Old 13-05-2016, 12:46   #15
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Re: Float mode on solar controllers?- why?

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Maine sail

Agreed about the state of charge screen on the monitors, so many separate things can upset that.

I guess it makes the most sense to people as they can relate to percentage, amps at 14.4v is great advice, thanks. I am getting around 4 or 5, so under the 0.5% rule guess i am fully charged, but that 4 or 5 amps is bugging me!

C.




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Do you have an 800 - 1000Ah bank??

.5% of 1000Ah = 5A

.5% of 800Ah = 4A

For a 630Ah bank full would = 14.4V and 3.15A

Remember 3.15A net acceptance at 13.4V is NOT the same as 3.15A at 14.4V...... Big difference and the .5% net acceptance needs to occur at 14.4V not float......
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