Is There a Point to Float with LA/Liveaboard/Solar

[Dsanduril]

The contention on most of CF seems to be that with solar boats rarely get to 100% state-of-charge (SOC), particularly on lead-acid (LA) batteries. Of course defining 100% SOC is one of the questions, I generally see something less than 1% battery capacity (C, measured in AH, while end-Amps is measured in A), with 0.7% and 0.5%C being pretty common at absorption voltage (as recommended by the battery manufacturer).

On our own boat, with a fairly large solar array and fairly small daily usage we find that we can get to end-Amps between about 2 and 4 in the afternoon (as long as we've had good, sunny days). So, the question is, is there really any point to dropping back to float for an hour or two before the sun goes down?

As liveaboards we know there will be usage immediately after sunset, and even if we're away for a few days the fridge will run things down some overnight. Given that usage (where I know we won't be better than 90% SOC in the morning) is there really any point to float? Since the contention is that we don't reach 100% SOC as often as we should, are we better off 'floating' at absorption voltage for an extra hour or two (at the most) and making sure things are really up at 100% when we can get them there? Or would that irreparably harm the batteries?

I understand float for boats that don't draw down much overnight (moored, docked, weekend use) but for the 'use power every single day' crowd would we better if we ran absorption even down to 0.1%C or something like that (when sunlight is sufficient) rather than dropping to float at the end of the day?

[CatNewBee]

Well, this is done by the controller anyway...
It depends on yout batteries and their inner resistance and capabilities I would say.

GEL batteries have a higher resistance due to the gel inside, the current are not as high as comparable AGM or FLA. That requires longer absorption cycles you probably wont reach during the day ( 6 or more hours ). AGM batteries are like wet batteries, they charge faster and need shorter absorption ( 1-2 hours are sufficient ). Same for SMF FLA. Too long absorption put some stress on the battery, they warm up and can potentially start boiling, building up pressure inside and release it through the safety valves. that leads to irreversible loss of fluids inside and shortens the life too. Open FLA can be re-filled with destillated water, and they do need some gasing at the end of the charge to balance the cells and prevent sulphation of the surface. From time to time they must be overcharged in a maintenance regime called reconditioning. Do not do it too often, this is also stress for the battery.

So there is no absolute answer for the problem. I would recomment do stay with the settings according to your battery type WET / AGM /GEL and not override the absorption cutoff point manualy.

[conachair]

Originally Posted by Dsanduril

Given that usage (where I know we won't be better than 90% SOC in the morning) is there really any point to float? Since the contention is that we don't reach 100% SOC as often as we should, are we better off 'floating' at absorption voltage for an extra hour or two (at the most) and making sure things are really up at 100% when we can get them there? Or would that irreparably harm the batteries?

I have float set to absorption as well , 14.8v on t105's. Get back to full most days but I think you're right, most controllers switch to float far too early certainly with default settings.
There was a paper release a while ago but unavailable now recommending even higher absorption voltages of 15.3 for off grid installations - less damage from high voltage than from not being really fully charged.
OutBack Power Technologies User Forum • View topic - L-16 recommended charging voltages

In general from the off grid forums the feeling is that charge them high a long is less damaging than not getting back to actually fully charged. Takes a loooong time..

[LeaseOnLife]

Originally Posted by conachair

I have float set to absorption as well , 14.8v on t105's. Get back to full most days but I think you're right, most controllers switch to float far too early certainly with default settings.
There was a paper release a while ago but unavailable now recommending even higher absorption voltages of 15.3 for off grid installations - less damage from high voltage than from not being really fully charged.
OutBack Power Technologies User Forum • View topic - L-16 recommended charging voltages

In general from the off grid forums the feeling is that charge them high a long is less damaging than not getting back to actually fully charged. Takes a loooong time..

I am in the same camp, with Lifeline AGMs, all solar controllers are set to Vfloat = Vabsorbtion. The only controller that does go to float is for my alternator in case I motor for a long time.

[Adelie]

If you stay at absorption voltage past 100% SOC then you are boiling off/electrolyzing water without adding any energy to the battery that you can later withdraw.
Sort of like your toilette tank shoreside, you can set the float valve so it cuts off above the top of the tank, but all that extra water doesn’t add to flushing force, it just gets the floor wet.

[john61ct]

Originally Posted by Dsanduril

we find that we can get to end-Amps between about 2 and 4 in the afternoon (as long as we've had good, sunny days). So, the question is, is there really any point to dropping back to float for an hour or two before the sun goes down?

Yes in that specific scenario.

But keeping Absorb going past endAmps doesn't accomplish anything. Some chemistries are sensitive to overcharging. And you need to remember to change the profile when not actively cycling.

[conachair]

Originally Posted by Adelie

If you stay at absorption voltage past 100% SOC then you are boiling off/electrolyzing water without adding any energy to the battery that you can later withdraw.
Sort of like your toilette tank shoreside, you can set the float so it cuts off above the top of the tank, but all that extra water doesn’t add to flushing force, it just gets the floor wet.

Is your floor getting a bit wet worse for the battery than rarely actually getting it full?

[john61ct]

If you really want precision, get charge sources that go to Float controlled directly from BM's monitoring endAmps.

But yes, getting reliably to 100% at least a few times a week is critical.

Going a bit over long at low amps does no harm to robust FLA, just top up water more often.

[Adelie]

Conachair: Was that meant to be funny or was that a serious question that inadvertently mixed the circumstances of my analogy with the details of the actual question asked?

[a64pilot]

A couple of things may be incorrect, like the assertion that an hour or two at absorption is enough for a Lifeline bank.
I don’t find that to to be true.
Then the thought that you’ll boil off your electrolyte. That’s true if it goes on for a Long time, longer than achievable by Solar, an ideal world is to have a charge source that will trip to float when a measured absorption is reached, I have one source that will do that, but my other three won’t, so it’s not common, but a feature Id like to have, it takes a shunt of course.

Finally I will leave you with the thought that for 660 AH bank, “full” is less than .5% of rated capacity, that’s 3.3 amps of course, for a Lifeline, others will vary a little.
3.3 amps or less spread out over a half dozen batteries weighing about 300 lbs of so isn’t enough heat to do much, (47 Watts) now if it continued for hours on end, yes it’s damaging, but an hour or two?

Worst case is probably an AGM battery or Gel maybe cause you can’t top up electrolyte, however AGM’s are recombinant gas, meaning by some magic that I can’t explain the hydrogen recombines with O2 to form water and as long as the vent valve doesn’t open, you don’t lose electrolyte?

I assume it works cause Lifeline recommends equalization, and that is a whole lot more voltage, held for a whole lot longer, and yet doesn’t harm the battery, or maybe what harm it does is less harmful than the sulphation it corrects?

Bottom line, in my opinion dropping to float at the correct time is the ideal situation, however maybe staying in absorption for an hour or two extra may be less harmful than dropping into float prematurely day after day, and if you have a flooded bank, then of course you can top off the electrolyte as necessary

[a64pilot]

I have a kilowatt of Solar, and had to leave the boat on a mooring for an unknown period, only having an Engel running, nothing else.
Then I set the Outback 80 to drop to float, figuring that the Engel alone wasn’t enough draw to prevent excessive overcharging, so it depends as does everything I guess.

[john61ct]

Originally Posted by a64pilot

A couple of things may be incorrect, like the assertion that an hour or two at absorption is enough for a Lifeline bank.

Yes that would very rarely be true.

OP did show awareness that endAmps is the true measure of Full.

Bottom line for Lifeline is Float should come after .005C trailing amps but hitting that a few times per week is enough.

Equalization is a relatively rare maintenance procedure, I would be wary of long overcharging on a daily basis.

Obviously the OP main point is valid, that while actively cycling, sundown prevents that.

[donradcliffe]

If I was the OP, I'd reset the float voltage to the absorption voltage, then monitor the water usage of the batteries for a few months.

[chris95040]

Originally Posted by Adelie

Conachair: Was that meant to be funny or was that a serious question that inadvertently mixed the circumstances of my analogy with the details of the actual question asked?

No, I think Conachair understood you perfectly. They were asking, is the battery analog to "getting the floor wet" (ie, continuing to float even when it's at 100%) causing bigger problems than perpetually undercharging the battery?

Its a good question, but I imagine there is no "right" answer, the answer depends on the degree of each problem in a particular setup.

[Adelie]

The OPs question asked whether ther was a problem leaving the charger at absorption voltage for several hours (2-5) after reaching 100% SOC? 100% SOC defined by having charging amps drop to about 0.5%C. This assumes no concurrent house loads.

If you continue absorption voltage for those 2-5hr/d then you will see increased water demand over time. So instead of checking water levels every month you need to be checking them every 2wk or even every week.

And if you leave the boat unattended for a period it will be overcharging all day every day unless you remember to switch charging to float. That’s the point where damage due to plate erosion or overheating can occur.

If the batteries haven’t gotten to end-amps then the controller shouldn’t be kicking into float voltage anyway and it should continue at absorption voltage until sundown.