Trying a New Bulk / Absorb / Float Charging Scheme with the Classic

[Matrix]

I have mentioned this before on several forums and even called Trojan.   Since my L16's went into service last Oct 17 I have not been able to get the SG to rise to the fully charged level of 1.277 - Unless - I use really hi absorb volts,  like upwards of 31.3v with a 24v bank.   (Trojans recommended is 29.6 - but i realize that is for AC charging).  But I seemed to have never been able to use anything lower than 30.8v (in the winter) and higher in the summer.  And using 2% as my end amp,  especially if the voltage is closer to the recommended absorb voltage also would leave me short of a full charge according to SG.   I would usually have to set the end amps to 1% or less.  And raise the voltage, currently up to 31.5v (but it never gets there because of temp comp).  Charge temps have been 33-max 33.5C this summer.

Trojan said even charging at 31.5 + was not really a problem.   That the L16H-AC batteries could handle 32 volts daily if needed.  (but that seemed like a way to sell batteries to me??)   But at lower volts I was not using any water at all at say  29.6v (although it was "boiling"). 

So up until this week I had been running 31.4v for absorb with temp compensation (for summer heat) and an end amp of 3.5 amps for a 435 Ah battery bank for about 3 hours or more  a day in absorb (this being after bulk had finished).  The CC would switch over to absorb at about 88-91% and then the charge would continue at this hi absorb voltage. 

Problem is,  the battery temps where getting hi.  Like 33.5 C sometimes in charging.  Has never gotten higher than that even with ambient temps of 95+ F.

Anyway,  I had this idea the other day.   Maybe it makes sense?   

To keep the bulk charge on as long as possible,  pushing the most amount of amps into the batteries for the longest amount of time (say up to 92-94% SOC) but without hi volts for long periods of time,   I decided to:
1.  Set absorb to 31.8v
2. But only allow the CC to stay in absorb for 15 minutes or less on the timer (setting the timer for 15 minutes),  and set the end amps (with WBJ) to 6.5 amps.  This terminated absorb really quickly, but allows the Classic 150 to stay in bulk much longer at lower volts.
- in bulk the volts build very slowly but the amps try to stay constant if the sun is there
- And the temps do not rise on the batteries as much - like 31.5C or less

3. Then Almost as soon as the CC hits absorb voltage,  it does not stay there but for a very short amount of time, about 15 minutes or less.

4. Then I set Float to 30.5v
5.  Thing is,  the CC hits float about 1 pm in N Central FL,  So then it will stay in float at this voltage the rest of the afternoon ... just like a nice long soaking absorb of about 4-6 hours (this time of year)

I have now been tweaking the float voltage (like it was absorb voltage) this past week to see where it needs to be set to achieve 100% SG.  Seems to be coming out around 30.5 v  or less (much lower than it was taking in absorb)

So I am basically just using abosob voltage as the change over point between Bulk and float,  but by setting the absorb voltage hi and the timer short,  the CC does not hit absorb until the batteries are nearly 92-95% charged. 

Result has been the batteries have charged cooler and it seems the system has had a much easier time getting to 100% SOC - albeit that it takes most of the day - but it seems to let the batteries just soak a whole lot more gently at lower over all voltage and daily charging temps (by about 1-3* C).  I am can see this on the MyMidNite page. 

Does anyone see  a problem with this idea and charging like this? 

Mike

[Matrix]

Why not just set absorb to 30.5 and set the timer for 4-5 hours you might ask?   Two reasons:

1. Bulk terminates much sooner,  thus charging amps start to tapper at an earlier SOC

2.  Heat,  I get less heat in charging with what is described above. 

[ClassicCrazy]

What do you use to check your specific gravity ?
Have you ever calibrated or checked that to make sure it is accurate ?

Larry

[Matrix]

Hi Larry,

While never actually calibrating a hydrometer with calibration fluid, I actually use 4 different methods. I have:

2 bulb type glass temp compensated float hydrometers

1 hydro volt

1 refractometer

They all 4 agree within 5 points +/-

I use the hydro volt as my go to for the large number scale and ease of use. and "knowing" that it is 5 points higher than my refractometer and glass units I simply subtract 5 points.  But I test it regularly against the other methods to confirm accuracy. In each of the 4 methods my SG levels are repeatable and consistent.

And as Vic often says, I rinse, rinse rinse immediately after each use.

That is the best I feel I can do

[ClassicCrazy]

ha ha - sounds like you have it covered !

[Matrix]

ha ha - sounds like you have it covered !

Never said I wasn't OCD. 

[keithwhare]

Any update on how this charging scheme is working for you?

Keith

[Matrix]

Any update on how this charging scheme is working for you?

Keith

Hi Keith,  Yes it does work.  But I never got anyone to dialog with me about "is it healthy".  So I still do not know.  But it seems reasonable based on  my experience of lower temps and lower charge voltage that it would be OK to do it that way.  Its not like my array in bulk charge can produce C5 for a charge current (435ah / 5 = 87amps)  The highest I get for a charge current is about 75amps and that is never sustained.  I average about 40 amps for a day during peak solar times.  C8 for my bank is 54 amps.  So not concerned about too much current and either way, if it was a problem temps would get hi which they are not.

So short answer,  Yes it does.  But,  it seems the shorter the days get,  I may run into trouble again using float as my absorb.  But if that happens I can always start tweaking the float voltage upwards ... or even just increase the time on the absorb clock to longer,  like 30 or 45 minutes. And then back to the lower float voltage. 

I think annually from season to season as the sun changes,  solar charging is always a knife edge balance and must be tweaked to keep up with the changes. 

But for now, (since I actually made this post),  I am having less issue.  at the time of this post I was discharging down to between 70-55% DOD.  I have since installed an auto transformer,  allowing my AC grid to help support the loads from the battery.   and now I discharge no more than 85% daily.  So fully charging has become less of an issue.    I still use the solar because of a diversion load.  My water heater,  which has not been connected to the grid at all since June.  And with the autotransformer, I get a full 40 gallons of water now almost daily at 160* F  which is plenty for us thru the nite.  So I am still making between 6-11 kWh of power a day,  but rather than it going to the batteries for charging,  now a good part of it goes to heat water.  Saves on the electric bill AND extends the life of the batteries .

Here is the discussion of the Autotransformer purpose and install

[keithwhare]

Thanks for the update. The point about battery temperatures caught my attention because I helped install a PV system at a clinic in Haiti. The ambient temperature there can be pretty warm, so a charging scheme that doesn't heat the batteries as much could be useful.

Keith

[Matrix]

Exactly.  Here in Mid-Florida,  we have seen daily temps around 95+ and not getting much below 76 a night.  Batteries even at night never drop below 26.5 C even with a fan.      So Yes that scheme will help with lower charging temps in my experience.  If you have a chance to try it,  let me know how it works out in Haiti