250 watt panel with Morningstar SunSaver MPPT 15 AMP controller

[Sunking]

but whatabout the max charge rate for an FLA batt. How does it affect battery health if exceeding C/8 for any length of time?

Exceed C/8 by how much? C/8 is not a hard fixed number.

[FloridaSun]

Exceed C/8 by how much? C/8 is not a hard fixed number.

'hard fixed' sheesh, no kiddin, when it comes to all the variables in batteries, well, that's why I depend on your knowledge, Dereck.
say pumping 15A into a 100Ah true deep cycle battery. How much of a surface charge is wasted before a longer term finishing charge really brings the batts to good health?
Feeling the batteries the heart of any (off grid) system I tend to work from there, planning everything else around them. I'm sure a more than C/8 charge would do no damage to my hybrids but thenagin they aren't going to last like the true deep cycle that I want to work up to.

[inetdog]

'hard fixed' sheesh, no kiddin, when it comes to all the variables in batteries, well, that's why I depend on your knowledge, Dereck.
say pumping 15A into a 100Ah true deep cycle battery. How much of a surface charge is wasted before a longer term finishing charge really brings the batts to good health?
Feeling the batteries the heart of any (off grid) system I tend to work from there, planning everything else around them. I'm sure a more than C/8 charge would do no damage to my hybrids but thenagin they aren't going to last like the true deep cycle that I want to work up to.

Since you get power from the PV when you get it, "wasting" some of it by using a higher than optimal charging rate is not going to hurt you at all as long as you do not damage the batteries in the process and do not have other loads you could be using the extra power for.
In terms of not getting the extra capacity in the first place, it comes down mostly to planning for the worst PV conditions and deciding how much you want to spend to reduce generator use at those times.

[Sunking]

say pumping 15A into a 100Ah true deep cycle battery.

That is not going to be a problem.

Let me try to explain. You can charge at as high of a rate as you want providing you stay just below the GASSING VOLTAGE of the battery. That is the point where over heating and damage starts to occur. Once you hit gassing voltage corrosion of the plates begin and the battery starts using excessive water because you are electrolyzing the water turning it into hydrogen and oxygen. For example when you apply a Equalization Charge you are into the Gassing Voltage stage. An EQ charge is a controlled over charge, and should only be performed when warranted because it does damage the plates with corrosion. Gassing voltage depends on battery type and chemistry. Since you never want to gas gels or AGM leaves flooded and the voltage depending on being pure lead, lead-antimony, or lead calcium varies from 2.3 to 2.45 volts per cell. Understand so far?

OK to reach gassing voltage depends on the battery internal resistance and how much current we are pushing through it. Remember Voltage = Current x Resistance. So let's take a typical Deep Cycle battery. It has been rested and the voltage reads 11 volts. We have a fully discharged battery right? The battery is 12 volt 100 AH. So we take a rectifier that can provide 100 amps of charge current that we can set anywhere from 1 amp up to 100 amps with a source voltage of say 17 volts.

So we connect the charger up and start increasing the current until we measure 14.4 volts at the battery post and note the current at which point the battery starts to gas. We notice the battery liquid is boiling and smell that rotten egg odor. We are gassing the battery and charging too fast. At that point we record the charge current and it reads 16.6 amps or a C/6 charge current. Armed with that we can determine the battery internal resistance is [14.4 volts - 11 volts] / 16.6 amps = .21 Ohms at 100% DOD. So now we back off the current to read 14.3 volts on the battery just below Gassing Voltage and notice the boiling stops and the smell goes away. We then note the current and it reads 15.7 amps or a C/6.4 charge rate. We just found out this battery can take a maximum C/6.4 charge rate. So we shop for a 15 amp charger and set Absorb/Bulk to 14.3 volts and Float to 13.6 volts. That will charge the battery as fast and as efficiently as possible without any damage or excessive water use.

OK the takeaway here is for True Deep Cycle Batteries a safe rule of thumb is a max charge rate of C/8 because True Deep Cycle batteries has one disadvantage, they have somewhat high internal resistance compared to starting and hybrid batteries. This is because True Deep Cycle batteries have fewer but much thicker and heavier lead plates than their starting and hybrid cousins which reduces surface area thus higher resistance. The gain of course is with thicker and heavier plates the battery has many more charge/discharge cycles.

Here is a trick which you will rarely find in Flooded Lead Acid batteries. Specifying internal resistance. They really cannot because it changes with state of charge and age. If they did you can then determine the maximum charge rate. Some of the higher quality batteries like Rolls will specify a maximum do not exceed charge rate like C/6 for most efficient charging which tells you the internal resistance at 100% DOD.

Hope that helps.

SK

Edit Note:

After thinking how this might come accross to some, which is somewhat logical, is to think OK I can exceed charge rates so long as I set the Bulk/Absorb just below gassing voltage and set Float to 13.6 I can get away with charging faster.

Problem is it will not work like that. Say for example you have that same 12 volt 100 AH battery with a 750 watt panel that generates 50 amp charge current or a C/2 rate. Th every instant you hit the battery with 50 amps the voltage goes above the Bulk/Absorb set point and switches to Float immediately because the charger is fooled into thinking the battery is fully charged when it is not. The batteries will still charge using a constant voltage current taper algorithm instead of the faster constant current method of Bulk.

[FloridaSun]

Since you get power from the PV when you get it, "wasting" some of it by using a higher than optimal charging rate is not going to hurt you at all as long as you do not damage the batteries in the process and do not have other loads you could be using the extra power for.
In terms of not getting the extra capacity in the first place, it comes down mostly to planning for the worst PV conditions and deciding how much you want to spend to reduce generator use at those times.

hmmm... 'wasted' in the sense that a battery charges more efficiently with a longer slower charge rather than cramming in a buncha amps in a short period. With my minimum of 4.6hr winter insolation and the price of panels I could care less (well, sorta) about wasting max energy coming from the panels. With the high price of $ @ watt a few years ago I can see the advantage of using them to their fullest but I'm thinking of max battery health and longevity right now, not exceeding C/8 charge and spreading the charge time over cloudy periods.

[Sunking]

hmmm... 'wasted' in the sense that a battery charges more efficiently with a longer slower charge rather than cramming in a buncha amps in a short period. With my minimum of 4.6hr winter insolation and the price of panels I could care less (well, sorta) about wasting max energy coming from the panels. With the high price of $ @ watt a few years ago I can see the advantage of using them to their fullest but I'm thinking of max battery health and longevity right now, not exceeding C/8 charge and spreading the charge time over cloudy periods.

Most efficient charging is at a high of charge rate as possible without exceeding the gassing voltage.

What make and model are your batteries?

[FloridaSun]

Most efficient charging is at a high of charge rate as possible without exceeding the gassing voltage.

What make and model are your batteries?

My present 'beginner' batts are the cheap walfart, group 29, marine hybrid, supposedly made by johnson I think. Impossible to find factory specs on them but store rated 120Ah @ 120 hr which puts them around 100Ah @ 20 hr rating I figger. I've never smelled any gassing even at the 31 volt equalize stage but incoming amps are low now at a lil over 16.5A from TS-45 PWM CC. (The cheapo MPPTs I was using before would put over 21A at times from my 580W of panels.) My underused 24v, 200Ah bank never wakes up in the morning with less than 24.6v and has never gone to 50% discharge or lost cell balance.
My solar experience all started last summer when a neighbor bought a harbor fright kit. ...oh yeah? I bought one too (on sale, $150) just to recharge a new 18v tool set I bought. Worked fine but.... durn it, started reading on this forum and got interested in building a decent system. Sold the 45w kit to my neighbor for a good price ($125!) and been playin ever since. After my $25 45w learning kit I went to 290w panels 12v, 200Ah batts and quickly went to 580w panels, 24v, 200Ah to keep the 4 batteries fairly well matched. PWM to MPPT back to a quality PWM and will go to a TS-MPPT eventually. haha, eventully I may build up a decent system when time allows. All fun n games for me, retired but taking care of three houses on almost two acres so plenty of other things taking up my time. When these batteries give out I'll go 6v Trojan in series but durn sure gonna know what I'm doing before that investment.

Hope that helps.SK

Thanks for the explanation Sunking.... Batteryking. That Edit Note: explained much! I think I'm getting a better grasp on this subject now.

[solar.t]

Using SolarT example. He has a 250 watt panel with a 15 amp current limit. With say a 5 Sun hour day the panel delivers 1 Kwh to the batteries. However if he had used a 20 amp controller he would have 1.25 Kwh, so he gets short changed. I understand over sizing a panel on a MPPT stretches harvest time, but but we are only talking sun minutes, not hours. Common design practice is to over size 10% PTC rating. SolarT is at 25% robbing himself, meaning he would get substantial more harvest if he used a 20 amp controller.

And I would buy a 20 amp MPPT controller if they were available. I could be wrong but looking at the quality brands of Morningstar, Outback and Midnite after the 15 amp Morningstar MPPT the next available option is a 40 amps Midnite at more than double the cost or 45 amp Morningstar at nearly double.

My need was for higher voltage since the panel will be a distance away from the battery bank. Being able to purchase a single higher voltage panel with free shipping and using the Morningstar 15 MPPT amp just prices out better than other alternatives. I guess I could have saved a few bucks with a smaller panel but after reading that information from Morningstar, better power harvest on cloudy days, I'm feeling good about my configuration.

From the Morningstar PDF, this viable reason applies to me:
• Viable reasons for using an array rated for a wattage greater than the controller maximum:
o The availability and lower cost of 60 cell modules (Vmp~30V) from the Grid connected solar
market.

If I had sized everything to perfection to get my 3-4 hours @ 15 amps to bring my 225 amp battery bank to a full charge each day, and taking into consideration the distance from array to battery requiring an MPPT controller, I think the cost would have been substantially higher.

[Sunking]

Thanks for the explanation Sunking....

You are welcome. You are going about it the right way. If those are Marine Group 29 batteries you can charge them at C/4 no problem. They are made to use the Alternator which likely supplies more than C/4.

As for the Trojans upgrade let's talk about that later when the time comes. Might have a better idea for you.

[FloridaSun]

You are welcome. You are going about it the right way. If those are Marine Group 29 batteries you can charge them at C/4 no problem. They are made to use the Alternator which likely supplies more than C/4.

As for the Trojans upgrade let's talk about that later when the time comes. Might have a better idea for you.

Ya, figgered the hybrids would go much higher than C/8. Would be interested in your better idea but that can wait.... tho some others on here may benefit from it at present.

[Sunking]

From the Morningstar PDF, this viable reason applies to me:
• Viable reasons for using an array rated for a wattage greater than the controller maximum:
o The availability and lower cost of 60 cell modules (Vmp~30V) from the Grid connected solar
market.

You did just fine. FWIW the justification for MPPT controllers is you can use Grid Tied Panels vs Battery panels for PWM. Battery panels are 36 cells and almost twice the cost of Grid Tied Panels. GT panels come in 54 to 80 cells ranging in power from 200 to 320 watts. At 36 cells yields 18 Vmp which is PWM voltages.

54 cells @ 27 volts
60 cells @ 30 volts
80 cells at 40 volts

So the controller you have can use any of them from 36 cells to 80 cells.