AGM Battery Charging with Solar

[CJM1973]

This is my first attempt ever at solar so I'm learning as I go.

I've recently installed two 100 watt polycrystalline panels on the roof of my RV. I have a relatively new bank of AGM deep cycle 12 Volt batteries from Weize (three 100 amp hours).

I purchased and use a Victron 712 battery monitor (which works great). I decided to buy a Victron Bluesolar 100/20 amp solar charge controller to harness the panel voltage. The problem I'm faced with is the recommended settings on that controller for my battery type and brand. I'm reading conflicting advice online. Weize has this stickered on each battery:

Cycle use: 14.5 - 14.9V
Stand-by use: 13.5 - 13.8V

I've read that AGMs should be limited to 14.4V during cycle use (I'm assuming this is another name for asorbsion??) and 13.2V for Stand-by use (another name for float charge??)

I've also read that undercharging weakens AGMs more than overcharging. Not sure if this a truly accepted statement.

I'm looking for ideal charging parameters for the longest life of these batteries. In addition, I did purchase the temperature sensor for the Victron SCC.

Any help with these controller settings is much appreciated.

[pianotuna]

CJM1973,

I think dc to DC units are the bomb! Which one do you have and is there a link?

You will want a way to measure the amps going into the battery bank. For three 100 amp-hour jars, fully charged would be an acceptance rate of about 1.5 amps. When that number is reached, it would be nice to move back to float voltage.

[time2roll]

I would leave the voltages, just adjust the absorption time.

My Morningstar controller has three settings based on battery start voltage.
So if the battery is 12.5+ in the morning the absorption time is short like 15 minutes for storage. Below 12.5 and the absorption goes to two hours. Below 11.8 volts and it holds absorption voltage all day. All is adjustable and automatic. Maybe Victron has similar.

(I have flooded GC2 batteries)

[CJM1973]

time2roll wrote:
I would hit the middle on those specs. 14.7 absorption, 13.6 float. Probably two hour absorption time... maybe a little more for heavy use and less for actual storage between trips. Not sure if Victron adjusts for this automatically.

Yes I agree undercharging is generally a bigger issue than overcharging.

Thank you. That's what I was leaning towards. The Victron is highly adjustable. Now you got me thinking though. I'm wondering if I should change float parameters when the coach is in storage (let's say longer than a month). The only loads would be parasitic loads from the CO /Propane detectors, radio, and battery monitors. THoughts?

[time2roll]

I would hit the middle on those specs. 14.7 absorption, 13.6 float. Probably two hour absorption time... maybe a little more for heavy use and less for actual storage between trips. Not sure if Victron adjusts for this automatically.

Yes I agree undercharging is generally a bigger issue than overcharging.

[CJM1973]

I didn't mention this but it may be relevant. I've installed a DC to DC charger running from the the TV's starter battery. So between that (transit times to different campsites) and the coach's onboard 55 amp 4 stage charger (used at times with an inverter generator and shore power), I'm not worried too much about keeping the batteries topped off. I more concerned about what the "end stage" paraneters should be for battery longevity. Pianotuna, I'm going to heed your advice. Thanks.

[pianotuna]

Hi 2oldman,

I have but 256 watts of solar. My battery banks are in two 278 amp-hour banks. While in storage on Dec 21 about 17 amp-hours are pumped back into the battery bank.

The OP has only 300 amp-hours--so if he is a "week ender" 200 watts may yield a solar harvest of 800 watt-hours or about 58 amp-hours per (perfect) solar day.

Assuming 5 days between trips, there should be no problem returning the bank to fully charged. The biggest issue is getting to a high enough voltage.

AGM's wish to be "woken up", in general--so that is why I suggested crowding the higher side of the recommended cycling voltage, given the barely big enough solar wattage. I've allowed for non tilting fixed installation in the math above.

[2oldman]

With 300ah of batteries and only 200w of solar, it could take quite a while for you to ever reach absorb, let alone float/charged, depending on your use of course.

[pianotuna]

Definitely follow the makers directions for cycling voltages. But do wire in a balanced manner. Not doing so may shorten the battery life.

I would "crowd" the higher number on the cycling voltage.

[wires]

I'm no expert but I have 1000W of solar on 600AH of Lifeline AGMs in my rig so I've spent a fair amount of time researching battery charging and maintenance.

If it was me I'd go with 14.5 absorb and 13.5 float settings in the charge controller. Since you have the temperature sensor your charger will compensate and the voltage setpoints will be close to the high end of the range you mentioned in colder weather.

[CJM1973]

pianotuna wrote:
Pretty much all lead acid jars need to be fully charged. It is a bit harder to "recover" an AGM that has been left partially charged.

It will also be important to wire the batteries in a balanced manner. For an odd number of batteries only method #3 from the link below will work.

correctly interconnecting multiple twelve volt batteries

Thanks for the advice. I did read that AGM's are temperamental with subpar charging parameters. This is why I'm trying to identify the ideal charging characteristics of these AGMs. I think I'll stay with the battery manufacturer's recommendations.

[CJM1973]

Dusty R wrote:
Michigan ranks 47 out of the 48 Continental States in available Sunshine.

Not understanding the response here??? Some people actually travel outside of Michigan on their RV vacations. In fact, 90% of our planned trips are somewhere other than my home state.

I'm wondering if I should RV vacation in the 48th state. 😉

[Dusty_R]

Michigan ranks 47 out of the 48 Continental States in available Sunshine.

[pianotuna]

Pretty much all lead acid jars need to be fully charged. It is a bit harder to "recover" an AGM that has been left partially charged.

It will also be important to wire the batteries in a balanced manner. For an odd number of batteries only method #3 from the link below will work.

correctly interconnecting multiple twelve volt batteries