Solar Battery Depletion vs Charging

[zcapozzi]

Hi all,

I originally sized my battery bank and system to be able to provide power to charge cell phones, laptops, tablets, etc. So it's not a huge system. The panel is 100W, 12V, the battery is 12V 34Ah. There is a Sunsaver charge contoller in between and 400W inverter connected to the battery. It's all set up and working, so that's great.

Now, I'm trying to figure out if there are any electronics that I can have permanently attached to the system in addition to the random cell phone or tablet charge. It seems that I've got excess capacity based on my calculations, so I want to make sure I've got a good idea what I could reasonably do. Last thing I want is to kill my battery by inadvertently discharging it too much. To that end, I've done a few calculations that I was hoping folks could poke holes in. My discharge calculations are below, but here is my theory.

With the panel connected, it's going to generate 5.29 amps (at full sunlight). I'm shooting to constantly power a 1A modem at all times, plus random phones tablets, etc. For the sake of this example, let's say my average draw is 1.5A for 24 hours a day. In full sunlight, the <b>battery will be charged because 5.29 > 1.5</b>. Let's assume that the battery gets to a full charge by sunset, using my table below, if I'm drawing 1.5A on average, then I'd have a little less than 12 hours before the battery is discharged to 50%. If I unplugged <b>all but the 1A modem overnight, then I'd have closer to 20 hours before 50%</b>. I believe I'm understanding the amp hour ratings appropriately.

Second piece of this is, if the battery were to be fully discharged to 50.1% overnight (so it never actually shuts off) and then the sun rose just before it got to 50% and started charging, <b>my charging time would be 17 AH / 5.29 = 3.21 hrs to get back to full charge</b>. This assumes that I have no load on during that time. It would be 17 AH/(5.29-1) = 3.96 hours if there was a 1 amp draw during this time.

I want to make sure I'm not misinterpreting anything here before I start to test this out. The charge controller has indicators that glow yellow when I get to 50%, but would prefer to know that I'll never get there. Hope this makes sense. Thanks for the thoughts.

Battery Discharging: 34 Ah battery (24 hr rate)

Amp Hours	Hour Rating	Calculated Amp Draw	Watt Hours	Available hours (discharged to 50%)
28	4	7	336	2
30	8	3.75	360	4
34	24	1.42	408	12
36	48	.75	432	24

[Sunking]

OK for one you have a bit of a mismatch between panel wattage and battery capacity. If the Sunsaver controller is MPPT with a 100 watt panel will generate up to 8.5 amps of charge current. If PWM 5 amps.

The point I am driving at charge and discharge current should match. For a FLA battery the max charge/discharge current is C/8, and minimum C/12. So assuming your controller is MPPT you want your 12 volt battery to be a minimum of 65 AH and no larger than 100 AH. 85 AH is perfect.

Another issue related to your current battery size assuming it is a FLA battery is the maximum discharge you want is again C/8. So for a 35 AH battery is only 4 or 5 amps. At 12 volts that is 60 watt inverter.

OK so here is a problem you have. You have an oversized panel for the battery. The panel can generate up to 400 watt hours of usable power each day (33 AH @ 12 volts). Most likely your battery is fully recharged early in the morning. So the rest of the day potential power is just wasted and has no place to go. So what you have is a panel capable of generating 400 WH each day but a battery that can only supply you with 100 to 200 watt hours per day. You got a heck of a mismatch.

[zcapozzi]

OK so here is a problem you have. You have an oversized panel for the battery. The panel can generate up to 400 watt hours of usable power each day (33 AH @ 12 volts). Most likely your battery is fully recharged early in the morning. So the rest of the day potential power is just wasted and has no place to go. So what you have is a panel capable of generating 400 WH each day but a battery that can only supply you with 100 to 200 watt hours per day. You got a heck of a mismatch.

Yeah, I figured that would be the case since the battery is so small. I am considering getting another battery to put in series and increase my amp hour capacity to harness that extra sun power. Seems like a waste to have this nice big panel charging my mini battery.

If it makes any difference, my charge controller is a PWM (not MPPT) and the battery is a Sun XTender AGM. I assume FLA is a flooded lead acid battery? I remember seeing that AGM is a more maintenance-free type, so hopefully I can safely discharge slightly more than FLA, even if 50% is not advisable if I can avoid it. Are there C/8 equivalent ratios for an AGM battery?

Thanks for the response.

[Naptown]

Actually AGM is good as it will take a higher charge rate
Still undersized for much more load

[Sunking]

If it makes any difference, my charge controller is a PWM (not MPPT) and the battery is a Sun XTender AGM.

Still like putting 10 pounds of chit in a 2 pound box.

[PNjunction]

.......To that end, I've done a few calculations that I was hoping folks could poke holes in.............. My discharge calculations are below, but here is my theory.... (snip) ........ Let's assume that the battery gets to a full charge by sunset, using my table below,

Ok, you asked for holes.

The one major thing is that you indicated sunset, and sunrise to sunset hours are not applicable to solar, but *solar-insolation* is. Solar insolation basically discounts the early morning and late afternoon hours as very weak solar energy and are not used in calculations, so your location is very important to find out just how many usable hours you have for solar collection.

You can find these hours by using the popular PVWATTS program online, or a simplified version can be found here. Use the winter hours:

http://www.bigfrogmountain.com/SunHoursPerDay.html

I'm glad to see you are using an advanced AGM battery, and not the typical ups-style agm. However, note that Concord/SunXtender agm's like to see a *minimum* of 0.2C for charging to keep the battery healthy. They can take up to 5C charge! (see Concord/xtender manual for more). With your current setup, I'd at least double up the panel in parallel. Your Sun Xtender can handle it no problem, and will also help if you have poor solar insolation. Note that this is totally unlike cheap ups-style agm's, where the MAXimum charge current is usually 0.25C.

Search for:
Sun_Xtender_Battery_Technical_Manual. pdf

You bought a great agm battery, one of the few that can actually handle an EQ if necessary so you'll want to treat it right. Consider getting a decent charger, like a 15A Iota DLS charger with IQ option. You are just barely meeting that minimum 0.2C now, and even less if there is a load on the system while charging, so the sun xtender won't be very happy with being tickled to death. More panel power needed!

Your 24/7 application is also critical now - most batteries in that kind of use should be drawn down no more than 20% from full, and not 50% if you want to get years of cycle life out of them - at least according to most battery manufacturer warantee cautions. Some advanced users may use a different algo, like 50-80% SOC usage, followed by a REAL full charge once in awhile.

Your solar-insolation hours *in winter* are crucial, and without knowing it, I'd just say to at least DOUBLE your battery capacity, (or maybe consider a "buddy" system which has another battery on standby backup for swapout) and double, maybe even triple your panel power to also accomodate a day or so of bad-weather autonomy. Get this wrong, and you'll just be in deficit-charge most of the battery's life and it will expire earlier than planned.

Also if you are considering taking things beyond 50% SOC in an emergency, your dc loads may not act properly once dropping below 12v, so you may not be able to much deeper than 50% SOC anyway - depends on how finicky the dc load voltage requirement is.