battery charging with very small panel

wxh3

I have an extra used 78AH 12V AGM battery. (It was replaced by larger batteries when I upgraded my panels.) I think it still has some life left so I decided to set up a 2nd very small system in my house with an extra inverter and charge controller that I have.

I consider this primarily an emergency backup system...the inverter will normally be shut off. Only rarely might I turn on a CFL light,radio, cellphone charger on the system for short periods. (I might do this a bit more often if I find that I can recharge the battery successfully.)

My question is whether I can do anything more than maintain the battery with an 11W panel? If I put no load on at all and have enough sunny days, can a panel this small bring the battery fairly close to being fully charged from say, being 20% depleted?

I have a larger panel I could possibly use (50W) but unfortunately this location is in a place in the front of my house where I might get some grief from my HOA if I had something very noticible.

Also, I must say I have been a bit disappointed with what I am getting out of the 11W STC panel so far...about half of rated output even with direct sun. It is a bit hot out so that might explain it. Also the 25 foot run of 0.75mm2 x 2 wire (18.5 gauge I think) probably isn't the best for getting the most out of the panel....

westbranch

Re: battery charging with very small panel

go to this link and use the calculator to see what you are getting/losing.

http://forum.solar-electric.com/showthread.php?t=29

HTH
Eric

BB.

Re: battery charging with very small panel

Measure the voltage at the panel connections and then at the battery--if you are getting a big voltage drop (15 volts at the panel and 12 volts at the battery--you need heavier wire)...

Regarding a 11 watt panel and 78 AH AGM battery...

The panel directly connected to the battery, with a blocking diode (if the panel does not have one) will probably not damage the battery as a trickle charger... You should also have a fuse to protect the wiring--chances are that a shovel or weed wacker could cause the wires to short--and you would set your home/landscaping on fire (assuming combustibles in area).

It would probably be better if you got a solar charge controller with AGM/Float setting--your battery may last longer...

In terms of getting any useful emergency power... If the panel is working well, you would be getting around 20 Watt*Hours per day... So that is how much you can pull day after day in an emergency.

-Bill

wxh3

Re: battery charging with very small panel

BB. wrote: »

You should also have a fuse to protect the wiring--

...

It would probably be better if you got a solar charge controller with AGM/Float setting--your battery may last longer..

-Bill

I am using a Prostar controller (also retired from my "main" system) and have fuses between the battery and both the controller & inverter.

I guess my concern was ability to charge bulk charge a battery with very small charge current and I guess you are saying given sun and enough time it should be ok.

BB.

Re: battery charging with very small panel

AGM's are supposed to be less affected by deep discharge / sitting uncharged / sulfation. And you can cycle them down to 20% state of charge (always check the specification for your brand/model of AGM battery).

You did not say what voltage the battery attains when charged---some small solar panels do not have high enough voltage to be used with a charge controller... Your panel probably needs to output upwards of 15+ volts to charge the battery to around ~13-14 volts (depending on temperature).

In the end, assuming ~20 watt*hours of power per day and a 78AH battery discharge to 20% state of charge (after your emergency usage):

78AH * 80% to charge / (20 watt*per day / 13 volts) = 40 days of sun to recharge...

In reality--the 11 watt panel is just a trickle charger. You will want to put it on a big panel and/or AC battery charger to get it quickly ready for use again.

-Bill

niel

Re: battery charging with very small panel

if it charges the battery it will take a very long time. the charge rate would be about 1.1%, but into a more efficient type of battery being agm, the current realtime capacity of which is unknown. my advice is to try it and watch what it does. if it reaches a consistant resting voltage (after sitting for 2 or 3 hours minimumly with no charge or discharge) of about 12.7v or 12.8v then consider it charged. if you don't see it reach the voltage points i mentioned after a month of charging, consider it unable to fully charge the battery. if it reaches that voltage quickly (like a week or less) you may have something wrong with it, but i can't say for sure as it depends on the aged/abused battery capacity and current charge in the battery.
i will add here that you can give it a bit of a bulk charge from your car's alternator (hour or less) and try to finish it off with the pv. this will speed things up for sure.

wxh3

Re: battery charging with very small panel

BB. wrote: »

You did not say what voltage the battery attains when charged---some small solar panels do not have high enough voltage to be used with a charge controller... Your panel probably needs to output upwards of 15+ volts to charge the battery to around ~13-14 volts (depending on temperature).

In the end, assuming ~20 watt*hours of power per day and a 78AH battery discharge to 20% state of charge (after your emergency usage):

78AH * 80% to charge / (20 watt*per day / 13 volts) = 40 days of sun to recharge...
-Bill

I see my voltage go over 14V at the charge controller when I am in absorb state. However, the Prostar meter seems to always read 0.0 amps at that point....I don't know if it is just at a very small <0.1 amps. I see 0.4 amps in full sun during bulk charging.

I am now at fully charged battery voltage ~12.7V (used my 50W panel to charge battery) but I haven't discharged the battery any significant amount yet at all to really see how my 11W panel charges.

I actually meant 20% discharge (battery still at 80%) If I really go below 50% battery charge I think I will be pulling out my 50W panel in a temporary set-up to charge.

mike95490

Re: battery charging with very small panel

What does your 11W panel look like ? Is it a even deep color, with parallel "hairlines" (conductive wires) every 1/2" across it ? If so, it's likely a type of panel that looses 10% capacity each year it is out in the sun.

wxh3

Re: battery charging with very small panel

mike90045 wrote: »

What does your 11W panel look like ? Is it a even deep color, with parallel "hairlines" (conductive wires) every 1/2" across it ? If so, it's likely a type of panel that looses 10% capacity each year it is out in the sun.

I don't think so. Aren't those characteristics of an amorphous, thin-film panel? Anyway, this little panel is from Northern Tool:

http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200306865_200306865

BB.

Re: battery charging with very small panel

Looks like a poy-crystalline panel--should be better than the amorphous panels.

-Bill