Solar Array Needed to Charge 8 Trojan L16 6volt Batteries

OffGridTom · February 2015

I have a small cabin with three 230 Watt Solar panels and a small battery bank of two 6 volt L16 Batteries connected to produce 12 volts. My controller is a TriStar MTTP 45, and I have a 1500 Watt Pure Sine inverter. The 690 watts provides me with more electricity than I need during the day, but I am wanting to have a lot more available after sunset, and on extremely cloudy days.
-Is my current solar power sufficient to increase the battery bank to 8 vs 2 L16's, or will I need to add solar panels in order to maintain a battery bank of this size? I have been using these batteries for two years now, so I realize I will need to purchase all new batteries for the larger bank. At least that is what I believe to be correct. Please correct me if it isn't.

Thanks for any input.

solar_dave · February 2015

Re: Solar Array Needed to Charge 8 Trojan L16 6volt Batteries

OK so a couple things:

1) any idea on how much more power you want to use? That would be the basis for the upgrade in the battery department.
2) Are you ready to upgrade your system to a higher base voltage, 8 batteries in 4 strings is not the way to go to keep the battery charge balanced.

Of course a voltage increase will require the replacement of the inverter, but you might get away with keeping the same charge controller depending on your consumption. That is what will determine how much solar you need. Texas is pretty good with solar insolation.

OffGridTom · February 2015

Re: Solar Array Needed to Charge 8 Trojan L16 6volt Batteries

When I build my larger cabin, I do plan on going with a higher voltage system and inverter. This was somewhat of a test cabin, and the 12 volt inverter I have is only a few months old, so I really want to stay with a 12 volt battery bank.
I must not understand the amp hour rating of solar batteries. The ones I have right now are rated at 766 minutes @25 amps. I thought I would be safe, for example, running a 75 watt TV for a couple of hours in the evening, as long as I had a full charge on the batteries.
Maybe you can help me understand better how much electricity I should be able to use with fully charged batteries. Maybe a bank of four would be sufficient for my needs. Right now, if I just leave the inverter on all night without any load, my volts go down from 12.7 to 12.3 or 12.4 by the time I wake up. I know you never want to discharge deep cycle batteries below 50%, and I was told that would equate to 11.9 volts. Is that correct?
I wold like to become much more educated on electricity before building my larger home, and maybe someone can recommend some good learning sources for this purpose.

Photowhit · February 2015

Lots of things;

First lets get away from the and energy draw type of rating, Trojan L-16's are likely around 350 amp hour batteries when they are new. Even this doesn't work with your 766 minutes @ 25 amps not sure where that came from.

Measuring battery capacity with voltage is like telling someone how far it is to town in minutes... with out finding out what type of transportation they are using. Voltage will vary dependent on what state of charging or discharging they are in, you could have a 50% 12 volt battery with a voltage of 12.7volts when charging, or a 100 % charged battery reading 11 volts with a very heavy drain. The best method of charging a battery is by measuring the specific gravity of the electrolyte in the cells.

If I were to assume you had a fully charged battery bank (2 L16's in series to make 12 volts) and the only draw on them was the inverter and they went from 12.7 to 12.3 over night, I might conclude that they had lost much of their capacity, or perhaps you have a cheap(Harbor Freight) or poorly operating inverter...
...but as I said voltage is a poor indicator of capacity. You should check the, state of charge, of your batteries with a hydrometer and figure out where they are at...

2-L16's each having a 350 amp hour at 6 volt rating will give you a battery bank of 350 amp hours at 12 volts, when batteries are added in strings/series the voltage adds and the amperage remains the same. 350 amp hours, should run a 100 watt load, 75 for your TV and 25 for your inverter, for quite a while. 100 watts per hour ÷ 12 volts = 8.5 amps per hour, so keeping in the top 20% of your battery, you should have about 70 amp hours to deal with or run the TV and just the TV for about 8 hours...

Follow threads here will help you with your education, pick up a hydrometer and measure and tell us, and we might help you with your system, If you have lost some capacity, Your batteries may benefit from being equalized. When you measure the electrolyte in your batteries you may find one cell, or a few, that are lower than the rest, equalizing may help bring that cell in line with the cells and increase your capacity.

BB. · February 2015

766 min @ 25 amps sounds like the automotive "reserve capacity" definition... Basically, if the alternator went dead on your car, how many minutes can you drive the vehicle before the battery goes dead. Not a a rating system that we see very often on Deep Cycle batteries. The conversion:

766 Min / 60 min per hour = 12.8 hour rate
766 minutes * 1hour / 60 minutes * 25 amps = 319 AH capacity at 12.8 hour discharge rate

Of course, discharging at a 20 hour rate or C₂₀ rate, the AH capacity will be higher.

-Bill

OffGridTom · February 2015

Thank you for the helpful information. The L-16's are 370 AH batteries as stated in the Trojan Data Sheet, which is the same place I obtained the minute capacity @25 Amps from. I downloaded the data sheet from the Trojan website.
I think my main problem is that I am going by what the TriStar meter is displaying inside the cabin with a partial load. I did check the specific gravity last month with a hydrometer, and the batteries checked out within parameters when fully charged. In addition, I equalized the batteries 8 days ago. Being a novice, I have probably overreacted somewhat when I saw the meter at 12.2, afraid of allowing the batteries to discharge below 50%.
The batteries are located under the cabin, which is well ventilated, but not heated, as I wasn't aware of how much the temperature effected the charge and discharge rate of batteries. I am getting ready to add a small room onto the cabin, and this will allow a place to house the batteries in a warmer environment.
All this said, would you recommend I double the size of the battery bank when I relocate them to a more protected location? From what I understand, my solar array of 690 watts is sufficient for maintaining four L-16's.
Thank you again for the helpful information you have given me.

BB. · February 2015

Batteries are complicated... Check the (temperature corrected) specific gravity if you have questions. Also, check the voltage (each cell or each battery) to make sure all is even (with 12 volt batteries--most do not allow you to measure cell voltage--You are left with specific gravity for flooded cell batteries).

Some reading for you:

http://www.windsun.com/Batteries/Battery_FAQ.htm
http://www.batteryfaq.org/
http://batteryuniversity.com/

Poster "leaf" has a really nice set of charts that compare battery voltage against different rates of discharging and charging (as well as resting voltage readings).

leaf wrote: »

Am trying to upload the charts I am using...

Attachment not found.Attachment not found.

[note charts are from: I believe those charts are from Home Power #36, August- September 1993. Lead-Acid Battery State of Charge vs. Voltage ©1993 Richard Perez.
Here is a link: http://www.scubaengineer.com/documen...ing_graphs.pdf

vtMaps]

I don't quite a agree with the resting voltage line (I think the voltage is a bit low)--But it shows how to estimate a battery's state of charge while operating.

Note, where the charts "flatten out"--the room for error estimating state of charge is pretty high.

-Bill

jcheil · February 2015

I have a new (4 months now) set of the Trojan L16-RE-B batteries, 16 of them, 2 banks of 8 each in a 48v system.
If you search on this forum I have posted a few threads about some questions/concerns/issues I have had.

One thing I know for sure, even with the EXCESS of solar that I have (8475 watts, in central Florida and 740ah of batteries), they will NOT charge properly at the voltage that trojan recommends. Again, even with generous amounts of Florida sun and me reaching 15% charge rate nearly every day (and sometimes higher - see that thread also).

Trojan says absorb at 2.35v - 2.45v. Even at 2.45v and a full day of sun, and my FNDC showing 100% returned into the batteries with a charge factor as low as 85% AND only 4 amps being returned into the batteries at "full charge" (which is about 1%), my SG's were still in the 1.26 range. Those L16's are 1.280 SG at 100%. I have played with this forever and finally increased the absorb to 2.48v and am getting much better results. still not perfect. I still get big variations in sg after about 2 weeks, but a 14-day EQ at the trojan 2.58v fixes that within a couple of hours. After I get one of mine replaced under warranty (definitely an issue), I will try 2.49v and 2.50v for a week each and see if that works better without excessive gassing and water consumption.

Which is a good point, you SHOULD be consuming water. If not, then that is a sign that they are not being charged properly.

And unless I am reading it wrong, you have a 12v system with 4 L16 RE's. That would be 740ah at 12v. And only 690w of solar?
One of the experts on here can figure out the exact numbers, but from what I see, even in TX, that is just a tad over the very minimum (5%) to just "maintain" that large of a bank. It won't charge it properly if you have daytime loads.

westbranch · February 2015

No one has mentioned that you should 'unplug' the inverter at night as it has a constant load on your batteries. Depending on the model it may have an energy saving mode but with only 2 L-16 cells of 6V, you do want to conserve a bit, so a disconnect will help... as well as 2 more cells but the new ones will degrade faster connected to 2 older batteries.. and btw the charge to capacity ratio is about ~4.8%, so it is just at the minimum 5% rate, the recommended rate is 10%, but you will do OK for now, but when you get more cells you should add 2 more panels minimum to achieve a good charge.

hth

Solar Array Needed to Charge 8 Trojan L16 6volt Batteries

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