# max number of batteries



## Explorer (Dec 2, 2003)

Is there a max number of 6 volt Trojan T-105 batteries in a 12 volt system? I am upgrading my system and will have an Outback 80 amp controller. I would like to have three strings of four batteries each, but am told 8 batteries is the max for a 12 volt system. I have never heard of that limit before.


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## TnAndy (Sep 15, 2005)

Haven't heard of it either. 

The rule I have heard is limit your strings to the least number you can, because the greater the number, the more wiring resistance and differences in individual cells in a string become a factor.

But something else here:

You're using a 6v T105, and on a 12v system. If you make a string ( which are batteries in series to build voltage ) of "4 batteries each", that's 24v.
How is that gonna work on your 12v system ?

Looks to me like 12 batteries is going to be 6 strings of 2 each....or am I missing something ?

One of the reasons to step on up to a 24 or 48v system, and use larger batteries, like L-16's, is to avoid issues like this, along with smaller wiring and fusing between the batteries and inverter(s).


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## Explorer (Dec 2, 2003)

Let me try again.

My current system is six T-105's wired in pairs to produce 12 volts with no increase in AH for each pair. The three pares give me the increased AH. The new one will still be in pairs, but have six pairs giving me considerable more AH.

The plan is to add two batteries to the current system which is 2 1/2 years old, knowing they will have a shortened life. The next expansion will be to twelve batteries of four per leg, maybe L-16's depending upon finances.

Each leg of four batteries may be switched so I can take one leg off-line if desired.

This expansion is more panels and a new controller along with a new battery box with cables.


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## swampyoaks (Jan 6, 2008)

When you have a bunch of batteries in parallel, what happens if one of them develop a short? A lot of amps can be sent through it from the rest of the bank. A lot of batteries in series and a short develops in one it only drops the voltage of the string. It doesn't create more load on the bank. At the very least I'd fuse each string in a parallel arrangement.


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## Explorer (Dec 2, 2003)

Yes, I was planing on fusing each string of four along with the switch. Probably a 1-200 amp fuse as I haven't decided that aspect yet.


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## 12vman (Feb 17, 2004)

My question would be how many watts in panel do you have to charge with? That's a lot of battery to charge properly.

T-105's have an aHr. rating of 225. 4 pairs will give you ~900 total. 25% discharge would be 225. Figuring ~4 hr. charge in the winter in Az., you'd need ~1kw. in panel.

The Outback will handle ~ 1.3k safely @ 12 volts. If you were to discharge your 8 batteries to ~ 50% discharge and had near 1.3k in panel, the resistance of the battery would put the near maximum load on the controller in mid summer.


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## Explorer (Dec 2, 2003)

Yes, 900 AH is what I am shooting for on this expansion. Currently I have 340W of panels and have another 250W ready to go for this expansion. The panels will be wired for 24 volt input to the controller. Everything will be set to add another pair of panels if needed.

I was thinking I got more than 4 hours of winter Sun, maybe six. However sometimes it is overcast for 3 or 4 days. That is why I want the extra battery power to carry me thru those overcast days. I have never discharged 50%, except occasionally in the winter.

The final expansion to 12 batteries is just a big maybe; however, I could add more panels to give me near 1KW.

I thank everyone so far as this help is going a long way in clarifying my thinking.


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## zant (Dec 1, 2005)

We had 18-2V C&D batteries wired series/parrell for 3yrs-no problems...sold them when we moved and new owner still using 6yrs later.We used Trace SW-12v inverter.


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## swampyoaks (Jan 6, 2008)

I have 1000 Ah of AGM 8D batteries and I'm charging them with 1050 watts of panels. I'm running a refrigerator and freezer through a Xantrex Prosine inverter charger. On a sunny day I can get them to float charge off the solar. Getting them to bulk is fairly easy but getting them through absorb can be a problem as the sun fades away. Seeing as how I have a lot of bucks in the AGM's I make sure they get a full charge everyday even if I have to use the grid to do it. I have another 860 watts I'll be mounting and I think with them I should be able to get through bulk a lot faster and that way get more absorb time. A primary reason for early failure in batteries charged by solar is the failure to bring them to full charge day after day.


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## Harry Chickpea (Dec 19, 2008)

Although I've not heard of limitations like this, it kinda makes sense. All parts in a system work best if sized in relation to each other. That limitation sounds reasonable for an 80 amp controller.

When you start going into more batteries at the same voltage, issues with overheating and wire size just keep mounting.


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## Explorer (Dec 2, 2003)

Depending upon how I wire the panels this controller will handle up to 7500 watts from the panels. As I plan to wire then it will handle 2500 watts @ 24 volts.


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## swampyoaks (Jan 6, 2008)

Only 1910 watts to go!


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## Explorer (Dec 2, 2003)

Don't I wish $$$$.


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## swampyoaks (Jan 6, 2008)

Some rough numbers. With 590 watts you'll get maybe 45 amps tops. Trojan recommends 10-13% of your battery capacity at a 20 hour rate for your charger. So you'll have 900 amp hours of battery bank and you'll need at least 90 amps. If you use 25% of your AH capacity a day that's 225 AH. You'll need to put 250AH back to bring the batteries to full charge. 250 divided by 45 amps = 5.5 hours. The trouble is even if you get full bright sun for 5.5 hours the FM 80 will go into absorb and the current will taper off. Getting that last 20% can take 3 or more hours. You may never see your setup get out of absorb. Chronic undercharging wrecks batteries.


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## Explorer (Dec 2, 2003)

Thanks. On average I use less that 30 AH a day now and maybe another 10 AH for my pellet stove in the winter. I want to power things for 5-7 days in the winter on occasion, so it looks like I will be OK.


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## swampyoaks (Jan 6, 2008)

30 AH is really good! My inverter pulls more than that with no load on it at all. You should have a good 2 weeks of autonomy then with 900 AH.


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## Explorer (Dec 2, 2003)

I want to thank everyone for their input again as it has greatly helped in clarifying my thinking. I am going to go with eight 6 volt Trojan T-105's which will give me 225AH each or 900AH @ 12 volts for the bank. Down the road in 3-5 years when I have to re-battery I will still have the option of going to L-16's for additional 400-620AH, if needed at that time and still use the same battery box. (I will build it to fit the L16's.)

Now I need to figure out the inter-connect battery cable size.


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## 12vman (Feb 17, 2004)

540 watts @ 12 volts of charge will give you ~ 32 amps of charge per hour on a good day during the summer. (latitude/season adjusted, fixed flat panel facing south) Figure ~5% less during the winter because of added distance from the sun.

In reality, your system could only be 50-60% efficient during winter because of the cloudy periods. These are the perimeters to design your battery from to assure a good charge.


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## Explorer (Dec 2, 2003)

I am at elevation 6000' if that makes a difference. With my existing panels, 340 watts, I am getting normally about 22-23A at mid day and the highest I have seen is 25 amps on a cool day (75F). Measured at the controller.


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## Explorer (Dec 2, 2003)

swampyoaks said:


> 30 AH is really good! My inverter pulls more than that with no load on it at all. You should have a good 2 weeks of autonomy then with 900 AH.


I screwed that one up, should have been 130 AH. So maybe I will get 4 days at the most.


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## swampyoaks (Jan 6, 2008)

Yep if there's little solar for 4 days, sometime in the fourth day you'll hit the 50% mark. Then if the sun comes back out bright it will take about a week for it to charge back up to 100% given your array size.


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## Explorer (Dec 2, 2003)

That is about my experience. I usually will go to grid power the third day for the duration. That is the main reason I am making this expansion, to outlast the 4-5 day overcast weather.


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## nadja (May 22, 2011)

Explorer, I have been using 18 trojen T-105's for about 16 years now, with little or no problems. Most of my friends around here are all using about 24 of them with no problems. Just make sure you can charge them on any normal day, or you will have problems in the long run.


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## ralph perrello (Mar 8, 2013)

Rule of Thumb: one 6 or 12 volt battery per each 100 watts of panels or you will not fully charge the batteries and shorten their life.


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