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We have 8 Trojan T-105 6V batteries configured as a 12V bank (4 strings of 2). Each battery provides 225Ah, so that should give us 450Ah at 50% DoD. Our cabin only draws 2.5A during the day and less at night. (I measured it at various times of day, and different days of the week, using a Kill-a-Watt on the inverter's AC output feed to the cabin). I'm new to all this, but shouldn't 450Ah give us a crazy amount of runtime? Like somewhere in excess of 100 hours? Our solar isn't enough to charge the batteries, so we end up running a 6500W generator for several hours a day. We run the genny for about 3 hours, and during that time the batteries measure 14V. When we turn off the genny, the batteries drop to around 12.3V, and we get 3-6 hours of runtime before they are down to 12.0 again (or lower). Clearly we are doing something wrong. I want to buy a whole new solar system, but not until I know what I'm doing. Otherwise I'll just kill the new one.
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Last edited by eric@psmnv; 08-14-2021, 12:51 PM. -
I am not an expert but something seems wrong with that charging voltage. I don't know which one of the three Trojan T-105 versions you own but if I use even the cheapest version, the charging on the data sheet says
Bulk charge = 7.41V
Float charge = 6.75V
Equalize Charge = 8.10V
Your 14V or 7V per cell is below the Bulk charge rate. My guess is that you are not charging these batteries even close to capacity.
I am not expert on Flooded batteries, so maybe someone else can chime in. BTW if you are changing the batteries get Lithium Iron Phosphate batteries.
The prices on the cheap ones are getting ridiculously low.
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The generator has a 220V cord that is broken out into 2 x 110V circuits, each one going to an Iota DLS-55 charger that converts from AC to DC and then into the batteries. In theory, each charger should be pushing 55A into the battery bank, but I don't know how to tell if that is really happening. I can tell you that I have rarely if ever seen the battery voltage go above 14.1. If it takes 14.8 to charge it,then I'm sure you are right. The batteries are not getting charged.Comment
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I also do not understand why your battery system is draining so quickly. My guess is...
1. they are not wired correctly so are not charging / discharging equally
2. your load is much higher then you think and you are draining the battery system.
3. one or more of the batteries is bad and is not holding a charge which may be hurting all of the others.
4. your battery charger is not sending doing the job.
Just a quick note. Anytime you wire batteries with more than 2 parallel strings you run the risk of unequal charge/discharge which will end up killing the entire bank.Comment
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That's a piece of the puzzle we didn't know! When sizing the bank, everyone advises you to total up your usage and then buy a battery bank with enough Ah to cover it. When you total your usage, you're working at 120V. When you size a battery bank, you're usually working at 12-24V. It probably sounds blatantly obvious, but not to us noobs. Nobody explains that you need to run a conversion like the one you just did. That explains a lot. Now I'll go back and look at the online solar calculators I've been using, and see if that's what they are doing.Comment
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1. I inherited the system from the previous homeowner, so I won't take credit for the way it is wired,but they seem to be wired in a standard way based on the diagrams I've seen. I checked the batteries a moment ago, and the voltages (when touching the multimeter leads to the battery terminals) are:
6.18 6.06
6.15 6.17
6.17 6.16
6.17 6.16
The voltages read lower when touching the multimeter leads to the copper lugs attached to the terminals.
2. I'm measuring the load using a Kill-a-Watt on the output socket of the inverter. It says it has been up for 186 hrs and has used 33.32 kWh.
3. That's possible. I apparently suck at determining the health of batteries.I've tried to measure the specific gravity, but the non-digital testers are hard to interpret. It's tough to tell where the needle is pointing,
4. That, too, is possible, I'll split out the wire and run one side of it through a clamp meter to see what's happening when the genny is running.Comment
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Don't feel bad, it's not THAT obvious
This is one reason you'll often see the calculations done in watt hours instead of amp hours. The watts going into, and coming out of, your inverter are about the same, it's just that going in it's 12V x 25A (300W) and coming out it's 120V x 2.5A (also 300W). When dealing with battery sizing specs it's always about the current out of the battery.
Edit: I just saw your numbers above. Someone double check my math here but it looks like 33320Wh/186h x 24h = 4300 Wh/day average, or 358 Ah/day. That seems like a lot for that size battery.
Also, it sounds like you're measuring the battery voltage under load since you're measuring something different at the cable terminal. Battery Voltage (or volts per cell) is used for rough state of charge measurements etc. but it's usually measured after a few hours of resting with no load.Last edited by sdold; 08-14-2021, 05:35 PM.Comment
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Your math seems right. Every real-time measurement I've ever taken (and I've taken many) has shown that we typically draw about 120-220 watts during the day. Average that to 170 and multiply by 24 hours and you get 4080, which is close to what you came up with. I'm going to say the Kill-a-Watt is right, and what we're actually drawing averages about 179 Watts (more during the day, less at night). That doesn't seem like much, but it really adds up!.
Still, I should be getting around 400-450 Ah from that bank, so it really should be able to hold us all day, assuming it gets a full charge, don't you think? I measured the current to the AC/DC converters. One is getting 8.8A (1056 W) from the genny, the other is getting 9.3A (1116 W). Converting that to DC, that works out to 88A and 93A. Since they are 55A converters, I assume the max that is going into the battery is 110A. So it should take like 4 hrs to charge the bank off the genny, and I should get 24 hours of runtime, right? But I'm getting closer to 6 hours.
Yes indeed. I'll disconnect the load for a few hours and test it again later.
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Have you tested your batts under load? While either putting a sizable load or charge on the battery bank, measure the voltage on each 6 volt battery. Whichever one is different from the rest is your problem. Putting four sets in parallel is what "you did wrong".BSEE, R11, NABCEP, Chevy BoltEV, >3000kW installedComment
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if you do what Solarix suggested, it would be interesting to also measure the current in each string with a clamp on meter to see how different they are.Comment
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I did the clamp-meter test as suggested and I think it's safe to say we found a problem. The picture below shows the battery wiring layout and the amps measured on each wire. This was while the generator was running, with about a 200W AC load on the inverter. battery_amps_problem.pngComment
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I tested the batteries under a small load. 6 of them registered 6.13 V. The one in the upper left measured 6.18 V. The one in the upper right behaved very differently. Upon first touching the leads, it measured 2.5 V, then it gradually increased over the next couple of seconds and finally topped out at 6.07 V.Comment
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I am still a little confused. Why do you have 2 chargers on the bank and why are they connected differently?
Chargers are pretty stupid and will react to the battery voltage so one could be confused by the output of the secondComment
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