Advice on RV Set up & Wiring?

[sensij]

Now on the discharge side you want to limit the rate to no greater than 5% voltage drop. 5% of 12 volts = .6 volts. So max discharge rate is ,6 volts / .012 Ohms = 50 amps. So for a Trojan T-105 you can charge as high as C/2, and discharge up to C/4 without issues of over heating or performance degradation.

Max Charge:
On the charge side heating is the limiting factor and for any Pb battery you want to limit thermal capacity to 0.05 watt per AH.

So if we use a single cell Ri (resistance) = .001 Ohms and the max thermal charge limit is the square root of [.05 (Thermal capacity Limit) x 225 (Rated Ah)] / .001(single cell resistance) = Max Charge Amps to reach thermal limit.

Trojan T105: Square Root of [.05 x 224] = 11.2 / .001 = Square root of 11,200 / Which = 105.83A Max Charge Amps is Thermal Limit/ AKA Max Charge Rate

To Turn it in to Charge Rating for the T105: (224(Ah) /106(Max charge Amps) = 2.11) or C/2


Thanks

This is great, in theory. Has Sunking (or anyone following along) ever actually charged T-105RE's routinely at C/2? I've never seen anything in Trojan's documentation to support that high of a charge rate. A quick email to their technical support generates this response:

Trojan Battery Response

Dear sensij:

Thank you for submitting your question regarding Technical - Charger.

Based on your comments below:

I'm hoping for guidance on the maximum safe charge rate for these, let's say at 30% discharged and 25 deg C. I understand they are lower internal resistance and will not build up heat as quickly as standard T-105's, but does that translate into a wider allowable range for charge rate? Thanks, sensij

Below is Trojan Battery's response:

Normal charge rate is 10-13% of the C20 rating. Maximum is 20% of C20 rating. Same as for the T-105.

Maybe they just want to legally CYA, but it surprises me the Sunking is willing to go so far out on a limb.

[Sunking]

My Only Question is where did the .012 Ohms on the discharge side come from?

Thx for the catch, my bad, I made an error assuming a 24 volt battery using T-105's. instead of 12 volts which now makes it .006 Ohms and 100 amps on paper but not design. Hang in there. A T-105 battery has a total of .003 Ohms. That means each cell is .001 Ohms and 3 cells in series for a 6 volt battery total .003 Ohms. So two T-105's in series is .006 Ohms. The rest is simple 5th grade math and Ohms Law. Voltage = Current x Resistance, and Current = Voltage / Resistance.

You want to limit voltage sag on a battery to some definable percentage to achieve an operational goal. For batteries somewhere around 2 to 5% loss. Otherwise if you take a lot of current from a battery to say an Inverter, if the voltage SAGS to far, your Inverter trips off line from Under Voltage as designed. You have both battery voltage sag and cable line voltage loss to account for. Otherwise you get nasty surprises like a fully charged battery will not supply power to your Inverter drawing 100 amps. Ideally to be functional you want to limit battery and cable loss to 5% between the 2. 5% of 12 volts is .6 volts total to work with. So use 3% for battery, and 2% for cable. 3% of 12 volts = .36 volts. So now we have two known variables to determine how much current the battery can supply. Again Amps = Voltage / Resistance. So .36 volts / .006 Ohms = 60 amps max discharge rate. So now we have the maximum current of 60 amps to be applied to calculate the size of the wire required to limit voltage loss to 2%. 2% of 12 volts = .24 volts. So now we now again from simple math using Ohms Law Resistance = Voltage / Current. We need to keep the wire loop resistance to .24 volts / 60 amps = .004 Ohms or less. So lets say the one way distance between the Battery Term Post and Inverter is 10 feet and the Fuse is 70 Amps. Go back to the chart above and select conductor.

If you followed all that you came up with a 6 AWG conductor and a 60 amp x 12 volt = 720 watt load is what a pair of Trojan T-105's can handle and expect to operate until the battery is fully discharged utilizing all the power.

[Carv]

So after weeding through all the fluff Sun King has provided some really good information here on how to actually figure out how much charge & discharge your batteries can take WITHOUT damaging the cells.

hybrid batteries like a Golf Cart battery come into play, or very expensive AGM batteries. They have much lower Internal Resistance, thus can deliver much higher discharge currents and much higher charge currents. Example a Trojan T-105 has an Internal Resistance of .003 Ohm's, and thus two in series would be .006 Ohms. On the charge side heating is the limiting factor and for any Pb battery you want to limit thermal capacity to 0.05 watt per AH. Bet no one here even knows that limit. So if we use a single cell Ri = .001 Ohms and the max thermal charge limit is the square root of [ .05 x 225] / .001 = 106 amps thermal limit.

Now on the discharge side you want to limit the rate to no greater than 5% voltage drop. 5% of 12 volts = .6 volts. So max discharge rate is ,6 volts / .012 Ohms = 50 amps. So for a Trojan T-105 you can charge as high as C/2, and discharge up to C/4 without issues of over heating or performance degradation.

Translated:
Here's how to figure out your max Charge & Discharge rates using a Trojan T105 as an example:
Trojan T105s have an Internal Resistance of .003 Ohm's (.01 per cell), and thus two in series would be .006 Ohms total resistance.

Max Charge:
On the charge side heating is the limiting factor and for any Pb battery you want to limit thermal capacity to 0.05 watt per AH.

So if we use a single cell Ri (resistance) = .001 Ohms and the max thermal charge limit is the square root of [.05 (Thermal capacity Limit) x 225 (Rated Ah)] / .001(single cell resistance) = Max Charge Amps to reach thermal limit.

Trojan T105: Square Root of [.05 x 224] = 11.2 / .001 = Square root of 11,200 / Which = 105.83A Max Charge Amps is Thermal Limit/ AKA Max Charge Rate

To Turn it in to Charge Rating for the T105: (224(Ah) /106(Max charge Amps) = 2.11) or C/2


Max Discharge:
Now on the discharge side you want to limit the rate to no greater than 5% voltage drop.
5% of 12 volts = .6 volts.

So max discharge rate is ,6 volts / .012 Ohms = 50 amps.

To Turn it in to Discharge Rating for the T105: (224(Ah) /50(Max Discharge Amps) = 4.48) or C/4

Without issues of over heating or performance degradation.




My Only Question is where did the .012 Ohms on the discharge side come from?

Thanks

[Carv]

Now back on Track

THANK YOU sensij for an actually answering my with some facts & logic behind it.

I can get both 250W 60s & 330W 72s for very similar prices and they take similar real estate so again it comes down to 1 330W or 2 250W panels and what the performance differences would be VS what I'd give up/have to do.

sensij in your opinion, would you say I'm better off with the 2 250W 60 cells because of the Voc limits on the CC AND the fact I'll have over capacity so if there are some inefficiencies it'll compensate for it them? (If my batteries can comfortably handle a C/5 charge rate and not the assumed C/8 (more than 30A charge) depending on what their resistance actually is, they may be able to use up to 45A charge, I'll research it and see)

Thanks

[sensij]

I never experience temps lower than 45-50 when the sun is up, It can get to 20s at night and the first 90 minutes of day break so not sure how that plays in to it (again that dang real world making problems)?
So 60 cell panels have a lower Voc rating and can handle a wider range of temperatures than 72 cells without pushing over the Max Voc of the charge controller?

Im probably looking at a morning star MPPT TS 45, do you know what their Voc max is?

Thanks for the help

Voc max of the MPPT TS 45 is 150 V. That means that you can generally get away with a string that is about 125 Voc at STC (standard conditions, as reported on the datasheet), but it depends on the actual temperatures you may see. That usually equates to 3 x 60 cell panels or 2 x 72 cell panels. In some warmer locations, a 3rd 72 cell will work, but not if you are in the 20's. You don't want to get too close to the limit... as J.P.M. has pointed out in other threads, the surface temperature of the array in the morning could actually be less than ambient temp, with open exposure to a cold night sky.

The first 90 minutes of the day is the most dangerous time for voltage damage to the CC. Full voltage is generated with only a little bit of sunlight, and that is the coldest time.

Now that is some real world information, so two 225PVw 60 cell panels would be better than 1 330PVw panel for avoiding hitting the CC Voc rating limits?

If I did 2 225W panels itd generate 37.5A and my batteries at a C/8 are 27A charge/discharge rate, so If I limited the charging amps on the controller to 30A, what would be the difference between a single 330W and 2 225W panels, just the amount of time I can supply the 30A?

Yes, with 450 W instead of 300 W, you'll be capable of generating 150% more power for every moment that you haven't maxed out at 30 A, and extending the amount of time you are capable of generating the full 30 A. I'd argue that 60 cell 260-280 W panels are the most cost effective to build around with today's panels... < 250 W panels take the same amount of space, don't really give you any more design flexibility, and aren't usually any cheaper per watt. 72 cell panels are worth considering if they fit better in the space, but usually they are more limiting for your design options.

[Wrybread]

I'd suggest the part about "nothing in the solar world" may refer to more than 12 Volt systems.

Not in an RV forum. But to be clear, I'm *NOT* talking about grid tied or large scale off-grid systems, just RV solar.

[J.P.M.]

This thread is in the "12 V Solar for RV" section of the forum, so I think the risk profile is somewhat different than for rooftop grid tie systems. I think both Sunking and Wrybread are right to point out that the stored energy, be it in gas or batteries, requires serious attention to safety.

This has all drifted quite a way from the OP's questions, though.

I'd suggest the part about "nothing in the solar world" may refer to more than 12 Volt systems.

[Wrybread]

Me thinks RV's have gas tanks.

Of course. But those are thousands of times safer than those plastic gas cans that people strap to their campers for generators.

And don't get me wrong, I'm not saying that the risk from a gas can isn't manageable, we've all carried them. But for someone who spends a good half his day talking about safety on this forum, you seem to be overlooking that massive risk pretty often, while recommending to people that they add that risk to their setups unnecessarily.

[Sunking]

Me thinks RV's have gas tanks.

[sensij]

I'd call the potential of getting zapped by several hundred Volts and then falling off a roof as somewhat comparable in terms of danger.

This thread is in the "12 V Solar for RV" section of the forum, so I think the risk profile is somewhat different than for rooftop grid tie systems. I think both Sunking and Wrybread are right to point out that the stored energy, be it in gas or batteries, requires serious attention to safety.

This has all drifted quite a way from the OP's questions, though.

[Wrybread]

Well Wrybread you are the only person on the planet who has never had a cloudy spell. Unfortunately for the rest of us have cloudy spells and without a genny or battery isolator are forced to shut down and wait a few days to recharge the batteries. You really expect anyone to believe your fairy tales? You gotta be pretty gullible to believe you have been doing this for 20 years and never had a cloudy spell and batteries that never get discharged to the point you went dark. Or never needed to EQ the batteries. You are are full of chit. You might fool a few folks with no knowledge of solar and batteries, but not folks in the biz, educated, with knowledge and real experience.

I never said I haven't had a cloudy spell, and I never said that in 20 years that my batteries have never run low. Of course they have, especially when my batteries are at the end of their life, and especially before the era of inexpensive solar, MPPT charge controllers, cheap LEDs, and the internet. I don't miss the days of spending $450 for 75 watt panels, or having to buy LEDs in truck stops!

But I still didn't need a generator.

You talk as if we all have someone on life support in our RVs, and if there's a single outage over the course of years that person will die. It's an RV, there just isn't that kind of urgency to the situation. And if someone wants to use AC power to equalize their batteries there's usually far easier places to plug in than a generator.

Remember, we're talking about real world RV installs here, not theoretical installations. You seem to have a hard time remembering that.

I'd call the potential of getting zapped by several hundred Volts and then falling off a roof as somewhat comparable in terms of danger.

If you've got several hundred volts in an RV install, I'd say you're doing it wrong.

Falling off the roof is of course a danger, but we don't need to climb on the roof very often, while the danger from a gas can exists whenever it's around.

[J.P.M.]

And, on the subject of safety, there's nothing in the solar world that comes close to the danger of hauling around cans of gas for a generator.

I'd call the potential of getting zapped by several hundred Volts and then falling off a roof as somewhat comparable in terms of danger.

[Sunking]

Well Wrybread you are the only person on the planet who has never had a cloudy spell. Unfortunately for the rest of us have cloudy spells and without a genny or battery isolator are forced to shut down and wait a few days to recharge the batteries. You really expect anyone to believe your fairy tales? You gotta be pretty gullible to believe you have been doing this for 20 years and never had a cloudy spell and batteries that never get discharged to the point you went dark. Or never needed to EQ the batteries. You are are full of chit. You might fool a few folks with no knowledge of solar and batteries, but not folks in the biz, educated, with knowledge and real experience.

[Wrybread]

Unfortunately most people (or significant others) want AC because they are too hot. So unlike you (who I admire living as basic as possible) a genny is a necessary evil and IMO can come in handy if needed when the batteries are low and you have no other way to charge them. It does happen sometimes to us less fortunate people that need creature comforts.

Small point: I have creature comforts! I'm *not* living as basically as possible. As the expression goes "any fool can be uncomfortable". I'm running multiple computers pretty much all day and much of the night, a lot of lights all night, music, heat, charging lots of devices, and I even power my fridge off electricity part time. Whatever I want, I hardly give a thought to power. Just common sense simple optimizations like switching to LEDs, no microwave, no electric coffee maker. If someone needs a lot of heat consider a Buddy heater or something similar that doesn't have a heinously inefficient fan (most are around 80 watts).

And by the way I'm far from the only one who noticed that with some optimizations they don't need a generator. Most of the people I know who installed decent solar systems have ditched their gennies, or just never use the thing anymore.

Then there's no stinky, noisy and flaky generator running all the time.

And, on the subject of safety, there's nothing in the solar world that comes close to the danger of hauling around gas cans for a generator.

[SunEagle]

That's *highly* debatable. I've never once needed a generator in my RV, and I've been at it for 20+ years. Now the first thing I do when I get a new RV is rip the gennie out. Makes a great spot for battery storage by the way.

Unless you're using an air conditioner, there's absolutely zero need for a generator in my experience, especially if you're not living in the camper. Solar is your friend, especially in an RV.

And despite the recent turn in this thread, there's no need to over complicate it. As long as you're keeping it safe of course.

Unfortunately most people (or significant others) want AC because they are too hot. So unlike you (who I admire living as basic as possible) a genny is a necessary evil and IMO can come in handy if needed when the batteries are low and you have no other way to charge them. It does happen sometimes to us less fortunate people that need creature comforts.