Charge controller for large 1766AH, 48V battery bank with 6 hour peak sunshine

[SunEagle]

According to Trojan's website, they clearly state that the battery charging process has to be done at a higher than nominal voltage. For a 48V battery, the charging has to be done even up to 59.3V during Bulk phase and 64.8V during EQ. So that means that the MPPT Controller will pump out current at the 59.3V... no ? What am I missing here? If I want to calculate the amount of current coming from the MPPT at any instant of time it will be: Array Wattage/59.3V. Look:
1.png

Technically you are correct. Although the real current value will be determine by the CC based on the voltage it is providing the battery.

I would go back and check what the manufacturer states is the higher input wattage input to that CC based on the battery voltage (12, 24 or 48). You should be able to get a quick wattage rating by multiplying the battery system voltage by the current rating.

So for a 48volt battery using a 60A CC that will come to 2880watts. Now the CC may state you can exceed the calculated wattage by 10 % which may get you to 3200 watts but I doubt it will state it can handle 3600 watts. More than likely it will clip some of the wattage so it does not exceed the 60A rating. All that does is over heat the CC and reduce your true pv generating wattage.

[BackwoodsEE]

So for a 48volt battery using a 60A CC that will come to 2880watts. Now the CC may state you can exceed the calculated wattage by 10 % which may get you to 3200 watts but I doubt it will state it can handle 3600 watts. More than likely it will clip some of the wattage so it does not exceed the 60A rating. All that does is over heat the CC and reduce your true pv generating wattage.

I've watched my TS-MPPT-60 in action with a real-time voltage-current tracing device I designed and built. What the charge controller does to limit battery current is pretty slick, and does not dissipate heat anywhere downstream of the panels. It simply adjusts where it operates on the PV array's IV curve so that less power is drawn from the array.

For example, right now my toy 3-panel array is producing a mere 110-120W to float charge the battery and run a few isolated loads. The PV current is hovering around 98-99V, well above the maximum power point. If I turned on more load (I've done this many times with big incandescent lights while watching the real-time plot), it would slide the voltage down to the MPPT and the PV current would rise dramatically. Occasionally, I've also seen it move the PV voltage up, toward a MPPT that was higher where it got left the last time it reduced power draw.

It's provided me hours of free geek entertainment watching the current voltage and current point slide left/up and right/down along the IV curve as the charge controller adjusts its production to match the inverter's consumption on a float-charging battery. A good MPPT charge controller is a sophisticated device indeed.

[Sunking]

According to Trojan's website, they clearly state that the battery charging process has to be done at a higher than nominal voltage. For a 48V battery, the charging has to be set to 59.3V during Bulk phase and 64.8V during EQ. So that means that the MPPT Controller will pump out current at the 59.3V... no ? What am I missing here? Look: 1.png



At what point in time does the MPPT controller actually sets the 59.3V charging voltage ? Early on in the charging process or near completion ?

You are looking at a voltage Set Point. Again READ THIS so you understand how a battery charges.

Example if your 48 volt 1766 AH battery SOC were 45% would have an OCV of exactly 48 volts. Such a monster of a battery would have an Ri of 0.0006 Ohms. To make that battery go from 48 to 60 volts on a charger instantly would require [60 volts - 48 volts] / .0006 Ohms = 20,000 Amps. At 20,000 amps x 60 volts is 1,200,000 watts. Do you have 1.2 Mw solar panel with a 20,000 Amp MPPT Controller?

MAX MPPT OUTPUT CURRENT = PANEL WATTAGE / NOMINAL BATtERY VOLTAGE

A 48 volt nominal voltage = 48 volts.

Again the Voltage of a Charging Battery = OCV + [Charge Current x Ri}

Any battery charger is not a pure Voltage Source with unlimited current. They are a Current Source up to the point where the loadcurrent is equal to or less than the chargers current rating. You can certainly set the charger to 60 volts. That does not mean the output voltage is 60 volts. It is only at 60 volts when there is no current.

A Solar CC is different than a commercial AC charger. A 10 amp commercial AC charger provides 10 amps until the Voltage Set Point is reached. Connect a 12 volt battery to a 10 amp AC charger and the current is 10 amps with a starting voltage of 12.1 volts. 10 amps x 12.1 volts is 121 watts. It continues to charge at 10 amps until it nears the Voltage set point of say 15 volts. 10 amps x 15 volts is 150 watts, Power went up and is variable in an AC charger. Current is constant in the AC charger.

Not so with Solar. Power is limited. Say you have a 120 watt panel with a 10 amp CC. Connect it to a 12 volt battery with an OCV of 12 volt, current is 10 amps. 12 volts x 10 amps is 120 watts. Now the battery voltage starts to rise say 13 volts, and panel wattage is still 120 watts means the current is now 120 watts / 13 volts = 9.2 amps. At 14 volts is 8.6 amps. If the Voltage set Point is 14.2 volts, and the battery voltage is 14.2 volts is ZERO AMPS. The battery is saturated and fully charged.

[SillySnakes]

Alright, I think I understand now. I read the thread Sunking pointed to.

So here is a table of my future panel output wattage as a function of time (thanks to PWatts/SAM simulation). The panel is a 315W STC panel. So in calculating how much total AH this panel will produce a day, I divide each Watts by 48V during each hour....and then sum up all the Amperage per hour to give me total AH of capacity generated at the end of the day, correct ? From this I can plan out how many panels/controllers I need to recharge my 1160AH bank properly per day
2.png

[Sunking]

You are making this a lot harder than need be. Gotta go for now.

[Sunking]

OK I have a few minutes. The design process is simple and straightforward. Here are the Cliff Notes from this Thread.

1. Determine worse case daily Watt Hour consumption. Example let's say 17 Kwh per day.

2. Determine Battery Watt Hour Capacity = 5 x Daily Wh. You need 5 day reserve capacity to give you 3 days without recharge (cloudy spells) and never allow batteries to go below 50% DOD. So in our example 5 x 17 Kwh = 85 Kwh

3. Find Panel Wattage. Determine worse case month Sun Hours. Say 5 Sun Hours. Panel Wattage = [1.5 x Daily Watt Hours / Sun Hours, or [1.5 x 17,000 wh] / 5 Sun Hours = 5100 Watts. You multiply by 1.5 to account for all system losses using MPPT Charge Controller. If you use PWM you multiply by 2 which would be STUPID.

4. Determine appropriate battery voltage based on Daily Watt Hours. You want to do this so you only have to use one charge controller. Largest charge controller out there is 100 amps like the Midnight Classic 150. In this example 48 volt battery is as low as you can go. So we select 48 volt battery.

5. Determine MPPT Minimum Charge Controller Size = Panel Wattage / Nominal Battery Voltage or 5100 watts / 48 volts = 106 amps. Pushing the limits of a Midnite Classic 150 but doable. It will clip at 96 amps.

6. Determine Battery AH Capacity = Total WH Capacity / Nominal Battery Voltage. In this example 85,000 wh / 48 volts = 1770 AH battery @ 48 volts

7. Determine Largest Inverter than can be used is simple, No larger than panel Wattage. You could use as small as 700 watts if the load is a 700 watt light bulb on 24 hours a day, or as large as 5000 watts.

[SunEagle]

I've watched my TS-MPPT-60 in action with a real-time voltage-current tracing device I designed and built. What the charge controller does to limit battery current is pretty slick, and does not dissipate heat anywhere downstream of the panels. It simply adjusts where it operates on the PV array's IV curve so that less power is drawn from the array.

For example, right now my toy 3-panel array is producing a mere 110-120W to float charge the battery and run a few isolated loads. The PV current is hovering around 98-99V, well above the maximum power point. If I turned on more load (I've done this many times with big incandescent lights while watching the real-time plot), it would slide the voltage down to the MPPT and the PV current would rise dramatically. Occasionally, I've also seen it move the PV voltage up, toward a MPPT that was higher where it got left the last time it reduced power draw.

It's provided me hours of free geek entertainment watching the current voltage and current point slide left/up and right/down along the IV curve as the charge controller adjusts its production to match the inverter's consumption on a float-charging battery. A good MPPT charge controller is a sophisticated device indeed.

I agree that a high end MPPT CC will be able to do a lot more then a cheap MPPT. But most 60Amp MPPT CC's will limit the output to no more than 1.10% of that 60amp rating and some max out at 100% of the rating depending on both the internal components and software.

[sensij]

I agree that a high end MPPT CC will be able to do a lot more then a cheap MPPT. But most 60Amp MPPT CC's will limit the output to no more than 1.10% of that 60amp rating and some max out at 100% of the rating depending on both the internal components and software.

Read what BackwoodsEE wrote a couple more times. He is not saying that the controller produces more than it's rating. He is explaining how the operating point of the array is adjusted to control the power. This is very basic mppt functionality... Cheap ebay junk might not have it, but most legit controllers are capable of controlling array power in this way, certainly the midnight controllers the op is telling about. They can handle arrays much larger than you are suggesting, with no extra heat dissipation at the controller.

For a system of this size, I would ignore sunking's rules of thumb for solar newbies and use better engineering tools like PVWatts or SAM to estimate solar output. The table the OP posted looks like the right approach.

Back to one of the original questions, cables and busbars are sold that are rated for these high currents. It is important that all terminations are made in a way that meets the rating... Hydraulic crimps, bolts torqued to the proper value, etc. Don't cut corners on here.

[sensij]

Alright, I think I understand now. I read the thread Sunking pointed to.

So here is a table of my future panel output wattage as a function of time (thanks to PWatts/SAM simulation). The panel is a 315W STC panel. So in calculating how much total AH this panel will produce a day, I divide each Watts by 48V during each hour....and then sum up all the Amperage per hour to give me total AH of capacity generated at the end of the day, correct ? From this I can plan out how many panels/controllers I need to recharge my 1160AH bank properly per day

Just keep in mind that the power estimate produced by PVWatts or SAM will have assumptions on losses, and the default loss values are for grid tie systems, not battery systems. It would be appropriate to use the DC power modeled by each system as a starting point to which charge controller and battery losses would be applied.

Another thing to keep in mind is that only the bulk stage of charging uses all available panel power, but that only gets you 80% charged or so. Once the controller moves into absorb stage at a fixed voltage, there current the battery can accept becomes limited. This means not all of the energy available from the array is used, and is effectively another loss factor to consider when sizing the array based on daily energy of Ah output.

[littleharbor]

Another thing to keep in mind is that only the bulk stage of charging uses all available panel power, but that only gets you 80% charged or so. Once the controller moves into absorb stage at a fixed voltage, there current the battery can accept becomes limited. This means not all of the energy available from the array is used, and is effectively another loss factor to consider when sizing the array based on daily energy of Ah output.

And a good reason to use opportunity loads if you can. Many off grid folks will delay laundry, vacuuming or other high load activities till afternoon when this otherwise wasted potential power is available.

[Sunking]

Read what BackwoodsEE For a system of this size, I would ignore sunking's rules of thumb for solar newbies and use better engineering tools like PVWatts or SAM to estimate solar output.

Look at his chart for that day. It tells him he receives 5.2 Sun Hours. No reason to make it anymore complicated. 1649 wh / 315 w = 5.23 Sun Hours. You use PV watts to find th eworse case month and use the Sun Hours. Works everytime.

[J.P.M.]

Look at his chart for that day. It tells him he receives 5.2 Sun Hours. No reason to make it anymore complicated. 1649 wh / 315 w = 5.23 Sun Hours. You use PV watts to find th eworse case month and use the Sun Hours. Works everytime.

Provided the user knows how to use PVWatts, or particularly SAM, get the inputs reasonably close to reality and applies some judgment to the results. GIGO.

[Sunking]

Provided the user knows how to use PVWatts, or particularly SAM, get the inputs reasonably close to reality and applies some judgment to the results. GIGO.

Agree and it is simple. Use a 1000 watt panel, 0% loss, input location orientation, and tilt will get you Sun Hours. Look for the month with lowest production. 1.5 Multiplier used takes care of the rest.

KISS

[J.P.M.]

Agree and it is simple. Use a 1000 watt panel, 0% loss, input location orientation, and tilt will get you Sun Hours. Look for the month with lowest production. 1.5 Multiplier used takes care of the rest.

KISS

Without getting into how off grid or battery system requirements (about which I know little) might impact, influence or change sizing or orientation considerations, I'd tend to stick with PVWatts for annual output estimates, and modify/iterate the array size and parameters to mesh with requirements of the after array equipment and application particulars. Maybe that's what your method does. But, respectfully, what you're suggesting seems to have more leaky variables in it than I'd use. I try to save my necessary fudge factors and their quantity until the end of the analysis. However, given my limited knowledge of off grid storage particulars, I don't have a better option to offer.

[Sunking]

Without getting into how off grid or battery system requirements (about which I know little) might impact, influence or change sizing or orientation considerations, I'd tend to stick with PVWatts for annual output estimates, and modify/iterate the array size and parameters to mesh with requirements of the after array equipment and application particulars. Maybe that's what your method does. But, respectfully, what you're suggesting seems to have more leaky variables in it than I'd use. I try to save my necessary fudge factors and their quantity until the end of the analysis. However, given my limited knowledge of off grid storage particulars, I don't have a better option to offer.

Sure if I am being paid to do it. Then I would take the time for all factors. But after doing several hundred designs, the difference is so small is insignificant.

With a grid tied makes no difference if you miss the mark over or under, it still works and every drop of power is used regardless. With a battery system there is only one side you can afford to miss on, the high side. Two designs have to be done. One for summer, and one for winter. You use the larger of the two which 90% of the time is winter. Come up short and you go dark, and destroy batteries, and/or use more generator fuel than necessary. If off-grid you had better have a generator to protect your battery investment.

So for me if I were being paid I run PV Watts, input a 1 Kw panel, 0% loss, orientation, and tilt. Click Go and see what the shortest days are. In the USA is going to be either December or January. So if 1 Kwh panel in December generates say 3200 watt hours. I know Sun hours is 3.2 is worse case. You cannot use yearly average as that is for Grid Tied only.

FWIW this is the method John Wiles and a few other pioneers came up with and tested at NMSU. At the time John and his grad students used a simple 100 watt panel with a MPPT Tracker and Dump Load to find Sun Hours. If that 100 watt panel generated 320 watt hours, Sun Hours = 3.2. That was well before PV Watts was ever conceived. John Wiles and his grad students then went on with JPL and NASA and developed PV Watts. No longer need to set out a panel for two months in winter. It is very simple and straight forward and a 5th grader can do it.

Yes on the Forum I multiply by 1.5 the daily watt hour usage. Works every time and never comes up short which is death. When I crunch the numbers once I know all the equipment, batteries, cable losses I have never ever came in lower than 1.3. For a DIY it is just not worth the time and effort to try to shave off 5 to 15% of panel power. Well unless you are talking something like a 2000 watt or more system. Otherwise 1.5 always works my friend, and 1.4 may fail my friend. Just KISS it.

Think of it this way JPM. You are the pilot of the plane. Your plane burns 13 gallons an hour. You calculate under current conditions will take 3 hours to get to your destination. As a pilot you want the plane to be light as possible and safe. So how much fuel do you have loaded, 3 x 13 = 39 gallons. Or do you follow the rules and make sure you have at least 52 gallons. DIY uses 39 gallons and crashes most of the time.

As my wife says, I don't have much of a butt. That is why I wear a belt and suspenders. No butt cracks or falling pants. .