Newbie Questions about Calculations
Ukesh
Registered Users Posts: 11 ✭
Hey Guys.
This is my First post here. Been reading about PV solar systems and have a few doubts. Just want to get it clarified with the experts.
1. How much Watts am I really getting from a Panel?
Say I have a 250W panel and using an MMPT charge controller for an Off Grid system. Derating factor is 80% and MMPT controller efficiency is 0.66
So Practically, Power = 250 * 0.8 * 0.66 = 132 W out of a 250W Panel. Is this correct ?
Also is there any other efficiency loss that has to be taken into account for ?
2. Time taken to recharge the battery?
Say, I have a 100 AH Deep cycle battery and its used till 50% DOD. Assuming Panel outputs 10 Amps (C/10) of current, how long will it take for the battery to get fully charged?
This is my First post here. Been reading about PV solar systems and have a few doubts. Just want to get it clarified with the experts.
1. How much Watts am I really getting from a Panel?
Say I have a 250W panel and using an MMPT charge controller for an Off Grid system. Derating factor is 80% and MMPT controller efficiency is 0.66
So Practically, Power = 250 * 0.8 * 0.66 = 132 W out of a 250W Panel. Is this correct ?
Also is there any other efficiency loss that has to be taken into account for ?
2. Time taken to recharge the battery?
Say, I have a 100 AH Deep cycle battery and its used till 50% DOD. Assuming Panel outputs 10 Amps (C/10) of current, how long will it take for the battery to get fully charged?
Comments
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Re: Newbie Questions about Calculations
Welcome to the forum.
Say I have a 250W panel and using an MMPT charge controller for an Off Grid system. Derating factor is 80% and MMPT controller efficiency is 0.66
So Practically, Power = 250 * 0.8 * 0.66 = 132 W out of a 250W Panel. Is this correct ?
The MPPT controller efficiency depends upon the particular controller, as well as how much power it is handling and the voltages at which it is working. Under most circumstances, I would expect to see a controller effeciency of about 95%.
Solar panel power derating depends upon many factors. temperature, insolation, atmospheric conditions, tilt of panel, etc. Your number of 80% is reasonable.
The moderator "BB." just posted some numbers to consider: http://forum.solar-electric.com/showthread.php?21634Time taken to recharge the battery?
Say, I have a 100 AH Deep cycle battery and its used till 50% DOD. Assuming Panel outputs 10 Amps (C/10) of current, how long will it take for the battery to get fully charged?
Do you mean panel output of 10 amps, or do you mean controller output of 10 amps?
Charging up to about 90% SOC is pretty efficient... Thus 10 amps (into the battery) for four hours would take the battery from 50 ah (50% SOC) to about 90 ah (90% SOC). Getting from 90% to 100% SOC will probably take at least another 2 hours... depends on the battery. Best way to be sure is to check the SG.
--vtMaps4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i -
Re: Newbie Questions about CalculationsWelcome to the forum.
The MPPT controller efficiency depends upon the particular controller, as well as how much power it is handling and the voltages at which it is working. Under most circumstances, I would expect to see a controller effeciency of about 95%.
Solar panel power derating depends upon many factors. temperature, insolation, atmospheric conditions, tilt of panel, etc. Your number of 80% is reasonable.
The moderator "BB." just posted some numbers to consider: http://forum.solar-electric.com/showthread.php?21634
Perfect. Exactly what I wanted. Thanks for the Link!Do you mean panel output of 10 amps, or do you mean controller output of 10 amps?
Charging up to about 90% SOC is pretty efficient... Thus 10 amps (into the battery) for four hours would take the battery from 50 ah (50% SOC) to about 90 ah (90% SOC). Getting from 90% to 100% SOC will probably take at least another 2 hours... depends on the battery. Best way to be sure is to check the SG.
--vtMaps
I Meant Panel output of 10 Amps. Is there a way to calculate the approximate time taken for the recharging?
Like, Would it be possible to recharge the battery from 50% DOD at 10 Amps during winter with 4 hours of sun ? But from your response I think I would need at-least 6 hours right? -
Re: Newbie Questions about CalculationsI Meant Panel output of 10 Amps.
Panel output (in amps) is irrelevant with an MPPT controller (but entirely relevant with a PWM controller). If your panel is 250 watts and your battery charging starts out at 12 volts (50% SOC), your amperage into the battery through an MPPT controller will be just a bit under 20 amps.
That would be a very high charge rate for a 100 ah battery (unless it is an AGM battery). If the battery can handle the charge it should go from 50% to 90% SOC in about two hours. The last 10% always takes longer... how long depends on the battery.... probably a couple of hours.
--vtMaps4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i -
Re: Newbie Questions about CalculationsSay I have a 250W panel and using an MMPT charge controller for an Off Grid system. Derating factor is 80% and MMPT controller efficiency is 0.66
So Practically, Power = 250 * 0.8 * 0.66 = 132 W out of a 250W Panel. Is this correct ?
It depends on insolation. With just the right conditions, you can get up to 300W. On a sunny day and good angle in the summer, it's about 230W. In hot weather may be 210W. On a cloudy day, may be from nothing to 50W - depends on the thickness of the clouds. Nothing at night, even with a moonlight. -
Re: Newbie Questions about CalculationsPanel output (in amps) is irrelevant with an MPPT controller (but entirely relevant with a PWM controller). If your panel is 250 watts and your battery charging starts out at 12 volts (50% SOC), your amperage into the battery through an MPPT controller will be just a bit under 20 amps.
That would be a very high charge rate for a 100 ah battery (unless it is an AGM battery). If the battery can handle the charge it should go from 50% to 90% SOC in about two hours. The last 10% always takes longer... how long depends on the battery.... probably a couple of hours.
--vtMaps
For Flooded Lead acid batteries charge rate should be around C/10 if I'm correct. For 100 AH battery it comes to 10 Amps and battery starts charging at 12 Volts (50% DOD). What would be the correct panel wattage ?
12V * 10A = 120 Watts. Is that Correct ?
Also Does rated Voltage of a Panel ie 12V or 24V has any part to play while using MMPT Controller? -
Re: Newbie Questions about CalculationsIt depends on insolation. With just the right conditions, you can get up to 300W. On a sunny day and good angle in the summer, it's about 230W. In hot weather may be 210W. On a cloudy day, may be from nothing to 50W - depends on the thickness of the clouds. Nothing at night, even with a moonlight.
I understand that NorthGuy. But just want an approximate value to make calculations. -
Re: Newbie Questions about Calculations
The actual math (rules of thumbs) would be:- 10 amps * 14.5 volts charging * 1/0.77 panel+controller derating = 188 Watt panel minimum
A 120 watt panel would (typically) output:- 120 watts * 1/14.5 volt charging * 0.77 panel+controller derating = 6.37 amps (typical noon time maximum output)
It is not pretty.. .and the actual math is different between PWM and MPPT type charge controllers... For for a typical system running 9 months of the year, the rules of thumbs are close enough to design a system around.
If you do things like mount a panel/array flat to roof (typical RV installation), and camp in the northern US/Canada in the winter--You need to change the tilt on the panel to near vertical (in deep winter) and usually need to use a generator for backup power (or use less power on days with poor sun)...
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: Newbie Questions about CalculationsI understand that NorthGuy. But just want an approximate value to make calculations.
A 250W panel in average climate (if such thing exists) is capable of producing about 300kWH/year as seen on the output of a charge controller (or GT inverter). -
Re: Newbie Questions about CalculationsThe actual math (rules of thumbs) would be:
- 10 amps * 14.5 volts charging * 1/0.77 panel+controller derating = 188 Watt panel minimum
A 120 watt panel would (typically) output:- 120 watts * 1/14.5 volt charging * 0.77 panel+controller derating = 6.37 amps (typical noon time maximum output)
It is not pretty.. .and the actual math is different between PWM and MPPT type charge controllers... For for a typical system running 9 months of the year, the rules of thumbs are close enough to design a system around.
If you do things like mount a panel/array flat to roof (typical RV installation), and camp in the northern US/Canada in the winter--You need to change the tilt on the panel to near vertical (in deep winter) and usually need to use a generator for backup power (or use less power on days with poor sun)...
-Bill
Bill, by doing the math for a 250W panel current comes around 13A.
250 watts * 0.77 (derating) / 14.5 volts = 13.2 Amps. Is that an overkill for 100 Ah battery ? -
Re: Newbie Questions about CalculationsA 250W panel in average climate (if such thing exists) is capable of producing about 300kWH/year as seen on the output of a charge controller (or GT inverter).
Thanks NorthGuy. Approx it comes around 0.8 KWh per day. -
Re: Newbie Questions about Calculations
More or less, the usual recommended charging current is ~13%... When you go above 13%, you should have a remote battery temperature sensor (available as an option for many solar charge controllers) to help prevent thermal run-away (and help maintain optimum battery charging voltages).
250 Watt * 0.77 panel+controller derate * 1/14.5 volts = 13.3 Amps
Going much above 13% for solar power systems is generally "over paneled"--The batteries will be full by noon (in many applications)... If you have poor sun due to shade or weather conditions, then more panels won't hurt.
Generally, going above 20-25% rate of charge is not recommended.
Check battery ratings... Some batteries, like many GEL, are rated for less maximum current (many GEL batteries in the US are limited to C/20 or 5% maximum charging current--That is really too low for many solar applications where you have a limited number of hours of sun per day).
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: Newbie Questions about CalculationsThanks NorthGuy. Approx it comes around 0.8 KWh per day.
Yes, on average in average climate. You can get 2kWh one day and then 0.05kWh the other day. -
Re: Newbie Questions about CalculationsMore or less, the usual recommended charging current is ~13%... When you go above 13%, you should have a remote battery temperature sensor (available as an option for many solar charge controllers) to help prevent thermal run-away (and help maintain optimum battery charging voltages).
250 Watt * 0.77 panel+controller derate * 1/14.5 volts = 13.3 Amps
Going much above 13% for solar power systems is generally "over paneled"--The batteries will be full by noon (in many applications)... If you have poor sun due to shade or weather conditions, then more panels won't hurt.
Generally, going above 20-25% rate of charge is not recommended.
Check battery ratings... Some batteries, like many GEL, are rated for less maximum current (many GEL batteries in the US are limited to C/20 or 5% maximum charging current--That is really too low for many solar applications where you have a limited number of hours of sun per day).
-Bill
Got it Bill.
Likewise is there any math to calculate time taken to recharge the battery ? Say I use 250W panel and put 13.2 Amps into the battery how long to get the battery back to 100% SOC from 50% SOC ?
The part I get confused while designing an simple Off Grid system is while selecting the panel. If I go for a 200W panel charging current is exactly at 10A but my KWh is reduced (Lesser than my requirement). If I go for 250W panel I get my required KWh but my charging current is around 13A which might be harmful for the battery life.
How do you guys about this while making calculations? -
Re: Newbie Questions about CalculationsLikewise is there any math to calculate time taken to recharge the battery ? Say I use 250W panel and put 13.2 Amps into the battery how long to get the battery back to 100% SOC from 50% SOC ?
A rough estimate--For 50% to 80% state of charge, the battery is near 100% efficient during charging:
30% recharging * 100 AH = 30 AH to recharge.
30 AH / 13.2 amps charging = 2.3 hours of bulk...
and about another 2-4 hours of "absorb" (charge controller holding 14.5 volts) or so to fully recharge the battery bank.
But remember, the solar array will only output near rated power during the middle of the day, in the morning and evenings, the output current is reduced (fixed panels, more atmosphere for light to travel through, etc.).
Another way to look at it...
13.2 amps * ~4 hours minimum/nominal sun per day = 52.8 Amp*Hours per fall/spring day (more hours of sun in towards summer).
Your flooded cell battery bank will take:
50 Amp*Hours * 1/0.80 battery charging efficency = 62.5 AH to recharge bank
62.5 AH per day (fifty percent discharge) / 52.8 AH per day = 1.2 "sunny days to recharge" (4 hour typical late winter/early spring sunny day).The part I get confused while designing an simple Off Grid system is while selecting the panel. If I go for a 200W panel charging current is exactly at 10A but my KWh is reduced (Lesser than my requirement). If I go for 250W panel I get my required KWh but my charging current is around 13A which might be harmful for the battery life.
How do you guys about this while making calculations?
The whole reason we go "balanced systems" and rules of thumbs to sketch out a system design. They usually work pretty well for "most" applications.
13% or 13.5% rate of charge is not going to negatively affect the battery bank.
Using somewhat more expensive charge controllers that support remote battery temperature sensors help reduce the chance for battery damage.
If you start doing something "different" (such as using lots of off grid power just during the day)--Then we have to start looking at different questions. Say you want to pump water--Then we may suggest a pure Solar Panel+Direct connect water pump and avoid a battery system altogether.
If you need to pump water even on a cloudy day, then you need a battery bank. And we usually need to have a larger solar array that can run the solar pump plus recharge the battery bank at the same time (next sunny day), etc.
It all depends. But the "balanced system" design are usually a good place to start.
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: Newbie Questions about CalculationsHow do you guys about this while making calculations?
Most go with ballpark estimates. For every kWh/day of loads you need 4kWh of battery capacity and 500W of panels. -
Re: Newbie Questions about CalculationsA rough estimate--For 50% to 80% state of charge, the battery is near 100% efficient during charging:
30% recharging * 100 AH = 30 AH to recharge.
30 AH / 13.2 amps charging = 2.3 hours of bulk...
and about another 2-4 hours of "absorb" (charge controller holding 14.5 volts) or so to fully recharge the battery bank.
But remember, the solar array will only output near rated power during the middle of the day, in the morning and evenings, the output current is reduced (fixed panels, more atmosphere for light to travel through, etc.).
Another way to look at it...
13.2 amps * ~4 hours minimum/nominal sun per day = 52.8 Amp*Hours per fall/spring day (more hours of sun in towards summer).
Your flooded cell battery bank will take:
50 Amp*Hours * 1/0.80 battery charging efficency = 62.5 AH to recharge bank
62.5 AH per day (fifty percent discharge) / 52.8 AH per day = 1.2 "sunny days to recharge" (4 hour typical late winter/early spring sunny day).
Thanks for the detailed write up Bill!
A general doubt. Most of the off grid solutions seems to be targeted at using the loads during the Night (from battery) and charging the battery during day time. Cannot find much info about simultaneously using the battery during day time itself while its charging.
Can the battery be used while charging? Can you elaborate on this ? -
Re: Newbie Questions about CalculationsMost go with ballpark estimates. For every kWh/day of loads you need 4kWh of battery capacity and 500W of panels.
Thanks NorthGuy! -
Re: Newbie Questions about CalculationsThanks for the detailed write up Bill!
A general doubt. Most of the off grid solutions seems to be targeted at using the loads during the Night (from battery) and charging the battery during day time. Cannot find much info about simultaneously using the battery during day time itself while its charging.
Can the battery be used while charging? Can you elaborate on this ?
Most battery-based grid tie systems only use the battery when the grid is down. The rest of the time they run from utility and whatever power is available from the PV array at the time.
Almost all off-grid systems charge while in use, meaning the PV has to be able to supply enough power for recharging the batteries while offsetting the load requirements. This is usually done with the simple expedience of targeting a higher peak current rate than is minimally necessary and knowing that the average constant load will not reduce that rate below the minimum. If you have particularly large and/or constant loads that need to be on while battery charging is taking place they must be accounted for in the array sizing.
But basically no one off-grid shuts down their system and waits for the batteries to recharge.
The first 'fudge' on this is using a 20 hour battery rate to calculate supply for 24 hours average loads. In this way you are looking at drawing all loads from the batteries so that, on average, the panels are really only recharging the batteries.
The second 'fudge' is to target a peak 10% charge rate, allowing "5%" to vary between minimum and maximum current level. This is to assure the recharge rate is never too low to be ineffectual.
The other important thing to do is to check the 'harvest' potential of the array (size * hours of equivalent good sun * over-all system efficiency) against the daylight Watt hour demand to make sure it is covered.
And then have a generator ready in case the sun decides not to shine. -
Re: Newbie Questions about Calculations
One of the reasons I avoid the "daytime use" system only design... Is usually even if the system is used 8am-5pm (office/store/work site/etc.)... My first question will be "Can you shut down the power use during cloudy weather?".
During the deep overcast/getting ready to storm days, I have seen my GT system (which always "harvests 100% of the available solar energy" every day) output drop down to ~5% of its normal harvest.
So--If you cannot turn off your power usage during the day for bad weather--Then, you pretty much have to design the system as if it was a daytime charge/nighttime usage (two days of "usable" storage and cut power/turn on genset the third cloudy day).
If the system is "truly" use power during the day only type system... Then I suggest looking to see if the power can be supplied by a solar panel only (no battery bank) system. A very common load of this type is daytime water pumping... Hot/dry/sunny weather--Need lots of pumping. Dark/stormy weather, virtually no bulk water pumping needed.
In the end, if you need a "day time" battery based system... You can probably get away with a smaller battery bank (may be 1/2 size of an off grid system)--But will still need a fairly large array. Because, as Marc typed, you need to support both battery charging and running the loads at the same time. Also, the battery bank has to be large enough to handle starting surges (pumps, motors, etc.) and run the power when a random cloud/bird/kid with balloons shade the array.
And there are secondary issues--Too large of Solar Array + MPPT controller can "overwhelm" a smaller/fully charged battery bank and take a 48 volt battery bank >72 volts and fault the controllers/inverters.
Balanced system design rules are there to make it "easy" to define a reliable off grid power system. You can cut corners if you are willing to experiment--But most people need their power to be pretty much 100% reliable. You don't want to lose power for several hours or several days per year--If you really need reliable power. Of course, a small genset is not a bad backup for utility or off grid systems--And helps you through poor weather.
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: Newbie Questions about Calculations
And there are secondary issues--Too large of Solar Array + MPPT controller can "overwhelm" a smaller/fully charged battery bank and take a 48 volt battery bank >72 volts and fault the controllers/inverters.
Clarification:
Not a problem if everything is set up right and the charge controller is doing its job. Voltage will be where it should be and current will not exceed spec.
What you would not want to do is put a 'full size' array on an MPPT controller (giving it maximum output current potential) and then connect it to a much smaller than maximum battery without programming a current limit as the array could push too much current to a battery in Bulk stage and cook it. Id est: 80 Amps out of a MidNite Classic into a 100 Amp hour 12 Volt battery would be very bad. -
Re: Newbie Questions about CalculationsCariboocoot wrote: »The first 'fudge' on this is using a 20 hour battery rate to calculate supply for 24 hours average loads. In this way you are looking at drawing all loads from the batteries so that, on average, the panels are really only recharging the batteries.
The second 'fudge' is to target a peak 10% charge rate, allowing "5%" to vary between minimum and maximum current level. This is to assure the recharge rate is never too low to be ineffectual.
Ideally If there arent any loads that have high surge currents like Motors, Pumps etc, but just few lights, fans etc , a system with 10% charge rate should be okay for simultaneously using the battery while charging. Correct Cariboocoot ?
On the other hand if the system does contain a heavy load like an AC or Microwave etc how would the design vary ? A higher AH battery ?And there are secondary issues--Too large of Solar Array + MPPT controller can "overwhelm" a smaller/fully charged battery bank and take a 48 volt battery bank >72 volts and fault the controllers/inverters.
Bill for 100AH battey and 250W Panel I'm looking at an 20 or 25 Amp Charge controller, since the max charge current is around 13 Amps. Should be fine right ? -
Re: Newbie Questions about CalculationsAnd there are secondary issues--Too large of Solar Array + MPPT controller can "overwhelm" a smaller/fully charged battery bank and take a 48 volt battery bank >72 volts and fault the controllers/inverters.
However, most charge controller will limit the voltage to a specified value, usually around 58V and will limit it lower to float level when the batteries are charged. -
Re: Newbie Questions about Calculations
It should be OK... Basically, the maximum recommended current would be C/4 or ~25% of the battery 20 Hour capacity rating.
The problem appears to be (I am purely guessing) is that an MPPT controller can run a "sweep" looking for the Maximum Pmp=Vmp*Imp -- and the controller needs to sink the current to the battery bank during the scans. If the current is too much (and the battery bank is fully charged), the excess current can pull the battery out of "regulation" (basically, the battery bank is the primary regulator in any off grid/ DC power system).
I am not sure, but the control loop of (some?) MPPT charge controllers is slow enough that they cannot fold back the current fast enough to prevent the battery bus voltage from exceeding the "Never Exceed" voltage one would expect for a 12/24/48 volt battery bank.
The control loops for a battery charger is quite different than a true power supply that can operate without a battery bank as a ballast. This is normal and not a design problem (in most cases). For example, if you disconnect the car's battery while the engine is running, the alternator can easily exceed the battery voltage by large amounts and blow the lights/computers in the vehicle. The Alternator's Regulator is not capable of holding 14.2 volts without the battery bank to buffer/stabilize the system voltage.
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: Newbie Questions about Calculations
I am not sure, but the control loop of (some?) MPPT charge controllers is slow enough that they cannot fold back the current fast enough to prevent the battery bus voltage from exceeding the "Never Exceed" voltage one would expect for a 12/24/48 volt battery bank.
Moral: don't buy cheap equipment.
The loading question depends on whether it's a matter of a sudden surge such as a motor starting or a constant draw. In the case of the former it's down to having sufficient battery capacity to start the motor (which you should have anyway); one 'blip' of high current will have no affect on over-all panel output. If it is a constant draw you want to have enough power available from the panel to offset the load and be able to maintain the charge rate.
Now a word about that rate: it is a peak potential rate, not a constant or even daily rate. That the panels can produce current of 10% of the Amp hour rating of the batteries is a short cut to designing an array that will be able to fully charge the batteries from about 25% DOD in one good day. It is not an absolute. You may never see that much current if he batteries don't need it, or you may need more if loads demand.
In most cases the batteries start charging at a lower rate when the panels first produce current but are not yet up to full power. As the output from the panels increases the batteries' needs are already decreasing. Thus you have a bell curve of power output available from the panels supplying a reverse curve power demand from the batteries. Toss in the loads and you have the basic problem of power systems everywhere, trying to even out supply and demand.
So you can use the rules-of-thumb to get a basic system design, then you have to look at your particular needs. The worst thing you can do is think you can be precise in all measurements and calculations. The world will not co-operate with such exactitude; weather isn't the same every day, loads aren't the same every day, and equipment will not function the same every day.
Margins are key to success. Err on the side of caution; no one ever complained about having too much power available, but lots of people complain about their batteries going dead or the refrigerator shutting off at 2:00 AM. -
Re: Newbie Questions about CalculationsCariboocoot wrote: »Moral: don't buy cheap equipment.
Unfortunately, this issue does not seem to "go away" when you pay for expensive MPPT controllers either.
As Marc says... You have have the choice of skating on the edge or in the middle of the rink. Everything may work fine--Or not.
-Bill "rules of thumbs are created by experience" B.
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: Newbie Questions about CalculationsI am not sure, but the control loop of (some?) MPPT charge controllers is slow enough that they cannot fold back the current fast enough to prevent the battery bus voltage from exceeding the "Never Exceed" voltage one would expect for a 12/24/48 volt battery bank.
I don't think it is possible to build an MPPT controller that it is so slow. Usually they try to make them as fast as possible to minimize the size of capacitors and inductors necessary for operations. -
Re: Newbie Questions about CalculationsI don't think it is possible to build an MPPT controller that it is so slow. Usually they try to make them as fast as possible to minimize the size of capacitors and inductors necessary for operations.
The venerable Outback MX60 had this problem and could be 'fooled' by conditions on either end of its connections into picking the wrong power point or letting Voltage/current go out of spec momentarily. The FM series and the MidNite Classic react so fast it amounts to continuous re-adjustment.
I think the Morningstar and Xantrex units could still have the sort of trouble Bill describes. There are some really cheap "MPPT" units out there that can barely regulate Voltage as well as your average PWM. I'd say what ones but I couldn't read the name after they burned up.
At any rate, a slight surge in current well above spec to a battery isn't much of a problem. A sudden over-Voltage condition can shut down an inverter - or possibly even pop a component. -
Re: Newbie Questions about CalculationsCariboocoot wrote: »At any rate, a slight surge in current well above spec to a battery isn't much of a problem. A sudden over-Voltage condition can shut down an inverter - or possibly even pop a component.
I charge at 64V (most do less so overvoltage is less of a problem for them). 72V is harmful for the inverter. This is 8V difference. They momentary dV/dI, which people often call battery internal resistance, is 20 Ohm. To go up 8V, you will need 8V /0.02 Ohm =400A! And this is on top of the existing charge+load current. There's absolutely no way this can be produced by arrays/controllers, which are rated 120A. Even if I doubled the array, it still wouldn't be possible. Even if I tripled my array, the probability of harmful voltage would still be very small.
Of course, there could be bad controllers which misregulate voltage. But they do this because the controllers have bad design/programming. This is not because solar arrays produce sudden current spikes. -
Re: Newbie Questions about Calculations
You're assuming the Voltage is always 'clamped' by the batteries. If this were so there would be no danger from stray Voltage which is extremely low current but very high Voltage. -
Re: Newbie Questions about CalculationsCariboocoot wrote: »You're assuming the Voltage is always 'clamped' by the batteries. If this were so there would be no danger from stray Voltage which is extremely low current but very high Voltage.
Sure, if batteries get disconnecred everything will go haywire. For example, if you wire a breaker to disconnect batteries from both charger and inverter at the same time, and it trips ... you can only hope that charge controller engineers thought about that
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