OK. So, thanks you to you kind knowledgeable folk on here, we are starting to understand motorhome electrics. I have a couple more questions if you would be so kind.....
1. Lets assume we have 2 x 80w solar panels on the roof. Total = 160w.
2. I have read on this forum (and others) that a good 'rule of thumb' to ascertain the daily amps that could be expected out of your solar panels is to use the equation 0.33 x Total Wattage (0.33 x 160w) = 52.8amps. This is based on average daylight hours etc. This is actually borne out by my solar controller (PL20) which stated that 55 amp hours were put in, so pretty accurate.
3. OK, I know that when I looked at the Solar Controller and everything else was switched off, the fridge freezer (Waeco 141 Litre) had a load of 6.3a. The only reference to power etc in the fridge manual I can find, is that is it a 75 watt fridge. Using my high school electrics knowledge (Power (Watts) is equal to Amps x Volts), I get 75 watts is equal to 6.25a x 12 volts, so again, pretty accurate. This also shows that there is no voltage drop between the fridge and the battery as this is what I expected the voltage to be at the battery.
4. The question I have is this; If, at the time I measured it, the fridge was drawing 6.3a, does it do this for 24 hours a day? If so, the draw from the fridge alone would be 151 amp hours. My solar panels only put in 55 amp hours, so my shortfall would be 96 amps a day, or in other words, one full battery. This can't be right surely? I understand that we do have a 'deficit' each day from the fridge, but how much is it really? Does the fridge 'cycle' between power needs? What is the REAL amp use of a fridge in 24 hours?
5. The only other question left remaining is; Given some cash restraints, what would you guys choose to have fitted to assist us? (We will be having a new large charger fitted, that's a given) A) An extra battery fitted (i.e 120 a/h AGM) to go with our two current 100 a/h lead acid batteries) or..... B) An extra solar panel (say 120 watts) to compliment the existing two panels and the existing two 100 a/h batteries
As someone else pointed out, I don't want to throw away two decent batteries if they can be fully charged up with a decent charger. Your thoughts are most welcome. Which would you go with, A or B??
Cheers,
Paul & Val
Paul, I have numbered your points so that I may attempt to address them in a way that is not too confusing.
1. I am confident that your total panel wattage is greater than 160w. Why..Because you told us that you were reading 9.5 amps charge, and the absolute best that you could expect from 160w via a series controller on a really - really good day at noon would be a little over 8 amps.
2. There is no rule of thumb that fits every situation. Why..
The maximum power voltage (Vmp) of a 12v panel is around 18v, The maximum power current (Imp) is equal to the rated wattage divided by that voltage, so for a 160w panel, Imp = 160w/18 = 8.9 amps, and that's under ideal test conditions. Once you factor in efficiency losses due to panel temperature, line losses etc, don't count on better than 8 amps.
A series or PWM controller such as the PL20 when it is in the on state connects the battery directly to the panels, this pulls the panel voltage down to the battery voltage. Now comes that catch, the battery is at say 12v. So you now have effective panel watts being W= volts X amps, = 12 X 8 = 96w. So your effective input power from the 160w panel is just 96 watts or 60%.
An MPPT controller does not connect the panels directly to the battery, and so the panel voltage is not pulled down to that of the battery. so now you have that 8 amps at 18 volts (144 watts) being applied to the input of the controller which then converts it to the battery voltage of say 12v.
Watts in must equal watts out, so the amps coming out = 144/12, = 12 amps. Although it will be somewhat less due to conversion losses which are dissipated as heat. The true figure will be closer to 10 amps.
Now allowing the "rule of thumb" that you get an average five peak sun hours per day, you multiply the above figures by five to get the daily Ah input from either type controller. ie series controller = 8 x 5 = 40Ah...MPPT = 10 x 5 = 50Ah. Of course, there will be differences to these figures based on time of year, cloud cover, present latitude, angle of sun on panels, orientation of panels, present battery Soc, etc. etc.
3. Not necessarily correct. You are basing your calculations on 12 volts whereas your battery will (I hope) be closer to 12.7v. So 75/12.7 = 5.9 amps. As the voltage falls, the current increases to attempt to maintain the wattage, and this causes more voltage drop. The only way to know is to compare the voltage at the battery against that at the fridge terminals when the compressor is running.
4. The fridge has an average duty cycle of around 50% in normal temperatures and internal heat load. This means that if the weather is not too hot, say less than 30°C, and the product inside is already cooled, and you don't leave the door open excessively, it will run for a total of 12 hours out of 24 (more often during the day and less at night). Alter any of those parameters and the duty cycle changes.
5. The solar panel would be the first option as there is only temporary respite in fitting more battery without sufficient recharge capability. I would be loathe to put an AGM in parallel with a couple of older flooded batteries as the charge/discharge characteristics are very different from new, and far different if they are aged. First step would be to fully charge the existing batteries and see how they hold up under load, then make a decision from there.
Hope this helps.
moblet said
07:08 PM Nov 12, 2013
brian wrote:
Wirroff wrote:
OK. So, thanks you to you kind knowledgeable folk on here, we are starting to understand motorhome electrics. I have a couple more questions if you would be so kind.....
1. Lets assume we have 2 x 80w solar panels on the roof. Total = 160w.
2. I have read on this forum (and others) that a good 'rule of thumb' to ascertain the daily amps that could be expected out of your solar panels is to use the equation 0.33 x Total Wattage (0.33 x 160w) = 52.8amps. This is based on average daylight hours etc. This is actually borne out by my solar controller (PL20) which stated that 55 amp hours were put in, so pretty accurate.
3. OK, I know that when I looked at the Solar Controller and everything else was switched off, the fridge freezer (Waeco 141 Litre) had a load of 6.3a. The only reference to power etc in the fridge manual I can find, is that is it a 75 watt fridge. Using my high school electrics knowledge (Power (Watts) is equal to Amps x Volts), I get 75 watts is equal to 6.25a x 12 volts, so again, pretty accurate. This also shows that there is no voltage drop between the fridge and the battery as this is what I expected the voltage to be at the battery.
4. The question I have is this; If, at the time I measured it, the fridge was drawing 6.3a, does it do this for 24 hours a day? If so, the draw from the fridge alone would be 151 amp hours. My solar panels only put in 55 amp hours, so my shortfall would be 96 amps a day, or in other words, one full battery. This can't be right surely? I understand that we do have a 'deficit' each day from the fridge, but how much is it really? Does the fridge 'cycle' between power needs? What is the REAL amp use of a fridge in 24 hours?
5. The only other question left remaining is; Given some cash restraints, what would you guys choose to have fitted to assist us? (We will be having a new large charger fitted, that's a given) A) An extra battery fitted (i.e 120 a/h AGM) to go with our two current 100 a/h lead acid batteries) or..... B) An extra solar panel (say 120 watts) to compliment the existing two panels and the existing two 100 a/h batteries
As someone else pointed out, I don't want to throw away two decent batteries if they can be fully charged up with a decent charger. Your thoughts are most welcome. Which would you go with, A or B??
Cheers,
Paul & Val
I'll run with Brian's numbering...
2. Actual performance will depend a lot on whether your panels are positioned in the sun or the shade. Were all of those 55 amp hours supplied by the panels, or were some supplied by the generator?
4. Yes, the fridge consumes 75 watts when operating. Your fridge's compressor only operates at 100% or off, it doesn't vary its output according to the conditions. The fridge manages the temperature by switching the compressor on when the internal temperature gets too high, and switching it off when the internal temperature is back down to target. You will probably hear it switching on and off, and running, if it's quiet indoors. The percentage of time that the compressor spends running is the "duty cycle" that Brian's referring to. The higher the ambient temperature (and sunshine hitting the fridge surrounds), the higher the duty cycle and hence the higher the fridge's average consumption over time. Waeco's product page for your fridge gives a couple of average consumption figures at different ambient temperatures, so you can see that the amount of refrigeration energy you require will depend on your location and the season. One energy management tip is to position your vehicle to avoid or minimise the sunlight striking the wall behind the fridge.
5. You haven't told us anything about your system's other loads (e.g. pumps, TV, computers, etc), or your travel habits or intentions. E.g. do you normally seek to park in the open or in shade? Do you stay put for long periods or move often? Does your power system need to sustain your load indefinitely or just for a few days at a time?
***
For comparative purposes, here's what I have, which is relatively minimal as I'm a former bicycle tourist and haven't kicked the habit of travelling lightly:
- Waeco CF35 fridge, which has published consumption figures almost exactly one third of yours. I don't have a freezer of any kind, the fridge is set to 4C.
- 80Ah deep cycle AGM battery (my battery travels inside a car so floodeds were never an option).
- The cheapest portable 80W panels and controller I could find on eBay, which I position to maximise capture (not that full sun is attainable at all camp spots).
- No alternative means of charging the battery. My fallback strategy would be to switch off the fridge or stay somewhere with power available, but I've never come close to needing to do this.
I tend to stay put for around a week at a time. The fridge stays in the car, which I park such that the fridge is not excessively exposed to the sun. I don't much like hot weather so I mostly stay in cooler places, and often at elevation, which will naturally improve system performance because the temperatures are lower and the sunlight stronger.
With this setup my battery voltage never drops below 12.6 unless I don't put the panels out for a day or place other significant loads on the battery (e.g. recharging laptop) when the weather is wet and the panels are mostly shaded. Even then, in ten months' use only once has the voltage dropped as low as 11.9 (under load). In summer the battery will be on float charge with less than two hours of morning sunshine, while in winter it'll take half that time or less. I don't need to move the panels during the day to keep up with my load, but I do anyway, as I like the feeling that the battery is fully charged when the sun goes down.
Wirroff said
08:33 PM Nov 12, 2013
Again, you have all been so helpful. We took the whole rig to a battery expert and he concluded that the batteries were 'rather old' and one side of them was indeed 'blown out'. Their capacity was also only 80a/h each. We have had fitted 2 x 110a/h AGM's now and a 25 amp charger (something tek?). We don't have enough real estate on the roof at present for another solar panel and will have to think about a 'mobile' panel or, eventually, replace the two that are there.
We have now spent $980 on batteries and charger and will see how we go, topping up the batteries with the generator for now. We run the TV/DVD at night and a couple of LED lights. The water pump is used when washing up only. We do have a 600W inverter on board which I use for my laptop about 1 hour per day.
Once again, many many thanks to all of you!
Ontos45 said
09:07 PM Nov 12, 2013
I use a Megapulse unit to monitor my batteries and keep them in shape. The unit tests the batteries every few minutes and shows via LED lights if ok.
Paul, I have numbered your points so that I may attempt to address them in a way that is not too confusing.
1. I am confident that your total panel wattage is greater than 160w. Why..Because you told us that you were reading 9.5 amps charge, and the absolute best that you could expect from 160w via a series controller on a really - really good day at noon would be a little over 8 amps.
2. There is no rule of thumb that fits every situation. Why..
The maximum power voltage (Vmp) of a 12v panel is around 18v, The maximum power current (Imp) is equal to the rated wattage divided by that voltage, so for a 160w panel, Imp = 160w/18 = 8.9 amps, and that's under ideal test conditions. Once you factor in efficiency losses due to panel temperature, line losses etc, don't count on better than 8 amps.
A series or PWM controller such as the PL20 when it is in the on state connects the battery directly to the panels, this pulls the panel voltage down to the battery voltage. Now comes that catch, the battery is at say 12v. So you now have effective panel watts being W= volts X amps, = 12 X 8 = 96w. So your effective input power from the 160w panel is just 96 watts or 60%.
An MPPT controller does not connect the panels directly to the battery, and so the panel voltage is not pulled down to that of the battery. so now you have that 8 amps at 18 volts (144 watts) being applied to the input of the controller which then converts it to the battery voltage of say 12v.
Watts in must equal watts out, so the amps coming out = 144/12, = 12 amps. Although it will be somewhat less due to conversion losses which are dissipated as heat. The true figure will be closer to 10 amps.
Now allowing the "rule of thumb" that you get an average five peak sun hours per day, you multiply the above figures by five to get the daily Ah input from either type controller. ie series controller = 8 x 5 = 40Ah...MPPT = 10 x 5 = 50Ah. Of course, there will be differences to these figures based on time of year, cloud cover, present latitude, angle of sun on panels, orientation of panels, present battery Soc, etc. etc.
3. Not necessarily correct. You are basing your calculations on 12 volts whereas your battery will (I hope) be closer to 12.7v. So 75/12.7 = 5.9 amps. As the voltage falls, the current increases to attempt to maintain the wattage, and this causes more voltage drop. The only way to know is to compare the voltage at the battery against that at the fridge terminals when the compressor is running.
4. The fridge has an average duty cycle of around 50% in normal temperatures and internal heat load. This means that if the weather is not too hot, say less than 30°C, and the product inside is already cooled, and you don't leave the door open excessively, it will run for a total of 12 hours out of 24 (more often during the day and less at night). Alter any of those parameters and the duty cycle changes.
5. The solar panel would be the first option as there is only temporary respite in fitting more battery without sufficient recharge capability. I would be loathe to put an AGM in parallel with a couple of older flooded batteries as the charge/discharge characteristics are very different from new, and far different if they are aged. First step would be to fully charge the existing batteries and see how they hold up under load, then make a decision from there.
Hope this helps.
I'll run with Brian's numbering...
2. Actual performance will depend a lot on whether your panels are positioned in the sun or the shade. Were all of those 55 amp hours supplied by the panels, or were some supplied by the generator?
4. Yes, the fridge consumes 75 watts when operating. Your fridge's compressor only operates at 100% or off, it doesn't vary its output according to the conditions. The fridge manages the temperature by switching the compressor on when the internal temperature gets too high, and switching it off when the internal temperature is back down to target. You will probably hear it switching on and off, and running, if it's quiet indoors. The percentage of time that the compressor spends running is the "duty cycle" that Brian's referring to. The higher the ambient temperature (and sunshine hitting the fridge surrounds), the higher the duty cycle and hence the higher the fridge's average consumption over time. Waeco's product page for your fridge gives a couple of average consumption figures at different ambient temperatures, so you can see that the amount of refrigeration energy you require will depend on your location and the season. One energy management tip is to position your vehicle to avoid or minimise the sunlight striking the wall behind the fridge.
5. You haven't told us anything about your system's other loads (e.g. pumps, TV, computers, etc), or your travel habits or intentions. E.g. do you normally seek to park in the open or in shade? Do you stay put for long periods or move often? Does your power system need to sustain your load indefinitely or just for a few days at a time?
***
For comparative purposes, here's what I have, which is relatively minimal as I'm a former bicycle tourist and haven't kicked the habit of travelling lightly:
- Waeco CF35 fridge, which has published consumption figures almost exactly one third of yours. I don't have a freezer of any kind, the fridge is set to 4C.
- 80Ah deep cycle AGM battery (my battery travels inside a car so floodeds were never an option).
- The cheapest portable 80W panels and controller I could find on eBay, which I position to maximise capture (not that full sun is attainable at all camp spots).
- No alternative means of charging the battery. My fallback strategy would be to switch off the fridge or stay somewhere with power available, but I've never come close to needing to do this.
I tend to stay put for around a week at a time. The fridge stays in the car, which I park such that the fridge is not excessively exposed to the sun. I don't much like hot weather so I mostly stay in cooler places, and often at elevation, which will naturally improve system performance because the temperatures are lower and the sunlight stronger.
With this setup my battery voltage never drops below 12.6 unless I don't put the panels out for a day or place other significant loads on the battery (e.g. recharging laptop) when the weather is wet and the panels are mostly shaded. Even then, in ten months' use only once has the voltage dropped as low as 11.9 (under load). In summer the battery will be on float charge with less than two hours of morning sunshine, while in winter it'll take half that time or less. I don't need to move the panels during the day to keep up with my load, but I do anyway, as I like the feeling that the battery is fully charged when the sun goes down.
We have now spent $980 on batteries and charger and will see how we go, topping up the batteries with the generator for now. We run the TV/DVD at night and a couple of LED lights. The water pump is used when washing up only. We do have a 600W inverter on board which I use for my laptop about 1 hour per day.
Once again, many many thanks to all of you!
www.megapulse.net/products/megapulse-green-energy.html
Peter
He might just get one now that the fridge is working.
I would rather you owe it to me for the rest of my life than do me out of it.
Good luck with it all.