Passing wiring through an external electrical hatch on a camper van

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Tom Cleary
Tom Cleary Solar Expert Posts: 37
Hello Everyone,

I wondered if I could ask for some expert help with a proposed design. I've posted before several times on NAWS, gone back to the drawing board more times than I can count, and the feedback from NAWS contributors has always been a help and a motivation to keep going. Without it I would not have gotten anywhere. I would like to ask you for your consideration once more, to render an opinion about whether my new proposed layout will work or not.

The situation is that I have a 1993 Volkswagen Eurovan. I finished constructing my first solar generator device, and while it does work, i have run across a new challenge, i.e., installing it into my camper van.

My solar panel is a 240 watt Kyocera panel. The panel attaches to two Group 27 100 amp hour deep cycle Costco batteries. The batteries are connected to a 1000 watt inverter with a trimetric Bogart monitor. The panel and devices work together and tested successfully.

The question now becomes how best to install the panel and devices into my small camper van, a 1993-year-old eurovan with pop-up.

It seems safe to me to keep the batteries outside the van and advantageous to do that because of the potential gassing problem. It would be nice to keep the inverter and monitor inside the camper van on the other hand, particularly the trimetric monitor, so I could read what is going on with the solar generator inside my camper van instead of having to traipse outside. Some of you have no doubt heard in the news that California has a drought going on, and since conditions are usually dry even in years when there is no drought, the batteries should be safe outside.

It seems it should be possible to run the wiring from the batteries outside the van through a wall in the van to the inverter inside using an electrical hatch installed in the van wall for that purpose. That way I could keep the batteries and panel outside together in one place, and keep the delicate electronics of the inverter and trimetric monitor device inside where they will not be exposed to the elements. Batteries I don't much care about, but the other electronics may suffer in the heat and cold. anyway, it would be nice to have them inside with me.

To provide an entry point into the van, a three inch aperture could be cut into the van, and covered with an electrical hatch over it, that would provide access for the cables and wires pass through the wall of the van. If the drought breaks, and we should get some rain finally, it should be possible to pull in the wiring (using Anderson quick connect devices) through the hatch into the inside of the van, and keep the van interior dry that way by closing the exterior hatch. The batteries could be placed in a box outside that can be closed during rainy weather.

I ran a voltage drop calculator for the wiring run from the batteries outside to the inverter inside. With six feet of number two wire between the battery and inverter, the percentage voltage drop for a 200 DC amp load would be less than 3 percent.

I have not seen this design anywhere else, which makes me think that possibly I am on the wrong track, being new, and this solar generator being my first. If you have any feedback, it would be appreciated as always. Thanks,
Tom Cleary

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Passing wiring through an external electrical hatch on a camper van

    Do you really need a 1kW AC inverter?

    A C/10 discharge rate would give you:

    200 AH * 12 volts * 0.85 AC inverter eff * 1/10 hour discharge = 204 Watts

    A 1,000 Watt AC inverter is a bit large (in my humble opinion) for a 200 AH 12 volt battery bank unless you need to power a (for example) skill saw or similar high surge/short time loads.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Tom Cleary
    Tom Cleary Solar Expert Posts: 37
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    Re: Passing wiring through an external electrical hatch on a camper van

    Thanks for the feedback BB. I was told the bigger inverter was the better choice than the alternative (a 300 watt inverter) when i purchased the components for the solar generator a couple of years ago. I have wondered about the need for the larger sized inverter; it may be too late to change the inverter now. Looking on the bright side, there will be room to add batteries and panels later I suppose if the opportunity presents itself.

    Do you think separating the panel and batteries from the other components will work using an electrical hatch as cover for the van opening? Best, Tom
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Passing wiring through an external electrical hatch on a camper van

    Tom,

    If your charge controller will accept a remote battery temperature sensor--I would get that. The batteries will be at a different temperature vs the charge controller, so the RBTS will be a big help in properly charging the batteries.

    Make sure you have fuses/circuit breakers at the battery + bus to all wires leaving the battery bank. A short circuit needs to pop the circuit protection device instead of turning your wiring red hot.

    Are you using Deep Cycle batteries or "marine" batteries. Ideally, I would be suggesting a pair of 6 volt @ ~220 AH "Golf Cart" batteries instead of marine type batteries (which are usually not very good deep cycle batteries). Also make sure you can get to the battery caps to check/add water as needed--Typically once per month. You want to make battery maintenance as easy & safe as possible.

    The size of battery bank you presently are planning for--It is a bit large for running from your 240 watt panel unless you have a good backup charging source. And the 1 kW inverter is really too large for the planned battery bank to drive effectively. Of course, if you never use more than 300 watts or so, then the battery will drive that inverter OK. Just check your losses from the inverter--TSW usually use 2x the "Tare Power" vs MSW--But MSW can be very hard on small transformers and AC power supplies (computers, chargers, DVD players, etc.).

    With the 240 Watt Kyocera panel, you are using a good quality MPPT type charge controller?

    Do you have a plan for a backup battery charger? The 240 Watt panel is about the minimum I would suggest for a 200 AH @ 12 volt battery bank:
    • 200 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 188 Watt minimum array
    • 200 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 377 Watt nominal array
    • 200 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 490 Watt "cost effective" maximum array
    Otherwise, you could look at a slightly smaller battery bank with a better 10% rate of charge
    • 240 Watts * 0.77 panel+controller derate * 1/14.5 volts charging * 1/0.10 rate of carge = 127 AH @ 12 volt battery bank
    With your system (and probably flat mounted solar panel), you really will need to have an adequate backup charging source. Running the van motor to charge the battery bank is hard on the engine/alternator. A Honda eu1000i generator + 15 amp Iota battery charger (get the IQ4 module if you will be using AC Mains power for charging too) would be a good backup/alternative charger (and not use very much fuel).

    You will need a hefty On/Off DC switch for the 1kW AC inverter. You should also look at the AC inverter specifications and find out what the "tare loss" is (wattage used just turned on with no load). The larger the AC inverter, typically the more "wasted power" there will be.

    Also, is this a TSW (true sine wave) or MSW (modified square/sine wave) inverter? I highly suggest a TSW inverter if you can swing the cost. The MorningStar 300 Watt TSW 12 volt inverter is a good match (I think) for your needs. It is pretty efficient and includes remote On/Off and a "standby" sleep mode that can be very handy if you are doing things like recharging your laptop computer/rechargeable batteries (if you have more than 6 watts of AC loads, the inverter will turn on). It will also supply ~600 watts for ~10 minutes if you need times of heavier power usage.

    Try to keep the heavy gauge cable short--6 feet (one way run?) is pretty long for a 12 volt power system. You need to keep cables short to the AC inverter. You can have about 1/2 volt maximum cable drop for good AC inverter operation.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Tom Cleary
    Tom Cleary Solar Expert Posts: 37
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    Re: Passing wiring through an external electrical hatch on a camper van

    Thank you for the thorough and detailed reply BB. Your suggestions were much appreciated. My apologies if this reply seems rough, I've been working a 15 hour shift, and just got home, but wanted to get this out soonest.

    It was interesting to learn that my two battery bank is too large for my 240 watt panel. Purchasing a Honda generator makes sense I suppose, everyone I have talked about the Honda generator says "just buy Honda, you will not be sorry." They are a bit expensive, however, and it may be cheaper to simply add another photovoltaic panel. I will look into both options in the future, after I have used the system for a while, and have seen how it's working out. It is not clear if more efficiency to charge the batteries is needed; if this need proves large, a generator may be the best way to go.

    The remote battery temperature sensor was not something I have previously considered, but will buy now. It seemed unnecessary to me given the fair Southern California climate, but since the batteries will be placed separately as you mentioned, and since it is a low-cost addition to the system and should be helpful charging the batteries, I will purchase one.

    I will double check circuit breakers at the battery for all wires leaving a battery bank; I believe all of the wires are presently fused with circuit breakers but it's worth checking again.

    Six volt 220 amp hour golf cart batteries get good reviews on this forum. Since I have already purchased batteries, I will stick for the time being with my two 12 volt deep cycle batteries from Costco. Perhaps batteries will be due soon for replacement though and I will look at the golf cart batteries then. This contingency seems likely, since I already nearly destroyed the Costco batteries when I stored them without trickle charging for a long period. It was a newbie beginner mistake.

    My 1000-watt inverter is of the pure sine wave type. The inverter has its own power switch. I assumed that the power switch on the inverter would be adequate. Do I need a second switch?

    One thing confused me. You gave a list of four battery banks with different rates of charging. I assumed that the reason for that was to point out that a slightly smaller battery bank with a better rate of charge would be more useful than my present battery bank. The specifics of the listings were not clear to me though.

    Thanks for the suggestion to obtain shorten the cable length if possible. Since you mentioned that, I think I found a way to shorten it from six to four feet.

    Thank you again for the suggestions. Your detailed critique was tremendously helpful; I plan to make modifications based on the comments. Best,

    Tom Cleary
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Passing wiring through an external electrical hatch on a camper van
    Tom Cleary wrote: »
    It is not clear if more efficiency to charge the batteries is needed; if this need proves large, a generator may be the best way to go.

    Batteries are a "Consumable" for solar power systems. Treat them "badly" and they may die in months. Treat them well, and you may get 3-5 years from them.

    One of the major killers of Lead Acid batteries is over discharging/under charging... Interestingly enough, another major cause of failure is probably over charging/not watching water levels.

    It does come back to your loads and how much power you can generate (solar/generator/etc.).
    This contingency seems likely, since I already nearly destroyed the Costco batteries when I stored them without trickle charging for a long period. It was a newbie beginner mistake.

    Yep, batteries do not store well. Flooded cell batteries need recharging ~once a month. You can buy a Battery Minder or Battery Tender brand of float charger (not cheap). Or get a cheap float charger and run it on a lamp timer (something like ~1 hour per day) to keep the batteries charged.

    If your van is parked inside/under shade for much of the time, you might want a good quality float charger anyway.
    My 1000-watt inverter is of the pure sine wave type. The inverter has its own power switch. I assumed that the power switch on the inverter would be adequate. Do I need a second switch?

    One switch on the inverter should be enough (with a fuse/breaker near battery to protect wiring).
    One thing confused me. You gave a list of four battery banks with different rates of charging. I assumed that the reason for that was to point out that a slightly smaller battery bank with a better rate of charge would be more useful than my present battery bank. The specifics of the listings were not clear to me though.
    Do you have a plan for a backup battery charger? The 240 Watt panel is about the minimum I would suggest for a 200 AH @ 12 volt battery bank:

    • 200 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 188 Watt minimum array
    • 200 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 377 Watt nominal array
    • 200 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 490 Watt "cost effective" maximum array

    Otherwise, you could look at a slightly smaller battery bank with a better 10% rate of charge
    • 240 Watts * 0.77 panel+controller derate * 1/14.5 volts charging * 1/0.10 rate of carge = 127 AH @ 12 volt battery bank

    There are several ways to "size" a solar power system... My suggestion:
    1. measure/estimate your loads (peak Watts, and average Watt*Hours of use per day)
    2. size the battery bank to the loads (1-3 days of storage/50% maximum discharge). For an "off grid" cabin, we would be suggesting 2 days of storage and 50% maximum discharge or a battery bank 4x your daily power usage... For an RV you may only do 1 day of storage (weight/storage space, etc.).
    3. Size the solar array/charging to give you a 5% to ~13% rate of charge... 10%-13% rate of charge is a very nice system. A 5% rate of charge is a minimal solar array and can have issues (i.e., if you use power during the day, you lose current that would have been charging the battery bank). As you can see, the larger the battery bank, the larger the array needs to be.
    4. Size the solar array based on the amount of sun/mounting/location of the van/installation. Both #3 and #4 are taken together to size the system.
    5. Size the genset/AC battery charger to (typically) 10% to 20-25% of the battery bank capacity (20 Hour Rate). This is your backup/power source when you need more power than the solar system can supply.

    So, just to give you an idea... If you camp in good weather 9 months of the year in the southwest US--You can get a minimum of 4 hours of sun per day (and upwards of 6+ hours per day in middle of summer). The estimated available power from a 240 Watt Array as 120 VAC AC power would be:
    • 240 Watt * 0.52 system eff * 4 hours of sun per day = 499 WH per day (minimum for non-winter months)

    Is 499 WH per day/night enough power for you? A single laptop computer plus wireless connection running at 30 watts would run:
    • 499 WH per day / 30 Watts = 16.6 Hours per day

    Another way to design a system is to base the size on the battery bank, or frequently for RV's, how much solar array you can fit on the roof (and where the RV will be parked). That was sizing the battery bank a 10% rate of charge based on a 240 Watt array. It gives you a smaller battery bank (127 AH, less expensive, less weight)--But if the battery bank does not support your needs--Then it is a non-starter.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Tom Cleary
    Tom Cleary Solar Expert Posts: 37
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    Re: Passing wiring through an external electrical hatch on a camper van

    BB,
    Thanks for the promotion from "New Guy" to "Not so New Guy." It seems silly of me to be this pleased about it, but there it is.

    Yes, I nearly totaled the two batteries by failing to use them or charge them for 18 months. An electrician friend resuscitated them by charging them at high voltage several times, using equalization type voltage. Thanks for the suggestion to purchase a cheap trickle charger and lamp timer. My electrician friend made the same suggestion after resuscitating the batteries. The combination has worked well so far..… Despite mistreatment, the batteries seem to have survived. The batteries hold voltage at least, whether they will hold voltage after a load is put on them still remains to be seen.

    The equation you used to find the product of 499 watts seemed clear, as did the other numbers employed, but for one question: what was the reason for using a 0.52 system efficiency number in that equation? Is that typical of battery based solar systems? I did not understand how that 0.52 number came to be chosen I guess.

    The product number 499 watts derived from the equation sounds like enough of an energy budget for my setup. I don't plan to use a laptop (the biggest part of my energy budget) for 16 hours a day. I plan to run a laptop and fan and a few LED lamps too, but my actual usage for all of the items will be closer to eight hours. Whether 499 watts a day will be enough remains to be seen, if anyone else has comments on how useful this much available energy is I would be thankful for any feedback you might have.

    I have heard that in the desert you have to make do, and that has been my plan. "Making do" seems theoretical to me, however, since I have not actually camped in the summertime here in California. If the weather got hot enough I could buy a generator and run an air conditioner I suppose, just dislike to inflict that noise on the neighbors.

    Thank you for the tips on sizing a solar system, this time around, I understood it better. It seems clear that pulling significant power from the batteries while simultaneously charging them with a PV panel means they are probably not being adequately fed by the PV panel. I also see what you mean about sizing the system based on how much solar array you can fit on the roof. A smaller battery bank with lesser weight and expense will be charged more efficiently by a smaller PV panel, that much seems clear. May I ask one question though?

    I fail to understand the reason a 200 amp hour battery system would fail to fall within the rate of charging range of 5 to 13 percent with a 240 watt panel. Can I ask how you calculate that? Might there be a link available explaining how to calculate that?

    I looked for the ideal rate of charging range in the NAWS forum and saw it mentioned a few times, but the descriptions seemed to lack detail on how the charging range is calculated. I can reduce the battery bank size to one battery if I don't get a rate of charging that fits within the five percent to 13 percent range.

    Thanks again for all of the help BB, you didn't have to go out of your way to help me, but you did...
    Tom

    P.s. Here is a link to a hatch cover I plan to install in my camper van this week to permit wiring connecting the batteries (on the outside) to the inverter and components (on the inside) to pass through the camper van wall.

    http://www.amazon.com/JR-Products-541-2-A-Deluxe-Electric/dp/B000BGJWA2/ref=cm_cr_pr_pb_t
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Passing wiring through an external electrical hatch on a camper van
    Tom Cleary wrote: »
    BB,
    You can call me "Bill" -- That is my real name. ;)
    The equation you used to find the product of 499 watts seemed clear, as did the other numbers employed, but for one question: what was the reason for using a 0.52 system efficiency number in that equation? Is that typical of battery based solar systems? I did not understand how that 0.52 number came to be chosen I guess.

    Solar Panel output Vmp/Imp are based on marketing numbers. Vmp falls as the cell temperatures rise. The "Test/Specification" is based on the cell temperature of ~75F. Basically a panel at room temperate "flashed" for a few seconds under a sun simulator to test/validate ratings. Place a panel on a hot roof under full sun for many hours a day--And the temperature of the cells rise 20-35C. That will drop the cell Vmp output voltage down to ~81% of rated Vmp. With average temperatures below freezing a field of snow in front of the array--Some folks here in Canada will get peak power that exceed their summer noon-time production. Of course the sun is up for only a few hours per day vs 12+ hours per day for winter.

    Add some dust, controller/wiring losses, less than ideal sun (haze/humidity), and such--Then we derate so that your actual average system power matches "real" performance 10 years down the road. The overall calculation is:
    • 0.81 panel derating * 0.95 controller losses * 0.80 flooded cell batt eff * 0.85 inverter eff = 0.52 overall system eff
    It is sad, but engineering is full of these deratings. In the building industry, is very common to use a factor of 10x derating to allow for flaws in the wood, concrete variations, etc...

    We use a lot of rules of thumbs here because they are quick and will, in general, give you a reasonably cost efficient system that will supply your requested power for years in the future (with aged batteries, some dust on the panel, Batteries that are less efficient when discharged at faster than C/20 discharge rate, etc.).

    If you have requirements that require different/more detailed calculations (power is used during day, not night; using DC power and no AC inverter; using AGM batteries, etc.)--We can play with those numbers--But in general, the differences are only 10-20% in system sizing--And since most people want to run some power on cloudy days and don't want to pay for expensive AGM batteries, the extra design work is not worth the effort. In solar power, if you are within ~10% of your design numbers--that is pretty much "dead on".

    Over the years, we too have been working against conventional wisdom to make for better performing off grid power systems. One of the biggest "old wife's tail" is that if your system does not provide enough power, is to add a larger battery bank.

    In reality, it was almost never a good idea to add more batteries. Batteries do have losses and require a minimum rate of charge (~5% to 10% depending on manufacturer and battery type). Adding more batteries and not adding more panels actually tended to make the system losses go up and batteries die even more quickly from under charging.

    The old days of solar panels costing $10-$20 per Watt are long gone. Panels are in the $2-$1 per what or less. And batteries are getting more expensive. Having more solar panels is almost always a better solution these days (more power on sunny days, less battery losses, as long or longer battery life, and when your battery bank does die--The bank is 1/2 the size and 1/2 the cost to replace). Add a cost efficient genset (Honda eu1000i for small systems, or an eu2000i or a bit more power) to carry you through that week or two of bad weather/no sun. No matter how many watts of solar panels you have, you are still not going to get significant power in very bad weather (5% of expected output I have seen in on very dark days sometimes).
    The product number 499 watts derived from the equation sounds like enough of an energy budget for my setup. I don't plan to use a laptop (the biggest part of my energy budget) for 16 hours a day. I plan to run a laptop and fan and a few LED lamps too, but my actual usage for all of the items will be closer to eight hours. Whether 499 watts a day will be enough remains to be seen, if anyone else has comments on how useful this much available energy is I would be thankful for any feedback you might have.

    A Kill-a-Watt type meter for measuring AC loads. A DC AmpHour/WattHour meter like this for "smaller" DC loads. You either have to measure your loads and design the system to support it... Or you simply fit the system in the space (and wallet) you have--And simply use the power available. Knowing your loads--You can then make plans to best use your available power (and when you plan to fire up the genset).

    Once you have a system installed--You can get a Battery Monitor (Trimetric is popular) to help keep track/plan your electrical usage.
    I have heard that in the desert you have to make do, and that has been my plan. "Making do" seems theoretical to me, however, since I have not actually camped in the summertime here in California. If the weather got hot enough I could buy a generator and run an air conditioner I suppose, just dislike to inflict that noise on the neighbors.

    Some folks simply use a small DC fan. Efficient and better than nothing. Yes, you can run an eu2000i with a small A/C (you have to pick the right unit that will run on a 1,600 watt genset). The Honda (Yamaha and a few other inverter-generator units) are quiet. And quite a few folks have run them overnight for A/C when needed. It is up to you if it is worth ~1 gallon or so of gasoline or not.

    Electrical usage is a highly personal set of choices... We just help people make their own educated choices. We do not claim that our solutions are the only ones out there.
    I fail to understand the reason a 200 amp hour battery system would fail to fall within the rate of charging range of 5 to 13 percent with a 240 watt panel. Can I ask how you calculate that? Might there be a link available explaining how to calculate that?

    The math looks like:
    • 240 watt panel * 1/14.5 volts charging * 0.77 panel+controller derate * 1/0.05 rate of charge = 255 AH @ 12 volt battery bank minimum rate of charge
    • 240 watt panel * 1/14.5 volts charging * 0.77 panel+controller derate * 1/0.10 rate of charge = 127 AH @ 12 volt battery at nominal rate of charge
    • 240 watt panel * 1/14.5 volts charging * 0.77 panel+controller derate * 1/0.13 rate of charge = 98 AH @ 12 volts "cost effective" rate of charge

    I looked for the ideal rate of charging range in the NAWS forum and saw it mentioned a few times, but the descriptions seemed to lack detail on how the charging range is calculated. I can reduce the battery bank size to one battery if I don't get a rate of charging that fits within the five percent to 13 percent range.

    5% is the minimum rate of charge (if given) for most Flooded Cell batteries. Some Mfg. actually recommend 10% rate of charge minimum (to better stir the electrolyte, open pores in the plates, etc.).

    Minimum equalization current (only done when needed--equalization is "hard" on lead acid batteries) is ~2.5 to 5% of battery capacity (again we use the battery 20 Hour rate for capacity numbers in our rules of thumbs).

    Other issues--The longer batteries sit at less than ~75% State of Charge, the faster they sulfate (soft fluffy grey lead sulfates turn into hard black crystals--permanently removing lead+sulfur from chemical reactions). Batteries "self discharge"--A battery near the end of life will discharge at ~1-2% rate over 34 hours--Which eats into your 5% for ~6 hours per day of recharging.

    So, after years of experince here by those living off grid and helping others--5% seems to be the real "useful" floor for recharging a battery bank. And if you need/choose that level of charging--You have to keep close track of your loads and battery state of charge--And probably have more generator run-time too.

    13% maximum is based on several reasonings. The first is that if you are charging at 13% rate of charge for hours per day--Batteries will get hot. Any more current and the batteries need active cooling and/or a Remote Battery Temperature Sensor (highly recommended) to reduce the chances of thermal run-away (hot battery, charging voltage drops, charger sees lower voltage and cranks up current, battery gets hotter, etc.).

    The second is that once the system gets your battery fully charged--There is no more place to send the energy (other than turning on loads--which some people do. Pumping water, washing clothes, etc.). So more than ~13% rate of charge is not cost effective for most folks. But with the dramatic fall in price of solar panels (mine where $10 per watt a decade ago, now $1 per Watt), over paneling is not nearly as costly as it used to be. Whereas $4+ per gallon gasoline is quite dear right now.
    Thanks again for all of the help BB, you didn't have to go out of your way to help me, but you did...
    Tom

    Everyone here is volunteering their time and knowledge. We work pretty hard to try and give people enough information that they can design/build their own system that works well the first time. Pretty much everyone here has made most of the mistakes we have written/warned about here. We aim to reduce the school of hard knocks form of education (which tends to be very costly).

    P.s. Here is a link to a hatch cover I plan to install in my camper van this week to permit wiring connecting the batteries (on the outside) to the inverter and components (on the inside) to pass through the camper van wall. [/QUOTE]

    That is intended to be a temporary "pass through". If you are making a permanent install, you will want to have a better seal. You don't want to draw in carbon monoxide while driving/running a genset-engine for recharging (as well as road grime).

    Good Luck,
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Tom Cleary
    Tom Cleary Solar Expert Posts: 37
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    Re: Passing wiring through an external electrical hatch on a camper van

    Hi Bill,

    Thanks for the help, it clarified the questions I asked, specifically concerning why the overall efficiency number was seemingly low at 0.52. You wrote the following:

    0.81 panel derating * 0.95 controller losses * 0.80 flooded cell batt eff * 0.85 inverter eff = 0.52 overall system eff

    It is sad, but engineering is full of these deratings. In the building industry, is very common to use a factor of 10x derating to allow for flaws in the wood, concrete variations, etc...


    I'm beginning to understand why you suggested a smaller inverter! I had no idea that the efficiency derating for an inverter was 0.85, that is a lot of lost power. I guess that is why I need a Kill-a-Watt measuring device, to see whether my actual usage exceeds 300 watts. I have purchased one and learned how to use it, so that is no problem. When my laptop and lights and fan are going I will see what the actual load is. It might be worth it to have a second 300 watt inverter when I'm running smaller loads.

    Thanks for the clear explanation of why the actual Vmp output is less than the marketing Vmp output. It makes sense that the cells will be less efficient after warming up, something not accounted for with lab tests using a "flash" of light instead of sustained output over several hours. That explained the 0.81 panel derating.

    The controller losses and the 0.80 flooded cell battery efficiency number sort of make intuitive sense, I get that energy cannot be transformed completely efficiently at %100 level even if the derivation of the actual numbers are not clear, it seems clear that energy will be lost to heat or inefficient chemical reactions in the case of the battery.

    The explanation of why too little charging (< 5%) degrades the soft battery gel into hard crystals and leaves less lead and less sulfur available to do the battery's work was easy to understand. It also makes sense that too much stirring and electrical discharge (>13%) causes the battery to get hot and affects its efficiency, and that there is no sense in producing surplus electricity this way.

    The reason for sizing the solar array to the batteries seems clear now. At a price of two dollars a watt it makes sense to add panels instead of batteries if turnover becomes quicker that way and the charge rate is greater. If my two battery bank is under charged at or near the 5% level, I will need to buy a second panel. I will measure this when I have the load hooked up and figure out the percentage charging for it.

    Regarding the monitor, thanks for the suggestion; I already have a Trimetric monitor. It has been amazing device, and adds a lot of fun to tinkering.

    Sorry it took me awhile to get back to you, it took me some time to process the information, and I haven't had much time to study lately.

    I've read so much on this forum that has helped me understand how to build and install a solar generator. Thank you BB and Caribou Coot and other contributors, because your input made this possible. Without the information I received in this forum, and without the help of my friend who is an expert electrician I would have never had the confidence or the information necessary to attempt this project. I'm still amazed it's possible for an amateur like me to do this at all, but for the help you gave so freely it would have never happened. It was fun to see it come together, and I would have never understood how solar power works without building one myself. Thanks very much,
    Tom Cleary

    p.s. Just to give a status update on my installation that I originally wrote the forum about three weeks ago, this week I installed the cover hatch with the help of my electrician friend who also had experience cutting metal. The task was not hard. With a circular metal blade and a drill, we cut a circle sized chunk of metal plate out of the wheel well to insert the main tube to contain the wires. The metal was about eight of an inch thick, but popped out easily. We drilled separate holes for the bolts, and inserted the hatch cover with the tube. This is the link for the hatch cover:

    http://www.amazon.com/JR-Products-541-2-A-Deluxe-Electric/dp/B000BGJWA2/ref=cm_cr_pr_pb_t

    The gaps are small, but they will be filled with caulking. Since the hatch cover is inside a wheel well, it should be protected from rain. One disadvantage may come up when the car has been underway since the tire will throw off road grime and some will accumulate on the hatch cover; hopefully this will come off easily when the driving is over.

    The van now has a hatch, through which 3 1/2 inch wide objects can be passed, including quick connect Anderson connectors and #1 wire. The quick connects only measure two inches, and will fit easily. One thing I need to do is learn how to use a quick connect on multiple wires of the Trimetric hookup to the shunt. The reason for that is that the shunt will be with the lead acid batteries outside, and the monitor will be inside with the inverter, so it will be important to use quick connectors for the monitor and inverter, so they can be connected and disconnected quickly when setting up and breaking camp. Ralph Hiesey at Bogart Trimetric has been helpful, he suggested that most 4 terminal quick connectors will work for a quick disconnect for theTrimetric 2025 monitor. The remainder of the project work should go quickly. Will send information update when I have more progress to report.