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Author Topic: Using PV to charge campervan leisure battery  (Read 5224 times)
skutter2k
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« Reply #15 on: April 03, 2016, 10:38:49 PM »

I've just been researching the various charge controllers and I'm getting into the whole pwm v mppt. From what I can tell the mppt give better harvesting at low and high temps. As I'm in Wales low temps are pretty much the norm, is the difference in cost proportional to the performance increase of mppt with something low ish powered like the 150w panel I am considering?
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Scruff
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« Reply #16 on: April 03, 2016, 11:00:26 PM »

Dear Biff, the suitcases have their uses.
The Maplins yolk has a blocking diode and a charge reg. ?!? I have to argue though that the current price of £3.85 per watt is rather ludicrous, and charging through a cigarette lighter port upsets me muchly. They are just trickle chargers, a motorhome imho requires a minimum of 50W for practical porpoises.  

13W can work on a system that is mostly idle and has no parasitic load. However if you put it off angle, behind a windscreen of a car then it's seriously going to have a hard time keeping up to some ECUs et al. Coach built campervans are notorious for parasitic loads. Water tank meters, perma-on inverters, battery indicators, smoke alarms, CO alarms, USB ports, powered tv antennae, etc...


Guy real MPPT has a massive inductor on the board.
If yer worried about frying them just check the water levels. I refilled 250ml between my 6 cells this year with a 14.8V + temp. compensation absorption setpoint.
{EDIT: I do turn the absorption down to 14.1V though soon as they hit SG 1.275 per cell...which usually takes aboot 3 weeks no load (other than the charging and monitoring system)}


Ritchie
None of the flush mount MS controllers are very good at flooded batteries. They tend to have a maximum charge setpoint of 14.4v for the AGM markets. If you want 14.7V or higher then you have to get up to a TriStar. I've never seen another charger of any kind charge as well as MorningStar and it's not for want of trying.
In rare circumstances the solar controller can confuse the alternator and throttle the charge by confusing the voltage sense so it helps to have a PV isolator. This is more prevalent from a heavily loaded battery with a factory fitment underwired alternator.
As a rule of thumb you only want one charging source operating at a time for proper regulation. There are exceptions like when all charging sources are working flat out with limited time.

For a week's autonomy I'd say minimum get 100W PV, a 200Ah+ battery and upgrade your alternator cabling and split charging (it's always fitted incorrectly). 150W is as big you can get a 36cell module but you can use as many in parallel as your controller can handle (it's recommended to only load a controller to 80% rating but you can push it if your modules are always off angle.
I wouldn't fit a "leisure battery" either they're renown for underperforming. 6V flooded deep cycles are the best bang for buck.

150W PV requires at least a 10A controller check the panel ISC. If you are making it tilt adjustable then I'd recommend 15A.

"To comply with the National Electric Code (NEC), the current rating of the
controller for solar charging must be equal or greater than 125% of the solar
array’s short circuit current output (Isc)."
« Last Edit: April 04, 2016, 12:51:56 AM by Scruff » Logged
Scruff
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« Reply #17 on: April 03, 2016, 11:05:33 PM »

This is the best appraisal of the situation I've seen.
It boils down to price per watt and the tie breaker is reliability.

Quote from: Ralph, Bogart Engineering
The "good" for PWM: It is simpler and lower cost technology. Under some common circumstances--it can actually deliver more amps to the battery. That could be when:

(1)days are moderate or warm, with few clouds.

(2) batteries are charging at over 13 volts, (in a 12 battery system) which they almost always are when actually CHARGING.

(3) Panel voltage is properly matched to the battery voltage, for example "12V" panels are being used with a 12V system.

PWM is actually more "power efficient" than MPPT--which means less total power loss in the controller itself. So heat sinks in the design can be smaller (and less expensive). Missing in most analysis of MPPT is that there is always a conversion loss with MPPT, which tends to be higher the greater the voltage difference between battery and panels. That's why PWM can actually beat MPPT under circumstances described above.

Some places that analyze MPPT assume that panels with 30V open circuit voltage are being used in a 12V system. Any good MPPT system will easily provide better performance in that case. They also may assume batteries are charging at 12 or even 11 volts, which is unrealistic. Lead acid batteries are typically below 13 volts only when discharging, or perhaps charging with very little charging current--meaning the actual potential gain in amps is not great.

The benefit for MPPT becomes apparent if you use panels not voltage matched for the battery. If they are not, MPPT will utilize more of the potential energy of the panels. For example, if you use 24 volt panels to charge a 12 volt battery system you must use MPPT, otherwise you would be using your panels very inefficiently. If you are trying to use PWM in that case, you are misusing the PWM technology.

Another potential benefit with MPPT is that if distance between panels and batteries is far, smaller wire can be utilized by running panels at higher voltage to the batteries. Running at twice the voltage reduces wire size to 1/4, which for a long run can be a significant saving in copper wire.

If temperatures are low enough, the slightly less power efficiency of MPPT will be compensated by the higher panel voltages, which will result in a little more battery current. But in actual measurements we made using a commonly sold MPPT solar controller, this would occur at temperatures less than 55 F degrees (in full sun, when charging at more than 13 volts), where there is a slight advantage to MPPT in my location (Boulder Creek, near the California coast). As temperature drops below that (in full sun) MPPT will get some advantage, such as could occur at high elevations in Colorado in the winter. Potentially this would be maximum about a 2.5% improvement in amps output for every 10 degrees F lower in temperature (or 4.6% per 10 degrees C colder. I'm using data from Kyocera KD-140 panels.)

There can be theoretically optimal situations (that I don't personally experience where I live) where MPPT could give some advantage: that is when solar current is present, but the batteries are quite low in charge--but because loads are high and even greater than the solar current the batteries are still discharging despite the solar current. Under these conditions the voltage COULD be at 12.5 volts, or even lower. Again, using data from Kyocera panels, ("Normal Operating Conditions") there is a theoretical maximum gain over PWM of 20% current assuming NO MPPT conversion loss and no voltage drop in the wires to the panels, at 20C (68F). With PWM, the voltage drop in the wires in this case would not affect the charging current. Now if in addition you lower the temperature to below freezing at 28 degrees F (while sun is shining) you might actually get up to a THEORETICAL nearly 30% gain while the batteries are discharging.

The only REALLY BAD part of MPPT, is all the hype surrounding it--for example one manufacturer advertises "UP TO 30% OR MORE" power harvested from you panels. If you are using solar panels properly matched to the batteries, 30% ain't gonna happen unless it's EXTREMELY cold. And your batteries have to be abnormally low in charging voltage--which tends not to happen when it's cold (unless you assume the battery is still discharging while solar is happening). Virtually all the analyses I've seen touting MPPT on the Internet ignore the conversion loss, assume really cold temperatures, assume unreasonably low charging voltages, assume no voltage drop in the wires from panels to batteries, use STC conditions for the panels (that the marketing types prefer) rather than more realistic NOCT conditions, and in some cases assume panels not voltage matched to the batteries.

The other thing that is misleading about MPPT, is that some manufacturers make meters that show both the solar current and the battery current. In almost all cases for a well designed MPPT type the battery current will be greater. The engineers making these know better, but it is implied (by marketing types?) that if you were NOT using MPPT you would be charging your batteries with only the SOLAR current that you read on their meters. That's not true, because the PWM BATTERY current should always be higher than the MPPT SOLAR current. It is the nature of the MPPT that maximum power occurs when the current is lower than the maximum, so they must operate there to get the maximum power. So to properly compare the two you need to compare MPPT with an actual PWM controller in the same circumstances.

Finally, the reason we went to PWM is that I was anticipating that panel prices were going to drop (which they certainly have over the last 5-10 years!) and that the small advantage of MPPT (under conditions where the correct panels are used for the batteries) would not justify their additional cost and complexity. So my thinking, for more total benefit per $, put your money in an extra panel rather than a more expensive and complex technology.


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skutter2k
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« Reply #18 on: April 03, 2016, 11:44:11 PM »

Thanks Scruff, some really useful information there.

I'm now thinking along the lines of 150w PV with TriStar and if budget allows a display to go with it so I can see what it's up to.

I appreciate the alternator cabling is always pretty poor but tbh I'd rather the battery was kept topped up by solar anyway. Am I right in thinking the split charger will only draw power from the alternator if required, thus hopefully meaning next to nothing if my pv has been doing the job?

The van has a "leisure battery" fitted already, under the drivers seat along with the van battery.  Typically this limits the capacity of the "habitation" battery, which I haven't had a chance to check yet (I would put money on it being 120ah or less). I suspect that later on when time and money allow I will end up fitting a second habitation battery somewhere else in the van.

My next mission is to come up with some scissor style mounting system so I can angle the panel any of 4 ways (I don't want to keep turning the whole van around!)

Ritchie
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Scruff
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« Reply #19 on: April 04, 2016, 12:21:33 AM »

The TriStar is definitely worth it for the data-logging and programmability imho.
I wouldn't bother with the TS display. It's a controller monitor not a battery monitor. I can recommend a few better by pm but Navi don't stock them. I'm currently testing a cheapo Chinese version to see if it cuts mustard.

I ran 35mm² from my alternator to my starter and 90mm² across a relay to my deep cycle. Most I've ever seen on the habitation side of a 150A alternator is 45A although it used to be 11A on the stock 4mm² to the starter battery and 10mm² thereafter.

Alternators are cr*p chargers but if you set it up right it'll hold a line so you never drop below 70% charge (lower the SOC more efficient the battery and higher the alternator output). I wouldn't be able to maintain my lot in the Winter without it. They're brilliant at high current charging but at >85% SOC PV rules the roost.

Standard fitment is someat like this.




It's not a split charger it's just a relay but the alternator will compensate with it's own regulator seeing all your batteries as one bank.
The use of the alternator indicator to power the relay means isolation between batteries when the engine is off. Personally I don't like it, it's not unidirectional or discriminating. I just use a switch and a 200A latching contactor when I'm touring and a SmartBank when it's idle.

There's dozens of ways to improve on that circuit layout too.

It's not a good idea to expand batteries they'll never balance and work against each other. Best swap the pack at once. If you spec. the cable right you can put another deep cycle wherever you want.

« Last Edit: April 04, 2016, 12:45:30 AM by Scruff » Logged
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« Reply #20 on: April 04, 2016, 08:04:52 AM »


My next mission is to come up with some scissor style mounting system so I can angle the panel any of 4 ways (I don't want to keep turning the whole van around!)

Hiya Ritchie, told you these guys were good, didn't I.

Just some thoughts on the mounting system, I've had a play with PVGIS an excellent site for estimating PV generation. I stuck a pin on my house, and 1kWp (just for simple comparison of the numbers, not the numbers themselves) of PV facing due south, and got the following figures for differing pitches:-

35d - 973kWh pa. Dec 30.4kWh. Jun 122kWh.
20d - 942kWh pa. Dec 24.4kWh. Jun 127kWh.
0d  -  829kWh pa. Dec 13.2kWh. Jun 126kWh.

So annually total gen doesn't matter too much. December gen takes an absolute kicking, but June generation, when the sun is high is fine in any combo. So whilst picking the side you want to point south, and having a single tilt of 20d might be worth it in the winter months, I'd suggest a simple flat mount, or fixed mount with a very slight tilt would be fine*. The advantage of a flat 'south facing mount', is that there is no such thing, everything is south (or north) facing, so no need to think ahead and ponder fitting angles, or parking angles.

*Can't fix mount at an angle forward or backwards, as wind resistance whilst driving will cause problems.

But do carry a soft cloth and squeegee for cleaning away the dirt and bird poo.

Mart.
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Just call me Mart.     Cardiff: 5.58kWp PV - (3.58kWp SE3500 + 2kWp SE2200 WNW)
Scruff
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« Reply #21 on: April 04, 2016, 08:32:58 AM »

I had the same plan with 4 way tilting. In the end I went with just one for simplicity. Definitely worth it if you intend on using it in the cusps of Winter. I've often gotten a fivefold harvest increase or more with some strategic parking and tilting. If you wanna get fancy you could put a linear actuator on it. Come April to September it's not necessary.
« Last Edit: April 04, 2016, 08:34:47 AM by Scruff » Logged
biff
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« Reply #22 on: April 04, 2016, 09:15:40 AM »

Dear Scruff,
           Richie,s original request was for info on a trickle charger to keep the battery topped up over the winter months,
  And yes, they have charge controllers, I bought mine way back in 2008 @ £60.00 each. I have no idea what price they are now.
  Mine connect directly to the battery but I will agree that apart from clocks and alarms mine have no parasitic loads.
  My emphasis is on safety and use of proper controllers to avoid fire damage or loss of life.
  It surprised me to find that the suitcase charger actually topped up the batteries of several different vehicles of mine from that same position behind the windscreen
  nevertheless, it worked and works fine.
  I will not argue electronics with you because your expertise ranks high above my own newbie status but I have cooked a few controllers in my time and boiled
  a few batteries dry and I felt the embarrassment as the black smoke waffed around the room for the second time in two Marchs and this black smoke and toasty roasty controller
  was the results of the said controllers not being able to handle the stated loads.
  It is about safety. I take your point about the Camper homes being notorious for parasitic loads, so it would be better if Richie got it seen to professionally.
  Better safe than sorry.
                           Biff
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An unpaid Navitron volunteer,who has been living off-grid,powered by wind and solar,each year better than the last one.
biff
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« Reply #23 on: April 04, 2016, 09:26:16 AM »

Hi Richie,
       My apologies for talking about you in the 3rd person,
     I would regard Scruff as pretty smart with electronics, he don,t know it all. He never met my X.
   However, without wanting to sound like a wet blanket and seemingly to steer towards this safety lark (the new Biff)
  I have to say that it would be better to get it looked at professionally than start interfering with the present wiring system in the camper.
  I assume that you have camper insurance and that your camper is quite a valuable one, so it makes good sense to think  ahead.
  If something went wrong and your camper went on fire (god forbid) you would have to fill in your insurance claims forms and tell them
  about any alterations to your camper prior to the fire.
   You say that you are a complete newbie to PV and electronics but of course you can learn, but you do not want to be like me and
  have the black smoke waffing around your nice camper.
  Whatever decision you come to,,make sure it is a safety first one and do not take chances.  Good Luck.,
                                                           Biff
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Scruff
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« Reply #24 on: April 04, 2016, 02:09:32 PM »

Sorry Biff, I don't mean to upset you or your suitcases. It was the wild camping comment spurred my rational.
I have a 0.5W parasitic load on my camper system and yet from November to Jan some days I net 6Wh from a 150W module so anything lesser is just going to be redundant for 3 months a year.
I certainly don't know it all I just have a tendency to meter things and gather my own data.
I've let the magic smoke outtov many a lecky box, but hey that's how I learn...still cheaper than lekytrix college...

As regards getting a professional to wire a campervan, unfortunately few and far between actually do it right. Certainly they can do it safely but rarely do they understand the concepts of battery charging. Things like acceptable voltage drop, effective split charging, reduction of perma-loads, not using an inverter for lighting and other unnecessaries, changeover circuits etc...
« Last Edit: April 04, 2016, 02:14:00 PM by Scruff » Logged
biff
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« Reply #25 on: April 04, 2016, 02:27:06 PM »

No need to apologise Scruff,
                     Your comments were all in order. Your input to the forum is always good and hopefully will continue that way.
   The "Get the professionals" is really necessary from a Navitron viewpoint and very often it is excellent advice but it is up to the beholder to
   follow through on this advice. To be perfectly honest. i never thought that I would see the day that I would be tying the top of the ladder
   ,making the lads wear their safety helmets, giving out to ones who were acting the goat, Insisting on toeboards, handrails and decent scaffold
   but here I am on Navitron, telling newbies to call in the professionals,, Grin. It could be worse.
                                                         Biff
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Scruff
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« Reply #26 on: April 04, 2016, 03:17:46 PM »

But, but, but Biff somebody has to climb the ladder to tie the ladder! stir

People often confuse professionals with experts. Professional just means someone is willing to pay them for services rendered.
Not enough banana distribution in the health and safety business if you ask me. ralph
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skutter2k
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« Reply #27 on: April 04, 2016, 03:27:40 PM »

Scruff, Biff & Mart,

Thank you all for your input, between you it pretty much evens everything out to cover off efficiency and safety.  I have upgraded from a 25 year old van to a 12 year old van. I'm not shy when it comes to working on them (in fact I ended up replacing most of the wiring on my old van) and just as Scruff has mentioned I too have seen a fair number of botched installs with remarkable price points.  I really don't like over paying for things especially if it's something I think I am capable of doing myself (safely).  Most importantly, I like to understand what I need to do and between you guys and Google I think I have a fair idea of what to get and how to set it up without breaking the bank :-)

I just need to work on a way to run a cable from the van to my house so I can pinch any excess power while it's parked outside now ;-)
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Scruff
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« Reply #28 on: April 04, 2016, 05:28:33 PM »

Ritchie

Seeing as you mention google. This is one of my gurus and a highly recommended read, just ignore the cynical tones his advice is sound.
https://handybobsolar.wordpress.com/the-rv-battery-charging-puzzle-2/

Using the van power in the house, I had that notion too...practically the 12V supply is very difficult to utilise from any distance and the capacity is not great, paralleling the battery with a domestic battery also has it's own can of logistical worms. For the devices it would power it's far more cost effective just use the grid sockets (says the guy who installed a 12VDC system in his house instead of using a buck regulator for the 48VDC system  whistlie).
There's two ways to do it that I can think of; one is install a boost regulator on the outgoing van supply ~60VDC to reduce cable losses and a buck regulator on the other end at the house for a DC supply or a humble inverter. Probably a 10% - 15% round trip loss + inverter loses.
The other is install a ceeform socket beside your charging plug that's powered by the onboard inverter and run artificial mains back to the house to a plugboard or somesuch. You can even get a remote controlled relay to turn the inverter off from the house (*Waiver...theoretically*) Round trip losses 10 - 15%.
The ceeform socket can be used as an external supply for things like the bangin' techno and electric knifes for the barbecue parties or a projector onto the body panelling for outdoor cinema nights. It'll need to be lidded however because ceeform is only IP rated while it's plugged in.  
« Last Edit: April 04, 2016, 07:14:24 PM by Scruff » Logged
skutter2k
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« Reply #29 on: April 06, 2016, 09:56:59 PM »

I'll stick with my original plan of just plugging my gadgets in to charge in the van for now then :-)

Think I may have over done the PV on my old van?! ;-)

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