Sizing a 12v Solar system for an RV

Re: Sizing a 12v Solar system for an RV

EurovanCamper wrote: »

The DC clamp ammeter arrived and at the 4A (1mA accuracy) range on the meter and running everything in the van at the same time I can’t get it to draw more than 1A.
I have the wire sandwiched between the jaws and the meter perpendicular to the wire. How long of a straight section does the wire need to be? (Photos) Otherwise my previous assumptions are generous.

Just to be clear--A single wire through the hole in the middle of the jaws, you do not "clamp" the flats of the jaws on the wire itself. The jaws must close completely. And the closed jaws encircle the wire/cable. The wire can be "floppy loose" floating around in the hole in the middle. It will not change the readings a bit.

Also, you only clamp 1 wire at a time... If you clamp two wire (such as the +/- wire pair to a load), the "sum of the current" adds up to zero amps, and the meter will read zero amps.

And, for DC measurements, you need to "zero" the DC amps to zero. It is the nature of a DC current meter readings to drift over a few minutes.

Try clamping the battery positive (or negative) lead. Turn on the head lights, you should read 10-15 amps load with the meter. Start the car and you should see 10-50 amps or so from the alternator charging (depending on battery charge, engine RPM, etc.).

One wire, clamp closed, is all you need to measure the current properly. How they work:

http://www.kew-ltd.co.jp/en/support/mame_02.html

So a 250 Watt array should give me
250Watt /14.5v charging * 0.52 system eff * 5hrs = 44.8 AH from the panel on a below average day.

Will a 105AH battery not accept more than 13% of the AH of the battery 13.6A?

On average--If you have a near dead battery and recharge it at more than ~13%, it can overheat without active cooling.

In general, batteries at less than 80% state of charge will accept very large current current and not overheat (charging is very efficient).

Once a battery is over ~90% state of charge, it will accept much less current and can overheat with too high of charging current... Even a 2.5 to 5% rate of charge going into a 100% charged battery (typically "equalization" charging), can easily overheat a lead acid battery.

The 13% "upper limit" is suggested for generic lead acid "rated" charging current to keep batteries relatively cool. Also, if you drain the battery bank by ~25% per day, on average, the battery will easily reach "float" charging (fully charged) by mid-day for most people. Putting a larger solar array on will not recharge the battery any more. However, if you have other loads (say you run a well/irrigation pump and other loads in the afternoon, you may be able to "cost effectively" use a larger array).

And, if you have >13% rate of charge, you should look for charge controllers that have remote battery temperature sensors. As lead acid batteries get hot, their charging voltage falls. This can help limit the chances of thermal runaway (battery gets hot, controller sees battery voltage "drop", things needs more charging current, battery gets hotter, voltage falls more, controller pumps out more current--etc.). Practically speaking, around 20-25% is probably a good limit for maximum charging current in most situations (size of wiring, limited ability of battery to accept higher currents, etc.).

Again, just generic rules of thumbs--There can be good reasons to do something different for special situations.

I was suggested to pair a 245 Watt SolarWorld Polycrystalline panel with a Midnite The Kid charge controller. Is this system overkill and permit the van to roam Arizona indefinitely during the zombie apocalypse?
Would a smaller panel and a Sunsaver 15A MPPT be the maximum my battery can accept anyway (or PWM)? Why was I suggested against using the same panel with the sunsaver and told it will only accept up to 200W solar panel?
Because 250Watt/14.5v * 0.77 panel +controller derate = 13.2 A max charge rate anyway

In round numbers, The "cost effective" maximum solar array for a 15 amp MPPT controller on a 12 volt battery bank:

15 amps * 14.5 volts charging * 1/0.77 panel+controller deratings = ~282 Watt array (roughly) maximum cost effective array

So, 1x 250 panel is find for the controller--Two on a 15 amp @ 12 volt controller would be way over paneled, and a waste of money.

Can you fit 2x 245 watt panels on your van roof? These are pretty large panels (and can the roof support ~100lbs for two panels?):

Pmax: 245 Watts
Voc: 37.5 Volts
Vmp: 30.8 Volts
Isc: 8.49 Amps
Imp: 7.96 Amps
Weight: 46.7 Pounds
65.94" x 39.41" x 1.22"

Some people have made "awnings" so they can get more panels on their RVs. Mounting so they don't blow off on freeway is not trivial.

I suppose this situation is low risk since I'm only short term camping and can run the vehicle when necessary. I would just like to avoid what has been happening that I'll have to start the van in the middle of the night on the second night when running the furnace.

Energy usage is a set of highly personal choices... Anything I suggest as "practical" or not for your needs would be pretty much worthless. You need to measure/make estimates of your power needs.

A kill-a-watt type meter for measuring 120 VAC loads. A DC AH/WH meter for DC loads. A Battery Monitor would be very nice--But probably a bit pricy for a smaller off grid system. A DC current clamp DMM meter can help estimate loads too.

The RV heater may be one of your "worst loads"... Some seem to draw ~8 amps for 1/2 the night. You might look at alternatives for lower power space heaters (if there are any).

If you will be needing more power--A Honda eu1000i or eu2000i inverter generator (900 or 1,600 watt rated respectively) is hard to beat. They are not cheap (around $700/$1,000)--But they are light weight and very quiet. Also, overall, much more fuel efficient vs running your van engine--Plus you can move the genset 50' away on an extension cord to keep the noise and fumes down.

-Bill

More or less for solar power--If the numbers are within ~10%--That is pretty much in the margin of error (measurement error, average solar production, variation in loads/usage, etc.).

For RV systems--Many times the larger battery system does not really give you much--The batteries typically die from aging/deep cycling or an "oops" (somebody left the lights on and killed the battery bank).

I would not worry too much--But I would probably tend to the smaller battery bank--That will give you a slightly better charging rate for the batteries, and hopefully lower costs (and weight).

But the differences are very small in the practical world.

-Bill