Solar PV expansion

Thanks, Frank.
From the transformers on each device:
V A W Hours to charge
MacBook Pro 16.5 4.6 75.9 4
Macbook 16.5 3.65 60.2 2
iPad 5.2 2.3 12.0 8
iPhone 5.2 2.3 12.0 2

I've attached a little spreadsheet which calculates the load based on the transformer details, with an estimate of how long it takes to charge each device. I think this comes out to 45.3 Ah.

The Bushman Fridge doco says that it draws 16 Amp hours a day at -4C at 25C ambient temp. This brings me up to 61.3 Ah, with a bit more from the LED light.

Does this give you enough info to work with?

Rob

Hi Rob,

I was going to explain how one might work out what you need, but I see you've already looked at the solar article by Collyn Rivers on this site.

I agree with John in his reply below that my use of the total watts of the power bricks was an overestimate, but it did give us a starting point. Perhaps the numbers would be closer to the mark if the power supply was charging a flat laptop battery and running the laptop as well, but once the battery is charged and the brick is just running the computer then clearly the power draw would be reduced.

You've given figures from your power supplies of about 76 and 60 watts for your computers and I think the same argument would hold. Collyn Rivers suggests 20 to 50 for laptops, but let's go with the worst-case scenario and use your figures.

I have a spreadsheet prepared by an electrical engineer for working out solar needs in the brand of caravan he and I own. Using it is much simpler that explaining the calcs here. I have attached it so you can play with it.

You make your inputs in the yellow cells. DO NOT CHANGE ANY OTHER CELLS, just the yellow ones.

Input your loads, your battery size, how low you are prepared to let your battery go, the size and number of solar panels and it will tell you how long you can camp, based on solar charging, for each season.

I have set all our caravan inputs to zero, and made a couple of dummy entries (Inverter and Redarc Relay) that make it total the daily load match your 61.3 amp-hours. The fridge is included in that, so the specific fridge entry is set to 0. I have assumed a 120Ah battery and that you're prepared to let it discharge to 50%. I think the peak sun hours are for Sydney.

Based on 180 watts of panels, camped near Sydney you would get 2 days in winter, 4 days in autumn, 5 days in spring and unlimited days in summer before that 120Ah battery dropped below 50%.

Play around with it, see how you go. I found it very useful.

(50% is the rule-of-thumb sweet spot between minimising battery wear and tear and practical use for regular discharge of a deep cycle battery)

The author wishes all readers to know that this spreadsheet grew from an original posted on ExplorOz some years ago.

Cheers

Rob,

John's right, MPPT is the way to go. I think I would go for a larger controller so that you have room to expand your solar if necessary.

Specs suggest you could get good results with something like this. (No affiliation, etc.) There are plenty of others, but these are modestly priced, reasonably compact, versatile and provide a reasonable amount of info on the display if you're technically minded and want to monitor what's going on.

Reading the Collyn Rivers article, he emphasises more solar in favour of more battery capacity, and considering that you are living in pretty close quarters, more or larger batteries might not be practical. Hence my suggestion for room for solar expansion.

Remember that if you increase battery capacity you must increase solar capacity to keep them charged, or you will risk sulphation and premature death of the batteries.

Running that spreadsheet suggests that increasing your battery capacity to 200Ah and using 120 watts of solar would limit your camping in all seasons.

However, if you stay with 120Ah of battery and have 240 watts of solar you get a couple of extra days in winter (not much change there), but unlimited time in spring, autumn and summer.

I agree with John about the flexible panels. You would get the best "flexibility" from your system with portable panels, eg 2 x 120 watt briefcase units so you can deploy them in the sun and keep the Troopy in the shade. Or with some panels fixed to the roof of the Troopy (with ventilation) and some portable and arrange your parking to suit the season, location and sun. Maybe 120 watts on the roof and 1 x 120 watt briefcase unit.

Cheers

Thanks, John and Frank

It's all very interesting, and still all black magic! It all depends on the estimates I make of usage amounts. i.e. how long it takes to charge my devices and how many Watts they use on average (not peak).

The spreadsheet is fantastic, and the advice about getting an MPPT controller is also great. I just need to work out what panel output I need, and then how to fit it into our very constrained set up. A large panel mounted over the cab is probably the main option, but the wife always likes to park in the shade!

I saw a vehicle recently which had a flexible panel mounted on his bonnet. I take your point about heat and lack of ventilation and cost. That option is OUT.

Back to the main point - working out how much wattage is being used each day. If I knew what the draw was, I could reduce it, or install enough panels to cover it. So, the question is, how can I work out my consumption.

I know of some hugely expensive and very 'sexy' Redarc devices, but I can't justify the cost. How can I easily measure my power generation and consumption, given that I can't directly measure my battery capacity?

Cheers
Rob
Marooned in Tasmania because there's no space on the 'Spirit' for 3 weeks.

Rob,

"I saw a vehicle recently which had a flexible panel mounted on his bonnet. I take your point about heat and lack of ventilation and cost. That option is OUT."

Real flexible panels are expensive. But you can make a thin fold-up set that could work, would be light, compact and easy to store.This company will make thin, semi flexible panels to a size you specify. You can fasten them to a canvas backing and depoy them loosely (with air gaps) on your car and fold them up into a compact pack for storage. I have four slimline 50watt panels of this type - they weigh only a few kilos. A friend has done the canvas thing with those panels with some success.

"Back to the main point - working out how much wattage is being used each day. If I knew what the draw was, I could reduce it, or install enough panels to cover it. So, the question is, how can I work out my consumption."

I suppose you have to measure it. In a camping situation I cannot think of any way other than an amp-hour counter, ie a battery monitor, but they're expensive, and once you get a decent solar controller with a decent display you won't need it.

A suggestion:

I presume you have a second battery in the Troopy - don't do this if you only have an engine battery. If you are committed to upgrading your system, you COULD buy a good MPPT 20 or 30 amp controller that will display amp-hours in and amp-hours out of the battery. Initially just hook it up to the second battery. Remove the feed to the "house" from the battery and connect that to the load terminal of the controller. It would then give you a daily reading of your useage in amp-hours. Chargeing will not be recorded. Once you have established your average to your satisfaction, use the spreadsheet or consult further to choose your battery size and solar wattage.

The Jaycar product I linked to does not appear to show amp-hours out, so you would have to choose something else. I was going to link to Derek's ABR Sidewinder site, but it says solar panels and controllers are sold out and discontinued. Something like this (again, no affiliation) could be suitable. There are plenty of others - Google is your friend.

"How can I easily measure my power generation and consumption, given that I can't directly measure my battery capacity?"

Once you get your panels hooked up a controller such as the Roc-Solid (there are others) will measure both generation and consumption. When the controller goes into Float mode (it will indicate that) the battery is fully charged.

There are worse places to be marooned than in Tassie :-)

Cheers

Rob,

We need to know your present battery capacity, and your energy requirements.

1) Battery – how big is it physically? Capacity is roughly proportional to volume. Is it similar to the one under your Troopy’s bonnet? If so, it’s probably about a nominal 100 Ah, in which case we should budget on it supplying about 60 to 70 Ah.
2) Your energy requirements – Your spreadsheet values are very high. We measure the current flow in our system as follows - the laptop draws about 2.3 amps, the tablet well under 1 amp and the phone very little. Applying these figures to your gear, I calculate about 20 Ah for the electronics, 16 Ah for the fridge. Allow 4 Ah for LED lighting for a daily total of 40 Ah.

The figure for the fridge seems a bit low, but overall this seems a fair ballpark figure. (I’m assuming that you are running the gear directly from the 12V supply, and not through an inverter?? If using an inverter you’ll need to add about another 3 Ah to cover inefficiencies.)

On this basis, to be totally reliant on solar I’d aim to harvest at least 50 Ah per day. Assuming say 7 hours sunlight with the panels all aimed reasonably well at the sun, we are looking for 7 amps from the panels. That will require 120W of panel capacity with a PWM controller such as your present one, or at least 100W if using a MPPT controller. Battery capacity to allow for a few days without full sunshine – I’d suggest 200 Ah, especially if you must keep working. An option is to augment the solar using the vehicle’s own charging system, in which case a dc-dc charger would be very nice to have. Adding that in would point straight at a Cteck D250S dual (or similar) charger which supplies the necessary switching, smart charging at 20A, and MPPT solar controller all in the one box. All of this is starting to cost significant money!

Measuring – I’d suggest investing in a suitable meter to measure the current actually flowing in and out of the battery. Jaycar is probably the most available supplier and they offer digital multimeters from $5! (A $5 meter would be a good place to start if you aren't too familiar with this stuff.) To measure current with a conventional multimeter it’s necessary to set the meter up to measure using the 10A scale, then temporarily disconnect one of the battery leads and reconnect it through the meter. (Hopefully there will be a fuse close to the battery, in which case I’d just unplug/remove the fuse and introduce the meter leads in its place.) I’d remove all loads and all solar panels first, then add each of them in individually to measure the actual current drawn or contributed by each.

Clamp meters also exist and are very easy to use, but they don’t have the sensitivity and accuracy for what we need here…..and they cost a lot more!

HTH

Cheers

John

Rob,

I just ran some lower consumption figures, about what John suggested, through the spreadsheet and concur. It's amazing what an unfavourable difference "adding a bit for mum just in case" makes.

Run some figures yourself to see the effect of battery size and solar wattage.

If you can carry 200 amp-hours of batteries then I agree it will give a good reserve for bad solar days, but just watch how long they stay down around 50%. You don't want them there for too long and they'll take a fair while to charge fully to prevent sulphation.

John,

Thanks for the link to Electricity for Camping - I hadn't seen it before, it's excellent!
A question - can you get a relatively cheap meter that will count amp-hours or watt-hours over a period? Multi-meters will give instantaneous readings but they won't accumulate - unless there's one out there that will??

Cheers

Frank,

Jaycar offer dc watt/amphour meters - catalogue numbers MS-6170 and MS -6172. I haven't used them, and at $70ish I won't be! Jaycar is a useful, but usually expensive, resource for lots of stuff.
Cheers

John

Thanks John.

Cheers

Lots of responses below.

Electricity for Camping was very useful, thanks. I note that the 2 links to 'how to use multimeters' both point to the same place, and this redirects you somewhere else. Perhaps you can amend this. I look forward to reading more about multimeters when I get the chance.

Your suggestion to run the multimeter as an ammeter through a fuse box was brilliant. So obvious, but I hadn't thought of it!

My new house battery is 105Ah, and there's no space anywhere for another one. :-(

One of the reasons we bought the Bushman fridge was its energy efficiency. Also easy to clean and well-designed stacking racks.

I agree that 40Ah/day is a good estimate of or use. The spreadsheet tells me that 120W of panels will be fine for 4-5 days in winter and overkill for summer, but of course, I don't have an accurate figure for current draw. I'll use the ammeter to get a rough idea of this.

I just unscrewed the Solar Charge controller from its mount, and it is a 10A one, so one simple solution is to just add more panels to the little distribution box my son and I made. (4 cables in from the 4 panels and 1 cable to the Solar Charge controller.

I'll look into the other gadgets you and Frank have suggested. A better MPPT controller is on the list.

I'll also look at the flexible solar panels. Something which can roll up would be better for storage on the roof rack. Don't the flexible panels use amorphous technology which still works when part of the panel is in the shade?

A DC-DC MPPT charger from the car would be great, but I don't think it will work with our old dual battery system. This has a big round Cole-Hersee manual switch which redirects the earth cable rather than the active. I think I’d need to completely rewire the dual battery system to use one of these.

I hope this thread is also useful to any lurkers out there!

Rob

Rob,

"I'll also look at the flexible solar panels. Something which can roll up would be better for storage on the roof rack. Don't the flexible panels use amorphous technology which still works when part of the panel is in the shade? "

The ones I linked to are semi-flexible. They are monocrystalline (not amorphous) on a thin aluminium backing with some kind of flexible, clear coating instead of glass. They are shade affected, the same as conventional panels. You can't roll them up but you can flex them and they won't break. My friend attached 4 of them in a grid pattern on a piece of canvas. When he uses them he drapes them on his car or something else in the sun and when he packs them away puts an old towel on them to stop face-to-face contact then folds them up into a pretty compact package for transport.

Cheers