Power System Requirements: Determining Capacity

[Scott Brady]

I have been researching batteries and some of the technical aspects of an auxiliary system, solar panels, etc.

The evaluation starts with determining LOAD, which is the total amperage draw for the accessories while parked (and in summer).

Load:
Fridge: 30 amps (12 hours x 2.5 amps)
2m Radio: 16.8 amps (24 hours x .7 amps)
Fan: 16 amps (8 hours x 2 amps)
Light: 1 amps (4hours x .25 amps) note: 12 watt, 120v light
Misc: 30 amps (Running computers, charging batteries, inverters, etc. 120w x 3 hours)

24 hour period: 93 amps!

My auxiliary battery Optima blue top (55ah) has a Depth of Draw (DOD) capacity of 25 amps (for max battery life). That leaves me 68ah short.

So, that is a big problem (at least in summer months).

There are ways to help mitigate the under-capacity, which includes running the car for a period of time, installing solar panels, etc.

Are my calculations correct? Any ideas?

I am looking into solar right now...

 

[Brian McVickers]

Hi Scott,

I am fairly new to the group here but thought I could share some ideas that may help in your 12v project. I just spent the last 4 years living on my sailboat that used a 12v system for elecrical power needs. 2 years were spent dockside plugged into 110v with a converter to charge the batteries and the last 2 years were spent on the hook while traveling and exploring. While on the hook we were completely 12v dependent using two isolated power banks. The first was a 1000 CCA battery for starting the aux diesel engine and the second was a 700 amp hour house bank made up of 6v batteries wired in series. Both banks were charged using the 120amp alternator on the diesel, 400 watts of solar panels and a wind generator. I designed and created this system from scratch for our particular boat, systems and application. That said I still only know the basics compared to what there is to know.

You are right on in determining your power needs. By taking the power that each system will draw in one hour of use and multiplying that by the number of hours the system will be used in a 24 hour period you determine your power requirements for one day.

In your case = 93 amp hours

With that much power needed, the remote locations you will be in and the importance of having your engine starting when needed; a dedicated starting battery isolated from a "house battery" would be in order.

The DOD of your battery is the next concern. You don't want to drain your battery below 50% of its capacity. Along with damage to the battery it will reduce the life cycle and capacity while increasing resistance recharge time.

Depending on your space availability either in the cab or in the bed of your truck I would go with an 8D battery. They range in AH from 170 to 200 ah and weigh in at around 135lb. Oh, and be sure to strap it down nice and tight! Using a 4D battery may also be an option with 120AH an about 100lb.

Wire it up to be charged from the engines alternator but isolated from the dedicated starting battery. Throw in a battery selector switch and you will now have a backup starting battery. If possible add on a small solar panel, maybe 30watts.

Solar panels are great but they are primarily for topping off with a trickle charge, at lease until you get into the 100's of watts of solar panel power. Also note that a good 55w panel costs about $250. Either way be sure to wire your solar panel with a voltage regulator. This will act as a check valve for the electricity, without it the solar panel will actually suck power out of your battery at night or when in the dark = two steps forward one step back!

Also be careful on location of the panels. Scenic Wonder Runner - locating the solar panels under the tinted glass of your sunroof will drastically decrease their output. This can be seen by hooking a solar panel up to a volt meter in direct sunlight and watching the output drop as you cast your shadow over it. I was amazed to see it happen when I did it.

Hope this helps, I have a bunch of books on 12v systems somewhere, I think in storage. You are welcome to barrow them if I can find them and happen to get together with you all for an adventure soon. If not, West Marine has a good selection of books on 12v systems.

Brian

 

[freightdog]

http://www.rvsolarelectric.com/inform.htm - informative site...

http://www.pplmotorhomes.com/parts/solar-power/rv-solar-kit.htm

I really like the 55watt kit they have!

But for me - a 1000 watt generator from Yamaha is really more practical and almost the same price. http://www.northerntool.com/webapp/...?storeId=6970&productId=200315659&R=200315659

Plus you can use it for so many other things!

 

[Scott Brady]

Brian! You lived on a sailboat for four years... You are my new hero

Great feedback on the 12v system

I should have expressed some limitations in my first post, the most important of those being weight. The single battery and mounting braket weigh over 50 lbs. If I add even one more, it will start to be an issue (though I am considering adding one more).

Researching this really is an eye opener. Even with a second battery I am only at 55% of my requirements per day. That is for only ONE day without running the engine.

In winter I am probably ok, as the fridge will run much less, and the fan won't be required, etc.

I have enough area for a maximum of 140 watts of solar, which is 11 amps per hour of sunlight. If I have 8 hours of sunlight that will give me 88 amps, and I am set! BUT, the entire roof will be solar panels and cost over $1000 to construct... :Wow1:

 

[goodtimes]

Scott, your fridge only pulls 30 watts? I figured it would be much higher....How much (% of time) will it realistically be running? (I have no idea...). What about your other power requirements that are listed? Are they going to be pulling the amount of power you have listed 24 hours a day? Or are you going to be shutting some of them off (like the VHF when you are away from the truck)? It sounds like you are building your system to support %100 of the *possible* demand....not the actual demand. Sure, we would all like to have a system that will put up with 10% more abuse than we can possibly give it...but do you have the capacity left to hold that much weight? Arent you already pushing GVWR (or are you beyond that already)?

I'm just wondering if you can minimize some of that load to get your system requirements down where they won't be so difficult (read:expensive) to reach, while still retaining your functionality....

 

[Brian McVickers]

That's a good point
Maybe take another look back at your calculated power consumption.
Only take into account the time each system will be in use with the vehicles engine off
and then the ammount of time you will actually be using the system.

After you drive to your destination and turn off the engine you probably depend on the vehicles electrical system less than you reoize, maybe?! I have a feeling that once you get to camp and turn off the engine you go running around exploring and apprecaiting your destination.

For example

Fridge: 30 amps (12 hours x 2.5 amps) - seems valid for at camp

2m Radio: 16.8 amps (24 hours x .7 amps) - Your not going to sit there a monitor your radio for 24 hours straight with the engine off.

Fan: 16 amps (8 hours x 2 amps) - seems valid for sleep time with a 12v fan in your tent.

Light: 1 amps (4hours x .25 amps) note: 12 watt, 120v light - insignificant

Misc: 30 amps (Running computers, charging batteries, inverters, etc. 120w x 3 hours) :Wow1: STEP AWAY FROM THE COMPUTER -- AND INTO THE WILDERNESS!! :luxhello:

You will probably require less than you think.

 

[BajaTaco]

Hmmm... fun topic :lurk:

Man, that is a lot of juice Scott

OK, I am a complete amateur at this, but here is my go at it:

Optima bases their AH rating on the 20-hour rate. This means they start with a 100% state of charge (12.7V) and discharge the battery over a period of 20 hours to a 0% state of charge (10.5V). They measure the amp-hours provided by the battery during this 20-hour period. In Scott's case, that number is 55. So, 1 amp per hour x 55 hours (or 2.75 amps per hour for the 20 hours tested). If you convert this to 24 hours, it comes out to 2.29 amps per hour (average available if battery is completely discharged). These are not exact numbers, as the faster a battery is discharged, the less AH it provides. But for a rough calculation, you get the idea.

So we'll say you have an avg. of 2.29 AH if you fully discharge the battery. However, you only want to disharge to about 50% (optimally) so we'll reduce that to 1.15 AH. If your loads are 93AH, that averages out to 3.88 AH (3.88 amps per hour) that you will use for a 24 hour period.

So here is a suggestion. Since you have a nice big storage compartment for your compressor and battery, I would see about making a mount for a 2nd Blue-Top Optima, that could be connected to the first one in parallel,(Effectively providing 110AH rated capacity at 12V) or maybe look at the TrollFury system by Optima. This would come in handy for your winch too.

This would give you a 2.29 AH "safe supply" yield of power from the batteries (per hr. for 24 hrs). Now if we consider augmenting the system with solar charging, then we can use the solar to make up for some of the lost power in the batteries.

If you use a 50W solar panel, then you have 50W/15V = 3.33A - 10% 3.0A. (I believe 15V is the approx. true output voltage of a solar panel) If you figure an average of 6 hours of good full sun to produce 3.0 amps/hour, you end up with a total of 18AH charge from a 50W panel. If you used 3 panels for a total of 150W of solar, you would have 54AH of charge. If you are consuming 93AH per day, and you get 54AH of solar charge, you are effectively using 39AH from your battery reserves each day. This may be considered a reasonable amount of discharge for almost 1.5 days before you would need to run the alternator to charge the batteries back up (if you insisted on not exceeding 50% DOD (depth of discharge)).

93 amps - 24 hours consumption
54 amps - solar charge
39 amps - battery discharge

If 2.29AH is safe yield for 24 hrs. (2.29AH x 24 = 55AH).

55AH divided by 39AH (daily discharge) = 1.4 days

Regarding your power consumption, I would actually increase the anticipated load from the fridge just a bit, to say maybe 35 amps. If you could cut your 2m radio use by 2/3 and cut your misc. consumption by 1/2, you would end up with around 73AH which would increase your "parked" time to about 3 days:

73 amps - 24 hours consumption
54 amps - solar charge
19 amps - battery discharge

If 2.29AH is safe yield for 24 hrs. (2.29AH x 24 = 55AH).

55AH divided by 19AH (daily discharge) = 2.9 days

 

[BajaTaco]

I should have expressed some limitations in my first post, the most important of those being weight. The single battery and mounting braket weigh over 50 lbs. If I add even one more, it will start to be an issue (though I am considering adding one more).

Scott, my suggestion above it obviously going to add a heck of a lot of weight, and cost - to your system. And I only suggested it based on what you perceive as your requirements. It's another classic expedition vehicle trade-off scenario. I made my compromise by going with a 75 lb. deep cycle aux. battery, and running much less power consumption (fridge being the worst). And also living with the fact that my battery will not last as long as it ideally could, due to deeper discharges on occassion. To me, it is worth the more frequent replacement expense of the battery, than carrying the extra weight for optimum battery life cycle. Also keep in mind that the Optimas deal much better with abuse (DOD and large charge currents) than conventional wet-cell lead plate batteries.

Another idea would be to only add the 2nd battery for the trips where you expect to be parked more than a day. This would require more battery maintenenace in that you would need to rotate the active battery, and keep the inactive one charged up and cycling periodically.

 

[freightdog]

Or you could buy a bigger truck!

Kidding -

I have three yellow top Optima's - with a multi/battery isolator...

We will be out nine day - I will get back to you!

:elkgrin:

 

[datrupr]

 

[BajaTaco]

What? I know - it's nuts... which part are you shaking your head about

 

[Scott Brady]

Scott, your fridge only pulls 30 watts?

Engel 45 power consumption: 0.7 - 2.5 amps

What about your other power requirements that are listed? Are they going to be pulling the amount of power you have listed 24 hours a day?

I tried to estimate the power requirements based on the number of hours I expected they would run. After further evaluation though, I do know that the misc. will be less, as I was using my BIG laptop as a gauge, which pulls 120 watts @ 120v. My smaller laptop pulls half that, so that is a good start.

 

[Scott Brady]

Misc: 30 amps (Running computers, charging batteries, inverters, etc. 120w x 3 hours) :Wow1: STEP AWAY FROM THE COMPUTER -- AND INTO THE WILDERNESS!! :luxhello:

You will probably require less than you think.

Good point about stepping away from the computer. I am always reviewing photos and doing daily logs for the articles.

I calculated that wrong. That would be at 120v, since those accessories run through the inverter. There is likely some parasitic loss through the voltage change, but we are still only looking at 4-5 amps. Big difference, and likely saves my from needing the extra battery.

BUT, this only works for one day without running the engine

 

[Scott Brady]

Man, that is a lot of juice Scott

OK, I am a complete amateur at this, but here is my go at it:

Great go at it, and completely correct from my review. Some reevaluation of my requirements have reduced the expected load.

I am really interested in solar though. And the main reason is redundancy! Even if my battery relay fails, or one battery fails, I could still charge the truck and get home. The challenge with solar is the cost and surface area required (expected). I really only want to cover the tool-box, but I have found (on the canyoneering trip) that I might even need to store gear on top of that.

So, that leaves the roof, which I have no problem using, but would require some engineering to protect the cells and mount the units effectively. Certainly a fun project though, and maybe one to tackle next spring.

 

[BajaTaco]

... I am really interested in solar though.

ME TOO!

...The challenge with solar is the cost and surface area required (expected)...

I don't dispute the cost challenge. But how about surface area - you don't care for my prior idea of portable panels that would stow under the tent? It would allow you to park the truck in the shade, and prop the panels into maximum sunlight for a multitude of scenarios.