Someone teach me leccy usage formulas?

Evenin' all,

So I have a plan for my expedition fender but I need help with working out the electrics usage and stuff.. This is my intention and I want to check it's viable;

So, major IT geek here (not so much since I got my landy) so for my expeditions I want to have my raspberry pi (small pc) running 24/7, I am writing some software so that I can plug my camera and 2 video recorders in at the end of the day and have it automatically download the images and copy them into a file structure splitting them first by which camera they were taken on and then the date and then the location. Once it's done with that I want it to then backup all that stuff onto a separate drive, then the raspberry pi will send my phone a notification to say it's finished and how much data it copied and then put itself to sleep.. Well done pi

But I am stuck at the first hurdle.. I don't even know if my aux battery can power the pi and my fridge and a few lights before the battery cuts out..

So, pi leccy specs;

5V, 700 mA (3.5W)

Battery is a odyssey pc1500;

.. No idea but presumably on this page of random numbers?

Odyssey PC1500

Cheers all!

ohms law and online battery capacity calc

Right, few things, that battery seems to be a starter battery, so will not work well for what you want, what you need is a leisure or marine battery, its only max 65A/Hr (ie will supply 65A for one hour, except its an AGM, so it wont)

An electric fridge will eat the battery VERY quickly, unless its a very expensive 12V compressor fridge.

To work out what you need to divide watts by voltage to give you amps drawn if you don't know them (one of these should be on the label on your units).

For example the minifridge I have is 60W so 60/12= 5A
so if I run the fridge for one hour it will use 5 AHr
on a 65Ahr battery do 65 x 0.8 = 52
then 52/5 = 10.4 hours.

The 0.8 calculation is to get an approximate usable capacity of the battery, a 65AHr is calculated as giving that power till voltage is completely dead, which causes battery damage, using 80% will give the battery a better life.

As a side note, not sure what voltage the Pi runs off, but you would need a voltage stabiliser in there, a 12V battery isnt a 12V battery, its voltage changes with its state of charge, and when charging can be as high as 14.4V

timtheenchanteruk said:

As a side note, not sure what voltage the Pi runs off, but you would need a voltage stabiliser in there, a 12V battery isnt a 12V battery, its voltage changes with its state of charge, and when charging can be as high as 14.4V

Click to expand...

Pi runs at 5v 700ma not tried it but it should run off one of the power socket to USB adaptor (make sure it's a good one with dual outlet and one of the sockets able to do fast charge) use 1 socket to power the pie and the other to power a USB hub that you can plug your camera equipment into

that should be fine, they have usually voltage stabilisation built in.

Bomber785 said:

Pi runs at 5v 700ma not tried it but it should run off one of the power socket to USB adaptor (make sure it's a good one with dual outlet and one of the sockets able to do fast charge) use 1 socket to power the pie and the other to power a USB hub that you can plug your camera equipment into

Click to expand...

I was thinking of wiring one if these direct to the battery http://www.rammount.com/Portals/0/Skins/NationalProducts/pdf/toughhub.pdf

Then keeping my 4 cigarette lighter sockets for the gps's and maybe a hard wired gopro on the car.. The hard drives for the PI would be run off a 2000w power inverter. The rest of the ports would be for phones, cameras and stuff.

Cheers

timtheenchanteruk said:

Right, few things, that battery seems to be a starter battery, so will not work well for what you want, what you need is a leisure or marine battery, its only max 65A/Hr (ie will supply 65A for one hour, except its an AGM, so it wont)

An electric fridge will eat the battery VERY quickly, unless its a very expensive 12V compressor fridge.

To work out what you need to divide watts by voltage to give you amps drawn if you don't know them (one of these should be on the label on your units).

For example the minifridge I have is 60W so 60/12= 5A
so if I run the fridge for one hour it will use 5 AHr
on a 65Ahr battery do 65 x 0.8 = 52
then 52/5 = 10.4 hours.

The 0.8 calculation is to get an approximate usable capacity of the battery, a 65AHr is calculated as giving that power till voltage is completely dead, which causes battery damage, using 80% will give the battery a better life.

As a side note, not sure what voltage the Pi runs off, but you would need a voltage stabiliser in there, a 12V battery isnt a 12V battery, its voltage changes with its state of charge, and when charging can be as high as 14.4V

Click to expand...

The battery is supposedly 'dual-purpose' so hope thats not the case.. I wanted a battery I would be able to use in case my starter battery failed that would be able to power all my kit over night before been charged up again the next day

I think I might turn the fridge on tomorrow night and just see how long it lasts..

Well accpording to this Battery Usage & Drain Calculator I will only get 7 hours out of my battery and pi and thats without the cameras charging and hard drives running! So where do people generally put a 3rd battery in a 90?!

why dont you buy a small genny

is that just for the Pi & drives? or with the fridge as well, if with the fridge, sounds about right.
65AHr is small thesedays for a battery, most use 110AHr

Thats around 47W??

so presumably with the fridge, seeing as a Pi is 700mA (0.58W) and that by itself wold gove over 500hours. I said they (frisdges) were power hungry, which is why campers/caravans the fridges work off 12V only when the engine is running. youd be better off buying a Waeco, or similar 3 way fridge that will run off gas overnight, and 12V with the engine running.

you never actually said what type of fridge is is, if it is one of those mini fridge jobbies, you may end up dissapointed anyway, Peltier coolers only cool to xdegrees below the ambient, most 10-15, so in the summer, when the temperature rises, if the interior of the landy ends up at 30 degrees your fridge will only manage to cool it to 15-20, not very fridgy...
a three way will cool down to 3-4 degrees.

No it's a proper compressor fridge, I got it based on the A++ energy rating and have a friend who has used it in 30 degree + weather and had no problems.Dometic WAECO International - Thermoelectric coolers

Could get a little generator but I think I would rather have a 3rd battery for the fridge :/

cartoonhead said:

No it's a proper compressor fridge, I got it based on the A++ energy rating and have a friend who has used it in 30 degree + weather and had no problems.Dometic WAECO International - Thermoelectric coolers

Could get a little generator but I think I would rather have a 3rd battery for the fridge :/

Click to expand...

That link is pelter 46 watt at 12volt and will need constantly on to stay cool

thats a peltier, albeit a decent one, seeing as it can manage 30 below ambient, it still be power hungry, 46W thats around 4A @ 12V
an equivalent compressor unit would use 35W, over an amp less, doesnt sound alot, but @ 12V makes a big difference.
Really, your battery is too small for the load you want to run, a 110AHr leisure battery will start a landy with no problems, the one I have in my caravan has a CCA of 680, and is about the same size as the one in the fender. (used it for a while as a battery in the fender, cos I think the one that was in it was out of a mini or summat)

how much storage do you need, SSD uses alot less power. having said that, a 2.5" HDD can use less than 500mA, your big requirement is for your fridge.

Apart from getting a proper fridge... you could get a proper battery management system - I have one on the D3 - it will equally discharge both batteries to the point that the starter battery is at a level which will still easily start the car, at which point it switches to just the leisure battery and will discharge that as far down as is safe to do so, then, it will stop powering all together.
When driving it manages a safe charge up of both batteries to prevent a huge draw on the alternator which could result in other significant problems (for a D3 if you draw too much from an alternator it starts switching other stuff off... like the air compressor for the suspension ) - in my Series if i draw too much the belt just snaps!!! (that's more to do with the alignment, or lack their of on my alternator belt though)

3rd battery wise, how about in the back on the wheel arch section in it's own "box" (with ventalation - don't seal a battery in completely)
If you need thick battery cable let me know.

I assume you are using solid state disks for your hard-drives, cos if not then that would massively reduce your power consumption of your IT equipment fitted.

Hmmmmm, I need to find out what a peltier fridge is!

No, 3tb (6tb including backup) is too large for solid state disks it will be 2 3tb spinning disks and I think I will fit a 3rd leisure battery for the fridge.

What battery management system are you running bump? I have a merlin smartbank and merlin alternator management system which do everything you mentioned so I 'could' put my faith in it and run the kit off both batteries if I really wanted too.

cartoonhead said:

Hmmmmm, I need to find out what a peltier fridge is!

No, 3tb (6tb including backup) is too large for solid state disks it will be 2 3tb spinning disks and I think I will fit a 3rd leisure battery for the fridge.

What battery management system are you running bump? I have a merlin smartbank and merlin alternator management system which do everything you mentioned so I 'could' put my faith in it and run the kit off both batteries if I really wanted too.

Click to expand...

Ah ok, sounds like your battery monitoring system is ok then - mine is a propitiatory system specifically developed for the D3 in Australia... So, i'd stick with your Merlin system

3rd battery! Get a yellow top?

Thermoelectric cooling - Wikipedia, the free encyclopedia

Peltier, is thermoelectric cooling, two different materials bonded together, when a current is applied, it draws heat from one material to the other, so put one side in a cooler, the other outside, and it cools the interior.
reverse the polarity, it works in reverse, (hence why they can also heat)

reliable, but needs a current across it permanantly, and not very efficient, efficiency can be inproved with heatsinks, and fans (cycling of fans on and off usually provides basic regulation of temperature.

Just a thought on the HDD, are they going to be running 24/7 as the Pii??? they aint going to last long if they are, at least not without some good damping.