How much battery power do I need?

GypsyBob · Veteran Member

How much battery power do I need?

Cruising Cruze · Guru

Hi Bob

just to give you any idea

as for your cpap 2.65 average an hr 10 hrs say 30 amp
Fridge 2.5 average an hr 24 hrs say 60 amp
external hard drive 5 watt an hr 2 hrs 1 amp
Radio 5 watt an hr 10 hrs 5 amp
Lights 24 watt 3 small fluro's 4 hrs 8 amp

as for the 800w inverter I don't know but some one will tell you who knows more than I do

so far an 120 w solar wont do it for the recharge you need

btw do you need a desktop computer or can you do with a laptop ( saving 80% power )

maybe an DC to Dc charger as well

this is not my gospel but just an idea of what you can expect

I would have a gas stove to make a coffee

I hope this will help you a bit Bob

Cheers John

GypsyBob · Veteran Member

Thanks for the reply John.

What will a DC to DC charger do for me? I've not come across one of these in my planning so far :(

If just pack some frozen milk and stuff in an esky along with other stuff I need for for 2-3 days I should be able to get by without a fridge. For lights I can use lcd's that happily last about 40+ hours on a fully charged set of AA rechargeables so I'll not need to allocate battery power to them.

I've been told that monocrystalline 120W solar panels can produce up to 9 Amps per hour so if I pick a figure of half that for 8 hours a day it still gives me about 36Ah being generated daily. So if I start of with 2 fully charged 120Ah batteries and use 30Ah per day for my cpap and 30Ah for computers it means that at the end of day 1 my battery levels will be down to 216Ah, after 2 days down to 192Ah and after 3 days down to 168Ah. Allowing a depth of discharge of 60% it suggests that I could go 5 or 6 days before needing to have mains power recharge the battery bank. Given that I was only planning on 2 or 3 days away from mains power it seems I should be alright?

I know that there's a school of thought that says "just try it and see" but if it can be planned beforehand, that would be nice.

Oxley · Veteran Member

Youre at the stage Im at currently with working out power requirements.

I sat down with an electrician friends and did some maths and then went into the shed and found it was all out the window.

What Im currently doing ( as I have 11 months before going full time) is doing a lot of testing in the shed to work out what uses what.

For example I worked out having the fridge set so its about 5-6 degrees gives me 4 days on a 100 amp hour battery compared to 3 days set to 4 degrees, but thats in winter, so I can assume I will lose a day or 2 during summer.

I can run my laptop and a USB powered external drive for 12 hours from a 300watt inverter without the battery meter dropping a single bar, yep I probably should be a bit more scientific about this, but.....

As I already travel a fair bit for work I have my music/audiobook setup with a JBL portable speaker (with its own internal battery) and a Nexus 7 tablet, which I can get 3 to 4 days usage before charging, which I can do from a Power Traveller portable solar panel, or a small extended battery.

So once I work out usage, then I will work out how to charge it.

My CPAP machine is not critical, I can live without it for a few weeks, but for some strange reason, when Im holidays for more than 2 weeks my weight drops, my sleeping improves and my doctor take me off diabetes medication, I wonder why?

Cruising Cruze · Guru

Hi Bob

As I told you this was only to give you an idea
I'm no expert at all had to learn the hard way to

maybe some one that has more knowledge can give you an better answer than me

enjoy your trips mate

Cheers John

rockylizard · Guru

Gday...

Perhaps this Excel spreadsheet (I obtained it a long time ago) may be helpful

Cheers - John

VKPORTABLE · Senior Member

GypsyBob wrote:
Yes, the "how long is a piece of string question" .....
I use a cpap machine, the current draw @ 12V is between 1.8 and 3.4 Amps per hour, I like to sleep for about 10 hours per night. I have an Engel MT27F fridge that uses 2.5 Amps @ 12V. I have a few cameras and I use them regularly and hence need to frequently charge camera batteries, I am a heavy PC user and usually need to power additional external drives, I like to listen to music all day. Maybe some hot water for a coffee would be a good thing.
TIA
Bob

Okay, well take a 7AH SLA battery for example. It will, as it says provide 7 Amps for an hour - well not exactly but near enough for arguments sake.
So going by this your CPAP machine would run for about 2-3 hours on a 7AH SLA.

Have you considered a small generator?
It may well be a good investment for you considering the CPAP. A friend of mine uses a CPAP and I left the generator running through the night for him, after killing the vehicle battery the night before using the inverter to run it.

As for hot water - one of those Koookaburra or Gasmate portable gas cookers from KMart are a great little investment for $25.
KMart also sell packs of the gas cans for $5 most times.

beiffe · Guru

Not nit picking but just a suggestion.
I think you will find the battery is a lead acid battery and therefore at best will supply about 50% of the stated 7AH as long as you don't draw power at too high a rate. The higher the draw rate the less Ah's available. They suggest .720Ah for 10 hours which I would consider sus and they suggest a 2.1 amp charge maximum, but I would expect it would take more than 3 hours to charge fully to the 7AH eg the charger showed a charge of 0A
So a 7Ah battery (Lead Acid) would give 3.5AH if new and fully charged. the next charge would be less and you would need a considerable time to reach the full charge.
A crank battery is normally accepted to only give about 20% in a house battery scenario. It gives high current for a short time to start an engine but would not run a sustained draw of about 5 amps to run a fridge and a few lights.
That's why the lead acid batteries have restricted the advancement of the luxury or comfort items expected or request by RV travelers for so many years.

Regards
Brian

kww511 · Member

Gypsybob I find this site an excellent resource for calculating power requirements and solar panel calculation
http://www.batterystuff.com/kb/tools/

-- Edited by kww511 on Friday 27th of September 2013 07:05:26 PM

Big Gorilla · Guru

GypsyBob wrote:
I am thinking a pair of 100-120Ah deep cycle batteries with a 800W inverter to power the 240V computer/camera gear. while the cpap and fridge operate on 12V. I can top up the batteries from a 120W solar panel kit. Does this sound about right? OTT? not enough? , Might I need a small generator for emergencies in longer term stays?
TIA
Bob

Bob, when you say one of the batteries is stuffed, I assume you have 2 house batteries, and one is stuffed ? You could start off buying one 120 AH deep cycle battery. These batteries are damned expensive, $400.00 plus for a good quality unit. Don't buy a "Cheapie". If you have room on the roof, add another 120 watt solar panel. I believe you can buy them now for about $150.00. Check the size of your PV controller. It should be at least 20 amperes, but a 30 ampere unit would be much better. Your outlay will be for one solar panel, one battery, one inverter and possibly a larger PV controller. If you are able to install these items yourself, it would save you a few Dollars. From what you have described, I'm sure what I have suggested will do the trick.

If you have 2 x 12 volt house batteries, they will be connected together in parallel. That is, there will be a wire connected from the positive terminal of one battery to the positive terminal of the other. Also there will be a wire connected from the negative terminal of one battery to the negative terminal of the other. There should also be another cable from one of the negative terminals to the metal frame of your MH. The battery under the bonnet will also have a cable connected to the negative terminal and to the frame of the vehicle. Now if you run a single heavy duty cable from the positive terminal of the battery under the bonnet to a positive terminal of one of the house batteries, when the engine is running, the Alternator will charge your house batteries at the same time as it charges the battery under the bonnet.

Hope that info helps.... Ken.

-- Edited by Big Gorilla on Sunday 29th of September 2013 11:37:17 AM