Is there a formula to work out amp usage

Cee · Senior Member

Is there a formula to work out amp usage

justcruisin01 · Guru

Cee, if you look on the back of each appliance there should be a sticker with either the amps or the watts printed on it.
The fridge will possibily be in amps so thats easy, the other smaller units will be in watts, tally up the watts & divide it by 12, as in 12volts, eg if you had 100 watts, divide by 12 ==8.4 amps.
The total will be amps per/hour usage of all the units are being turned on at the same time. As you possibily dont use all at the same time you can calculate each unit seperatly using this system.
To calculate the amount of days you can stay out is a hard one due to your usage & the weather conditions as to how much solar you are getting per day.
The fridge will be the biggest user, as in running as a fridge or a freezer, the amount of times you open it & if you add more food/drinks etc, pack as much as you can in fridge while you are on power before heading out.

Happywanderer · The Master

This might be a good time to mention the smart charger.
I think that is what my sister was telling me about today when she rang from NZ. She said her friend has this little thing not much biggert than a handbag and it has a handle to carry it with. She said if the battery runs low she can charge it up. I might be wrong and it might just be for starting the car if that battery runs low as she said you can attach the jumper leads and start the car if the battery goes flat.
Maybe it is different to a smart charger. You can get them from Supercheap Auto and not that expensive.
I am interested in what others have to say as will be doing the same. Mainly free camping. Hope we meet up at the same site cee.

justcruisin01 · Guru

HW, two entirely different units, the one with the jumper leads that you speak of is just a small battery with leads to jump start.
The smartcharger is a proper battery charger that requires 240volt power.

Happywanderer · The Master

Thanks for that justcruisin, she explained what it did but not what it was called.

jimricho · Guru

G'day Cee,

The short answer is yes, there are formulas or equations that can be used to estimate this But.....

Unfortunately you need to know the amps drawn by each item and have a good estimate of their usage times. This makes it a "How long's a piece of string" question. Any calculation is only as good as the data you "feed into" the calculations.

Most likely the best information will come from someone with a similar set up who will pass on their experience.

It is important to understand that deep cycle batteries are rated in Amp Hours not Amps. Your battery is not 100 amps, it is 100 Amp hours. This distinction is not just being pedantic. "Amp hours" is an amount of electricity. "Amps" is the rate that the electricity is flowing out of the battery at any point in time. It is important to understand this distinction.

A bit like a water tank....Litres (amphours) is the amount of water in the tank, Litres per hour (amps) is the rate of water flowing out of the tank.

In theory, a 100 amp hour battery will discharge in 20 hours if the current being drawn from it is a constant 5 amps (C20 rate for the techies). In the real world life is not quite so simple. Also deep cycle batteries should not be discharged by more than 80% of their capacity depending on the type of battery.

Jim

Cee · Senior Member

Thanks for the info. I'll have to read the posts a few times to absorb it all, but I will definitely spend the weekend following up on the advice given.

As always I appreciate the time taken in replying to my questions.

Keep Smiling smile

jimricho · Guru

Just to add to my previous post....

Amphours equals amps multiplied by hours........ Hours available equals Amphours divided by (average) Amps

In the real world you'll have to allow for the fact that not all the Amps from the charger will be converted into Amphours in the battery and that not all the Amphours in the battery are available for use. (Life wasn't meant to be easy!)

However....

I really think that the only way you'll get a really good idea of how long you can go without hooking up to ground power at a caravan park (or wherever) is from your own experience over time as there are too many unknowns (or variables).

I assume you have an effective circuit wired from your tow vehicle to the van to enable the van batteries to be recharged from the tug while you're travelling.

-- Edited by jimricho on Wednesday 23rd of February 2011 07:44:30 AM

03_troopy · Guru

Cee wrote:

Hiya All,

Sorry, but I need to tap into your knowledgeable minds again.

I have 2x100amp deep cycle batteries & 2 x 80w solar panels in my preloved van.

As I plan on free camping quite a lot I am trying to work out how long my power supply would last before having to pull into a caravan park to recharge the batteries.

I was hoping that there some sort of formula to work out the power supply from the batteries.

I run a 240/12 volt Waeco fridge/freezer, tv,dvd, lights, & cd player on 12 volt, any feedback would be greatly appreciated as always.

I would prefer not to run a generator if possible.

Keep Smiling :)

Not sure if there is a protocol for posting links to other sites, but a bloke by the name of Richard on campertrailers.org has made up a spreadsheet to work it all out for you. I haven't used it fully, but after a quick play with it, it seems pretty good. I'll post the link, but if mods have a problem with it, feel free to edit/delete.

http://www.campertrailers.org/solar_spreadsheet.htm

Gary and Kerry · Guru

We have 3 x 110AMPH batteries and 2 x 80W panels fitted to the van, we also have a portable 120W panel that can be plugged directly into the batteries.
Our record so far is 5 weeks between plugging into power and not having to use the portable panel and that was in tassie where sunshine was somewhat lacking.

When working out what we needed the following calculations were used;
2hours TV a day = 10A
1/2 hour a day water pump=2.5A
lights/radio = .5A
total daily usage 13Amps.

80% of 330A = 264A
264A divided by 13A gives us about 20 days battery usage (before getting below 80% battery capacity) without any sunshine.

Hope this helps.

-- Edited by Gary and Kerry on Friday 25th of February 2011 07:29:01 PM