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If you have a Camper trailer, Caravan or other RV and you like to spend time away from a 240 volt power source, this article will give you some answers that the sales people avoid. It will allow you to estimate the size and number of batteries that will suit your camping.
As always there is a little explanation required to ensure that we are at the same starting point. The aim is not to make it complicated, but to explain it so it makes sense.
A blank calculation sheet can be downloaded from the end of this article, however we recommend you spend a few minutes reading why and how it works.
See also a related article: "Everything you need to know about installing solar panels."
The amount of electricity used to run a device or appliance may be defined as watts or as amps. If you know one you can calculate the other, provided you know the voltage.
We want to deal in Amps, as that is how the Deep Cycle batteries are rated. It is widely accepted at this stage that Deep Cycle batteries are the best option for RV use. In the past we recommended the AGM style (Absorbed Glass Mat) as the best for any Recreational Vehicle, but in more recent times as Lithium Batteries have become more affordable we believe the benefit is worth the initial setup costs. If you are looking for a battery for a new setup or needing to replace an existing Deep Cycle battery take a look at our related article.
A 100ah battery should provide 1 amp for 100 hours, 2 amps for 50 hours, 3 amps for 33 hours etc. It would be nice if this equation held true all the way up to 100 amps for 1 hour, but there are some limits to the maximum rate of current draw, and how much of that 100amps you can actually use without destroying your battery. We will get to those after you understand a few more things, but for now we want to know how many amps stuff uses.
Below: A simple example, but heavy user of power is a 12 volt hair dryer rated at 10 amps. A 100 ah battery should give you (100/10=10) 10 hours of usage.
If you use it for 5 mins a day, it could last 120 days. Happy wife, happy life.
If a device specification does not include amps but does include watts, the formula you need is:
Below: An example where we need to calculate the amps. A very small user of power is a 1.2 watt LED light running on a 12 volt power source will use 1.2W / 12V = 0.1 amps. Therefore a 100ah (amp hour) battery will last for 1000 hours.
A slightly different example is a 60 watt fridge running on a 12 volt power source uses 60 /12 = 5 amps, but only while the motor runs.
60 watt fridge ....divided by....12 volt battery....uses 5 amps of electricity (when running)
But....the fridge motor does not need to run all the time, in fact this model indicates it only needs to run about 15% of the time. 5 amps by 15% = 0.75ah/h. This fridge can operate over a period of 133 hours, or 5.5 days if the guidelines** are meet.
**Guidelines may be exceeded or not reached. For example this chest fridge used at Thredbo in winter may last 300 hours, but in Darwin in summer only 72 hours if you keep exchanging the beer!
Amp Hours (ah) is the rating used by Deep Cycle battery manufacturers as a way to compare batteries, but we cannot just measure how many Amp Hours are left in a battery. We can use a voltmeter to get the voltage, and this can be used to get a good approximation. You need to know a few things first.
First, you cannot use 100% of a battery for anything useful. To understand this, look at the following chart comparing a conventional Wet Cell battery and an AGM battery where the voltage reading shows approximate charge left.
Figure 1: Voltage indicating charge left.
If you have a 12V battery system, then one of the most useful items you can have is a Voltmeter. An effective one we sell is shown below. It plugs into a cigarette lighter socket and shows the voltage of your batteries. If you already have a multimeter it will also work.
When you have finished charging your battery you may get a reading well over 13 volts, however this will be a surface charge and will settle to a real reading of around 13 volts in an AGM battery or 12.6 volts in a Wet Cell battery. If you have a small load on your battery, say a couple of lights, the reading you get from your voltmeter when compared to the table or chart above, will show approximately the remaining battery charge. A moderate or heavy load will give a distorted reading until the batteries have rested and the charge equalises.
Looking at Figure 1 above you will notice that 12.1 volts AGM, or 12.1 volts for Wet Cell indicates you have used 50% of the capacity of the battery, you should have 50% remaining.
However, once you get to around 30% remaining the voltage will not be sufficient for most appliances, you may get a light to work but your water pump will not work very well. Using a battery below these levels will also severely reduce the number of recharge cycles you get from your battery.
Battery chargers like the SETEC (there are others) control the current draw from the battery as well as charging the batteries. These will prevent you drawing current from the batteries once the voltage drops below the ideal minimum level.
Most battery manufacturers indicate you will get the longest life from a battery by only discharging to 50%. This maybe the ideal case, but in an RV when you have to buy an extra battery, find storage space and pay for vehicle fuel to cart extra batteries that weigh about 30kgs each, I believe it is not unrealistic to allow for discharging to 40% or even 30% on those longer stays or when your other charging options don't work out. Yes, you may need to replace your battery a year earlier, but you have made savings along the way. Just one night away from a caravan park will save $30 plus extra weight for ever kilometre you travel.
Again it is important to note that the voltage shown will only be accurate if there is at least some load, but definitely not a load exceeding 5% of the batteries capacity. If the battery has just been charged or heavily discharged, it must be given time to equalise before taking a reading.
Using a water example, imagine 6 tanks of water (like 6 cells in a battery) each joined together with a straw. If you fill one end tank using a garden hose or drain the other end using a garden hose it will take some time for all tanks (or cells) to be at equal levels.
The time it takes to equalise is something you will get used to, you certainly don't panic if your voltage drops down while you use the microwave for a couple of minutes (inverters can consume quite a bit of power while in use). A low load that is drawing power no faster than the battery can equalise will give a reasonable value for the Charge remaining, by reading the voltage.
Be aware also, that a battery under no load at all may still show 12V, but have little or no amp hours left.
There are a few advanced gauges that will calculate your capacity based on actual recent usage, but the cost is still beyond most recreational users. They are more like a mini computer taking and recording sample data and making predictions of capacity remaining and how long it will last if you continue to use it at the present rate. See Below:
Other more economical voltmeters are shown here:
As can be seen in Figure 1 the battery type e.g. Wet Cell, Gel or AGM can give different voltage readings for percentage of charge left. While the volts is indicative of how much power is left, it is not like a fuel gauge. Petrol left in a fuel tank would show up as a straight line from 13 to zero in the chart above.
The AGM Deep Cycle battery plots closer to straight than the Wet Cell battery. They accept charge easier and faster than wet cell batteries. They are able to recover better from a lower discharge. Yes, they cost more than Wet Cell batteries, but with correct usage can last twice as long.
Not everyone will benefit from using deep cycle batteries. If your predicted usage is much lower than would be supplied by these larger AGM Deep Cycle batteries, it can be a cheaper option to use a Wet Cell battery. If you leave 240V for only one or two days at a time, and you use only LED lights and say a water pump, you should manage quite well with a cheaper Wet Cell battery.
Caravans Plus does not sell Wet Cell batteries as these are harder to ship and are readily available in most locations. Wet Cell batteries need to be upright and located in a ventilated area that is not in the living area. AGM batteries are completely sealed and can be used at any angle. They can be placed under your bed without risk.
Battery capacity decreases as the rate of discharge increases. Batteries (say 100ah battery) have been tested at a given discharge rate of 20 hours.
That means a constant use of 5 amps an hour for 20 hours (5 x 20 =100). The specifications should indicate this.
The bad news is that if you exceed that discharge rate quoted you will see a significant drop in capacity. If you need more than 5 amps per hour you should consider having two or even more batteries.
The good news is if you use less than the 5 amps per hour, you can get more than the quoted 100ah.
Figure 2: Battery capacity decreases as the rate of discharge increases.
When charging your batteries from a good battery charger over a number of days, you can get to 100% charge. However a vehicle alternator will only manage to get to around 70%. Also if you are charging your battery from your solar panels, you need to ensure your solar regulator is up to the job.
Voltage drop is one of the biggest problems in getting the battery in your Caravan or Camper trailer fully charged from your vehicle alternator. You can never charge the battery more than the voltage that reaches it through the copper cables. The longer the cable, the more the voltage drops. This can be compensated to some extent by increasing the cable x-section but even this has limits.
With your trusty voltmeter you can check what voltage your alternator is producing by starting your vehicle and connecting your voltmeter to the battery terminals. Ensure electrical accessories are NOT switched on. This should read from 13.8V to 14.8V.
Figure 3: Less charge due to voltage drop
Next test your voltage at the rear of the vehicle with the engine still running. This may be at an Anderson plug or the auxiliary wire in the trailer connector. Record this voltage and note any voltage drop. My vehicle has heavy duty cables and only dropped 0.1 of a volt.
Next take a reading where the auxiliary cable goes to the RV battery (or battery charger). My caravan has lighter wiring and the voltage loss was another 0.4 volts. The total voltage drop was 0.5 volts. When you look back at Figure 1, you will see that a reduction 0.5 volts pushing a charge into your RV battery can severely reduce the total usable amps until you top it up again.
There are two solutions to this problem. First you can increase the size of your copper wires so that the voltage drop is less. Second you can add a voltage booster that will step up the voltage to compensate for the loss. This must be placed between the alternator or vehicle battery and your RV battery so that current coming in will reach the battery at a higher voltage and thus increase the speed of charging and get more amp hours into that battery.
Figure 4: Increase amp hours stored with a booster
When all the above is taken into account you will still only get about 70% of your battery capacity as usable power when charging from your alternator.
When combined with manufacturers recommendations to get the most life out of your battery by discharging regularly to around 50%, then as a rule of thumb you will need twice the amps hours that you will calculate next. This rule of thumb allows for greater discharge on occasions, e.g. lower solar input or longer stays than normal or greater usage of 12V appliances. If you don't want this buffer then 70% usage will be the maximum you can use and any excess will greatly reduce your battery life and not operate all types of devices.
Examine the following chart to calculate the number and size of batteries you will need. You can download a blank calculation form and enter your battery size to determine how long it will last.
Notes explained:
1) The water pump has a rating of 5.2amps, I estimate it runs for about 12 full minutes per day. 12 mins is 1/5 or 0.2 of an hour.
Three fridge options are provided, select one.
2) The gas fridge has a 12V fan that comes on when needed, estimated 6 hours per day, selected here.
3) The upright compressor fridge has a quantity of 0 (zero) so it is not used in this calculation. It uses too much power.
4) The chest fridge would be a suitable fridge for a camper trailer but unselected for these calculations.
5 & 6) Would be optional when power gets low.
7) We do not use a TV when camping, but some people may. They do use more than you think.
8, 9 &10) Are all LEDs and are very power conservative. The specifications for the LEDs were provided in watts so they are divided by 12V to gets the amps/h.
21) Just having an inverter switched ON uses power. Switch off when not required.
22 to 25) All essential 240V appliances for some softies. The watts need to be divided by 240V to get the amps/h needed for our calculations.
When the amps/h for all devices are added up we find we use around 13.5 amp hours per day.
Anything in the amps column that exceeds 5 amps will cause your battery to deteriorate very quickly, so I've elected to go for two batteries.
This gives me ample capacity. It is only the inverter extravagances that have taken me over the limit, so for many people a single AGM 100 to 120ah battery will be enough unless you have an upright 12V fridge, or need your TV when away from power.
You can download an Excel sheet or OpenOffice Spread Sheet to fill in your values.
See also a related article: "Everything you need to know about installing solar panels."
Great service, easy to find and order part required, fast and efficient delivery. Thank you.
Prompt delivery Thanks
Quick delivery. Easy to deal with.
42 Comments
Hugh
Nothing forgotten. Just another bloody good article. It is probably covered elsewhere, but hand in hand with battery capacity is the ability to recharge them efficiently/timely, especially if using solar. Some people over estimate their ability to recharge. Also the capacity of the 240V charger if fitted. Was recently on a trip where someone had a camper trailer fitted with 2 x 100 AH batteries - plenty of power. however the 7 amp 240 V charger took some time running on generator to replenish the batteries after they had been run down considerably.
Zarian
Rechargeable rate for battery's is 20% of capacity or 200 amp/hr = maximum 20amps 2000 amp/hr= 200 amps so a battery bank of 1000 amp/hr can be changed by a solar array of 2400 watts or 200 amps DC anything more than that will result in excessive heat electrolysis buckling of the plates and general deterioration of the battery cells similar to extreme discharge rate is a 180 cold cranking amp battery in a 909 Kenworth or over cranking the starter motor in a 4x4 to get it home lol yep that's what a mate did to my 1300cold cranking battery in my 4x4 $1200 dollars of battery 🤬 PS a great battery charger is an old welder you can dial up the amps and the voltage 24/36 48 96 voltage and 200-400amp DC
john
How about a diesel heater
John
Diesel heaters like the Chinese (Espar) heater that I have, consume approximately 5 amps of draw for 3-5 minutes while the glow plug warms the heater and ignites the diesel fuel. After the fuel become ignited, power draw deceases to I assume 1-2 amps continuous to run the pump and fan, while the glow plug turns off as the fuel could get ignited from its own heat source.
Ian
While you advise that only about 70% charge is available from a modern smart alternator, it is entirely possible to get the required 100% charge into the deep cycle batteries with the use of Ctec D250S Dual and Smartpass combination. I have this setup in my 2009 Mercedes-Benz Sprinter with 180 amp alternator charging 3 deep cycle AGM batteries for a total storage of 330A/hr. This further info should be included in your advice column.
Mark
Great article but your excel spreadsheet will not work on current version of excel.
Rex Walker
After reading the info in these article about what 12 volt battery should do I think now I understand why my 6 year old 4x4 battery cant cope when powering the caravan Very good article regards Rex
ALLAN
Knew virtually nothing about correctly wiring 2 x solar panels with 2 x AGM D/C batteries. Still not an auto elect, but I do now know the correct procedure and also the importance of having a good regulator. Great article to read and easily understandable.
Jay
I am a little puzzled by the calculations when it comes to 240v devices. 1. On the website, in your notes to items 22 to 25, you say the Watts need to be divided by 240v to get the Amps/hr. But you appear to have divided by 12v. And it is then unclear how you reach the Total Ah per day. For example, the Microwave at 1800W, divided by 12v and adjusted for 85% inverter efficiency, gives 176.5A; but when used for 0.3 hours, how does it come to 2.25 Ah per day? This seems too small a power draw for such a hungry device. 2. The Excel spreadsheet also divides by 12v (contrary to the notes for items 22-25 on the website). If I put 0.3 hours usage opposite the Microwave here, I get 52.9Ah per day - which makes more sense to me. While dividing by 12v (in the case of the calculation on the spreadsheet) gives a result that makes sense, in both cases I am still puzzled as to why we need to divide the Watts by 12v and not 240v.
Scott
You're not using the microwave for .3 of the day, only .3 of one hour of the day, so divide 176 by 72.
Casey
The reason for dividing by 12 instead of 240 is because we are referencing the amp hours we can run off of a 12 volt battery, not a 240 volt battery. Amps x volts = watts. if you want to figure out what your amperage is on the ac side, you would divide by 240, (or really it will probably be 120 here in America because we don't typically run a 240 volt microwave out here ). That amperage will be much lower hence why the wires on the ac side of the inverter are much smaller than on the D.C. Side. Still to figure out how much that will talks off of the amp hour capacity of your battery it needs to be converted back to 12 volts
Peter
Have battery isue twin 120 amp hour batteries charged to 12,8 volts drop to 11 volts in van within 2/3 hours with ony 4 led lights on all systems checked twice by auto electricion batterys all test ok answers nedded batterys oniy 18 months old
Chris
Hi Peter I have almost exactly the spa problem although I only have one battery. Were you able to resolve the issue?
Tony
How do I measure the voltage drop caused by the draw of 3 way fridge running on 12v while being charged by the vehicle while traveling. Great info. Thanks Tony
Keith
Excellent article - I wish I'd seen it before I fitted my van out. I understand electricity from high school physics at an excellent (high academic standard) school in the 1960s (Wollongong High). I did it all the hard way, nutting it out for myself. The only thing I would do differently is take the 240v load (the inverter) from the load terminals on the solar regulator. Notes for the 12v calculation sheet - 1. preset all the 'Watts' columns with 0 (zeroes) and then tailor it to your numbers (either Watts OR Amps) 2. in the green Batteries section, it is preset to 2 x 120Ah batteries and only discharge to 50% of capacity (so change it to your setup and/or use it for capacity planning as input to your installation requirements) 3. Usage is hours and minutes on this version of the spreadsheet 4. Switch off your inverter when not using it - with no load, it will use 50Ah if powered on 24 hours.
Denis Kilner
I have 1x100 amp hr agm deep cycle battery in my van battery 4 years old, never discharged below 50% , can run tv, lights, range hood whilst cooking, fridge on gas, a few other bit's and pieces, fully charged 1x180 solar panel in a couple of hours, not always full sun. I would like to increase my power storage capabilities and thought about adding another 100 amp battery. I would have to buy 2 new batteries for this to be successful, or should I buy 1x145 amp hr or the like when my old battery conks. Would this be a cheaper option, and would I get benefit from this option. Thank's for such an informative article, Regards Denis.
Stretton
Extremely enlightening and presented most succintly especially for someone like me who is new to all things camping.
Lena
Playing devil's advocate here, I wonder is it worth bothering with all that wiring for light use? Say I just glue a couple of battery-operated LED lights into my van, chuck the AAA batteries into a battery charger when at a caravan park and good to go for another month or so? Obviously not a slick solution and the rest of the van would have to use gas for most things.
Peter
Could you please advise on the Pros and Cons of a Lithium Fe system against an AGM
David
LiFe batteries are 4-5x the cost for the same capacity, but you can discharge much deeper without shortening lifespan, so don’t need as much total capacity.
Steve
How long can I run 400 watts with a 140 amp hour battery any help please 🤔
David
400 / 12 = 33.3 A. 140 Ah battery has a standard 20H rate of 140/20 = 7, so the capacity at the higher 33.3 A rate would be much lower. Let’s say 100Ah. 50% of that is 50 Ah, so you could run continuously for 1:20 or so. (Someone check my math). I wouldn’t recommend running continuously at that rate though.
james
Good article but lost you at the end when you introduced 24 volts. I thought we were calculating the battery needed for a 12volt system? Totally lost you there.
Michael
Great article for a solar novice. Still unsure about the battery voltage reading on my panel. Usually shows 11.5 '- 13. I have two batteries so does this reading show a combined voltage or am I down on power? Thx for the info on inverters, explains a lot.
john
more of a question then comment .i live in my caravan on a permanent basis and the battery in my van is now 5 years old and i use a Projecta 12 volt 25 amp charger for my battery, when would you recommend replacing the battery, as if the main power goes out my 12 volt led lighting goes as well
Reeyan
Thanks for your great article. My questions is... I’m planning to install 12v 3000w Inverter with between 100ah-200ah Lithium Battery, directly connecting to the Starter Battery on my car. and i’m going to use some power tools around 1500w 240v such as high pressure. Mostly its gonna be under 1500w, definitely not over 2000w in total at a time. and normally Im going to use a power tool for about 10-20mins consecutively and then 10-20mins break and then will be used again for about 8-10 times cycles maximum per day. Do i need 200ah lithium battery to cover for my usage? Or 150ah or 100ah could be enough?
Ken
Best explanation I have ever read. Thank you!
Frank
I have a 3.5 amp 12v small compressor fridge which will run on the car battery while we are driving.What min size battery and solar panel would be required to charge the battery through daylight hours to keep the fridge/freezer on line through the night. thanks
Alaska
I have been told anytime from 100 watt on up would work great. A i00 ah batter will have 50% usage pulling 3.5 amp Divide 3.5 into 50 so 50 divided by 3.5 will get you about 14 hours not being charged by anything
Rob
Mines similar : have a barn down the back and want to run a waeco and a couple of LEDs. What do I need ??
Sara
Novice here and found the article very helpful and much more confident that my one battery will be fine for me Thanks x
Trish
So I have a pool pump that says its 115V and 12amps....based on what I read...Im guessing Im going to need 2 100ah deep cell batteries instead of just the one I ordered in order to run the pump 8 hours a day? I have an inverter and 2 solar panels too.
Martin
hello i am after information on wiring in a 2000 w inverter. do i need to disconnect from 12 v system when i wish to use 240 v or do i just turn off inverter?
Henry
I have 2 new 8v golf cart batteries. I just bought a 1994 van with a small solar cell on the roof. It is connected to a Solar Charge Controller and the empty battery storage area output is 12v. Can I hook the two 8v batteries to this controller and feel safe running a 3500w Pure Sine Wave 12V dc to ac inverter? Thanks for your advice.
Mark
Excellent article, lays out all the details needed to avoid that fubar in the outback.
John M
Very well presented. I'm looking at buying an older campervan that needs a bit of upgrading. I spotted a Parmco 88litre Underbench Fridge/Freezer that really appeals. It is 240V only and has a rating of 263kWh. How would this be worked in to the calculations. Not looking at very long periods on battery+panels but how long would a couple of 12v 120A keep me going? Cheers
John M
Nutted it out - I think. The 263KWh is the annual figure so divide by 365 to give 720watts per day. divide by 24hours and 12 volts gives an amperage of 2.5A. This should be quite Ok. Also found a Tankless Water Heater from the US for only NZ$63.14 - says 10amp / 3800watt. but running a shower this would only be for 10 minutes so works out feasible. Thoughts?
Thanks for explanation in easy way
Thanks for explanation in easy way
David
Very helpful information and simplified the issues I was struggling to get my head around. You've given me some homework. Thanks again. Cheers, Dave
Zarian
Lol a Honda Eu1000i generator will run at about 800w for about 8-9 hrs on 2.6 Lt of petroleum fuel so that's a washing machine or LCD screen and gaming desktop computer
Zarian
A Honda EU 2000i will run a 40amp battery charger and a microwave oven and a old fashioned tv or a air conditioner and a TV and use 5lt of petroleum fuel in a 10 16 hr
Tony
Thank you so much for simplifying a complex issue (well it was for me). I now have far more confidence in choosing what I need for installing a solar-based system without the huge expense and being told BS by those trying to profit from my inadequate knowledge. I appreciate what you've done very much. Thank you Tony van Maanenberg