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Calculating How Big Your Battery Needs To Be

Article by Peter & Rob Smith - Caravans Plus

How long will my RV battery last?

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."

How will we measure power?

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. We recommend the AGM style (Absorbed Glass Mat) as the best for any Recreational Vehicle, but you can research this more if you want.

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.

10amp Hair Dryer

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:

Watts / Volts = Amps per hour

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.

LED bedside light


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.

12v chest fridge

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!

How many Amp Hours do you really get from a 100ah battery?

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.

1) Low voltage cut off

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.

Voltage indicating 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.

Projecta - Digital 12/24V voltmeter

Projecta - Digital 12/24V voltmeter

4 In Stock Now.
$43.00
View

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.

Setec ST20-III Power Supply, 20Amps, 8 fuses

Setec™ ST20-III Power Supply, 20Amps, 8 fuses

1 In Stock Now.
$443.00
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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:

Setec Drifter Complete Instrument Panel

Setec™ Drifter Complete Instrument Panel

2 In Stock Now.
$540.00
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Other more economical voltmeters are shown here:

Voltmeter 8V- 16V, BEP 12V

Voltmeter 8V- 16V, BEP 12V

$110.00
$92.00
View
RV Electronics LCD 1 Water Tank Level Indicator and Voltmeter

RV Electronics™ LCD 1 Water Tank Level Indicator and Voltmeter

17 In Stock Now.
$89.00
View

2) Battery Type

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.

Camec Deep Cycle 100ah AGM Sealed Lead Acid Battery

Camec™ Deep Cycle 100ah AGM Sealed Lead Acid Battery

1 In Stock Now.
$256.00
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RITAR 12V 120ah AGM Deep Cycle Battery. RA12-120SSpecial Price

RITAR 12V 120ah AGM Deep Cycle Battery. RA12-120S

2 In Stock Now.
$576.38
$450.00
View

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.

3) Excessive Draw Rate

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.

Capacity effected by Discharge rate

Figure 2: Battery capacity decreases as the rate of discharge increases.

4) Getting Less Than A Full Charge

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.

Voltage loss

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.

GSL: 12V to 12v, Voltage Booster Battery Charger 22A

GSL: 12V to 12v, Voltage Booster Battery Charger 22A

4 In Stock Now.
$291.00
View

Voltage Booster

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.

Calculating Your Amp Hours

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.

Calculating the usage

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."

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