Confused? Here we hope to unravel the mysteries of volts, watts, amps and ampere hours.
Series or parallel? Volts - Amps - Watts - Ampere hours - Using inverters
12 volt or 24 volt battery systems?
To connect 2 batteries in series a link lead of a suitable grade must run from the positive (+) terminal of one battery to the negative (-) of the other battery, this will make a 24 volt battery system.
To connect 2 batteries in parallel a link lead of a suitable grade must run between each battery's positive (+) terminal. The battery negative terminals can either be linked in the same way or both batteries earthed to a suitable spot. Linking 2 batteries in parallel will keep the battery system/bank at 12 volts and double the Ah output, i.e 2 x 110Ah batteries linked in parallel = 220Ah
Volts explained.
The pressure or electrical force between two points. The longer the distance between the two points (i.e. the cable/wire length) the greater potential for voltage drop.
Amps explained.
The unit used to measure electrical current as it flows past a specified point.
Watts explained.
The rate at which electrical energy is used. Watts are a combination of pressure and flow i.e. volts and amps.
Ampere hours explained.
The ampere-hour rating denotes the amount of energy that can be taken from a battery before the terminal voltage falls below 10.8 volts. This test is usually carried out over a 20 hour period (20 hour rate). Thus a 50 ampere-hour battery can be discharged at 2.5 amps for 20 hours before the voltage drops below 10.8 volts (i.e. 20 x 2.5 = 50). With leisure batteries ampere-hours is often abbreviated to amps (e.g. 85 amp leisure battery).
How much 12 volt power (ampere hours) do I need?
The simplest method of working your power comsumption is as follows:
watts divided by volts = current in amps. Thus on a 12 volt electrical system a 120w pump will take 10amps (i.e. 120 watts/12 volts = 10 amps). In theory, using the above equation, you might think that an 80 amp leisure battery would be adequate to run the pump for 8 hours, but in practise by the pump will fail to run properly as the battery becomes completely discharged so a safety margin of around 25% should be applied with regards to the battery ampere hours required. See the equation below:
i.e 10 amp pump to run for 8 hours = 80 amps + 25% safety margin (20Ah) = leisure battery of at least 100 ampere hours is required to run the pump efficiently.
It is also possible to do the equation the other way round by multiplying the volts by the amount of amps, i.e 12 volts x 10 amps = 120 watts.
Ok, let's show off. Here's another way 120 watts divided by 10 amps = 12 volts.
Get it now?
How much power will I use when converting 12 volt to mains voltage with an inverter?
Say, for instance, you plan to use a 300w inverter to run 240 volt appliances; it follows that the equation becomes 300watts/12 volts = 25 amps. Most quality inverters have an automatic cut-out facility which will operate before the battery gets too low on charge thus ensuring that no damage is done to the appliance being run from the inverter and (if running off the vehicle starter battery) that the battery will still have enough power to start the engine. Although this facility is extremely useful you should also bear in mind it will also cut down the appliance running time (see ampere hours).
about | contact
