12V Installation

[SteveCrawford]

I was wondering if anybody can help. I'm looking into installing a 12V system and was wondering how to go about it. The system comprises of a 120 A/h battery, x3 double 8W light fittings, x1 300W Inverter (12v d.c. to 240v a.c.) and a supply to an accessory socket. My problem is on how to work out suitable values for the cabling and protection of each individual circuit. The battery also has a charger in parallel with it to maintain the charge.

My ideas are to supply a mini fuse board from the battery using 6mm2 2 core cable, protecting this would be a 50A fuse inline with the positive. Then each circuit would be protected by values listed below :-

Circuit 1 = Lighting, total power 3 x 2 x 8 = 48W = 4 A
Circuit 2 = Inverter, total power 300W = 25 A ??
Circuit 3 = Acc Socket, total power 80W = 8 A

Cabling for above

Circuit 1 = 2 core 1mm2 - kept in free air
Circuit 2 = 2 core 2.5mm2 - kept in free air
Circuit 3 = 2 core 1mm2 - kept in free air

If anybody could let me know if I'm on the right line, I'd appreciate your feedback.

Also does anybody know how to work out how long it would take to charge a battery ?

Many thanks

Steve

 

[Jattie]

Seems fine to me.

Charging the battery would all depend on statring condition of battery and the type of charger and it's maximum current capability.

If you have a 120 A/H battery it means that the battery can supply 120A for one hour, so assuming it's a lead acid battery you can charge it at that same rate (in most types in any case) unless it's charging current is specified as well, so assumin 12V you can charge is at 10 Amps and that would take an hour to charge of at zero volts. The charge current will decrease as the battery voltage increases.

Hope that helps!

 

[dhwilliams]

Just a couple of thoughts...

I think you might be underestimating the current for your light fittings and inverter, they won't be 100% efficient. Check the specifications for these.

I'm not sure whether this applies to Lead/Acid batteries but I believe 120A/H usually means 12A for 10Hrs. I bet there's a web site somewhere that will have a graph of current against time for batteries. Usually, the higher the current the worse the error with respect to the A/H rating.

Your system will draw about 40A (guess!) so would flatten the battery in less than 3 hours. Due to the inefficiency of the battery, you need to put in about 1.4 times the energy you have taken out so, if it is flat you would need about 17A for 10 hours to fully charge it.

Would the charger have to charge the battery and provide the 40A to your load at the same time or would the load be disconnected while you are charging? If the load stays connected you would need nearly 60A from your charger when charging a flat battery. You would also need some way of setting the initial charge current. At lower power levels (like in intruder alarm systems) the batteries are often charged at constant voltage, about 13.75V, and the charger, battery and load are all in parallel. I don't know how you could do this with your load (but I'm sure someone else will!

Hope you can get it to work.
Dave