An electrical question reverse wiring a 240v-12v converters.

[matnoo]

I have a boat which has a 12v DC circuit running LED cabin lights 600mA, radio 10A, masthead light 5A and sonar 2A. All fused. All run off one battery.

When I am on the pontoon, I run a battery charger overnight and run my 12v appliances by day, but this is probably wearing the battery out, and means me crawling through the aft cabin every morning and evening and so is a bit tedious and awkward.

Question1:
Can I simultaneously charge the battery and use my 12v appliances from that same battery, or will the juice from the charger fry my 12v circuit?

Question2:
I have a 12v DC to 240v AC converter (to run mobile phone chargers while at sea), can I wire this up the other way round? Effectively plugging it into a 240v supply in reverse and so on the pontoon would provide me with a 240vAC to 12v DC enabling me to completely bypass the battery all together running my entire 12v circuit directly from the mains...

I know how DC-DC and AC-AC converters work, and reverse wiring one of those would be no problem, but AC-DC converters I dont know how they work...?

3. I have several 240v to 12v chargers, (from various appliances over the years) but they only give out up to 2amps. If I run somthing like my LED lights (600mA) would this blow them? or does it need to be exactly the right ampage?

Cheers.


Mat.

 

[davel]

1. Should be OK depending on the rating of your charger. Worst case should be a shut down of the charger if current pull is too much.
2. No, this won't work. The 12v to 240 conversion is a one way process.
3. You're OK to power a 600ma load from a 2 amp charger. You'll only have problems if the load is rated higher than the charger capacity and you've indicated this is not the case. Voltage should be matched of course.

 

[cliffb]

Question 1:- Yes. This is what I always do. And I surmise most people... or am I missing something here?

Question 2:- No. Not unless you like fireworks!

Question 3:- If the load is 600 ma and the charger will output 2a, it's not a problem. The 2a quoted will be the maximum draw the device can handle. Any load upto that is just fine. (although it might get a bit hot if you're pulling 2a continuously)

 

[William_H]

Your statement that you might wear out the battery by using it is a bit wrong. If you have a battery being charged by a charger while at the same time discharging by running lights the total current into or out of the battery is what matters.
Battery chargers are a bit of a mixed bag. Most modern chargers are regulated to 14 volts or have a charge stepping arrangement both designed to reduce the chances of cooking the battery with over charge.

You can use some small 12v plug packs as a charger. If it is the type with a switch mode power supply then it will not be much good for battery charging. That is because they put out a precise 12volts. They are characterised by having an allowable input voltage of 120 or 240Volt and being light weight. One could be used for powering your lights direct without a battery. But there is not much point in that.

The other style of 12volt transformer or plug pack has a transformer in it. This is heavy and can only be used on 240V. (or what it is designed for) The transformer has what is called poor regulation. Thhat is it will produce about 14 volts or more with no load falling to 12volts at full rated load. This can work in your favour in that it willl put a small current into the battery if the battery is low but as the battery charges it will put less in. It will protect itself by reducing voltage at higher current until the voltage falls to where there is less current. (this does not always follow for a very flat battery or high usage load at the same time.

Anyway it sounds like you have a battery charger so the best way is to leave the charger connected at all times and use lights etc from the battery. It will not harm the battery. This assumes that the charger has regulation. You should check the battery voltage or charger current with a meter when the batteries are reckoned to be fully charged. The current should be fairly low less than 1 amp and the voltagee around 13.75 to 14. If there is signs of boiling of non sealed batteries it may be a non regulated charger. This type simply pump in current until you turn it off. Good for fast charge but not a permanent connection.

A 240V Ac to 12v dc converter is relatively simple compared to going the other way around. In fact some inverters have been made and sold to do both jobs. ie UPS poer supply fo computers.

The Ac is usually put into a transformer to produce a low voltage
this is put though a rectifier to produce DC. This DC comes out as lumps at 50 or 100 times per second. It rises and falls with the alternating cycle. This lumped DC can be fed straight to a battery where it has some advantages that the higher voltage of the peak pushes current into the battery while the average current is still manageably low. So while current will fall as battery becomes charged it will still push current in when battery voltage rises to charged volatage.
A regulated charger (or a transformer pack) will have a capacitor across the output which acts like a mini battery to absorb the peaks and fill in the troughs. The output however becomes that of the peaks when there is no load and falls rapidly with current drain.
A regulated type has an electronic system which reduces the voltage when it is high to the required voltage and automatically allows more current to maintain the correct voltage under load.
This is the kind that is best for your purpose of using it both as a supply for the lights and a battery charger.
A variation for modern battery chargers is to increase the voltage out to push more current into the battery until it senses it is charged when it reduces voltage to give a small charge current and to provide any current drain for lights etc. olewill