Thread: 12 volt power efficiency.....

Hi Drivesafe,

That Jaycar device is almost certainly the same device, but with the diodes reversed, ie, with the load coming from the junction of the two, and each other end of the diodes going to the two batteries.Thus the load will draw from whichever battery has the highest voltage - except that 0.6 ~ 0.9 volts will be lost across the diode.

Basically not much use at all!

Cheers all,

Paul

For sh*ts and giggles I tried this today.
Plugged inverter into cigarette lighter and battery charger into inverter.

According to my clamp-meter it drew 3.1 amps from the socket to provide 1.7 amps at the battery posts.

Hi Doudal, very interesting and it would seem to be highly inefficient BUT need a few more details, such as the voltage at the cigarette lighter, the voltage at the battery posts and what is the size of both the inverter and the battery charger ( in watts if possible ).

Cheers.

Originally Posted by drivesafe

Hi Doudal, very interesting and it would seem to be highly inefficient BUT need a few more details, such as the voltage at the cigarette lighter, the voltage at the battery posts and what is the size of both the inverter and the battery charger ( in watts if possible ).

Cheers.

Not highly inefficient, 1.8/3.1 = about 60%.
For my experiment the charging battery and the source battery are the same.
The inverter I was using is only 300 watts but when drawing 3.1 amps is only running at a fraction of it's capacity.
I need to split out a single wire (either phase or neutral) to be able to measure the AC current from the inverter. I didn't do that.

The battery charger is rated at 4 amps but I've never got more than 2.2 amps out of it. It's a trickle charger, the sort that tops up a battery overnight. It can't compete with an alternator for stoking up a battery.

Originally Posted by Dougal

For my experiment the charging battery and the source battery are the same.

I suspected that.

This idea WHEN USED WITH TWO BATTERIES, can be very useful for topping up camp fridge batteries over night, using the vehicle battery without the risk of flattening the vehicle battery.

Originally Posted by drivesafe

I suspected that.

This idea WHEN USED WITH TWO BATTERIES, can be very useful for topping up camp fridge batteries over night, using the vehicle battery without the risk of flattening the vehicle battery.

How can you top up battery #2 by using battery #1 without flattening battery #1?
Remember you're taking 1.5 times the juice from #1 that you're putting into #2. Asking #1 to still start the car in the morning sounds a little much.

IMO you need a 12v split charging system which works when the engine is running.

Originally Posted by Dougal

How can you top up battery #2 by using battery #1 without flattening battery #1?
Remember you're taking 1.5 times the juice from #1 that you're putting into #2. Asking #1 to still start the car in the morning sounds a little much.

IMO you need a 12v split charging system which works when the engine is running.

If you have the equivalent of a 100 A/H cranking battery, you could safely hook up the your inverter to the cranking battery, run a 240 VAC extension cord to the fridge battery and your battery charger and let it run over night.

Say you connected up at 6 pm and let it run to 8 am, even pulling the full 3.1 amps, you cranking battery would still have about 12.2 volts which is heap enough to start even the most stubborn diesel motor on the coldest morning at lest a couple of times.

If you had a smaller battery then yes it would pay to have an SC40 Dual Battery controller, which allows the cranking battery to be discharged down to 12 volts before isolating.

This has been covered before, elsewhere on the site but I can’t remember where plus, I am getting a tad of subject so I’ll leave it at that.

Cheers.

The idea of smart starting is to ensure you primary battery starts you in the morning. The standing voltage of a battery doesn't mean much. You can have 11.8V in a battery and it might still start the car. You can have a surface voltage up to 13.6V in a battery (directly after charging) and it might -not- start the car. Voltage is -not- what starts your vehicle. The cold cranking amperage is what's really important. It requires around 40Amps per cylinder to crank an engine to firing speed (very roughly speaking). If your battery can do that, great. The killer is the fact that on most engines it needs to put out those amps and -still- have a voltage above 9.6V...why? So your ignition system works! Common problem on old (VK-L) commodores as their computers wouldn't work under 10V, due to computer design. Manufacturers have fixed that fault since then.

The simple answer, if you want to start in the morning, dont drain your primary battery overnight. All it takes is the right heat/chill combination to crack the plates of a battery. They can fail, at the drop of a hat. An alternator will cope...if it doesn't, get it looked at. (Provided you're not driving around with several hundred watts of accessory lighting or some other rediculous power draining equipment.)