Thread: 12 volt power efficiency.....

Hello Lyndon just to add my 10cents
I use one of Drivesafes sc40 charger systems but instead of the output going directly to the aux battery I have it connected to a dc dc convertor... 8-12v in 13.8v out at 20 amps. When the main starting battery falls below its presets it cuts off the dc dc charger but the starter battery allows me to start the Rangie.
Like you I wanted a faster charge to the aux battery and this way does it.
It will continue to charge when I have turned ign off until the starter battery voltage has dropped enough to disconnect the sc40 therefore disconnecting the charge. This works for me and I have had it like this for over 2 years.
My only problem is the aux battery is deep cycle semi calcium and still does take a long time to recharge when flat.
Brad

I guess it all depends on what you want to achieve.

The energy output (volts x amps) of the alternator is fixed (for any given situation), therefore any losses in heat etc mean something, somewhere is losing out. Inverter (losses) to charger (losses) to battery will mean there is reduced charging efficiencies, which might affect the rate of charge of the starter, auxillary or both.

Brad's system is set up to hold more charge in the aux than is usual, but reads to me like it would preferentially charge the starter battery while on the move, then move charge across to the aux when stationary, maintaining 12 volts minimum in the starter. Did I read this right, Brad?

A system that directs current, rather than converting it to AC and back to DC (or even DC-DC) will be more efficient overall (i.e. faster charging both batteries to full) but it all depends on what you want/need to achieve.

Originally Posted by scrambler

Brad's system is set up to hold more charge in the aux than is usual, but reads to me like it would preferentially charge the starter battery while on the move, then move charge across to the aux when stationary, maintaining 12 volts minimum in the starter. Did I read this right, Brad?

Yes correct
Brad

Originally Posted by Feral

With the dual battery setup once the engine is stopped, so has your charging ability. With the inverter it will keep charging until it reaches the low volt cut out. If thats an hour longer or whatever is yet to be determined.

If I understand this correctly, you want to be able to charge one battery from another. Is that correct? If so, why?
In terms of the total energy stored, you will actually be worse off after charging because some energy will be converted to heat during the charging process. From what I can find, chargers are around 80% effiecient. That means that there is a 20% loss as heat!

Originally Posted by Feral

From the 'BS' info that I have seen it appears that because of the specifications of the battery charger wether it be 2, 3 or 4 stage charging it charges the aux. battery quicker than the alternator.

Depending on how clever the charger is, that may be so. It is only the fact that charging is controlled in a multistage charger. In terms of brute charging ability, an alternater should be just as effective. How fast do you need to charge the battery?

I feel that I am missing something here. Whether I am or not, the inverter still seems unecessary. (not to mention the potential hazard suggested by drivesafe)

Why why why ?


Why would you want to do this? Have you considered the extra heat generated, the safety issues, the aditional space required, and the expense?

If you want to do it obviously you'll do it , but it just sounds stupid.

To add to my previous post
Caravans use this system, so that they can use 240v appliances (USING AN INVERTER)from there batteries and a charger to charge the batteries, when plugged into 240v. Caravans dont just do it for the sake of doing it.

What you have to remember is that to recharge any flat or low battery you have to replace the same quantity of power taken out. For example , if you have a battery capable of supplying 100 amps/hour and you flatten it. you have to replace the same amount used. Say you run an appliance that draws 20a/h. then the battery would be flat in 4 hours. To recharge using a battery charger supplying say 5a/h would take 20 hrs. If you used an inverter on your main battery to drive your charger, how long do you think your main battery would last? 20h? I think not. I run dual batteries in my defender and on occasions I have forgotten to turn off my fridge at night. So the aux battery is rather low the next morning. However after a few kms the battery charge has been replenished from the alternator no problems. Jim

Originally Posted by Feral

OK, this is all good but I am not convinced either way yet

This is now going to get very technical and very messy and we still wont be any better for it but here goes to address some of your concerns.

The inverter does not need a constant high voltage to work. It will generate power wether it is at 11v or 14v. It has a low voltage cut out so it will shut down the draw from your main battery at its setting or if that is too low, you can purchase other cut outs. This will protect the main battery.

With the dual battery setup once the engine is stopped, so has your charging ability. With the inverter it will keep charging until it reaches the low volt cut out. If thats an hour longer or whatever is yet to be determined.

The inverter and battery charger all have there own protection systems. This means that they are all fused, circuit breakers or whatever internally. If the event of a problem they will shut down. This would happen if I use these in the van or even at home. They are safe to use in the shed but are not safe to use in a vehicle??? Thousands of campers have inverters!
You could even wire it up to have the inverter active only when the engine of the vehicle is running, but this then knocks out the advantage of charging when the vehicle is not used.

The whole system would be protected by circuit breakers the same as the dual battery setup so this would protect the vehicle so there is no issue here really.

What I am looking at is the efficiency of charging a seriously depleted battery.

I have read too many reports of people with flat or stuffed aux. batteries because they do not get enough charge into them quickly. I have read reports of having to drive hours just so they can recharge the battery and who wants to do that. If I can recharge the aux. battery in 2 hours instead of 5, isn't this better?

Like I said it is to do with the state of charge. I have yet to find some actually statistics in relation to hypothetical situations where say your battery is down to 50% capacity, how long would it take to bring it back to 100% or its maximum using the alternator or even a charger. Its all open to speculation and heresay.

From the 'BS' info that I have seen it appears that because of the specifications of the battery charger wether it be 2, 3 or 4 stage charging it charges the aux. battery quicker than the alternator.

Maybe I am expecting too much in an imperfect world

Cheers.

You haven't been convinced of the yet, despite the good advice!

Inverters and chargers are not very efficient. Yet you want to use them to charge an aux battery from your starting battery!

So whatever energy you can take from the starter battery, without drawing it down too low, is deminished by the considerable losses in both the inverter and the charger.

Now compare this to the SC40 dual battery unit that Drivesafe makes.

When both the aux and starter batteries are charged and the engine is stopped. The SC40 connects both batteries together and allows you fridge camp lights etc to draw power from both batteries. When the voltage of the starter battery drops to a set value that will still allow the engine to be started, the SC40 disconnects the aux battery from the starter and you can continue to run the fridge until the aux battery is flat. If you get a second SC40, it can be installed so the fridge will not draw the aux battery down so low that it becomes stuffed.

When the engine is stopped, this will give you more hours for running your fridge from the 2 batteries than using the inverter and charger. It will cost much less and will be safer.