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Well, I'm not sure I understand what the issues are here. However I just installed a second battery and one of drivesafes systems. We just had a 5 week trip, with our fridge and camping gear running off the aux. The beauty of this system is I didn't think about it for 5 weeks... it just worked. How good is that. :)
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:eek:
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.
Yes correctQuote:
Originally Posted by scrambler
Brad
If I understand this correctly, you want to be able to charge one battery from another. Is that correct? If so, why?Quote:
Originally Posted by Feral
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!
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?Quote:
Originally Posted by Feral
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
You haven't been convinced of the yet, despite the good advice!Quote:
Originally Posted by Feral
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.
Hi again Feral, you have a few things a tad wrong and I’ll just follow on from all those who have posted above.
For a starter, to be able to charge a battery quicker than the alternator can, you need a battery charger with at least a 20 amp capacity, even with an 80% efficiency rate, this means you need 25 amps to be drawn off the main battery.
Now this does not yet covered both the size and efficiency losses of your inverter and as the VERY BEST inverters are 93% efficient, your still looking at at least 30 amps being pulled from your Cranking battery. How long do you think your cranking battery can both supply that sort of current for and how often do you thing it can do that before you need a new battery.
It’s all self defeating.
I have posted in a number of other threads how you can use a SMALL inverter and a SMALL battery charger to top off you auxiliary battery WHILE YOU ARE CAMPED, someone might know the thread and post a link to it but as far as getting more stored capacity goes, you are far, FAR better off just fitting a second auxiliary battery then to use the inverter/battery charger.
With 2 auxiliary batteries, your alternator will easily replace the amount of amps used by the two batteries quicker than even a 20 amp charger could ever hope to do.
Add to this the cost difference, a charger and an inverter of this size would run to at least $500 to $600 and we haven’t covered the cost of heavy cable, about $100, which will be necessary if you want to stop the efficiency of your set up getting any worse.
Then there is some form of battery protection to stop over discharging the cranking battery, another $100.
Your now looking at around $800 for a basic set up that will, in your mine, allow you to charge your, say, 100 A/H auxiliary battery quicker that the alternator will.
This will mean that you will have about 100 A/Hs of total power available, taking into account that you take you 100 A/H battery down to 25% SoC and you add 25 A/H buy pulling 35 amps from your cranking battery.
For about $500 you could add the heavy cable, another 100 A/H battery and if you installed an SC80, you would have a total available capacity 135 A/Hs. This is based taking the auxiliary batteries down to only 50% Soc and the 35 amps from the equivalent of a 70 A/H cranking battery, is available as 35 amps at the auxiliary batteries.
Now if you were to take the auxiliary batteries down to 25% SoC the you are now looking at around 185 A/Hs.
That’s nearly double the power for just over half the price and based on a dollar to dollar comparison, that’s well over a 100% improvement over the inverter/battery charger set up.
And all this extra power without risking electrocuting anyone.
Just something to thing about.