Dual Battery Options and Fitting

[drivesafe]

A DC-DC will charge to 100% on a AGM and float the last 20%, the old school dual charging, never really got pass the 80% full mark

Hi Bryn and this is just one of many deliberately misleading “advertising facts” put out by those selling DC/DC devices
This 80% is just a number plucked out of thin air and has nothing to do with any form of reality.


If you drive long enough, your alternator will easily fully charge your auxiliary battery. It all depends on how long your drive for, as to what SoC the battery is in at the end of the drive.


A short drive might have the battery up to just 30% SoC, while a long drive might have the battery over 95% SoC.


So if you had an auxiliary battery in a low State of Charge ( SoC ) and you only drove long enough to get the auxiliary battery to 80% SoC while charging it from your alternator.


Then if you had the same auxiliary battery, at the same low level of charge, and you drove for the same length of time, but this time you were using a DC/DC device to charge the auxiliary battery, you would be very lucky if the battery got to 50% SoC.


This is one of the FACTS these sellers very conveniently forget to tell you, a DC/DC device will take at least twice as long ( twice as much driving time needed ) to do what an alternator can do and with an auxiliary battery like an Optima, with it’s a very high recharge current capability, a DC/DC device would take 4x as long to recharge it over what your alternator can do.

As for fuel consumption on a DC-DC, it wouldn't be much, it would be like not using the Air con, due to increased fuel use

Not talking about on the open road, this is about driving around town, particularly in heavy traffic conditions.


In STOP/START events, a DC/DC device will cause the cranking battery to discharge faster while the motor is off and will mean the motor will be turning on more often. This is both an additional strain on the cranking battery and the starter motor, meaning short operating life spans for both, plus the continual need to restart of the motor early, while stopped in a STOP/START event, will use up more fuel.

Also the most DC-DC have MPPT solar regs with auto switching while driving and parked up, they also have different charging profiles for different volt's as well as steeped charging.

If you have a portable solar panel and only use it while you are camping, then yes, this is an advantage.


But if you have your solar panels permanently fixed to your vehicle, while not that commonplace, but using a DC/DC device means you loose the additional charging capability of the solar while driving.


But if you have a separate solar reg, then you now have even faster charging while driving.

It looks like it would stress the alternator from the way i read it, and it isn't 4 times faster charging, in your website its 3.5 hours vs 5 hour approx?

I prefer to be honest when I supply data and with some battery types, in a worst case scenario, it would be exactly as I posted up on my website, but as I posted above, with some high current handling batteries, like new Lead Crystal batteries, you would easily charge them in half the time a DC/DC device could and an Optima, it would take around a 1/4 of the time to charge the battery.


And note, these are REAL facts that have been proven in real world use.


BTW, there is no such thing as “STRESSING” an alternator.


You can not overload an alternator, by their very design of how they operate, they protect themselves from current overload. Just one more myth.

[Transporter]

So the factory dual battery is just a standard lead acid. Does it have a redarc style isolator to prevent backflow and discharging the main battery? Is there a switch to charge the main battery if it discharges? Basically is it like a traditional 4wd dual battery set up?
M
This is not much info about the isolating part.

The AGM battery is not just your ordinary old style lead acid battery. The AGM batteries recharge much faster, should last longer if treated correctly.
It’s possible that it’s set up to help start if the main battery is depleted. At least Touareg with 2 batteries has that function.
You need to get the SSP (self study program) pdf to find out if it’s the same.

[Sunny43.5]

Hi Bryn and this is just one of many deliberately misleading “advertising facts” put out by those selling DC/DC devices
This 80% is just a number plucked out of thin air and has nothing to do with any form of reality.


If you drive long enough, your alternator will easily fully charge your auxiliary battery. It all depends on how long your drive for, as to what SoC the battery is in at the end of the drive.


A short drive might have the battery up to just 30% SoC, while a long drive might have the battery over 95% SoC.


So if you had an auxiliary battery in a low State of Charge ( SoC ) and you only drove long enough to get the auxiliary battery to 80% SoC while charging it from your alternator.


Then if you had the same auxiliary battery, at the same low level of charge, and you drove for the same length of time, but this time you were using a DC/DC device to charge the auxiliary battery, you would be very lucky if the battery got to 50% SoC.


This is one of the FACTS these sellers very conveniently forget to tell you, a DC/DC device will take at least twice as long ( twice as much driving time needed ) to do what an alternator can do and with an auxiliary battery like an Optima, with it’s a very high recharge current capability, a DC/DC device would take 4x as long to recharge it over what your alternator can do.


Not talking about on the open road, this is about driving around town, particularly in heavy traffic conditions.


In STOP/START events, a DC/DC device will cause the cranking battery to discharge faster while the motor is off and will mean the motor will be turning on more often. This is both an additional strain on the cranking battery and the starter motor, meaning short operating life spans for both, plus the continual need to restart of the motor early, while stopped in a STOP/START event, will use up more fuel.


If you have a portable solar panel and only use it while you are camping, then yes, this is an advantage.


But if you have your solar panels permanently fixed to your vehicle, while not that commonplace, but using a DC/DC device means you loose the additional charging capability of the solar while driving.


But if you have a separate solar reg, then you now have even faster charging while driving.


I prefer to be honest when I supply data and with some battery types, in a worst case scenario, it would be exactly as I posted up on my website, but as I posted above, with some high current handling batteries, like new Lead Crystal batteries, you would easily charge them in half the time a DC/DC device could and an Optima, it would take around a 1/4 of the time to charge the battery.


And note, these are REAL facts that have been proven in real world use.


BTW, there is no such thing as “STRESSING” an alternator.


You can not overload an alternator, by their very design of how they operate, they protect themselves from current overload. Just one more myth.

I looked into the dual battery thing last year but for just a handfull of weekend Moto Cross weekends decided it wasn,t worth the excercise . But out of interest I would love to hear your theory on nuclear power as that explanation was fantastic , I don,t know how you manage to remember all the tech stuff great job . {No not a sarcastic response either just fascinated by detail }

[Transporter]

By stressing the alternator, I’m sure that the the poster ment “loading” the alternator and yes if you’re to instal bigger battery or additional battery, you will increase the load on the alternator. You can use the scanner tool to check/verify that. When I’ve ordered my last van, I paid an extra for the option 220W alternator instead of standard 160W, since I was adding a second battery to the van later.

[drivesafe]

Hi Transporter and I think you mean 160A and 220A.


Your 160 amp alternator would have easily handled all the power you needed to run your Transporter’s electrical needs and charge a bank of batteries.


But having a 220 amp alternator is never a lose.


As I posted earlier, you can not overload an alternator.


In any DC circuit, with a couple of exceptions, as the voltage applied to an appliance drops, so does the current that appliance tries to draw.


It a current load is applied to an alternator that is greater than the current output of the alternator, the alternator’s regulator simply reduces the VOLTAGE output of the alternator until the current output of the alternator equals the current load being applied.


While not so obvious now days, because of the much larger alternators being installed in most new vehicles, but not that long ago, if you had a current load applied to an alternator that was greater than the alternator could produce, because alternators were much smaller, the alternator’s voltage would drop so low that the headlights would go yellow and dull.


This was particularly commonplace when driving in night time traffic in winter, where lights, heaters and sound systems were all being used at the same time, but the motor was not up at normal revs because of the traffic conditions.


Even in these common conditions, you never burnt out an alternator, it just lowered it’s output voltage, which dulled your headlights.


I regularly supply gear for Land Rover Discovery 3s and they can have as many as 2 x Optima Yellowtops under the bonnet and up to 3 x 120Ah house batteries in a caravan.


A D3 has a 140 amp alternator and they have no trouble charging all these batteries while on the move and never burn out alternators because of the high current load.

[Transporter]

Yes, should be 160A and 220A. The 220A alternator will recharge battery in shorter time and it will be less strained, if you have more appliances connected, incl. DC to DC charger.

In dual battery set up the dcdc chargers will maintain the second battery much better than the battery isolator, especially if the battery is a different chemistry. Though the VW second battery option is a good deal, since the 75A/h AGM battery will cost on its own around $400.

[drivesafe]

Hi again Transporter and I don’t know where people get this “STRAINING” of alternators from. The only time you might harm an alternator is with a dead short and even then, it will, in most cases, self protects itself. This one of the great advantages alternators have over the generator.


As for charging batteries of different chemistries. This is a potential problem when using a DC/DC device, where you can cook batteries if the wrong voltage setting is used.


But there is no such problem when using an alternator as the primary charging source. Alternators never cook batteries, and you can have a mix of battery types connected together, and all of them will be charged correctly.


And as pointed out earlier, an alternator will charge batteries much faster than a DC/DC device could hope to do, and that includes mix battery types, all at the same time.

[Transporter]

More load you put on less amp alternator hotter it gets, since it has to work harder to deliver higher current, if it wasn’t like that the alternators wouldn’t have internal fan, would be also fully enclosed.

[drivesafe]

More load you put on less amp alternator hotter it gets, since it has to work harder to deliver higher current, if it wasn’t like that the alternators wouldn’t have internal fan, would be also fully enclosed.

I think you have missed the point of why there is a fan. The fan is there to allow an alternator to produce high currents without risking being harmed by getting to hot.

[Transporter]

I think you have missed the point of why there is a fan. The fan is there to allow an alternator to produce high currents without risking being harmed by getting to hot.

Isn’t that in reverse what I have just said?

Please do yourself a favour and connect the scan tool and check what happens when you turn on headlights on high beam and the heather on, then check the alternator temperature at the same time. More current, more heat produced. The heat kills the components under the bonnet incl the alternators.

The DCDC chargers are far better for charging the second battery.

Just don’t use more than 20A-25A DCDC charger for the batteries up to 120Ah. The 45A DCDC charger should be used when the second battery is over 200Ah.