Opinions Needed on an Alternative Dual Battery setup

[deanchristensen0001]

I have experimented with an alternative Dual
Battery Set up.

I have used a 300w continuous rated inverter, run from the Accessorie Circuit from the main battery to power my Ctec 7000 charger and charge two 100 A/hr AGM batteries in parralel.

The Ctek is 240volt 1 amp input so I dont see any problems.

It seems to be working fine and the batteries are remaining charged while running around town with a 25 lt Waeco

Does any one see any potential dramas?

[-Scott-]

Each voltage conversion will be wasting power, but that's probably not a huge concern.

300W is a bucketload of current from a 12V system. I presume you have the inverter somewhere near the front, so your 25A line is as short as possible.

So I guess your 240V line runs past/under/through the passenger cabin. I hope it's well protected, as few people expect to find 240V in passenger vehicles - that would be my largest concern.

I imagine it would keep the batteries better charged than most traditional dual battery controllers - if it's working well, I don't see a pressing reason to NOT do it.

[drivesafe]

Hi Dean, this idea has been floated from time to time on some camping and caravan sites but as posted by Scott, the first thing that comes to mind is the safety factor.

If your were to have an accident, your fault or not, emergency personnel or good Samaritans coming to your aid would have no warning of or be expecting to have to check for lethal voltages in your vehicle before they try to help you.

Next, I’m not sure what you are trying to achieve. The Ctek 7000 has a maximum 7 amp output while each AGM battery, depending on brand, is capable of being charged at 25 to 30 amps.

This would mean that if your batteries were at 50% capacity when you started driving, you could put the bulk of the charge ( note, not fully charge ) back in the AGMs in about 2 hours of driving with the batteries connected directly to the vehicles charging system and this is with or without fridges running.

To achieve a similar charge level using the Ctek, you would have to drive for about 15 hours and that is WITHOUT have the fridges on.

Just my idea of your set-up’s operation.

Cheers.

[deanchristensen0001]

I have the two AGM batteries in a custom cabinet behind the rear seat. I have two waeco fridges (a small CF-25 as a center console and a CF-110 in the rear) I also have 2 x 500w rms amplifiers. I figured I needed a fair bit of power for the two fridges when on longer trips, hence the two auxiliary batteries.

I had hoped by using the Ctek that I could ensure that the batteries are kept in the best possible condition as I have read lots of confusing info on Wet cell/AGM Combinations and even alternators damaging AGM batteries.

Info from Opposite lock indicates that the only way to properly maintain this setup is to "independently charge each battery" this is the problem as independent chargers cost huge $$$$$

I did not count on a huge bill to keep them charged. Any suggestions that maintain there charge and look after these batteries would be helpful.

[drivesafe]

Hi Dean, these two diagrams will give you some idea of how you can set up to charge and use your batteries the way you want.

In the diagrams, the battery in the rear of the vehicle can be two batteries in parallel as you are planning.

Cheers

[chimpboy]

Yer. The best device in your vehicle for charging 12V batteries is your alternator. I would reserve the inverter for more useful tasks, and be charging the 12V pack with the alternator.

The safety aspect wouldn't really loom that large in my mind, but that could just be me As far as charging performance goes, though, your system sounds like it will work, but is way below what can be achieved with a less complex setup.

[chimpboy]

Info from Opposite lock indicates that the only way to properly maintain this setup is to "independently charge each battery" this is the problem as independent chargers cost huge $$$$$

I can't see a single problem with charging them both in parallel from the alternator, preferably with an automated dual battery system to isolate them from the main battery when the engine isn't running. There isn't really any practical way to charge multiple 12V batteries "independently" in any meaningful sense in a car setting... at least, not if you want to use the batteries as a combined power source at any stage.

[-Scott-]

Info from Opposite lock indicates that the only way to properly maintain this setup is to "independently charge each battery"

I agree.

I've been thinking about typical dual battery installations/controllers, and there must be a better way.

Hi spec (mains powered) battery chargers monitor the voltage of the battery they're charging, and adjust output accordingly - constant current mode, bulk charge, float etc.

Most vehicle "dual battery" installations use the output of the alternator, which is regulated according to the terminal voltage of the cranking battery. If the cranking battery is fully charged, but your auxiliary battery is not, the auxiliary battery won't be receiving as high a current as it could use. As the aux battery attempts to draw more current the voltage drop in the cabling increases, so the voltage at the aux battery terminals drops and the charge current drops. A self-limiting charge system. This is why an alternator regulator senses voltage from the battery, and not the alternator output - they're different voltages at high charge rates.

Dean's setup is at least charging his aux batteries independently of the starting battery's voltage.

I think Rotronics produce a charger which sits between the alternator and both batteries. It has "sense" wires for both batteries, and two output terminals - one for each battery. I presume this means it effectively switches the alternator between batteries, so theoretically the full alternator output can go to recharging your auxiliary battery - but NFI what happens to your starter battery while this is happening.

So, there must still be a better way - but I don't know what it is.

[DamTriton]

I have always wondered about the feasability of individual switchmode regulators hanging off the alternator, one for each battery. They could be mounted at the battery end of the wiring and actually regulate at the battery, instead of at a distance via a sense wire.

Benefit would be you could run the alternator a bit harder, may be even flat out (higher voltage 16-18 volts? using the switchmode regulator to downconvert a bit), and also ensure correct charge to all batteries no matter their location/distance, as voltage drop over distance would be less of an issue. Power feed would be "raw" alternator output.

[drivesafe]

Hi Scott, sorry but that theory couldn’t be further from reality.

The alternator can not tell if there are batteries of different states of charge connected to the system and even using your analogy of having a fully charged cranking battery on-line and this supposedly would cause the alternator to think that any battery connected to the system, was fully charged.

The hole in this theory is that the best quality battery, at 100% State of Charge ( SoC ) has a voltage out put of, at very best, 12.9 volts but is usually 12.7 to 12.8 volts and this is over a volt below the average regulator’s setting of 14 to 14.3 volts. So as you can see, the actual SoC of any or all of the batteries will have no independent control over the alternator.

Putting it very crudely, the way a vehicle’s electrical system works is all based around the joint operation on the alternator and the voltage regulator.

The regulator sets the voltage level of the system and the alternator produces enough current to keep the voltage up to that level, which in turn, automatically supplies enough current to cover the current draw of any devices that are connected to the system.

If an extra device is added to the system, like turning on your headlights or air conditioner or if a control device connects an additional battery to the system, the systems overall voltage level will drop.

As soon as the regulator / alternator set up detects the slightest drop in vehicle voltage, the alternator immediately starts to produce more current to get the voltage back up to the set regulated voltage.

The greater the load applied to the system, in other words, the more things turned on or connected to the system, the greater the voltage drop, the higher the current output of the alternator will be.

The amount of current produced by the alternator is directly proportionate to the voltage drop being detected.

The operation of any vehicle’s electoral system is basically as simple as that.

There is nothing high tech about how the system works, such as monitoring everything connected to the system.

The alternator / regulator set up has absolutely no way of knowing what is connected to the system and drawing current, they just work together to try to keep the vehicle’s common voltage at a pre-set level.

Just to give you an example of how your theory is actually the opposite of what really happens, the new Discovery 3 and Range Rover Sports have what is probably the most advanced electrical monitoring system of any 4x4s on the road today.

Their system is set up so that if the system detects that the continuos current draw is higher than what the alternator can maintain, the system starts to shut down high current devices such as the air suspension compressor, the sound system and so on.

The idea is that the engine management system is given priority over everything else, so as to allow these vehicles to get home.

One of the side effects of this system is that conventional dual battery controllers can causing incompatibility problems with these vehicles and the vehicles are shutting down air suspension compressors and so on and making the owner think the system has failed when the system is actually working exactly how it’s designed to.

Even with the advanced electrical monitoring that these vehicles have, they still have absolutely no way of knowing what is causing the additional current draw, they just act accordingly as does your vehicle and everybody else’s, in there own way.

One more point, because the battery is monitored independently, any dual battery system that disconnects the cranking battery in a D3 or Range Rover Sports will cause heaps of supposedly unrelated problems because the vehicle’s power monitoring will see that the battery’s voltage is lower than the alternator’s output voltage and the vehicle’s computers will go into overdrive trying to charge the battery.

Furthermore, except where a vehicle has a very small alternator, there is absolutely no reason to disconnect the cranking battery at any time from any vehicle, new or old. The alternator in most 4x4s is capable of charging 2,3 even 4 batteries at the same time and in many cases, depending on how low a number of batteries may be, disconnecting any of them will only mean that it takes longer to charge them all.

Cheers

drivesafe.

[-Scott-]

I have always wondered about the feasability of individual switchmode regulators hanging off the alternator, one for each battery. They could be mounted at the battery end of the wiring and actually regulate at the battery, instead of at a distance via only one sense wire.

Benefit would be you could run the alternator a bit harder, may be even flat out (higher voltage 16-18 volts? using the switchmode regulator to downconvert a bit), and also ensure correct charge to all batteries no matter their location/distance, as voltage drop over distance would be less of an issue.

That's what I was thinking today!

Issues I can see:

Current capacity! Modern vehicles are using more and more electrical power, and it's only going to get worse (until they up the system voltage...) Most of the time, the alternator is supplying this power directly, so a system such as you've described would need to be capable of supplying the same current.

So, the next best solution (to my mind) would be to leave the OEM system alone, but install a DC-DC converter at each battery - input somewhere in the 12-14V range (from alternator), output 13.8V and up(according to charge mode for battery.) I guess something capable of delivering a regulated 20A or so should suit most applications, with a little extra effort the design could be capable of adding extra current capacity.

[drivesafe]

Hi DAMKIA, such devices are already on the market.

A company near Port Macquarie brings in a range of hi tech chargers and some work exactly how you were suggesting but they can work out expensive.

Hi again Scott, from experience, they need to be at least capable of delivering a minimum of 50 amps to be of any real advantage.

Cheers

[deanchristensen0001]

I figured it out!!!

I was about to install a 2nd AC pump for air duties and I can install a 2nd alternator directly above the AC. then I will use a dual battery setup to charge the batteries independently and re wire them, one for the stereo, 25lt waeco and Aux lighting, and one for the 110lf waeco and some additional 12v outlets.

problem solved, I think?

this leaves the starting battery and vehicle electronics in standard form

[drivesafe]

Hi Dean, how big is your existing alternator.

If it’s 90 amps or bigger, it will do the job anyway. Shouldn’t need to go to the expense of fitting an extra alternator.

Cheers

[-Scott-]

Hi Dean, how big is your existing alternator.

If it’s 90 amps or bigger, it will do the job anyway. Shouldn’t need to go to the expense of fitting an extra alternator.

Cheers

Yes, it may do the job - eventually.

Tim, I think I wrote my earlier post a little too fast, and probably didn't explain it too well.

I understand how a vehicle charging system works. I agree with you that it monitors the (starting) battery voltage, and increases the alternator output to maintain this voltage at an appropriate level, to charge/keep charged the battery, and supply current to any/all accessories. It monitors the battery voltage, and not the alternator output, to eliminate any resistive losses in the cables between alternator and battery.

But it doesn't charge an auxiliary battery at an optimum rate.

Consider a constant voltage charger:

Apply a suitable voltage to the battery, and it will begin to charge at some current which is effectively set by the battery's state of charge. If the battery is close to 100%, the charge current is low. If the battery's SOC is lower, the charge current will be higher, and drops off as the SOC increases.

This is what's happening with most auxiliary battery systems. The charge voltage is the system voltage, regulated by the voltage regulator, which is measuring the voltage at the starting battery - not the auxiliary battery (or even the alternator output.)

When the auxiliary battery has a low state of charge the (nominally) fixed output of the charging system is applied to recharge it. Because the battery has a low SOC, it can draw a large charge current, and all is good.

But because there is real-world resistance in the cables to the auxiliary battery there is a voltage drop. The actual terminal voltage at the auxiliary battery is LOWER than the full system voltage. Because the charge voltage applied to the auxiliary battery has been reduced, so too is the charge current it accepts. So the auxiliary battery is not being charged as quickly as it could be. As the battery slowly charges, and the terminal voltage slowly increases, the charge current drops, and along with it the cable losses. So the battery does receive a full charge - eventually.

My suggested solution was a DC-DC converter, at the auxiliary battery. When SOC is low, and charge current is high, the voltage drop in the supply cable means the input voltage to the converter is below system voltage - but it doesn't matter, because the converter will regulate to the battery terminal voltage.

A 50A charger? How many auxiliary battery systems will push 50A through the auxiliary battery? Particularly if it's in a camper trailer?

Dean's proposed solution of a second alternator will work better, as the auxiliary regulator can generate whatever voltage is required to provide a decent charge voltage at the terminals of the auxiliary battery bank. Provided he runs a separate sense line from aux alternator to aux batteries.

[drivesafe]

Hi Scott, you have pretty well answered exactly the same thing I posted but there’s two subjects in my posting I will clarify.

I also agree that a second alternator would do a better job but you have to seriously consider the cost factor of setting up a second alternator, because it’s not just the alternator that is needed. You need an entirely new and very heavy cabling system to make this any where near useful. Why not just install heavier cabling on the existing system.

DC - DC converters will work but the largest common ones on the Australian market is only 20 amps and this is only advantageously useful for batteries up to 70 A/H and by the time you get to a 100 A/H battery, this device will actually take longer to charge the battery than the alternator can do ( depending on how low the battery is when you start charging ).

There are much better devices, like the Stirling Chargers, that start at 50 amps and go up to ( I think ) 200 amps, and at these current levels, would be a real advantage over the alternator ability to charge but again there is a cost factor that has to be considered which would include fitting a decent size alternator and cable.

Again, Yes, a 90 amp alternator would take longer than say a 120 or 150 amp alternator would, but to quote Dean “ I did not count on a huge bill to keep them charged. Any suggestions that maintain there charge and look after these batteries would be helpful. “
Hence why I have been suggesting the least expensive ways of achieving what he needs.

Now back to the charging operation. The voltage drop that occurs between the cranking battery and the auxiliary batteries is going to be influenced, in the great part, by the size of the cable running between the front and rear batteries. But even if this cable is very thin, it will still cause a voltage drop at the terminals of the cranking battery, which will in turn cause the alternator to produce more current.

You don’t need a voltage drop of a volt or more for the alternator to be required to produce additional current, just a few millivolts will start an increased charging procedure.

If your analogy was true then the auxiliary batteries would be pulling the voltage off the cranking battery and the cranking battery would be correcting the voltage and as such, then there would be no need for the alternator to produce more current.

The auxiliary batteries WILL pull voltage off the cranking battery but it will be supplemented by the alternator producing more current to the cranking battery as a means of raising the cranking battery’s voltage back up again and this additional current is going to flow onto the auxiliary batteries and charge them, no matter how long it takes, but the alternator will continue to produce as much current as it takes to keep the cranking battery’s voltage up. As the cranking battery is not requiring this current, the additional current is going to go to the auxiliary batteries.

Cheers.

[deanchristensen0001]

My second alternator is a while away as the guy helping me make up the brakets and move some AC pipes is vey busy so I have been lookin at independent charging system to tide me over.

http://www.projecta.com.au/catalogue/cid/24/asset_id/56

This system from projector seems to be similar the the $700 rotronics unit (RDA12) but is less than $100.

Can anyone see any problems using this system to independently charge the rear batteries, is there any known problems with diode systems?

I will have to run the same cabling any way and this may work, I hope!

[-Scott-]

Suits Vehicles with Externally Sensed Alternators
Common to all diode isolators is a voltage drop between the input and output terminals, it is 0.5V on the Projecta unit.

This voltage drop means that the charging voltage on the battery will be 0.5V less than the voltage coming from the alternator, which results in batteries being undercharged.

Vehicles with externally sensed alternators can connect the sensing wire directly to the battery. This makes the alternator produce an extra 0.5V on its output to compensate for the diode isolator.

If you have an externally sensed alternator it may work OK.

[deanchristensen0001]

If you have an externally sensed alternator it may work OK.

What is this? I have a GU 3.0ltr Patrol, does anyone know if it has an externally sensed alternator?

[drivesafe]

Hi Dean, I don’t know the exact system you are talking about but if it’s a diode type isolator then as Scott posted, unless the alternator is actually sensing the voltage that is going into one of the batteries, because the diode type isolator cause up to a .7 volt drop in the supply voltage coming from the alternator, neither of your batteries will ever be charged properly, no matter how long you drive for.

Cheers.

[drivesafe]

Hi again Dean, to get around the problem for now, why not just fit a cheap solenoid and switch till you get the second alternator on.

Cheers