In an effort to try and understand a bit better what is involved in maintaining my van batteries both from the tug and solar I read all sorts of stuff on the net.
In the process I have come across a post on a forum that in essence states that by using a DC to DC charger from your tug alternator/battery system you can in some circumstance due to the current draw by the DC to DC charger reduce your alternator voltage to the extent that it can affect the ability of the alternator to charge the tug battery, and advices using a voltage boosting diode.
This is part of the post in question with some sections deleted for clarity.
What no one has addressed yet is the alternator output.
if you now add a DCDC charger out putting another 20A (remember a DCDC charger at this voltage will draw around 30A) and assuming the alternator has enough reserve your going to drop the charge voltage available from the alternator down to around 12.5V or lower and we haven't even considered running any other accessories.
If think by now you'll see why they now make low voltage input versions of the DCDC chargers for the latest vehicles, the salesman never bother to mention though what affect they will have on the cranking battery.
Bottom line is you really need to use a booster diode to ensure the cranking battery is properly charged.
Referring to the voltage booster diode: The increase voltage will have little effect if any on the life of the alternator.
For example, an the alternator rated at a 100A will develop its maximum output current at quite a low voltage ie 12.5V. At this loading the voltage regulator will be driving the rotor coils at 100% with or without the diode fitted, ie the diode will have no affect on the maximum current output.
Ignoring battery charging for the moment, and looking at the additional loading on the alternator at higher voltages. At a 50A load, a increase from 13.8V (690W) to 14.2V (710W) equates to 20W increase in load on the alternator, the equivalent of a couple of interior light globes, turning on the headlights is an increase of 120W.
The higher voltage will cause a slight increase on engine load but then so will adding any accessory.
Regarding the affect on the alternator from a battery charging perspective, yes the increased voltage will cause more charge into the battery therefore more load on the alternator but then that is why we are fitting the diode.
By comparison, for the same equivalent charge current into a battery a dcdc charger will consume around 10% - 30% more power from the alternator than just having the battery connected straight off the alternator.
Conclusion:
Fitting a booster diode will have little or no effect on the longevity of the alternator, having an overtightened drive belt, dirt & mud would be of more of a concern.
My question is; is this correct? And if so what exactly is a Booster Diode? And where should it be fitted in the tug generator system to ensure that the tug batter receives the correct charging voltage when the generator load also includes a DC to DC charger charging the van batteries.
Cheers, Tom
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I had a thought but it got run over as it crossed my mind.
Yes Tom, it's a very common practice. It can only be done on a "3 wire alternator" which is one that has a sensing wire which is a skinny wire coming out of the back of the alternator. This wire senses the system voltage and adjusts the charge going to the battery accordingly. Putting a diode in series with the sensing wire makes the alternator think the system voltage is lower that what it really is because of the voltage drop across the diode. A silicon diode will reduce the "sensed" voltage by .7v, so the charge voltage will be increased by that amount.
If you google 'how to booster diode alternator' or search the same on youtube you'll get plenty of hits. If you take the time you'll probably even find a forum where someone has done it on a vehicle the same as yours. 4 wheel drive and off-road forums are the best place to look as it's so common for them to have dual battery setups.
If your vehicle has an alternator fuse you can even buy a diode built into a fuse... the only problem is you'll pay a lot more for that than what a diode will cost at Jaycar.
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Cheers, Steve.
"Any day above ground is a good day... unless you're a spelunker :)"
Thank Steve, appreciate that advice and I will certainly look at that whole situation a bit more in depth now that I know it is practical to do. I guess the only thing to be real careful of is not to increase charging voltage too much to the tug starter battery so as not to cook it by overcharging when the DC to DC charger is not drawing any current.
Cheers, Tom
-- Edited by greyhoundtom on Sunday 13th of September 2015 08:15:38 AM
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I had a thought but it got run over as it crossed my mind.
Yes Tom, if you have an old school flooded lead acid starting battery you don't want to increase it too much. It's not about the current the DC to DC charger is using, putting a diode in will increase the voltage that the alternator will want to charge to, regardless of current draw so you also want to make sure your alternator is up to it. If you want to you could put a switch on the dash that is wired across the diode to short it out when the den isn't attached.
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Cheers, Steve.
"Any day above ground is a good day... unless you're a spelunker :)"
I have a 40amp DC--Dc charger in my Navara, this is connected to an AGM battery in the tub and this is paralleled with the caravan batteries when the van is hitched on. The 40 amp charger keeps my Aux battery and the caravan batteries fully charged and there is no effect on the Starter battery.
Some modern vehicles have computer controlled alternators which will not recognise an Aux battery and so will not fully charge it, this is where the dc-dc charger comes into the equation.
When I read the article you quoted I did note some inconsistencies with respect to the terms used eg "voltage" when referring to current. Someone who knows this subject would not make that mistake. I also note that the article states that at max output of 100 amps the voltage will be quite low around 12.5v I doubt very much that 100 amps would flow at such a low voltage. Most battery charging is done at 13.6v or above.
The alternator in my vehicle has a 180amp output so 40 amps for aux batteries will not worry it at all.
I would just fit the dc-dc charger and see how it goes before doing anything else. I am confident that is all that will be needed to charge Aux batteries.
frank
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Avagreatday.
Kathy and Frank currently at Home near Quirindi NSW
Some modern vehicles have computer controlled alternators which will not recognise an Aux battery and so will not fully charge it, this is where the dc-dc charger comes into the equation. frank
Some people erroneously believe that alternator regulators monitor the state of charge of the batteries they charge. The following is a diagram of how the older ones work.
You will note the regulator sensing just picks up the voltage across the battery. If the state of charge of the battery was to be monitored then there would also be the requirement to monitor the actual current passing into the battery. The function of the battery is not to supply the power demand of the motor and all the accessories, it is the alternator that does that. After the initial battery charge all the alternator has to do is to maintain the power to tun the vehicle and maintain the voltage high enough to make sure the battery does not discharge (or in other words supply current to run the vehicle.)
The modern alternator systems that are controlled by the engine control systems supply enough high voltage to charge the battery in the first minute or three to charge the battery. After that they concentrate on supplying the vehicles demand.
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PeterD Nissan Navara D23 diesel auto, Spaceland pop-top Retired radio and electronics technician. NSW Central Coast.
Most of the voltage boosting diodes that I have seen referred to in the newsgroups are incorporated in an auto wedge fuse. The vehicles they are used in (Toyotas in particular) have a fuse in the earth side of the regulator. Those who use them remove that fuse and replace it with the replacement item with the inbuilt diode.
I think there would be more problem with alternator voltage drop in vehicles with direct battery charging than there would be with vehicles fitted with DC-DC chargers. When you first commence to charge a well depleted battery the charge current drawn via a very heavy cable is far heavier than a DC-DC charger will draw. In fact if you read back in some of the postings submitted by Drivesafe he makes the claim that direct charging systems work better than DC-DC chargers because of this (and he sells the direct cabling systems.) Vehicles with the direct charging systems have been around far longer than those with DC-DC charging systems and they have not suffered problems whether voltage boosting diodes are being used or not.
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PeterD Nissan Navara D23 diesel auto, Spaceland pop-top Retired radio and electronics technician. NSW Central Coast.
Again brilliant information and a great insight for me into the function of a vehicle alternator that I had not been aware of.
The only real advantage that I can see in using a DC-DC charger as against the direct charging method is when the auxcilery battery is of a different type to the cranking battery as the DC-DC charger allows you to better target the required charging voltage.
Cheers, Tom
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I had a thought but it got run over as it crossed my mind.
DC-DC Chargers are an advantage when you're trying to charge a battery that is fitted to a van or a trailer (with the charger located close to the battery to be charged), as they overcome the inherent voltage drop in the cables. Of course, the heavier the cable being used, the smaller the voltage drop will be. But sometimes running very heavy cables all the way to your van or trailer batteries simply isn't practical, and with variable voltage alternators the voltage at the van battery will be useless. The added bonus of a DC-DC charger is by using one with a solar input, like the Redarcs, you have a built in MPPT solar controller as well.
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Yes I am an agent of Satan, but my duties are largely ceremonial.
Sorry but I do not understand this voltage boosting diode, a diode will permit current flow in one direction but not the other with current flowing through a diode there is generally a 1 volt voltage drop across the device so I am unable to see how the diode can boost voltage - I am referring to normal diodes and not to the other diodes that limit voltage to a set level AKA Zener Dodes or those used in some so called chillers that use the Peltier effect. I can only assume that the device provides additional voltage drop in the sensing circuit that causes an increase in the alternator output ( Voltage ) . So what is the proper eplanation
You'd be assuming correctly then Ian, the diode goes on the sense wire so the alternator reads the system voltage as lower that actual and thereby 'boosts' it's output by a .7v (silicon junction diode).
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Cheers, Steve.
"Any day above ground is a good day... unless you're a spelunker :)"
Again brilliant information and a great insight for me into the function of a vehicle alternator that I had not been aware of.
The only real advantage that I can see in using a DC-DC charger as against the direct charging method is when the auxcilery battery is of a different type to the cranking battery as the DC-DC charger allows you to better target the required charging voltage.
Cheers, Tom
I see two additional advantages with my setup
1. Is the dc-dc charger also isolates the Aux batteries from the starter battery
2. My redarc has an inbuilt mppt solar reg which connects the 2 solar panels to the aux battery when the engine is shut down.
things to consider I suppose
frank
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Avagreatday.
Kathy and Frank currently at Home near Quirindi NSW
Sorry but I do not understand this voltage boosting diode, a diode will permit current flow in one direction but not the other with current flowing through a diode there is generally a 1 volt voltage drop across the device so I am unable to see how the diode can boost voltage Ian
Ian, the diode does not actually boost the voltage. It is used to change the calibration of the alternator regulator. The diode is inserted into the regulator sensing circuitry so the regulator increases the alternator output voltage. The effect is a little like changing the potentiometer setting in a power supply to raise or lower the output voltage.
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PeterD Nissan Navara D23 diesel auto, Spaceland pop-top Retired radio and electronics technician. NSW Central Coast.
