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The output of the alternator (D4D anyway) is controlled by the engine computer. The ECU reduces the output voltage of the alternator once the computer decides that enough energy has been put back into the start battery. This can cause two problems, the first is the solenoid which charges the auxiliary battery senses the lower voltage and assumes the engine has been switched off and therefore it in turn switches off. This is because the isolators are meant to stop the main battery going flat when the engine is off. The second is the isolator may still work but the alternator voltage isn't high enough to put a decent boost charge into the aux battery. So the auxiliary isn't charged properly. The diode (yes it is a diode) fools the ECU into thinking the alternator is putting out a lower voltage than it really is so the ECU 'bumps up' the output to a value it is happy with. This of course means the alternator is now maintaining a higher voltage than it otherwise would. This in turn keeps the isolators happy and gives the aux battery more volts to work with.
The discussion in this thread is about whether DC DC chargers are worthwhile. In this set up there is no solenoid isolator but a DC supply battery charger which is just connected to the main battery. These work just like a 240V smart charger only they work off the main battery. A DC DC charger uses a trigger wire from an accessories supply to tell it when to work and when to shut down. Again to ensure the main battery isn't flattened charging the aux when the engine is off. These chargers are much more expensive than the solenoid isolator + a boost diode. There is also a school of thought which states that the DC DC charger can take too much out of the main battery on short runs and that the alternator cannot keep the main battery charged. The DC DC systems can pull between 20 and 40 A to start boosting the auxiliary so that is hard on a battery that has just kicked the engine over. Usually there is a delay before the DC DC charger kicks in to let the main battery recover.
My view, for what it is worth, is that unless you are running a calcium or AGM battery then the more simple system is adequate. Also you need to monitor ALL your batteries and give them a top up charge with a smart AC charger whenever they need it.
I run an AGM battery and so have a DC DC charger but our Prado does very few short runs and in any case I keep a close eye on both my main battery and the AGM in the back. My system is the Redarc BCDC1225 and I have seen it run at 25A boost when I commissioned the system.
As for solar some of the DC DC chargers (mine for example) incorporates an MPPT controller and by using a changeover relay the solar panel can provide energy to the aux when the engine is off, start the engine and the solar panel is isolated. With my previous Prado I used to leave the solar panel connected all the time. This isn't the smartest thing I have ever done! I now realise that the charge from the alternator could have burnt out the MPPT controller that was in the solar panel unit, so I have woken up and fixed that this time around.
It would be possible to charge the main battery with the solar system but I suspect that that would trigger a conventional solenoid isolator and it would be possible that the main battery would br no better off. Most if not all just use the solar supply to put charge back into the aux because that is the battery that is doing most work running lights, fridge, phone chargers etc.
Michael
The discussion in this thread is about whether DC DC chargers are worthwhile. In this set up there is no solenoid isolator but a DC supply battery charger which is just connected to the main battery. These work just like a 240V smart charger only they work off the main battery. A DC DC charger uses a trigger wire from an accessories supply to tell it when to work and when to shut down. Again to ensure the main battery isn't flattened charging the aux when the engine is off. These chargers are much more expensive than the solenoid isolator + a boost diode. There is also a school of thought which states that the DC DC charger can take too much out of the main battery on short runs and that the alternator cannot keep the main battery charged. The DC DC systems can pull between 20 and 40 A to start boosting the auxiliary so that is hard on a battery that has just kicked the engine over. Usually there is a delay before the DC DC charger kicks in to let the main battery recover.
My view, for what it is worth, is that unless you are running a calcium or AGM battery then the more simple system is adequate. Also you need to monitor ALL your batteries and give them a top up charge with a smart AC charger whenever they need it.
I run an AGM battery and so have a DC DC charger but our Prado does very few short runs and in any case I keep a close eye on both my main battery and the AGM in the back. My system is the Redarc BCDC1225 and I have seen it run at 25A boost when I commissioned the system.
As for solar some of the DC DC chargers (mine for example) incorporates an MPPT controller and by using a changeover relay the solar panel can provide energy to the aux when the engine is off, start the engine and the solar panel is isolated. With my previous Prado I used to leave the solar panel connected all the time. This isn't the smartest thing I have ever done! I now realise that the charge from the alternator could have burnt out the MPPT controller that was in the solar panel unit, so I have woken up and fixed that this time around.
It would be possible to charge the main battery with the solar system but I suspect that that would trigger a conventional solenoid isolator and it would be possible that the main battery would br no better off. Most if not all just use the solar supply to put charge back into the aux because that is the battery that is doing most work running lights, fridge, phone chargers etc.
Michael
(i was just trying to answer djd's question) I doubt Toyota would ever find it and I'm sure Leigh has sold plenty and equally sure someone would have said if there was a problem. Practically the diode cannot make the alternator work any harder, ok maybe some immeasurable amount if the alternator is designed around the voltage profile but i doubt it. The power output from the alternator at any given speed is fixed. Increasing the voltage will reduce the "maximum" current that can be output but it is unlikely that will be an issue. V=I*R and all that. Can someone with the diode post an in situ pic please?
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