Advice for out-of-ordinary battery charging setup

[damien]

Hi all,
I'm setting up a split dual-battery charging system that appears sound to me, but i haven't seen it done before, and my main concern is how the alternator will react, and thus whether the source of the S wire should be changed....

The original vehicle is a Toyota LandCruiser, 1HZ motor with a 3-wire plug on a 60amp alternator (IG, L, S). I'm running a 96Ah silver-calcium crank battery and a 240Ah AGM auxilliary battery.

I have upgraded to a new alternator, a Denso based M2K 270amp with a 4-wire plug (IG,L,S,M).

To maximize the life of the batteries, and charge them both rapidly, I am installing a Sterling 120amp DC-DC charger at each battery, both fed directly by the alternator with upgraded cables.

So the 270amp alternator will ONLY be supplying both 120amp chargers (240amp total), which will each vary their current demand. Importantly the alternator won't be connected to a battery, but the electrics going to the alternator regulator (IG, L, S) will be sensing voltages just from the crank battery (if i leave everything as is).

So given that at times, the crank battery could be fully charged and showing that to the alternator regulator, while the auxillary battery charger could be at full capacity, I'm wondering if the S wire going to the regulator should be taken from the alternator side of the chargers. This location would show zero volts at start up.

Another consideration, albeit less important, is whether the L wire for the dash charge sensor should also be moved.

Cheers,
Damien

 

[Deleted member 26818]

The L terminal on Toyotas, is I believe just to light a warning lamp if the alternator ceases to work.
Not too sure about the S wire.
Normally it’s connected as close to the battery as possible to allow for volt drop.
With two batteries, would you have to run an S wire to each battery or perhaps switch it between batteries using a relay?

 

[HandySparks]

I'm not an expert, but I'd be surprised if a car alternator will work properly if directly supplying only two DC-DC converters without at least a small capacity battery connected to the output. I would expect the system to be unstable. Do let us know how you get on.

 

[damien]

The L terminal on Toyotas, is I believe just to light a warning lamp if the alternator ceases to work.

yep, so i was thinking if the DC-DC charger is between it and the alternator then it serves no purpose except to come on when the DC-DC charger drops its output for whatever reason in the charging cycle.
so if i run it from the common post where it splits to both DC-DC chargers and after the 500amp fuse, it will serve to alert if the alternator fails or the fuse has blown.

Not too sure about the S wire. Normally it’s connected as close to the battery as possible to allow for volt drop. With two batteries, would you have to run an S wire to each battery or perhaps switch it between batteries using a relay?

As i understand it, the S wire senses the battery voltage for the regulator to determine the charging required. if this is behind one of the DC-DC chargers, then it really won't have much to do with the reality of what both DC-DC chargers combined require from the alternator. If i switch to the position i described above for the L wire, I have no idea how the alternator will react to a 0.1V reading at start up.

 

[damien]

I'm not an expert, but I'd be surprised if a car alternator will work properly if directly supplying only two DC-DC converters without at least a small capacity battery connected to the output. I would expect the system to be unstable.

yes, this is my concern also. I've done a fair amount of reading, but have not come across any documentation about similar situation that would indicate how the regulator will react... possibly why i never found an example of what I'm trying to do!

I had thought of relaying the S wire to either side of the DC-DC charger for before and after start up, but again, I'm stabbing in the dark as to whether the alternator will play nicely.

I really don't have any room for another battery, but i understand that would solve my issue... how small a battery do you think I could get away with that would handle 270amps?

 

[HandySparks]

I really don't have any room for another battery, but i understand that would solve my issue... how small a battery do you think I could get away with that would handle 270amps?

I don't know. Personally, I'd use a conventional setup with the alternator supplying the start battery and the DC-DC converter between the two.

If you really need this unconventional setup, then try one converter and battery first. If that works (which I doubt) add the second ones.

If the voltage sense wire is connected to a battery, then I guess the output voltage of the alternator feeding the converter will rise to its maximum, since it's getting no feedback from the sense wire. I'd be concerned that the alternator voltage would exceed the safe input range of the converter.

Try Googling "running car alternator without battery", which is close to what you're trying to do.

 

[damien]

I don't know. Personally, I'd use a conventional setup with the alternator supplying the start battery and the DC-DC converter between the two.

If you really need this unconventional setup, then try one converter and battery first. If that works (which I doubt) add the second ones.

If the voltage sense wire is connected to a battery, then I guess the output voltage of the alternator feeding the converter will rise to its maximum, since it's getting no feedback from the sense wire. I'd be concerned that the alternator voltage would exceed the safe input range of the converter.

Try Googling "running car alternator without battery", which is close to what you're trying to do.

Many thanks sir!... I think you've just convinced me to go back to a conventional system and saved me some headaches. After more reading along the lines you suggested, my initial concern and reason for posting has now transformed into the realization that I could well fry my alternator at some point if I go down this route.

My initial reason for wanting the DC-DC charger on the crank battery was to A. charge it better via stage charging suited to the chemistry, B. isolate the existing vehicle wiring from the 270amp current outputted by the new alternator and C. have a split system where the auxillary battery is always being charged, not waiting for the crank battery to top up... given the now apparent risks to my alternator, none of those appear sufficient to go any further.

Thankfully i had wired everything up to be able to by-pass the cranker DC-DC charger by moving one wire from one post to another in the event the charger failed, so little effort lost apart from mounting it under the bonnet. I have plans for another auxillary bank so the second charger will not go to waste.

Maybe one day someone will develop an auto alternator that will directly run DC-DC stage chargers without a battery.