Blocking Diode advice

[cat007]

Hey all

I have a question regarding a blocking diode I have purchased

It's a HEHR branded one - like this: http://www.hehr-international.com/isolators.htm

It's rated to a 130amp alternator (I'm putting a 120amp alternator in)

The idea being you can charge two batteries separately without either batteries talking to each other. So if one goes flat from lights etc. the other still is fine.

I want to know - if one is flat - the alternator is going to go balls out to charge it. But that would mean that the other battery that is charged is going to be overcharged?

[Stealer Of Souls]

My understanding of all this (and it is definitely not exhaustive or complete) is that yes... with just a simple blocking diode system you will tend to overcharge one battery. This effect can be minimised by having two identical batteries.
I believe it is to do with the internal resistance of the batteries. Supposedly the best way is to buy a controller that can ensure the correct voltage/current is applied to each battery.

[johntramp]

If it works like this you will have no problems if one battery is flat.

[cat007]

If it works like this you will have no problems if one battery is flat.

It is like that - but that's not my point

I was wondering if 1 is flat and 1 is charged, will the charged battery become overcharged while the flat one is being charged. A reply from 4GA on NZICE was what I wanted to hear:

The alternator does not really put out current to the battery, the battery draws current from the alternator. So when one battery is flat and the other charged, the flat battery will draw a lot of current and the charged battery only a little.

If you were to measure the current from the diode to the battery for each battery it would be different and the current on the line from alt to diode would be the sum of the two.

[cat007]

Next question would be - does anyone know if its possible and how easy it is to change the internal regulators reference feed to an external source?

[johntramp]

...as I said, you will have no problems if one battery is flat.

If you are talking about the voltage regulator on the alternator, one of the pins is a reference.

[cat007]

If you are talking about the voltage regulator on the alternator, one of the pins is a reference.

Oh really? Sweet!

I thought it might be internal somehow. I assume the reference wont require very chunky wiring as it wont be carrying many amps?

I'm wanting the reference for the alternator to be from after the diode so the alternator can compensate for the voltage drop.

I've just bought a 120amp Holden VY alternator. I'll try and make up a bracket this weekend and swap the pulley over.

Thanks

[matt dunn]

No it wont overcharge a battery if one is flat.


they do not split the charge equally, the flattest one will take the most current.

[Stealer Of Souls]

I'm wanting the reference for the alternator to be from after the diode so the alternator can compensate for the voltage drop.

Hmmm... won't this cause issues?
If you're wired after one of the diodes, you have no idea what the other battery is doing.
If you wire to each battery then you've just parallelled the batteries together. Yes? No?

[cat007]

I'm wanting the reference for the alternator to be from after the diode so the alternator can compensate for the voltage drop.

Hmmm... won't this cause issues?
If you're wired after one of the diodes, you have no idea what the other battery is doing.
If you wire to each battery then you've just parallelled the batteries together. Yes? No?

hmmm that is true??

Without running a dual sensing alternator I'm unsure how to overcome this....

[matt dunn]

3 studs on the blocking diode,
Batt 1
Batt 2
And charge /alternator input.


Run a wire from the Charge/input wire through an inline diode, 6A or similar, and then to the sensing wire on the alternator.

Will make the alt charge slightly higher to compensate for the Voltage drop across the diode.
so say normal charge is 14.1V, may take it up to 14.5 or so.
and you should end up with about 14.1 coming out to the Batt 1 and Batt 2.

[cat007]

I have it wired so it goes - alternator to diode then to batteries then from the main batt to van fuses and from house batt to other fuses etc

Like that it would mean I would have to put the sense wire after the diode otherwise it wouldn't be able to compensate for the diodes voltage drop or be able to sense what volts the batterie are putting out?


Or am I looking at it all wrong? Is the alt supposed to sense what it is putting out or what the batteries are putting out?

[matt dunn]

Or am I looking at it all wrong? Is the alt supposed to sense what it is putting out or what the batteries are putting out?

looking at it all wrong.

Batteries dont "put out" when charging.

The alternator needs to sense what itself is putting out.
Adding an inline diode in the sense wire should increse it enough to compensate for the dlocking diodes.

[cat007]

Ah ok
Thanks for clearing that up!

I'll go to dick Smiths tomorrow. A 5 or 6 amp diode should be ok? Any particular sort?

[matt dunn]

Ah ok
Thanks for clearing that up!

I'll go to dick Smiths tomorrow. A 5 or 6 amp diode should be ok? Any particular sort?

just a nomal diode
http://www.dse.co.nz/dse.shop/4a3763780 ... View/Z3229

[cat007]

just a nomal diode
http://www.dse.co.nz/dse.shop/4a3763780 ... View/Z3229

For some reason that link doesn't work. Just takes me to the dick smiths site.....

[Boosted_162]

just a nomal diode
http://www.dse.co.nz/dse.shop/4a3763780 ... View/Z3229

For some reason that link doesn't work. Just takes me to the dick smiths site.....

Search Z3229 in the search bar at the top...

[cat007]

Ignore that last post - refreshed the page and it came right

Thanks for that - I didn't get a chance today - but I'll see if my local DSE store has one....

cheers!