It's all over the Internet in many forums and YT vids.
that 'fuse' you speak of is a regular fuse with a diode attached in series. Looks like this......
-----|--~~~
The fuse facilitates placing it in the circuit. The Diode drops voltage when placed in the circuit by it's physical nature. We are basically using what has most often been considered a negative aspect of the diode(voltage loss when inserted in circuit ) to solve a negative created by Toyota ( voltage regulation too low for AGM ).
They are used everywhere in electronics and have been forever.
In this scenario the voltage drop fools the Toyota electrical scheme into thinking it's too low, and it continues to deliver more voltage.
There is a threshold that needs to be crossed for the AGM battery. This diode voltage drop 'trick' basically facilitates crossing that threshold.
It's not a one size fits all perfect thing though.
Silicon diodes ( most common these days ) drop about .6V. It's just the way they are.
So for instance if you had a 12V circuit and needed to power a 9V device.
You put 5 diodes end to end dropping .6 each and you have 9V.
+12>>> --|--.--|--.--|--.--|--.--|-->>9V
So this 1 Diode 'trick' for the 4R just happens to get us inside that AGM window. Maybe you get 14.1, 14.2, 14.3, or the ideal 14.4. What we don't want is to go over 14.9.
So whatever we fool Toyota with it adds back. We fool it with .6v drop, it adds that back. So if we had 13.5 before, now we have 14.1 and the AGM says ok, I can deal with that.
All this is very coincidental and convenient at the same time. Not all diodes are alike. If you put an LED in there. It might try to drop 2Vs. That is their nature. Each color has a different drop from about 1.5V to 3.5V. Still a diode, just a different kind.
So it seems like we need like a Silicon diode and half. Well no such thing. But... there are Germanium diodes.. older and not as common these days.
They tend to drop -less- voltage. So if we could find the right combination of silicon diode ( 10cents ) + a Germainum diode ( $2 )in series. Maybe we could get .8 or .9 volt drop. Now that 13.5 from Toyota would jump up to 14.4 or the ideal AGM voltage. AND.. it would still be in safe range for everything else, and it would still be fused as normal.
So the trick is to find just the right diode pair for your truck. There are a million different diodes out there and they do publish their specs but it's for a specific temp / volt / amp. So a diode that specs on paper at 1V might measure .7
But this is what everyone is doing and most people are fine with being 'close and good'. No matter if they are doing it for 4W or Audio or whatever.
I'm just trying to find a way we could have a tuneable setup to dial in that extra or lesser .25 volts to create as close to ideal for the AGM. Otherwise you still aren't getting your complete money's worth from it.. unless you condition it from time to time with something like that CTEK.
Unfortunately the part we use to do it is a finicky part. You can have a bag of the same part and many different results.
But even with just two diodes we have a lot of options.
Silicon - S
Germanium - G
S --- works for a lot of people, not ideal
S + S --- too much voltage - will ruin system
S + G --- could work and maybe be ideal
G + G --- could work and maybe be ideal
G --- likely too little change to meet AGM needs
Only issue is a lot of trail and error. No one size fits all.
If we could get a 14.5 MAX and maybe drop into 14.2, I think that would be a good range based on results I have read from many people.
had to pick it up at the post office because it required signature. Good instructions, so far so good. And just in time! I had just finished up upgrading the stock battery to a northstar 31(M) AGM with Pelfrybilt battery cage (and extending positive cable, negative was fine granted you flattened out the stock terminal). Also put in an ARB Fridge Wiring Kit.
