Voltage modifier for a charge controller

I have a charge controller for 12v automotive type batteries. It currently limits the charge voltage to 14.4v but I need it to charge at 15.5v as I'm not using standard batteries. The controller has a voltage sense wire that picks up the voltage from the battery. I need to put something in this wire so that I can change the voltage going in the sense wire to trick the controller into giving more voltage.

I am very much a noob at this so any help would be appreciated.

I have read that I can use a diode to increase or decrease the volts by about .7v, but from what I know this depends on the amount of current going through the diode compared to its rated current. The problem it there is no current going through the sense wire of the controller so I cant use this method. Is there a component or circuit that I can built to increase or decrease by up to 2 volts whatever the battery currently is? Something that can be adjusted with a variable resister or something would be even more helpful.

Thanks

Any diode added to the sense line will have a -2 mV per deg C temp coefficient.

Within the charge controller there is likely a resistor divider network that take the battery voltage and divides it down the to the reference voltage that the controller IC actually uses. This is typically 1.25 or 2.5 vdc. Best way to change the resultant charge voltage limits is to change this resistor divider network.

You might get away with something as simple as a potentiometer across the battery terminals with the wiper going to your sense wire. If you use like a 1K pot it may be low enough to "fool" the divider in the charger. Set the wiper to 93% of the battery voltage and tweek it from there. If that doesn't work you can buffer it with an op amp - just a little more complicated.
You need to make sure your batteries are compatible with lead acid battery charging algorithms. What kind are they?

Within the charge controller there is likely a resistor divider network that take the battery voltage and divides it down the to the reference voltage that the controller IC actually uses. This is typically 1.25 or 2.5 vdc. Best way to change the resultant charge voltage limits is to change this resistor divider network.

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Unfortunately the charge controller is sealed in epoxy so I can't get at the insides.

You might get away with something as simple as a potentiometer across the battery terminals with the wiper going to your sense wire.

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I thought of trying this before but dismissed it for some reason (I cant remember why). I'll give it a try this weekend.

You need to make sure your batteries are compatible with lead acid battery charging algorithms. What kind are they?

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All the batteries I have are lead acid, one bank of flooded cell and one bank of AGM's. I need to be able to change the charge voltage as the AGM's don't take as high a charge voltage as the flooded cell.
I also want to be able to equalize or degassing the flooded cell meaning I need almost 16v.

You can only make the voltage go up using this method. What voltage do you want for the AGM's.

Something wrong about your statement about the charge voltage for AGMs vs flooded cell batteries. AGM need a charge voltage of ~14.7V @ 25C, while flooded cells need only ~14.2 to 14.4V @ 25C.

AGM need a charge voltage of ~14.7V @ 25C, while flooded cells need only ~14.2 to 14.4V @ 25C.

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Your right about the AGM's. Charging them at more than 15v for more than an short time will damage them. But deep cycle flooded cells will take considerably (cant remember how much right now, up around 40% i think) more amps during charge if the volts are up to 15.5v compared to 14.5v. Its also good for them as the small amount of bubbling mixes the electrolyte. Then an equalization charge is 10% higher then that yet.

Testing post

I guess that means i can post now...