Overvoltage Protection without Shutting Off

[dwalker2020]

Hey all,
I'm integrating an off-the-shelf wifi hotspot into a solar installation.
I've got a huge AGM battery (13.8 V nominal) that sometimes reaches charging voltages of 19VDC in full sun.
I want to power a cradlepoint IBR650, which takes 9-18 VDC at 1.5 amps.
I'm worried that overvoltage might hurt it in peak sun, but I don't want it to shut off if there's overvoltage.

I've considered zener clamping or regulating down to 10V, but neither is a great solution.
I've heard of regulators that pass through power under a certain voltage but regulate above the threshold as well, but I don't know of any or what to search for.
Do you have any suggestions for ways to deal with this problem? Thanks for your time!
-devin

[Seekonk]

There are plenty of charge controllers under $15.  19V on a battery will destroy it over time.

[dwalker2020]

Sorry if it was unclear, but I am using a PWM controller called the Morningstar SunSaver which charges at 19V at times. It doesn't let the battery charge above it's limits; it's a 4-stage charge controller and during the bulk charge stage the controller dumps 100% of the solar energy into the battery. During this stage, the raw panel voltage is applied and I can see voltages up t 19V at the battery terminals. My hotspot doesn't like that!

[Seekonk]

I've used several Morningstar controllers and I have never seen that condition.

[hamdi.tn]

19V on battery terminal is too much, you battery will die if not damaged.
There is plenty of cheap DC/DC converter that will stepped down to suitable voltage, so you may use them.

[brumbarchris]

You need something similar to this:

http://www.linear.com/product/LT4356-1

Regards,
Cristian

[rx8pilot]

I am using the LT4356's in a few of my designs. Its a great little chip to deal with OV and OI situations. For OV, you program a setpoint and a timer. The timer start when the voltage is above the set point and if the timer runs out it shuts off. For current, it puts the MOSFET in linear mode to limit the current while a timer runs down. Again, if that timer runs down, it shuts off the output. While the situation is over-voltage or over current, the MOSFET is linear and will get very hot. May or may not be great, they are intended to deal with transient situations, not really continuous operation.

In your case, you may need something like a Sepic converter that can take voltage under and over the target voltage. For best efficiency, you would need a more complicated setup that enables a buck converter and switches to it only when needed.

I can't think of a quick and dirty way to deal with OV while continuing to operate.

[Seekonk]

I hoped someone else would explain it to you.  You have wired something terribly wrong and/or it is time to use the Morningstar warranty.  Sounds like the controller is shorted.  Google charge states of a 12V batterry.

[mikerj]

If it's had 19v across it for prolonged periods, you'll be needing a new battery as well as a charger.  In fact either the battery is already on it's last legs or your solar panel is pushing a massive amount of current through the battery to see 19v across it.  Could you be measuring something incorrectly maybe?