# Voltage-controlled-switch (or similar) to discharge battery when voltage exceeds 14V?

#### LMF5000

Joined Oct 25, 2017
87
I would like to create a simple circuit that will discharge a battery through a 50W load (resistor or bulb) if its terminal voltage goes over about 14V.

The circuit should stop the discharge when the voltage goes below 14V and switch on the bulbs again any time the voltage is over 14V.

How could I implement this with the lowest possible part count and cost? Can I do it with a single electronic component for example?

#### ericgibbs

Joined Jan 29, 2010
11,944
hi LMF
One option would be to use a dual comparator to create a window comparison, Vmin Vmax.
A LM393 dual comp, drive transistor and a relay would be minimal.
E

#### LMF5000

Joined Oct 25, 2017
87
Hi Eric, thanks for the suggestion. I thought of something like that, but I'd need a reference voltage source, the actual switch (transistor?) and probably resistors and capacitors to make it all work right? I'm a mechanical engineer so I'm a bit lost when it comes to making a complete circuit with a comparator.

Is there a way to do it with one component? Or a ready-made circuit board for $5-$10?

#### LMF5000

Joined Oct 25, 2017
87
Thank you NSA - that's almost exactly what I need. Pity about the 2A limit, I was hoping for something that can switch at least 5A at 12V (circa 50W) without needing an external relay.

Any other ideas with different approaches?

#### ericgibbs

Joined Jan 29, 2010
11,944
hi,
Use the option 'A' with the module , post #4

#### LesJones

Joined Jan 8, 2017
2,972
I think if you have exactly the same switch on and switch off voltage it will be switching on and of quite frequently. This due to the fact that the battery voltage will increase when the load is removed. (And decrease when the load is applied.) I would think you would need to switch in the load at say 14.1 volts and switch it off at 13.9 volts. You could use a zener diode and scale the voltage with resistors to a suitable reference voltage to the comparators. You could also use an LM431 adjustable precision shunt regulator fo the reference voltage.

Les.

#### crutschow

Joined Mar 14, 2008
26,440
without needing an external relay.

#### LMF5000

Joined Oct 25, 2017
87
Hi Les, I agree that some hysteresis would normally need to be implemented, but in this application it's not essential.

Basically what I'm trying to do it get a battery to absorb an unlimited amount of energy. I have a hobby charger with regenerative discharge function. Normally a hobby charger is powered by a car battery and charges a lipo pack for an RC model. When you discharge said lipo, typically the charger must dissipate that as heat, but mine has a function to put that energy back into the car battery (hence "regenerative") and the permissible discharge power flow is much higher since it's not being lost as heat (for my charger it's 20W for internal discharge vs 300W for regenerative).

The problem arises when the car battery gets full and can't absorb much power - at that point discharge slows to whatever the car battery can absorb (maybe 0.3A at 14.4V). Hence why I'm trying to add something to the car battery (like lightbulbs) to continue discharging at a high rate even when it gets full.

Battery voltage is normally 12.6-13.5V, and regenerative discharge boosts that to a value I can set in 0.1V increments, say 15.0V. So if I can get my circuit to power the lightbulbs at 15V they'll only run when the charger is in regen mode, and won't discharge the car battery when the charger finishes the operation.