Electronics > Power/Renewable Energy/EV's

efficient way to disconnect battery when voltage drops?

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ja421:
We have an off grid water pump that runs off a deep cycle battery and is charged by solar.
It receives a wireless signal from the thank when the tank is 1/2 way empty and to start pumping. (on 3/4 low, an alert inside the house is given so we know there's a problem)

This is all working well, except... When the battery is drained too much, the solar charger locks it out as a bad battery.
So what I want to do is just have something that monitors the battery voltage, and if it should drop (say below 11 or 10.4v) it will not engage the pump.  I think most of the time, we can go a day or two w/o it pumping, and that should give the solar panel (and sun) enough time to recharge the battery.

I have toyed with a zener diode & transistor to energize a relay to cut the battery in/out, but this seems to waste a lot of energy (I have not tried to use a mosfet instead of a relay).  I am thinking about using an attiny85 & sleeping as much as possible...

It seems there might be a better / lower power way to do this. Any thoughts?
thank you

Seekonk:
11V is way too low.  I don't go below 12.6V.  You could just switch the control circuit on and off, that would be low current.  Even an opto isolator may be sufficient.  I use the little arduinos for everything at only $2 each in my solar applications.  A TL431 is an alternative for discrete.  Do you really need to pump at night?

wraper:

--- Quote from: Seekonk on August 08, 2016, 07:04:17 pm ---11V is way too low.  I don't go below 12.6V.

--- End quote ---
Really??? What's the point in the battery which will get cut off even before discharged by any sensible amount?

ja421:

--- Quote from: Seekonk on August 08, 2016, 07:04:17 pm ---11V is way too low.  I don't go below 12.6V.  You could just switch the control circuit on and off, that would be low current.  Even an opto isolator may be sufficient.  I use the little arduinos for everything at only $2 each in my solar applications.  A TL431 is an alternative for discrete.  Do you really need to pump at night?

--- End quote ---

I'm not sure I agree with 12.6 -- deep cycle batteries can provide a lot of current at lower volts, and 12.6 seems like a battery just sitting idle, so it might never get used... Either way, the cutoff/in voltage shouldn't be the primary focus; it's the method for monitoring the voltage I'm after a fairly effecient way.

"could just switch it on off" -- yes, I could; but that requires walking a fair bit up/down a steep hill to the pump house.
The pump does run at night if it needs to (drops to 3/4 low); no one likes waking up and finding out the hard way that there is only enough water for 1/2 a shower.

I guess I'll experiment with arduino more and see how power compares to the zener method.

Ian.M:
If its a Lead Acid battery, don't worry too much about getting ultra-low power consumption.  The typical self-discharge is 5%/month, which for a 100AH battery is equivalent to about 7mA continuous load.  It wont even notice a 1 or 2 mA load.

You do however need to add some hysteresis to make the turnon threshold sufficiently higher than the cutoff threshold so that the rise in terminal voltage when the load is removed doesn't turn the pump back on causing it to short-cycle.

Probably the best option would be a TL431 driving a high side big P-channel MOSFET.  Stick a NPN emitter follower on the TL431 cathode resistor to boost the +ve going gate drive, with a reverse biassed Schottky diode B-E so the TL431 can pull the gate -ve quickly.   Put a high value resistor from the MOSFET drain to the tap on the potential divider feeding the TL431 ADJ terminal to add hysterisis.  You should be able to get the quiescent current down to 1.1mA without any problems.

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