automatic battery cut off switch?

Steven Lake

Hi again all. I'm working on the peliminaries for a hydro power system (this will be built at a friend's house) and one of the things that kept coming to mind during the design was some things you'd mentioned before about never letting the battery bank get below 50% capacity. That got me to thinking, so I started looking for automatic battery cut off switches that could be set to automatically cut the power to the inverter if the bank dropped below 50% capacity, then reestablish the connection as soon as power rose above that magic number.

Now I'm not sure if that'd be possible, given that there'd be a constant 14.4v flowing into the bank (it'll be a hydro power solution, so it'll have 24/7/365 charging capacity) at all times, meaning that it likely couldn't sence the voltage and know when to cut off. So I'm wondering if there's a simple way to prevent the system from drawing down the batteries more than 50% in a situation like this. Given what I found while researching, I'm gonna say no, but as always, it's better to ask and be certain than not and wish you had. Plus it's always nice getting feedback on ideas from what arguably has to be some of the brightest and best minds in the off grid world.

H2SO4_guy

Re: automatic battery cut off switch?

What you want is a LVD (Low Voltage Disconnect) That is one of the options of the Moringstar (Available from NAWS) charge controllers. It will do up to 60 amps, which may not be enough, so check it out to see how much amperage you are going to need to put into the loads

Good luck and let us know.

unicornio

Re: automatic battery cut off switch?

most inverters have a configurable parameter to disconnect the battery when it is a given low voltage.
but for this protection might act in a state of charge of the battery (SOC), you need to have installed and properly configured a good battery monitor that can interact with the system ...

vtmaps

Re: automatic battery cut off switch?

Steven Lake wrote: »

Now I'm not sure if that'd be possible, given that there'd be a constant 14.4v flowing into the bank (it'll be a hydro power solution, so it'll have 24/7/365 charging capacity) at all times, meaning that it likely couldn't sence the voltage and know when to cut off.

Voltage doesn't flow. Current flows. Voltage is what pushes current and makes it flow.

You will NOT have a constant 14.4 volts. When the battery is discharged below about 85% SOC you will have a voltage less than 14.4 volts regardless of what your hydro is doing. When the voltage first comes up to 14.4 volts it means that bulk has ended and the current into the battery will start tapering off.

Steven Lake wrote: »

set to automatically cut the power to the inverter if the bank dropped below 50% capacity, then reestablish the connection as soon as power rose above that magic number

You don't want this either. If, for example, you want to disconnect the inverter at 12.0000 volts, you don't want to turn on the inverter at 12.0001 volts or it will be turning on and off 100's of times per minute.

The proper voltage for a low voltage disconnect is a whole other subject. by the way, how are you going to regulate the battery charging voltage?

--vtMaps

mtdoc

Re: automatic battery cut off switch?

All modern quality charge controllers have some type of low voltage disconnect.

The bigger issue with a hydro system is that you'll need a diversion load of some sort. You do not want to completely unload your turbine! Even without ever approaching a low voltage cut off situation you will need some way to keep your turbine loaded during absorb and float stages of battery charging.

BB.

Re: automatic battery cut off switch?

There are a couple of Battery Monitors that offer external alarm contacts that can be configured to (for example) turn "on at 50% and turn off at 80%).

http://www.victronenergy.com/battery-monitors/bmv-600s%20and%20bmv-602s/
http://www.xantrex.com/power-products/power-accessories/linkpro-battery-monitor.aspx

There may be others too...

You should also read all you can about batteries. There are a couple different methods to get the best life/useful total power from battery banks--And dozens of ways to kill battery banks:

http://www.windsun.com/Batteries/Battery_FAQ.htm
http://www.batteryfaq.org/
http://batteryuniversity.com/

-Bill

chrisc

Re: automatic battery cut off switch?

Hi all,

Don't mean to hijack the thread, but Bill, do you have a link to an example relay you might hook the alarm contacts to? I'm looking to cut-off battery power to an 12V inverter this way, so I would need a 100-150 amp switch.

Thanks,
Chris

Cariboocoot

Re: automatic battery cut off switch?

chrisc wrote: »

Hi all,

Don't mean to hijack the thread, but Bill, do you have a link to an example relay you might hook the alarm contacts to? I'm looking to cut-off battery power to an 12V inverter this way, so I would need a 100-150 amp switch.

Thanks,
Chris

Possibly not so much current; the control can operate the ON/OFF of the inverter, rather than try to switch the full power on the battery wires.

Best bet in a situation like this is to use a good inverter that has programmable LVD and reconnect (two different numbers to prevent oscillating). The Voltage measured under load will not be a direct indication of battery SOC; actual charge will be higher. This is good because it "automatically" prevents discharging too deeply.

For a simple adaptation a programmable Voltage controlled switch/relay (examples: http://www.solar-electric.com/volcon.html) can be used similarly.

BB.

Re: automatic battery cut off switch?

BlueSea makes some interesting relays:

http://www.bluesea.com/products/category/Remote_Battery_Switches/ML-Remote_Battery_Switches/featured

The bi-stable are very interesting--The only take power when changing states. However, the draw ~7 amps when changing, so you would still need a buffer relay/electronic switch to drive these large relays. Even the auto releasing require 7 amps to switch and 0.013 amps to hold.

Otherwise, a standard solonoid relay draws something like 1 amp to hold on. ~12 watts (12 volt system) is a fair amount of power to "throw away" (and need buffer relay too).

You can find DC solid state relays too:

http://www.powerstream.com/battery-isolator-solid-state.htm

This one draws around 15-25 mAmps to operate its logic section--perhaps you can drive this directly (I have not read the detailed specs to know for sure. But it does not appear they state the control voltage/current). You probably need to call the vendor for more information.

With Solid State relays (AC and DC) in general, they usually require some pretty good heat sinking. (0.22 volts * 80 amps = 17.6 watts of waste heat at full rated current).

I am not in the business, so others here may have some better suggestions. But, in general, switching heavy current (AC or DC) is difficult to do (cheaply, reliably, with small control voltages/currents).

I have not used any of these products. The BlueSea is probably elector-mechanical and the second one is solid state / MOFSET.

-Bill

BB.

Re: automatic battery cut off switch?

I should add, some AC inverters do include remote on/off control logic (MorningStar 300 watt TSW is one). That can make "switching" a much easier a much easier project.

-Bill

chrisc

Re: automatic battery cut off switch?

BB. wrote: »

BlueSea makes some interesting relays:

http://www.bluesea.com/products/category/Remote_Battery_Switches/ML-Remote_Battery_Switches/featured

The bi-stable are very interesting--The only take power when changing states. However, the draw ~7 amps when changing, so you would still need a buffer relay/electronic switch to drive these large relays. Even the auto releasing require 7 amps to switch and 0.013 amps to hold.

Otherwise, a standard solonoid relay draws something like 1 amp to hold on. ~12 watts (12 volt system) is a fair amount of power to "throw away" (and need buffer relay too).

You can find DC solid state relays too:

http://www.powerstream.com/battery-isolator-solid-state.htm

This one draws around 15-25 mAmps to operate its logic section--perhaps you can drive this directly (I have not read the detailed specs to know for sure. But it does not appear they state the control voltage/current). You probably need to call the vendor for more information.

With Solid State relays (AC and DC) in general, they usually require some pretty good heat sinking. (0.22 volts * 80 amps = 17.6 watts of waste heat at full rated current).

I am not in the business, so others here may have some better suggestions. But, in general, switching heavy current (AC or DC) is difficult to do (cheaply, reliably, with small control voltages/currents).

I have not used any of these products. The BlueSea is probably elector-mechanical and the second one is solid state / MOFSET.

-Bill

Thanks, I'll start digging into these products. Appreciate the help.