Voltage Sensitive Relay

You might also be interested in a device known as a battery isolator. It allows you to have a standby battery that charges in parallel with the main battery, but disconnects the load and the standby battery when its voltage drops below some preset value.

They come in many different control strategies, some preset, some variable, some just cut the load off of a single battery at a certain voltage point. Just remember that discharging an ordinary automotive battery to 9V on a consistent basis will greatly shorten its life.

The vehicle already has a dual battery , dual battery charge controller and isolator

The problem is that if the vehicle is parked up for extended periods the device mentioned will pull that secondary battery as low as 2.2 volts , whereupon the CTEK 250 battery charge controller will not commence charging ( requires at least 3 volts for the CTEK to kick in)

I want to turn off the accessory device if the battery falls down to 9 VDC as this will protect the battery better than if it falls to 2.2 VDC.

No problem except for semantics, the voltage sensitive relay is still called an isolator because it "isolates the load" from the battery when the voltage drops to a preset level.

Here's one for 15USD. Cuts off at 10V but I suspect you can tweak it (and it's better for your battery).

Don't think that'll work - it's for use with an AC supply. Think it's intended to prevent deep discharge in UPS applications.

The ELK-965 instructions can be found here, but they're not very clear.

thats what i thought too . i have emailed ELK technical to ask.

yes

looks like an accurate product and it will work here although there might even be an equivalent in a smaller cheaper item.

looks like it would last a million years without skipping a beat

As Ramconsult has already correctly said (we are both assuming that its a lead acid battery), taking it as low as 9 volts (even going below 12.6 for long periods will also) damage it, slowly but surely! Some car batteries will not even last a week from new if so treated as you plan to.

If it has to be LA (Lead Acid) then it must be of a specific type that can accept deep discharges, called leisure batteries, (though some are called deep discharge I believe) they are more expensive and cannot produce the huge cranking amps for car usage that a true car battery can make, but you only need 7.5 amps anyway....

The size of the battery dictates how long it will take at 7.5 amps discharge, to get to the low voltage setting, that you haven't mentioned (at least when reading down from the top, you may have mentioned this "below" where I am, if so sorry!). You need to tell us how long it must take to get to 9 volts!!

Ramconsult has also mentioned that these battery monitors can be used for your problem. Also, some are adjustable for the drop out/in voltage.....or even build one yourself, its not difficult, with minimum electronic knowledge.

MOST important is to have some significant "Hysteresis", to prevent "Cycling".

What happens is that as the device is going off, the 7.5 amp load is removed, the voltage climbs, the device switches back on and off and on and off and on!!!

That can be deadly for many types of electronics....... I am sure you want to avoid this!!

Assuming that you have a leisure battery, assuming that the battery is required to drop off at say 9 volts, then I would set the "switch on" voltage to at least 11 volts. That will help to prevent cycling.....assuming its not a "tiny" battery of course....

Now capacity!

Capacity, may I suggest, to be at least 50% of what you need, so that as the battery "ages" over the next year or so, it still works for you. Picking the lowest capacity will mean that within a short time, a few months maybe, the capacity will be too low.....you need new battery.

Batteries age - small, cheap ones quicker that quality "oversized" ones......

out/in voltage.....or even build one yourself, its not difficult.

MOST important is to have some significant "Hysteresis", so that as the device is going off, the load is removed, the voltage climbs, the device switches back on and off and on and off and on!!!

That can be deadly for many types of electronics....... I am sure you want to avoid this!!

Assuming that you have a leisure battery, assuming that the battery is required to drop off at say 9 volts, then I would set the "switch on" voltage to at least 11 volts. That will help prevent cycling.....

Now capacity! Capacity, may I suggest, to be at least 50% of wat you needs, so that as the battery "ages" over the next year of so, it sill works for you. Picking the "correct" capacity will mean that within a short time, a few months maybe, the capacity will be too low.....

Best of luck!

OK?

Best of luck!

GA

All good advice.

The auto stores will have a relay for paralleling another battery that prevents discharge of main battery, but it will cutoff at 12V6 iirc.

Gajanan Phadte

Cheapest way would be to DIY it, but it would depend on:

• Your abilities with a soldering iron
• You having time and inclination to build it
  
• Finding someone with the ability, time and inclination to design it for you
  
• Trusting whoever designs it for you
• Being able to get hold of the bits (should be the least of your problems in Oz)
  

hoping to buy one off the shelf if i can .

time is money

:-)

While still bad news for your battery, if your only goal is to stop battery voltage from dropping under 3V, so to allow battery charge controller to kick-in, and 2.2V is where discharge stops, just feed the "device" through a couple of beefy silicon diodes in series. That should drop about 1.3V and your battery will stop discharging at 3.5V min. All devices made to work on automobile electric system are should be tolerant to wide voltage variations and maintain basic functionality well below (and above) normal voltage, so mabe more diodes in series are OK. Anyway advices given hereby grant you no warranties expressed or implied and there's also no warranty for fitness for a particular purpose. etc etc. S.M.

The accessory is a security device that doesnt function properly under 9 VDC , so i would like to limit the drain below that level and extend the life of the $ 280 deep cycle battery plus protect the device from damage.

thanks for thinking

:)

Undervoltage damage in an automobile rated electric device is unheard of. Anyway you should also consider that by the time the battery reaches that 9V+1.3V that is 10.3V (at minimal load) as I suggested, it is some 95%+ discharged, well below the level I would recommend for decent battery life. S.M.

Window comparator.

That's a bit better at about $63 (Oz)

Thankyou kindly That will do me just fine . Problem solved .

They are called overvoltage/undervoltage relays they are widely spread in europe. I used to buy them 220v AC . I'm not sure if 12v DC version of these relays do exist.