Battery backup swtching

[originator]

I am assuming that a relay is a pretty solid method of switching in a battery on power failure. I am stacking two 12v batteries that have a single 24v charger. I don't like the idea of having the relay on forever powered my the mains, I feel like one day there could be a failure with it. I'd much rather have the battery power the relay and deal with the added drain, which I am not so concerned about. I am using a fat cap that holds the system up for at least 5 seconds, so the timing of the relay is no big deal.

Which way would you do it: hold the relay on for 10 years+, or leave it NC, and let the battery turn it on maybe once every year or so on average.

The other thing I don't like is the battery life, what are they good for, a year or two reliably? Considering they wont be drained hardly ever, maybe that could prolong their lifespan.

 

[itsmoked]

The less a battery is drained the longer it will live.

BUT!! By faaaaaaaaar what controls the life of the battery is a function of the price/quality of the charger. Chargers kill more batteries than anything else.

At any rate, the typical life of a floated Lead Acid Battery is about 5 years.

Let the relay sit on...


Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[BrianG]

If the equipment is normally mains powered why do you need a relay to switch the battery into circuit? Unless your normal current drain is very high the usual way to do this is for the "charger" to also provide the normal low voltage requirement for the system as well, so that when the mains fails - instant battery, no relay, no chance of contact failure.

In these sort of systems the charger usually has a voltage stabiliser to keep the float voltage within close limits so that the battery does not get overcharged. As itsmoked says, "Chargers kill more batteries than anything else".

 

[VEBill]

As Brian has stated, why switch? Just use the DC power supply to also float the battery and then simply connect the load across the battery. (Hint - the battery does not normally supply the current in this configuration.*)

* Some people seem to think that the current goes into the battery, turns around, and then comes out (on the same conductors) to supply the load. It isn't like that.


A 5 year life is a bit pessemistic. Even in a car they last about 8 years and the environment conditions are terrible. In an office environment they'll typically last 10 or more. I've one in my alarm panel that must be at least 12 years old and it still works fine (including amp-hours, holds up the panel for many days).

 

[VEBill]

PS: you need to consider the load current PLUS the charging current (when the AC comes back on). If the battery is flat, and you want to optimise the functionality, then you might need to use some diodes and a resistor to allow the power supply to feed the load immediately even while the flat battery is still being charged.

 

[originator]

"the usual way to do this is for the "charger" to also provide the normal low voltage requirement for the system as well"

In a case of the max current draw of 300 milliamps, why not just let the charger be the main power for everything, supplying the 24, 5 and 12 regulators.

If I understand this correctly, leaving the battery "in" all the time means that with the additional load of the system, the battery will be charged all the time?

Iddeally I'd like to have a lower level threshold to turn the system off rather than completely drain the battery. I am not sure a mthd to do that using a comparator, since both the battery input and the reference input to the comparator will both be on a sliding scale duer to the drain. I can nly imagine a standalone battery just for the purpose of supplying the reference, so that when the main battery gets below X volts, the entire systenm shuts off, since below 17 or 18 the motors will turn off anyway, no sense draining the thing dead. Then begs the question, what turns it off, a relay that was held on by the comparator? Or a transistor preferably.

 

[itsmoked]

To turn things off you just monitor the battery voltage. Since the charger is no longer running (no AC) when the voltage get down to some voltage turn off the load.

What voltage? (lots of arguments here). My rule is if the battery is going to be recharged within a few hours of being drained then you can drain the battery down to whatever voltage corresponds to an unloaded voltage of 12V if we are talking 12 LA. When draining the battery the voltage across the terminals will be less because of the battery's internal resistance and so might read 11V but if you drop the load then it pops up to 12V. That is how I do it. Checking by trial and error. Or by looking at the battery fully discharged verse drain rate charts.

Oh and you have to deal with this with your shut off detection. Otherwise as soon as your system shuts off it will try to restart as the battery voltage jumps up.

Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[ScottyUK]

Most batteries are rated at a certain discharge rate down to a certain voltage per cell. A 2V lead acid cell might be rated down to 1.70 or 1.65V for instance. At this cell voltage the load should be disconnected otherwise the battery will can be permanently damaged.

Use a reference for your comparator - a 6.2V zener is fairly stable thermally so it would be a good choice - and compare your falling voltage to that constant reference. You would obviously need to choose your resistors to scale the voltages appropriately for the comparator.


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I don't suffer from insanity. I enjoy it...

 

[originator]

Thanks fellas for the good advice as usual. Why go to school when there is eng tips dot com.

itsmoked said "Checking by trial and error. Or by looking at the battery fully discharged verse drain rate charts."

I will have to pass on the drain rate charts. Does smoke come out of it? If not, then it is working in my book


I just picked up a 24 DC floating charger with built in leds to indicate fast(>250ma) or slow(<250ma). I connected two 12 volt 7.8AH batteries which were already mostly charged. I plugged in the charger to the batteries and ran the terminals to the circuit board and everything seems to be running great. The lights indicate fast right now, and is reading around 26 volts already with the load on it, hopefully it will go to slow soon, this is as simple as it gets I supose.

Now to go smoke a few zeners to get the load shut off working (hopefully around 18v) and I am calling it a day battery backup.