SLA battery backup

[1-3-2-4]

I'm following this design here: https://shdesigns.org/gcellchg.html

only two changes I plan to do is because this is for a LED emergency lighting kit It's being connected to a 12V 17 Ah battery, with 3 LED's driven by a 350 mA driver.

OK so the relay as long as the charger is on the leds will be off, power fails and they come on.. that's simple... however I don't want to run the risk of having the SLA go under 11.6 V so how can I have it automatically disconnect the load when the voltage hits that point?

I was thinking about adding a analog panel voltage meter to the project box.

 

[Joe G]

how about a voltage divider feeding a comparator w/ a tl431 ref?

 

[1-3-2-4]

I know I have a quad comparator here but not too sure how to do a voltage divider

 

[KeepItSimpleStupid]

Voltage divider calculator: **broken link removed**

The reference datasheet that was suggested is here: https://www.electro-tech-online.com/custompdfs/2013/02/TL431-DPDF.pdf Basically a 2.5 to 35 V reference.
You would need a reference voltage below your threshold, say 10 V or less.

You would then divide the 11.6 so it is equal to the reference voltage. divide by 11.6/10 or 0.86

Pick the two resistors such that they don't draw appreciable current in series. 100 uA to 1 mA. Pick resistors such that the sum ~= R = V/0.5 mA. You can move the reference to get a better trip.

You could say do a divide by 2 with 1% resistors and easily and adjust the reference.

Not good at explaining things tonight. Sorry.

 

[MikeMl]

Check out Post #35 of this thread.

 

[1-3-2-4]

Is that the only way to do this? I was trying to limit myself from having to order parts online or find what I can locally which almost seems impossible these days.

What about this?

**broken link removed**

I assume P1 does not matter what value it is?

 

[Joe G]

The tl431 can be had for free from : computer mother brds, tv pcb's to name a few, maybe also the resistors needed for Mikes circuit. If using a comparitor circuit you can get a Lm339 at Radioshack. Using new items though is always better (you know what you have). And good things do come to those who wait

 

[1-3-2-4]

I do have one of those Lm339 here but going that route seems a bit much for a low voltage cut off? over the design in the link I posted given his design offers SC and OC protection, plus with the LM339 all the unused pins you have to do something with them so they wont effect the rest of the circuit, right?

 

[Joe G]

the unused pins (+/-) I just connect them to gnd.
This is a pretty informative read **broken link removed**

 

[Mr RB]

Alarm systems and such just use a fixed voltage (ie 13.2v to 13.8v) from something really simple like an LM317. If you want to limit peak currents below the 1.8A current limit of the LM317 you can put a series resistor before the LM317. In your case with a 17Ah battery a 1.8A max current would be fine, and would only occur for a short time when charging after a long blackout period.

Sorry that does not help with the other point about stopping deep discharge, but if drawing only 350mA from a 17Ah battery it should be good for 40 hours of lighting or so, do you expect it to get worst than that? Normally emergency lighting is only used at night for 10-15 hours at max and the power would probably be back on the next day anyway.

 

[1-3-2-4]

Yeah I was thinking I would have power back the next day after huricane Sandy came around.. I found myself out for at least almost a week

 

[KeepItSimpleStupid]

Hey, put 4 1n4001 diodes in series with a current limiting resistor and you'll end up with an aprox 2.4 V reference. You can use an OP amp to amplify it with a bit of variable gain.

 

[1-3-2-4]

uhmm wow I saw a huge error it's pretty funny since it was a huge part not connected.. the + output was not connected to the cap :hmm: I guess that happens with nearly two days without rest..

So without a load the output voltage is 13.5V

So now I have these two SLA that I know are dead.. for some reason they are so weak touching the terminals together does not spark them but unloaded they show like 12.6V

Anyways figure I can use this for testing before putting a known good one to test. First thing I noticed when any type of load is connected, be it a tiny festoon led bulb I went from 13.5 to 9V when the load is removed it goes back to 13.5V I also tried something very simple I have a 50 pack of 3 mm white leds I connected one and sure enough it lit and the voltage dropped from 13.5 to 2.8

I don't really have the right pot on I have a 100K on instead of 5k but still adjusting it near that range should not give me results like this right?

 

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I have a question so as the LED lights when it's at the max charge current On which voltage level will the max current be reached?