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Topic: Battery minimum voltage question (Read 7429 times) previous topic - next topic

5errated

Jun 17, 2012, 08:08 pm Last Edit: Jun 17, 2012, 08:21 pm by 5errated Reason: 1
Hi!

I'm using an analog input along with a voltage divider to keep an eye on battery voltage.  I'm using a 12v AGM deep cycle battery.

I've read that 11.9v is a good minimum voltage to stay above.  My question: I know that battery voltages drop when under a load.  Should I stop at 11.9v under load, or will I be able to go lower than 11.9v if it's under a load?  I haven't actually tested to see how much the voltage changes when I put the battery under load yet, but I'm guessing it'll be fairly substantial. (5 amp load)

I'm making a thermostat and low voltage controller for my coleman thermoelectric camping cooler.  I have two 55 amp-hour AGM batteries that I wish to power it off of.  I'm working on the low voltage detection part right now and just needed to know if the 11.9v minimum is open circuit voltage or if that's an absolute minimum, regardless of load.

Thanks!!

winner10920

I believe 11.2 is absolute dead so 11.9 isn't an absolute stopping point, heck you could stop at 11.3 but with negative affect on the batterys health,
for long term reliability of the batteries id say stop at 11.9 even if under load, and remember that you need to have it remember it dropped below 11.9, because it will drop to say 11.8 under load, and then recover to a higher voltage, and I imagine you don't want it cycling everytime it recovers

5errated

yeah that's a good point.  i hadn't though about the fact that it could cycle the relay on and off in a loop from the voltage being over the theshold open circuit and under the threshold under load.  i'll definitely code around that.  thanks for tipping me off on it.  easier to implement things like that now than to have to add it in later.

jackrae

Some good reading and sensible information at   http://www.windsun.com/Batteries/Battery_FAQ.htm


5errated


Some good reading and sensible information at   http://www.windsun.com/Batteries/Battery_FAQ.htm




that's actually where i got the minimum voltage number from--it just didn't mention if that was open circuit or if I could go below that value if I'm under load. (assuming it's still above 11.9 if I were to remove the load)

jackrae

11.9 will be the open circuit voltage.   Dip on load can be considered as a transient effect and is a function of internal resistance which is principally indicative of battery capability (capacity) rather than depth of discharge. (As a battery ages, through use, its fully charged capacity reduces and its internal resistance rises)
Certainly a battery half charged will show a greater dip than one fully charged but it is the overall condition, rather than state of charge, which governs internal resistance.

5errated


11.9 will be the open circuit voltage.   Dip on load can be considered as a transient effect and is a function of internal resistance which is principally indicative of battery capability (capacity) rather than depth of discharge. (As a battery ages, through use, its fully charged capacity reduces and its internal resistance rises)
Certainly a battery half charged will show a greater dip than one fully charged but it is the overall condition, rather than state of charge, which governs internal resistance.


so are you saying that I can safely stop at 11.9 open circuit voltage (even if it's lower than that during load)?

jackrae

yep !    But like all things in life, recommendations are the opinion of the writer. 

MarkT

I'd agree with that, but also enforce the 11.3V absolute minimum as well.  If you have an estimate of current and the internal resistance you can try to estimate the no-load voltage from the with-load voltage.   When a battery discharges slight variations between cells mean that one cell will start to dip voltage first - that's the one that will be damaged by over-discharge, so being a bit paranoid about detecting under-voltage is a good thing.  Perhaps make that 11.3 an 11.5V
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

jaydie

This is interesting.... (definitely will follow the this discussion).

Just a suggestion.
Since you're already using Arduino I believe it's not a lot of work to add something in your circuit that would put a load ( a bulb or a resistor) on the battery when you make a voltage check on it. It will off course eat some of the batteries power so you just have to be conservative in doing it.


5errated

Awesome!  thanks for the info!  i'll probably stop at 11.5v just to be completely safe.

The arduino will actually be controlling a relay that will power the cooler (5 amp peltier [thermoelectric plate]) on and off, so I can read the voltage with and without load with no problems.  I just don't want to have to switch the relay any more than I need to since relays seem to be a weak link in reliability.

winner10920

Use a mosfet? Then its not a physical connection to go back after arcing a million times, and no click noise

5errated

#12
Jun 19, 2012, 03:34 pm Last Edit: Jun 19, 2012, 03:38 pm by 5errated Reason: 1

Use a mosfet? Then its not a physical connection to go back after arcing a million times, and no click noise


I considered this, but decided against it because it would involve having to get a high current mosfet, and also having to possibly cool it.

I also considered a solid state relay, but it seems most of these are for switching AC.

Plus, I like the idea of the high current side of things being isolated from the microcontroller side of things.  (I have a 2amp fuse between the 12v battery and the arduino, and a 15amp fuse between the battery and the cooler)

EDIT: I guess I could use optical isolation going the mosfet route.

winner10920

I think the stp40nf12 is 1.25 US from digikey, I bought about ten also because they are logic level
they'll handle that 5 amps without getting warm, considering they are rated for 40 amps, could probably get away with 10 amps before you need a heatsink

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