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Old 21st December 2010, 05:48 PM   #1
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Default Battery cut off circuit

Im looking for a simple circuit to discharge batteries to around 33% (14v) and the cut off. . Current will be an amp or less. It would be nice if the circuit ran off of the battery.

I was thinking of using a large 12v relay with some resistors in series with the coil and sort of have it release once the voltage gets low enough.
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Old 21st December 2010, 06:08 PM   #2
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RCGroups.com: The ABCs of Radio Control - Aircraft, Boats, and Cars!
Start there....They have all kinds of battery circuits to choose from.
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Old 22nd December 2010, 01:32 AM   #3
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I found this:

Team Toad: Battery Discharger

If i can find a relay with a reliable/repeatable release voltage This should be pretty easy.
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Old 22nd December 2010, 01:42 AM   #4
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Years ago I read that smart batteries were coming, particularly for video camera use. They don't seem to have materialized.

Ideally each cell would record how much went into it, how much went out, and also factor in a few other things such as time passed and temperature, all taking virtually no power, so that you could have a reliable 'fuel gauge'.

Looking at how unreliable the indicator on my mobile phone is, and considering how much spare processing power it's got, this seems to be an intractable problem.

w
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Old 22nd December 2010, 02:16 AM   #5
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Or how the ipod estimates battery power by how many mp3's you played and at what bit rate, instead of a current or voltage monitor.
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Old 23rd December 2010, 04:16 PM   #6
k9swc is offline k9swc  United States
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I was thinking of the relay idea; by using a sensitive relay with a forward-biased zener in series with the coil, and the zener voltage selected to match the desired lipo cutoff voltage (i.e., 9v for 3S, 12V for 4S, etc.) then the relay drop-out should be able to be accurately controlled - easier than finding a relay that happens to drop out at the desired voltage. A properly-sized resistor in series with the coil and zener would limit the current through the coil (and zener) to appropriate levels. Simple - only three parts. As long as the lipo voltage is above the zener voltage, the zener conducts, pullling in the relay which passes the power to the output (contact ratings selected appropriately) and as soon as the voltage falls below the zener voltage, the zener stops conducting and the relay drops out, opening the contacts.

Workable? Or will I need a relay driver (transistor) to actually run the relay coil, with the zener controlling the driver?
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Old 23rd December 2010, 05:22 PM   #7
qusp is offline qusp  Australia
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Quote:
Originally Posted by nightanole View Post
Or how the ipod estimates battery power by how many mp3's you played and at what bit rate, instead of a current or voltage monitor.
ermm, are you sure about this? I find this highly dubious, because otherwise the battery power would never go down at all if you dont play anything and that simply isnt the case.

and wakibaki, the 'fuel gauges' you speak of do exist, linear tech have a large range of coulomb counters and charge managers with sense lines (accurate resistive dividers) for multiple cell batteries that will initiate a routine (via logic or pull down) at a certain level of your choosing and will trigger a warning (this signal can be logic level) which can be pirated to trigger a relay, LED warning light etc and then at a level of your choosing it will initiate a safe shutdown procedure. all powered off the battery with VERY low power consumption.

they are the market leaders and the whole system is highly integrated. I did lots of research for my portable buffalo II headamp project and noone else even comes close. the LTC4150 and LTC2953 which is the one I chose due to also having a momentary switch driver for forced manual shutdown. this one is more of a charge monitor/shutdown manager with push button driver.

I dont use any of their stuff for charging though, for this I use a RC battery balancer, which is what is referred to above. designed for large high current packs for RC planes etc, these will do what the OP wants, you can set the level it will discharge to (per cell) and the rate it will discharge at. most balance taps will go up to 2A or higher for discharge, for fast charging they can charge the LiFePo4 packs in a matter of a few minutes.

Last edited by qusp; 23rd December 2010 at 05:25 PM.
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Old 23rd December 2010, 09:44 PM   #8
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This is all good news, but my mobile phone is far from being old, and it goes from indicating full to empty in a small fraction of the time that it spends indicating full, and that is true of my digital camera too, so uptake of these devices among manufacturers is evidently slow. It was the battery manufacturers themselves who were promising this technology, built into the battery itself, nearly 15 years ago by my recollection, I still don't see any evidence of it in the marketplace.

w
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Old 25th December 2010, 02:32 AM   #9
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Sorry back in the day when the ipod played mp3's (up to like rev 5 i think) they estimated bat life with an algorithm of how many and what bit rate mp3's you played. They even had a faq of reseting the counter.
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Old 27th December 2010, 06:54 AM   #10
qusp is offline qusp  Australia
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aha, right sorry nightanole, I do remember the syndrome of which you speak, but I cant say i've experienced it for a while, I had thought you were talking about more modern units, the 3g was like 8-10yrs ago now. the ipod/ipad/iphone do not operate like this today, but i'm sure there are still some devices that do still lag behind (as evidenced by wakibaki's situation), unfortunately there is no cure for bad design.

wakibaki, these devices are indeed often built into large batteries of cells
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