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Replacing a 3.7v Lithium with a Fixed Power Supply?

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Wirth's Law, Jul 16, 2017.

  1. Wirth's Law

    Wirth's Law New Member

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    Normally, I have one of those dash camera systems in the car. In addition to drawing power off the car's 12V lighter port, the recorder contains a 3.7v 1100 mAh lithium ion battery.

    I suspect our recent wave of hot weather damaged the battery, because a couple days ago I found the recorder indicating an error; and when I popped open the cover, I found the lithium battery swollen and feeling like it was maybe 42 to 48°C. Not good. And the recorder doesn't seem to want to do much of anything without a viable battery present. So until I come up with a replacement, I have a non-functioning recorder.

    So my question is: is it feasible to remove the battery, and instead connect wire leads to a voltage regulator of 3.7 to 4.2 volts? I ask because I have no real information on the charge controller inside the recorder, or whether it will tolerate a connection to a "perpetually charged" load that never sinks any current. So you could say my real question is: could this cause any damage to the charge controller?
     
  2. JLNY

    JLNY Active Member

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    Can the recorder run off the battery alone or does it need both the battery AND the 12V in to work?

    If it can be run from just the battery, then I don't see why not. As long as the voltage you are feeding in at the battery terminals is within the acceptable range of the charger controller, I don't think that it would get damaged. As long as it is a fixed voltage with a forward current from it, I don't think it would "know" whether it is running from a battery or a DC supply, and it would just behave like it had a constantly full battery. I would just be sure not to try to connect a charger with it running on the fixed supply at the battery, as most voltage regulators don't work with a net reverse current.

    If you can, it might be a good idea to examine the circuitry a little and see how it is running from the battery: it might just run directly from the 3.7V, or maybe it has a boost converter that raises the voltage to 5V or some other voltage. If you see any ICs that look like protection chips or charge controllers, you might try searching the part numbers to try and find datasheets.

    Is there some reason you would prefer to add an external supply rather than just replacing the battery?
     
  3. Pommie

    Pommie Well-Known Member Most Helpful Member

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    If you replace the battery it will lose any settings every time it's powered down - time, date position etc. It may also need the battery to finish writing files when power is removed which could corrupt the SD card.

    Mike.
     
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  4. dave

    Dave New Member

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  5. Wirth's Law

    Wirth's Law New Member

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    Right you are. When the 12V power is cut, the recorder will go into a soft shutdown within a few seconds, or (optionally) continue recording as long as the battery holds out.

    I did consider buying a replacement battery on eBay; but I've always been a little off-put by how hot these systems can get in the summer (not just this model). Mostly I wasn't sure this wouldn't happen again on some future hot day. The battery includes a 3rd pin that seems to be wired to an internal thermistor (NTC 10k @ 25°C or similar) and the recharging function will shut off above 45°C. I think if this regulator approach looks promising after a week or so, I'll try using it to switch to a different battery chemistry or other heat-tolerant delivery method.

    Well, I was able to get the system powered up using the wired power supply approach. The mock battery is a LM317 Adjustable Linear Regulator with a 1N5817 Schottky diode placed inline between the regulator output and the recorder's positive input to prevent any current being forced backward into the regulator by the recorder's battery charger. There are also input and output capacitors, plus a red LED. The temporary battery is a Duracell DURA12-1.3F (12V, 1.3 Ah) sealed lead acid battery. This was kind of thrown together, so the system output is 3.89V, compared to the 4.0V I intended, but there are no apparent problems so far.

    [​IMG]
     
  6. Wirth's Law

    Wirth's Law New Member

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    Well, it's been a few months so I suppose I should share how this test went.

    First off, I removed the original lithium battery altogether. Given that the system is meant to go in my car, where I've caught temperatures near 46C in extreme cases, I've really gone cold on the idea of having more lithium batteries inside than I need.

    Second, I removed the mock battery power supply I mentioned in my above post. The system actually worked fine as a power source, but it had a couple of problems. For one, since I was using the LM317 linear regulator (heatsinked), the assembly would get VERY hot, to the point that you could smell the hot FR4 material as the video recorder began to draw power out of the mock battery. This was okay, since it was really meant as a stop-gap solution to use until other parts could arrive, but I was suprised that the recorder didn't use it's 12V supply from the car more.

    As a long-term solution, I created a new circuitboard using Express PCB. This board would pull in 12V from the car, and feed it into a switched voltage regulator (LM2596). I also added an Electro Static Discharge (ESD) supression diode to the 12V input as a bonus safety measure. From there, I connected the regulator output to the input of a linear regulator (I used LT1584, however another LM317 could have been used). Adding the second regulator was an intentional over-kill to make certain the video recorder wouldn't have any trouble with any switching noise from the initial regulator. The technical details on the recorder itself are limited, so I figured it was best to err for caution here. To protect against accidental short circuits, I added a series resettable fuse near the output, which was set to trigger near the max current rating of the voltage regulators. I also threw in a metal Radio Frequency (RF) sheild I had seen on Digikey. Basically, the switching regulator and all its peripheral components are under the shield. Finally, I added series diode to the final output to prevent the video recorder from attempting to charge my mock battery.

    Overall, the mock battery power supply worked out extremely well; with no over-heating. I've had the whole setup running since my last post, and it's really reassuring to have my dash cameras back in working condition :D . I attached a photo of the final assembly, but if there's enough interest, I can share a few more details on the build.
     

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  7. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    Very professional looking, WL :cool:.

    And your attention to safety issues are commendable.

    Details would be a appreciated.
     

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