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While reading this question, where a user is having difficulties with his dashboard camera battery, I noticed that one of the answers suggests (or seems to suggest) to replace the existing battery with a converter attached to the car battery.

Another way is to use the un-switched (i.e. always-on) 12V from your car, regulate it to 3V, and put this into your cam. But make sure you do not drain the battery and also use fuses, as there's a lot of power behind the 12V in a car.

This seems like a good idea - why bother replacing the battery with a capacitor (or another battery) if you have a huge one just laying there - the car battery. I happen to have the same dash camera at hand (G1W) and also have a 12V to 3V DC-DC converter just lying around.

Hence, the question. Can the replacement be done in-place or would an additional circuit be required to prevent the device attempting to charge the car battery through the converter. This is what happens during normal camera operation when attached to switched-on 12V car power supply - it only uses the battery to shut down safely and maintain its memory for settings. The original battery is a 150mAh 3.7V rechargeable battery.

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  • \$\begingroup\$ As you've noted, is that YES... at least a blocking diode on the original battery would be required to prevent converter current for passing back into it. And this may cause other problems. 3.7V means its likely a LiPO cell. the camera might very well see your 3V (or 3.7 with a diode drop) as an indication or undercharge, and shut down to protect the battery. So you might need something better than a 3V supply. \$\endgroup\$ – Randy Jun 15 '16 at 14:25
  • \$\begingroup\$ Keep in mind that the nominal 12 V of a car battery may vary wildly, especially when cranking. This is why DC-DC converters for automotive use have huge input ranges and special features (load dump protection, for one). \$\endgroup\$ – uint128_t Jun 15 '16 at 16:22
  • \$\begingroup\$ @Randy, the converter is actually adjustable. The output voltage and current may be adjusted via potentiometers. \$\endgroup\$ – predi Jun 15 '16 at 16:29
  • \$\begingroup\$ OK... that's good, as it will prevent the camera circuit from thinking there is an overdischarge problem. BUT... you still have the problem of the supply feeding into the LiPO cell. You probably know that charging a LiPO cell without a proper management circuit is at worst dangerous, and at least not good for the battery. How is the battery normally charged? Separate USB port? \$\endgroup\$ – Randy Jun 15 '16 at 17:36
  • \$\begingroup\$ @Randy, the goal is to completely eliminate the existing LiPO battery and rely only on car battery via the converter, but only during shutdown. The existing battery is not intended to be removable and is charged by the device from the same source as the device itself - a 12V adapter. To clarify - charging is governed by device's circuit and only happens when the car is "turned on". \$\endgroup\$ – predi Jun 15 '16 at 18:00
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If you are able to set your regulated supply to 4.2 volts, this will mimic the max recommended safe charge voltage of the camera's built in LiPO battery. Based on my experience with a few charge management ICs typically used in such applications, it is a safe bet that at this voltage there should be, at worst, only occasional VERY low current "standby charge" energy feeding back toward your supply. I believe it would be insignificant and could safely be ignored.

You'll probably want to place a capacitor in the circuit, maybe in the same physical place as the battery, and maybe 220uf (just a guestimate). This should further help fool the camera's battery charge management circuit to "feel" like a real battery is there. I'd also recommend sticking with a simple analog regulator rather than any kind of buck regulator. Such a device, though more efficient, can add high frequency noise which could leak into the sensitive analog electronics in the camera.

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  • \$\begingroup\$ Thanks, but.. What about the case when the device's circuit attempts to charge the now missing LiPO battery? Should I not guard against that? My understanding of your answer is that it only expects the device to be powered through the converter. The fact is that there will still be two power sources (technically a single one in two modes) - the car "turned-on" and the car "always-on" power - and the device will still switch between them, like it did before. \$\endgroup\$ – predi Jun 18 '16 at 10:49
  • \$\begingroup\$ As I explained, at 4.2V, any charging attempt should be insignificant and can safely be ignored. And by the way, since the battery you're replacing is normally "always there", you WILL want to wire the source of the regulator to a point where your car supplies 12V all the time, even when turned off. \$\endgroup\$ – Randy Jun 20 '16 at 13:34
  • \$\begingroup\$ I seem to have misread your answer. Oh, well.. Thank you. \$\endgroup\$ – predi Jun 20 '16 at 13:50

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