Putting an auxiliary battery for a dashcam in car

[nh3]

This is probably a very dumb question, but I have a dashcam that is surprisingly power hungry (about 500 mA at 12V during parking mode) and I would like to install an additional battery for it to keep it running longer than it does on the car's own battery. I was thinking of installing one of those small AGM batteries (e.g. 9 Ah) in parallel with the car's own battery in the engine compartment, and this is the wiring scheme I had in mind. Is it okay to wire it this way? Mostly I am unsure about the diode I added to prevent the auxiliary battery from being used for cranking the engine (otherwise I would have to install quite thick cables for the auxiliary battery to accommodate the cranking current?). And of course, by installing the auxiliary battery in parallel with the car's main battery, I'm looking for it to be charged whenever the car is running and charging the main battery. Am I completely off here, or is this plan okay?

[Ian.M]

No. The dashcam will continue to drain the main battery via the diode.   Get a DC-DC auxiliary lead acid charger and install it and the auxiliary battery as per the DC-DC charger's instructions.  For such a low battery capacity you'd probably need one described as a trickle charger not to exceed the battery's max. recommended charging rate.

Alternatively, if the car's main battery is getting up there in years, simply install the highest capacity one that will physically fit so running the dashcam off it doesn't drain it excessively if parked for 12 hours!

[nh3]

No. The dashcam will continue to drain the main battery via the diode.   Get a DC-DC auxiliary lead acid charger and install it and the auxiliary battery as per the DC-DC charger's instructions.  For such a low battery capacity you'd probably need one described as a trickle charger not to exceed the battery's max. recommended charging rate.

Alternatively, if the car's main battery is getting up there in years, simply install the highest capacity one that will physically fit so running the dashcam off it doesn't drain it excessively if parked for 12 hours!

Oh yes, I forgot to mention that I'm ok with the dashcam draining the main battery too to a certain level. I have a battery guard installed in front of the dashcam already and I'm intending to put that also here. It cuts the battery off at 12.10 V, which is still enough to crank the engine. I don't know how I forgot this from the opening post 

Something like this is what I had in mind:
https://fulbat.com/wp-content/uploads/2023/09/FT_FP12-9_GeneralPurpose_2023.pdf
https://fulbat.com/product/fp12-9/

But but, it might be just simpler to put in a bigger capacity main battery. I was just looking for cost savings as that AGM costs like $20 and a new main battery goes over like $150 

[Ian.M]

I can understand not wanting to replace a main battery that's got plenty of life left in it.

You generally don't want to cycle a SLA (of any type) too low as the life will be drastically shortened.  Also unless you can guarantee a good long run after each parking session, it wont get fully charged every time, so only about 40% of the capacity is available for regular cyclic use.  A 15AH battery would be the bare minimum to survive regular 12H 500mA discharges, and 20AH would be a better choice.   

[themadhippy]

chuck a relay in there with coil switched from the ignition then the main battery will be disconnected from the backup battery if the engines off

[nh3]

I can understand not wanting to replace a main battery that's got plenty of life left in it.

You generally don't want to cycle a SLA (of any type) too low as the life will be drastically shortened.  Also unless you can guarantee a good long run after each parking session, it wont get fully charged every time, so only about 40% of the capacity is available for regular cyclic use.  A 15AH battery would be the bare minimum to survive regular 12H 500mA discharges, and 20AH would be a better choice.   

Thanks for the great feedback. I have to consider the options now. Still one question: I have a maintenance charger for car batteries that can be plugged in 24/7, would it be okay to connect the charger to the main battery in the diagram's configuration? I'm thinking is it okay for the charger as it has some "intelligent" features, when there are two batteries in parallel (the main car battery and the dashcam aux) and it would be charging them both?

chuck a relay in there with coil switched from the ignition then the main battery will be disconnected from the backup battery if the engines off

This was one option I was pondering at some point, but I was unsure if I had the skills to build it  But it wouldn't be too hard really, I guess

[Peabody]

You need to keep the main battery fully charged as much as possible.  If it stays half-charged most of the time, it will sulfate up and die early.  I've learned this the hard way as the owner of a Honda vehicle, all of which have the infamous two-stage charging system.

Assuming you have available some indicator that you are in parking mode, maybe one of those power multiplexer IC's would switch from the main battery to the auxiliary battery automatically.  Something like a TPS2121, but in a more hobbyist-friendly package.

[u666sa]

I think you need a separate battery, a UPS or motorcycle, and a separate charging relay. Stock alternator should suffice, unless you have audio system.

[Ian.M]

I did suggest a DC-DC charger for the auxiliary battery.  The charger will provide isolation so no diode is needed, and most such chargers can be configured to shut down if the main battery isn't on charge, by sensing the input voltage.  Also by providing an appropriate charging profile, the auxiliary battery should deliver its full cyclic life, rather than failing prematurely due to being alternately undercharged on short runs and over charged on long ones.