These were great help to me. I am looking at a 12v 12ah SLA with AGM technology as anything bigger is too big in size. I would enjoy the LifePo4 but I don't want to invest the money for that but may in the future come up with a better kit on my own. I am an engineer.. electrical with a lot of experience in different areas but of course I can miss something. I try to think simple when I can. I like the link about powerstream and the cut off and all.
+1 for Powerstream, but try this one out:
http://www.powerstream.com/d-sine-dc-ups.htm
If you want to use your own battery, use this one:
http://www.powerstream.com/DC-UPS-1212.htm
I also like this here:
Or if I use this 12Ah SLA battery it would still give me 144wh which is double capacity of Cellink for only $75 incl. charger
https://www.ebay.com/itm/291830531509
I have laid out a basic schematic here and I am using an F770 which has an internal cutoff if the voltage drops too far.
So if I usa a 12v 12AH SLA/AGM I should get 30+ (about 33 assuming 80% efficiency and nominal 12v though if higher then the hours would go up), hours given then F770 with a rear camera draws 3.5Watts according to the docs. Now.. if I toss a relay (12V 30/40 amper) on a accessory fused line with add a circuit.. like the radio.. I can disconnect the aux battery while the car is off and and not drain the main battery. When I start, the car the radio shuts off and thus disconnects, or at least I need to check that it does for sure and not just a control signal, and thus the aux battery is not being used to start the car. But after it starts, the battery reconnects via the relay and it charges again.
The F770 dashcam will turn off after whatever voltage I set as it is wired to the aux battery.. and the car is off so the relay is off so when the F770 shuts off then no problem. Thus I dont need anything fancy I think. $5 relay and a SLA from what I can see. Cellink B too pricey and only good for 18 hours maybe.. and for another $200 or something you get 40 hours woohoo. I have spent a lot of time researching but still hard to spend $400 for 40 hours of parking mode.
Summary:
Acc connects to the relay and the F770. The relay shuts off when the car is off and when starting.. ideally.. and I will make that work. The F770 shuts off when the aux battery hits the lowest level. I can set it down to 11.8 Volts or less and not have to worry about it. Start the car and start charging again.
I did this today and it worked. Although that gizmo is pretty cool to disconnect and all.. its just already built into the F770. If you dont have that then of course you need that item or one of the other voltage or time controllers. I had let my battery accidentally get down to 9.6 volts while testing and ya it was dead but the F770 was still running.
This can be refined I am sure but I thought it is worth a test. The $50 setup from powerstream is great, but I dont need to draw from the main batteries while the car is off.
This does work for me at least and I verified the radio fuse shuts off on crank.. and the auxiliary battery is independent of the main battery when the key is off. The battery cost me $34.95 + tax.
Hope this helps someone.
Cheers.