9h1bw wrote:Thank you both for your replies. Which mirror each other.
Thus my idea is a non starter.
Thank you for your help.
I disagree! You just need to go about it differently. With one more 12 volt (deep cycle) battery, a backup system using the car works quite well. By running a small load over along period of time one wastes nearly 500 watts/hr running the cars systems in ready mode. So a small load is dwarfed by the (overhead) waste. The key is to power the small load from a deep cycle battery external to the car. If you are using an inverter to power the load you can use the audible alarm on the inverter as your reminder to put the car in ready mode for a while and recharge the deep cycle battery.
Using canion and testing this out, a low (alarm sounding but inverter still powering load) deep cycle 12v 100ah battery will charge from the DC to DC (to 80%) at around 1,100 watts which works out to a bit less than 65% overall efficiency. Sounds pathetic efficiency wise but think about it compared to a gas generator which would be more like 15% with engine efficiency factored in. Besides while efficiency is important, it's importance is far from the most important factor in emergency back up power.
Lets say your ham radio needs 100 watts/hr (Chosen arbitrarily). The external deep cycle battery could run it for 5 hours between (and during) 45 minute high amperage recharges. Running the external deep cycle battery between about 30% SOC and 80% SOC gives best efficiency for the entire system. So the car is only connected to and charging the external deep cycle less than an hour out of each 6 hours of run time for the radio.
A fully charged traction battery could run such a load for several days using this strategy!