Backup Battery

Well the battery needs to be charged, which can only happen when the engine is running, or maybe you plan to charge it separately with a battery charger, but this seems rather awkward....I would just hook the batteries together in parallel, but then comes the problem of finding a place to mount the battery, this depends on the type of vehicle....but most would be in the trunk...so all you need is an isolation switch, which basically breaks the neg line from the battery...Then you disconnect the spare battery while parked and operating the stereo system, when you are ready to leave and start the car you then energize the spare battery for starting and leave it on until the system is fully charged again...I would use marine batteries which can tolerate discharge much better than regular car batteries...

https://www.amazon.com/CZC-AUTO-Disconnect-Terminal-Isolator/dp/B08C7BSM5H/ref=asc_df_B08C7BSM5H/?tag=hyprod-20&linkCode=df0&hvadid=485521586392&hvpos=&hvnetw=g&hvrand=1025442557434944260&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9011819&hvtargid=pla-1128332863841&psc=1

If on the other hand you only plan on operating the stereo and other stuff while the engine is operating, then you don't need another battery, but you might need a larger alternator...so you need to determine the total load with everything on, and check the amperage output of your alternator to see if it is meeting the demand...or you could just calculate the extra load you are adding and add that to the existing alternator output so maybe go from a 45 amp alternator to a 65 amp or larger...

https://autoguysland.com/how-to-tell-what-amp-your-alternator-is/

Alternatively you could just check voltage output of the alternator, probably around 14 volts, and turn everything on and see if the voltage drops...if the voltage doesn't drop then you are good to go...

The simplest way is to use a prepackaged multi-purpose product like the one shown below. All you may have to do is plug this to your cigarette lighter to recharge..

or like this...

The set-up you want sounds the same as leisure batteries used in motorhomes. They charge from the engine & operate lights, coolers etc. but ensure that the car battery is not drained by any of these utilities.

There are several ways to do this. The easiest is to get one of those hot start jumper set ups. The issue with them is this; there are two types of batteries, Start and Deep Cycle. Start provides high current for a short period. Deep Cycle, lower current but for a longer period. Start batteries will be destroyed if the voltage drops below 10.5. D.C. can go to 10.0. The best way is two batteries, and two high power (100 amp), on-off switches.* These are available on line under marine electrical. A local R.V. outfitter may also have them.

You will need to remove the existing Red battery lug and connect it to one side of both of the switches. The existing start battery Red post is then tied to the other terminal on 1 switch. The new battery Red post is connected to the open terminal on the other switch. Leave both switches "ON". When you get to your destination turn the Start battery switch to "OFF". Upon leaving, turn the Deep Cycle switch OFF and the Start battery ON. After the vehicle is running smoothly, turn on the Deep Cycle switch to charge that battery. Leaving both batteries on line.

Depending upon the state of discharge of the D.C. battery and the drive time, you may need an Aux charger for the D.C. battery.

* If you have a diesel, best to have a friend with experience do this. The wiring can get quite involved.

Cheaper to just get a small generator to carry around...

Search for "dual battery system kit" and take it from there. Automatic relay disconnects (double-pole double-throw?), voltage monitor and separate isolator switches seems to do the trick. The local 4x4 community that likes to tackle Africa all have this, to power fridge/freezer, sound, camping lights, etc... (but also more seriously, it is fitted to game conservation, emergency and similar vehicles). It is available off-the-shelf here from outdoor places. Judicious positioning of fuses are important for protection against chafed-through wires and other accidental shorts circuits!

You can charge them in parallel but one is connected to the starter and the other is connected to the other loads. Isolation can be done with diodes, no complicated switching needed. I'd still use a switch, however, for when you don't need to operate any equipment (you're not in your car but the radio's on).

It sounds so simple so why aren't car companies doing it? I'm not sure but I think you'd need a larger alternator to charge the two batteries. That extra battery is also double the weight of a single one.

So, having a double battery setup may result in poorer fuel economy for our car.

regards,

Vulcan

It all comes down to price, manufacturers will not do anything which cuts their profit. It is standard in motorhomes because it is a good selling point.

BSR is on the money.

One caveat : You should install an overcurrent limiter between the relay and the leisure battery (LB) in order to prevent damage to the LB, should the main battery have become too discharged , as this condition might result in too much current to flow from the alternator.

Well, that's exactly the way a narrowboat is wired. The domestic batteries power the lights and the radio while the engine battery turns the starter. The idea is that, if the lights go out overnight due to a flat domestic battery, the engine battery is still able to start the thing in the morning.

Happy ditch-crawling!