Non-standard 2nd battery install wih isolator

[digitboy123]

The threads I've seen asking about installing a 2nd battery concern large after-market sound systems to be listened to while the car engine is off, connected directly to the auxiliary battery. Most respondents suggest just to buy a larger battery. My situation is different, however.

I need the ability to use the in-car accessories while the car is off without worrying about being unable to start the engine later. I'm not talking about just listening to the radio. I need to operate a CPAP machine overnight for 6 to 8 hours--yes, I am sleeping in my car--and be certain the car will start in the morning. The CPAP is rated 6.67A at 12v. Based on my calculations, this would drain the stock battery within 8 to 10 hours.

This auto (2003 330i) has the second battery bay in the engine compartment. My inclination would be to install a second battery with an isolator. Before you say it can't be done or won't work, the installation admittedly would be modified from the usual battery+isolator configuration... details below.

My real question is, other posters have focused their concern on the added load on the alternator of such modifications, which I don't quite follow. Do these concerns have merit? If so, can you think of any other way to address my needs?

The following is how I am envisioning the setup I would need, along with my understanding of the isolator's function. I am considering one of the relay-type isolators such as the Powerstream 12v Smart Battery Isolator. In other words, NOT a diode-based isolator.

First, the isolator has two output poles, one connected to the main battery (and thus to the alternator) and one connected to the auxiliary battery. While the engine is running, it automatically isolates the batteries so that the main battery is charged first. Once the main battery is charged, then the isolator "de-isolates" the batteries, putting them in parallel to initiate the charging of the auxiliary battery. If the main battery gets depleted by a certain small amount, due to the normalization of the batteries in parallel, the isolator will sense this and isolate the batteries once again until the main battery is fully charged. This is the normal function of the isolator.

Second, when the engine is turned off, the isolator once again isolates the batteries from each other. Loads placed on the main battery, i.e. the internal electrical components of the vehicle, run off the main battery. This includes the vehicle starter. In the typical setup, loads to be used when the engine is off would be placed on the auxiliary battery, and there would be no concern about the ability of the main battery to start the engine. But I want to be able to use the vehicle's internal electrical components while the engine is off--without doing a major rewiring of the electrical system. This is not the typical setup, as I won't have any loads on the auxiliary battery.

Since I will be running from the main battery overnight, leaving the auxiliary battery charged, I would install a switch to temporarily connect the batteries in parallel during the ignition process. Once the engine is started, the override switch should be turned off (a normally-off push button would be ideal), in order to return the isolator to its normal engine-running operation.

The override switch is not placed inline with a battery cable, which would need to handle a very large current. The override switch is placed inline between the (usually) yellow override pole on the isolator and the auxiliary battery. This overrides the isolator's function "de-isolating" the batteries during the engine starting process. This circuit does not need to handle a large current (<1A at 12v) because it is only changing the position of the switch within the isolator, and not carrying the full current from the battery.

Please tell me what's wrong with my proposed setup, and what my solutions might be. Thanks for reading and thanks in advance for your response .

[Tgarret]

Any update did you do it and if so how did you proceed ?

The threads I've seen asking about installing a 2nd battery concern large after-market sound systems to be listened to while the car engine is off, connected directly to the auxiliary battery. Most respondents suggest just to buy a larger battery. My situation is different, however.

I need the ability to use the in-car accessories while the car is off without worrying about being unable to start the engine later. I'm not talking about just listening to the radio. I need to operate a CPAP machine overnight for 6 to 8 hours--yes, I am sleeping in my car--and be certain the car will start in the morning. The CPAP is rated 6.67A at 12v. Based on my calculations, this would drain the stock battery within 8 to 10 hours.

This auto (2003 330i) has the second battery bay in the engine compartment. My inclination would be to install a second battery with an isolator. Before you say it can't be done or won't work, the installation admittedly would be modified from the usual battery+isolator configuration... details below.

My real question is, other posters have focused their concern on the added load on the alternator of such modifications, which I don't quite follow. Do these concerns have merit? If so, can you think of any other way to address my needs?

The following is how I am envisioning the setup I would need, along with my understanding of the isolator's function. I am considering one of the relay-type isolators such as the Powerstream 12v Smart Battery Isolator. In other words, NOT a diode-based isolator.

First, the isolator has two output poles, one connected to the main battery (and thus to the alternator) and one connected to the auxiliary battery. While the engine is running, it automatically isolates the batteries so that the main battery is charged first. Once the main battery is charged, then the isolator "de-isolates" the batteries, putting them in parallel to initiate the charging of the auxiliary battery. If the main battery gets depleted by a certain small amount, due to the normalization of the batteries in parallel, the isolator will sense this and isolate the batteries once again until the main battery is fully charged. This is the normal function of the isolator.

Second, when the engine is turned off, the isolator once again isolates the batteries from each other. Loads placed on the main battery, i.e. the internal electrical components of the vehicle, run off the main battery. This includes the vehicle starter. In the typical setup, loads to be used when the engine is off would be placed on the auxiliary battery, and there would be no concern about the ability of the main battery to start the engine. But I want to be able to use the vehicle's internal electrical components while the engine is off--without doing a major rewiring of the electrical system. This is not the typical setup, as I won't have any loads on the auxiliary battery.

Since I will be running from the main battery overnight, leaving the auxiliary battery charged, I would install a switch to temporarily connect the batteries in parallel during the ignition process. Once the engine is started, the override switch should be turned off (a normally-off push button would be ideal), in order to return the isolator to its normal engine-running operation.

The override switch is not placed inline with a battery cable, which would need to handle a very large current. The override switch is placed inline between the (usually) yellow override pole on the isolator and the auxiliary battery. This overrides the isolator's function "de-isolating" the batteries during the engine starting process. This circuit does not need to handle a large current (<1A at 12v) because it is only changing the position of the switch within the isolator, and not carrying the full current from the battery.

Please tell me what's wrong with my proposed setup, and what my solutions might be. Thanks for reading and thanks in advance for your response .

[JPWheelr]

I'm confused as to why you're not using an RV battery isolator - configured to how you're intending. It seems like it would be a plug and play solution.

As far as alternator issues, Re-Spun High Output alternators are not cheap ($400), but they're not expensive either compared to OE. At 17 years, you might be due for a new one, anyway.

I've come close to the situation, and I feel for you, and I always keep "you can live in your car, but you can't drive your house" in the back of my mind.