Originally posted by drkodin the alt can keep up if he did what i told him. the deep cycle is a must because system draw is at a minimum. bmw cars take what power they need for the electrical components and then let you use what is left over. the deep cycle wont do anything but help him due to the extra amperage on tap in the batt. i'm not saying add another batt, i'm saying replace the batt with a deep cycle.
note: i have personal experience with this. my car is basicly the same as his, his is just better. both are still the e36 platform.
The deep cycle is not a must, where do you get this flawed logic that because system draw is at a minimum that you need a deep cycle?
You tap into a vehicle's power supply, you tap into it. The current demand from the amps is going to effect the entire vehicle, there is no point blank way of isolating it as the supply is tied into the source. Supply and demand still applies, and you will still have a voltage drop situation if you have to fall back on the battery(ies).
Do you understand what a deep cycle's properties are?
Do you realize what the primary intent of the deep cycle battery is, and why it is NOT ideal for starting and maintaining a vehicle as the primary power source?
You do realize that a deep cycle battery by itself does not offer more amperage than a starting battery? In fact a starting battery on average can displace more current over a shorter period of time than a deep cycle. The deep cycle can
recover from a
deep cycle discharge state to a complete state of charge many many many more times than a starting battery, hence it's name. But it also as a rule cannot provide as fast of a recharge/discharge state as a starting battery can. There are gives and takes to the various types.
I have plenty of firsthand experience with charging systems also, and what you are saying about batteries (saying deep cycle is a must) rings incorrect at this point. The only reason to go with the yellow top is because optima will not provide warranty on the red top if you have a car audio system installed in your vehicle, and this is because kids liked to deep cycle them too often and damaged the internals which caused them to no longer be able to maintain a charge. You can do that with any standard starting battery if you like to use your system for extended periods of time with the engine off. Several repetitive severe discharge states on any starting battery can destroy it.
A standard deep cycle battery is fine under most conditions, but you move out of california to the arctic tundra once and you'll find that the slower recharge rate may leave you stranded with a dead vehicle. It takes you longer to bring a deep cycle battery back to full charge, and that can lead to issues if you don't have your car running for extended periods of time.