duell battery systems again

if you've totally flattened the deep cycle then the alternator will not properly charge it anyway. Also parallel charging a starter and a deep cycle battery is not ideal, but it should usually work to some extent.

Alternators are designed to bring a starter battery from 80% charged to 90% charged and they don't cope well with other situations. I have a Rotronics independant charge and find it works well but I don't flatten my deep cycle below 11.5 volts if I can avoid it. I also regularly top up the deep cycle with a 3 stage mains charger. I can usually run the fridge for 2.5 days before the battery gets down to 11.5.

Diamond.
The charging system on nearly all vehicles works by controling the output voltage of the alternator, the piont on the cct where the voltage is measured in order to control the alternator is usual at the ignition switch.THis means that resistance in the wiring between the alternator and the battery can be used to limit the charging curret of you standard battery to about 8 amps, if you connect a second battery to the first battery it can only share that 8 amps, if you supply your soleniod directly from the terminal on the alternator you will get the full charge into you second battery. Its that easy. Eric.

Not on the vehicles that we see here...

On a vehicle alternator, you have internal regulation. Voltage drop is compensated for by a 7800 voltage regulator. The rectification of the AC waveform generated in a Denso alternator comprise of four 100A diodes in a full-bridge arrangement. They are accompanied by a 7813 voltage regulator switching a series of SCR's to get the required voltage.

As load increases, the voltage output from the regulator increases to keep the voltage correct, so you don't blow anything up! Additionally, you should never run your car without a functional battery, as the voltage regulator could stuff-up from an open/short circuit condition.

The circuits like the Piranah and rotronics are actually very un-sophisticated relays, allowing the connection to the starter battery after it sees +13.8 volts or higher and an ignition source. This way, you won't parralel the batteries if you just switch your ignition on, without starting the engine. Rememer that your alternator will be produce 13.8v or higher, so that it can charge your batteries.

In my GUII I have three batteries. I have the starter battery,(HCD), a deep cycle 78AH for the fridge and a gell-cell for the alarm and emergency radios. These batteries all charge off the main 100A alternator without any problems, but the trick with this is to use a high-quality relay, some voltage sensing circuitry and very large cross-section cable.. 400AMP welders cable (2GA) is usually good for battery connections...