The relay isolator pros are that it’s relatively small, which makes it easy to locate, and there’s no voltage drop across the contacts. When the ignition is switched off, the relay opens, and the secondary battery is isolated from the alternator and starter battery. Like a vehicle’s trailer battery charge circuit relay, a relay isolator is closed when the vehicle ignition is switched on, allowing both batteries to charge. However, the alternator field sense line can be relocated to an isolator output, and the alternator voltage regulator will adjust charge voltage accordingly.Īn isolator relay also installs between the alternator and batteries. Its cons are that its heat sink is relatively large (especially for high-current models), and the voltage drop across the diodes can reach ~0.7V, which could result in undercharging the batteries. The diode isolator pros include being easy to install, all passive, no moving parts, and therefore very reliable. Medium- and high-current diode isolators for one alternator and two or three batteries are readily available. This configuration allows for loads to be connected to the secondary battery without discharging the starter battery. A diode isolator installs between the alternator and the batteries the built-in diodes allow the vehicle alternator to charge both batteries but otherwise keep them electrically isolated. More common solutions employ a diode or a relay isolator, each with its own pros and cons. This may not be enough to charge several batteries and run other major DC loads, such as a three-way refrigerator. A possible drawback, however, is that trailer battery charging circuits are limited to 30 A. Modern trailer battery charging circuits are deactivated when the vehicle ignition is switched off, so the starter battery is isolated from the secondary battery and can’t be discharged by outside loads.Ī Y- adapter would still allow a trailer to be connected to the tow vehicle. One solution is to plug an extension cord into a vehicle’s seven-way trailer connector and use the battery charging circuit to charge the secondary battery. I do not desire a switch.Most of our vehicles are equipped with a single starter battery at the factory, but some of us may want to add a secondary battery or two to power a winch, a compressor, extra work lights, an audio system, or to augment our camper batteries – or some combination of these – all without idling the engine.īut to do so, we usually have to take steps to isolate the starter battery from the secondary battery so that our vehicle will start when it’s time to head home.Ī larger alternator is sometimes required as well. If you want a switch you can install one. There is no switch necessary for it to work. With a 1 wire alternator this does not work as all the voltage sensing is inside the alternator with no feed back from the battery. 7 volts low it simply turns on more field current and the voltage at the battery is the same as before you installed the isolator. The 3 wire alternator should have battery voltage feeding the Voltage regulator which feeds the field wire on your alternator an if the voltage at the battery is Anytime the engine is running both battery are being charge and when you shut the engine off the batteries are isolated. If you have a 3 wire alternator then the battery isolator works perfect. VSR charges the starting battery until it reaches a certain voltage like 13.8 volts then it kicks in the second battery and charges both. 7 volt higher regulator or instead use a (VSR) Voltage sensing relay. In this case you would want to change your voltage regulator to a. Example if your voltage regulator is 14.8 volts then you will charge at 14.1 volts at each battery. So if you have a one wire alternator your batteries will charge slower. There is a voltage drop acrossed the diodes of about. These high output diodes are pretty expensive.įor just a few dollars more you can buy a battery isolator which is just 2 diodes with a large heatsink to keep them cool. The current only goes one way on this alternator output line. The diodes would need to be able to handel the full current output of your alternator and a little more for saftey. You connect from the alternator output to each battery. Re: how to wire up 2 batteries using diodes
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