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I want to install a secondary car battery for a system/project that I am working on. However, somehow I need to be able to charge it while car is running, but not let the car modules use secondary battery?

Is this possible?

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closed as off-topic by Leon Heller, placeholder, Nick Alexeev, Matt Young, JYelton Oct 7 '13 at 2:29

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    \$\begingroup\$ When the car is running, all the car systems are powered by the car generator, not by the battery. \$\endgroup\$ – johnfound Oct 6 '13 at 20:52
  • \$\begingroup\$ Google "split charge relay". X-Eng in the UK sell a nice heavy-duty one with a downloadable wiring diagram. \$\endgroup\$ – John U Oct 7 '13 at 8:42
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Yes, you can use a diode between the car's electrical system and the secondary battery. In the worst case, this diode needs to handle the full output current of the alternator, so make it a beefy one.

Note that the charging voltage that the secondary battery gets will be slightly lower than optimum, but in practice, it doesn't make all that much difference.

However, if it worries you, and you have an alternator on which you have access to the actual windings, you can connect a second full-wave (6-diodes) rectifier directly to the windings, and then both the primary and secondary batteries will get the same charging voltage.

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The proper way to deal with this is to get a battery charging splitter. These are commonly available at RV equipment stores. These are a box that attaches in your engine compartment near the alternator. The box contains two huge diodes connected to three bolt lugs. The anodes of both diodes go to a common terminal which is wired to the alternator output. The cathodes go to the other two lug terminals. One of those goes to each of the main and aux batteries. The two diodes keep the load on either of the batteries from stealing power from the battery but also allow both batteries to charge in parallel from the alternator.

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