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I have a van for summer travel and I am considering wiring up a car battery directly off of a 12V cigarette port which is rated at 20A by the car company. If both batteries were well charged at the time of connection, would it probably fry the fuse of the cigarette port very easily? The cigarette port is switched off by a relay every time I put the car into sleep mode.

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    \$\begingroup\$ Why do you want to do this? \$\endgroup\$
    – Andy aka
    May 2, 2021 at 9:28
  • \$\begingroup\$ The audio of the camping car is very tinny, so I would like to have a decently powered amplifier and speakers at the back, perhaps 300W RMS briefly, which is higher than the cigarette lighter jack can deal with. \$\endgroup\$
    – bandybabboon
    May 2, 2021 at 9:34
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    \$\begingroup\$ If there is voltage difference between the batteries, e.g. one is empty and one is full, the full one will always charge the empty one until potentials are equal. There could be more than 20A flowing between batteries into any direction when you plug them in, limited only by the resistance of the wires and fuse, and the lighter socket is not meant to have batteries connected to it, and the current surges could weld the relay contacts if it has to switch large currents. \$\endgroup\$
    – Justme
    May 2, 2021 at 9:46
  • \$\begingroup\$ 300W RMS - that is a nonsensical mix of power and RMS quantities - RMS applies to voltages and currents and not watts (no matter what you may have read on whatever site) \$\endgroup\$
    – Andy aka
    May 2, 2021 at 11:12
  • \$\begingroup\$ @andyaka ok, lol. Even www.electronicdesign.com › ... What's the Difference Between RMS and Peak Watts? | Electronic Design 17 Jan 2020 — Root mean square or simply RMS watts refers to continuous power handling of a \$\endgroup\$
    – bandybabboon
    May 2, 2021 at 11:25

2 Answers 2

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The issue will be after you have run the audio and discharged the second battery, then the alternator will charge the second battery possibly exceeding 20A... pop goes the fuse.

The solution is to fit a caravan or split-charge relay.

Available at many places and they come with instructions.

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  • \$\begingroup\$ perhaps i'd need a current limiter to limit the charge if the relay swtiches on and lets 40A through. \$\endgroup\$
    – bandybabboon
    May 2, 2021 at 10:06
  • \$\begingroup\$ @aliential last one I fitted had wiring for 100A... \$\endgroup\$
    – Solar Mike
    May 2, 2021 at 10:09
  • \$\begingroup\$ That's for a direct battery to battery connection, I was hoping to use the 20A cigarette lighter at the back of the car, the best would be a current limit of about 5A. \$\endgroup\$
    – bandybabboon
    May 2, 2021 at 10:23
  • \$\begingroup\$ @aliential the direct battery connection is the point, avoids overloading the vehicle wiring. Used to like customers like you - lots of work to replace or repair the loom... \$\endgroup\$
    – Solar Mike
    May 2, 2021 at 10:39
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    \$\begingroup\$ @aliential Do you find “No” a difficult concept? We tried to help with sensible suggestions based on experience to save you grief, but do what you want. \$\endgroup\$
    – Solar Mike
    May 2, 2021 at 12:00
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The fuse should be fine as long as what ever you plug into the cigarette lighter plug does not draw more than 20A from the socket. Something like a cigarette lighter or a phone charger will not draw much more than about 1 or 2 amps, probably even less for a phone charger.

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    \$\begingroup\$ Did you read the question? It's not about a phone charger... \$\endgroup\$
    – StarCat
    May 2, 2021 at 12:21
  • \$\begingroup\$ Did you read my answer? I am using a phone charger as an example as they are the most common things amongst cigarette lighters that are plugged into those ports. \$\endgroup\$
    – Artichoke
    May 3, 2021 at 7:39
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    \$\begingroup\$ Maybe, but the question was about connecting a 12V lead acid battery which is something completely different. I.o.w. I read your answer and it does not seem to be relevant to the question. \$\endgroup\$
    – StarCat
    May 3, 2021 at 8:05
  • \$\begingroup\$ Okay. Apologies. I was in fact referring to a lead acid battery as that is the type of most car batteries. Just trying to help :( . Ngl this community isn't very nice to newbies. \$\endgroup\$
    – Artichoke
    May 3, 2021 at 10:23
  • \$\begingroup\$ My apologies once again, I posted the question before the OP said it was for the audio. I did not yet read the comment (the guide lines do say you should edit the question to add information, not comment. I will edit my question accordingly \$\endgroup\$
    – Artichoke
    May 3, 2021 at 10:31

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