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I store my second car for 4 to 6 weeks at a time. There is no access to electricity, so a battery tender is not practical.

The vehicle electronics slowly discharge the battery so it goes dead in a few weeks. Is there any way to hook a second battery in series so I can go longer before draining the batteries completely?

I’m guessing some sort of diode to isolate, but what size?

Is it even necessary to isolate or should I just go direct to the terminals? I’m using AGM batteries. This set up would be temporary and only used during storage. Thank you for suggestions.

Addendum: Agreed, parallel is what I meant. Problem is that with a sophisticated sports car, disconnecting the battery creates many troubles that require resets and on some systems, hours or many miles for the cars computer to readjust to. Also, the trunk, where the battery is Located is operated by power. The emergency release requires the rear seat removal. How convenient! Maybe the best solution is to visit more often...


Agreed, parallel is what I meant. Problem is that with a sophisticated sports car, disconnecting the battery creates many troubles that require resets and on some systems, hours or many miles for the cars computer to readjust to. Also, the trunk, where the battery is Located is operated by power. The emergency release requires the rear seat removal. How convenient! Maybe the best solution is to visit more often...

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closed as off-topic by DoxyLover, laptop2d, pipe, Warren Hill, Elliot Alderson Jan 15 at 14:07

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – DoxyLover, laptop2d, pipe, Warren Hill, Elliot Alderson
If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ In series is a bad idea. I parallel - maybe. But why not simply disconnect the battery when stored? \$\endgroup\$ – Eugene Sh. Jan 8 at 15:18
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    \$\begingroup\$ hook a second battery in series That's not a good idea! An extra battery would be needed in parallel. If you connect in series you risk 1) damaging electronics when both batteries are charged as you get up to 28 V instead of 12 V. 2) one battery could "reverse charge" the other (empty) battery. That will damage the reverse charged battery. \$\endgroup\$ – Bimpelrekkie Jan 8 at 15:21
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    \$\begingroup\$ Would simply disconnecting the negative terminal (and bagging the cable end) cause any "theft protection" issues with your vehicle? \$\endgroup\$ – jonk Jan 8 at 16:08
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    \$\begingroup\$ Solar cell on the roof, and a cheap charge controller. \$\endgroup\$ – Brian Drummond Jan 8 at 22:37
  • \$\begingroup\$ You could also probably pull a fuse, though that might be less convenient than just disconnecting the battery. \$\endgroup\$ – jcaron Jan 9 at 10:29
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Disconnect the battery. That should give it some longer life by disconnecting any small power drain of your turned off car (radio memory, remote sensors, ECU, etc).

Other options are to Pull the battery and store it where a trickle charger can be used like motorcycle users do, or use a solar trickle charger.

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I simply disconnect the battery- a few things have to be reset, but so far the car starts fine after several months sitting so the self discharge is not too deep.

Your 4/6 weeks, when disconnected, should be no problem at all for a fully charged AGM battery in good shape.

Disconnect the negative lead only, and take care that nothing can short the battery.

IN theory two identical batteries in parallel could last twice as long but there is danger if batteries with different characteristics or charge state are connected in parallel.

You could also consider a solar battery tender.

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    \$\begingroup\$ Any particular reason for disconnecting the negative lead only? \$\endgroup\$ – Simeon R Jan 10 at 10:17
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    \$\begingroup\$ One is enough, and if the wrench shorts to chassis on the negative lead nothing happens, whereas bad things can happen if the positive lead shorts (and perhaps welds) through a wrench, and even worse if a ring is in the current path- it could get red hot in an instant, damaging the finger. \$\endgroup\$ – Spehro Pefhany Jan 10 at 12:44
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If you can't hook up a battery trickle charger, you are better off disconnecting the cable from the negative terminal. You could do parallel batteries (red to red, black to black) but they should be the same voltage and the same capacity, a new identical pair would be the safest choice. However, a battery can go dead in a couple weeks if left unattended so the pair might still be dead by the time you get back to your car.

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