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I want to add/charge a lead-acid battery UB1250 with the 12v system in my car. I put a battery quick disconnect directly to the battery so that long term, the main battery does not die. Problem is when you disconnect the battery all the settings on the radio get wiped out. So I want to add this battery to the radio memory wire so that it will keep the settings. Question is, do I need anything thing else like a resistor. I plan to put a diode so the battery does not back feed the entire system.

Do I need anything else?

enter image description here

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    \$\begingroup\$ "Universal Battery" is the brand and is irrelevant to your question - particularly the title. Change it to "lead-acid battery". \$\endgroup\$ – Transistor Nov 1 '17 at 19:25
  • \$\begingroup\$ The problem is the title. It's obvious what you're talking about once you get into the post - although the edit is an improvement. \$\endgroup\$ – Transistor Nov 1 '17 at 23:53
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It's complicated.

If you connect just a diode to this UB battery, and the radio to the UB battery, the UB battery would indeed keep the radio memory when you disconnected the main car battery.

But there's a problem here: when the main car battery is connected, and the engine is running and the alternator is supplying energy to car's system, and if the alternator has a voltage 0.6V (this if for the diode voltage drop) higher than the battery maximum, the alternator would keep feeding energy to the UB battery, and maybe wouldn't be safe keep feeding UB battery after it's full charged (especially this chinese battery on the photo; I wouldn't rely that it would deny the extra charging). If this battery explodes, remember that your car has a reservatory full of petrol...

Automotive batteries are designed to keep high voltage, in a few minutes they get to the alternator voltage and so they stop beeing charged. A few minutes ago I answered another question saying exactly that automotive and deep-cycle batteries are so different.

If you use a passive componente to limit the UB battery charging, doing the "trickle charging", this wouldn't help, too: if the engine is on, and the radio is on, the radio would "eat" more energy and the UB battery would be empty very soon.

I think that would be safer if, besides the diode to keep the UB battery to feed back the entire system, there were some control circuitry to watch the UB battery voltage and stop the charging if it is full charged.

EDIT: Maybe would be simpler and safer if you keep two direct wires (can be thin wires) from the automotive battery to the radio memory circuit. Put a fuse in the positive wire, and, after connecting those wires, disconnect the radio from the vehicle main power and from the negative chassis (from now on the radio will be powered for those wires).

Doing this way, you don't need to add this new battery to your vehicle, but keeps the radio memory even when the battery is not connected. It could work, unless the radio's memory drains so much battery (I already heard about hungry car's stereos even in standby mode).

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    \$\begingroup\$ "... keep a direct wire (can be a thin wire) from the automotive battery positive pole ...". Best safety practice is to disconnect the earth strap first. That way a slip with a spanner to chassis when undoing the positive lead doesn't lead to a flash. \$\endgroup\$ – Transistor Nov 1 '17 at 20:52
  • \$\begingroup\$ @Transistor Sure! :-) I was talking about WHAT could be done: use a thin wire to keep the radio memory. Disconnecting the earth is about HOW to make this connection, or whatever connection the OP does. There are others "HOWs", like: 1) the wire should be very well isolated; 2) the wire should pass away from hot parts; 3) the connectors used should be well tightened; 4) there should be a fuse in this wire... Thinking about it again, putting a fuse in this wire is even more important than disconnect the earth while doing the connection, isn't? \$\endgroup\$ – mguima Nov 1 '17 at 21:48
  • \$\begingroup\$ My point is that your wire from the radio to the battery positive won't save him when the earth strap is disconnected. \$\endgroup\$ – Transistor Nov 1 '17 at 21:59
  • \$\begingroup\$ @transistor You're right! I edited the post. \$\endgroup\$ – mguima Nov 1 '17 at 22:03
  • \$\begingroup\$ The diode is the obvious solution. Just top off the battery once in a while to the full 14.4 V and you should be good to go. Modifying your radio with a lithium coin cell battery or super capacitor comes to mind too. \$\endgroup\$ – winny Nov 2 '17 at 6:35

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