Idea for a straight forward backup battery option

[stioc]

I don't have a fridge, winch etc so I don't need the typical starting/house battery setup. Instead I'm looking for a basic battery backup in case the alternator quits - where a small, lightweight backup battery will give me a few extra miles. I already have a cigarette lighter plug in the trunk of my SUV, there's also a cubby in the side panel which should hold a small AGM or a motorcycle battery. Can I keep the battery connected to the cigarette plug permanently (the plug's only on with the ignition) and then when I need to use the backup battery I could remove it and use use the jumper cables. I guess the question I have is with a small battery it may get charged sooner than the main battery but the alternator would keep providing the 14.5volts until the main battery is fully charged thus overcharging the smaller backup batter?

I already have an AGM battery from a jumpstart power-pack but don't have the room or need for the 40lbs jumpstarter box with its inverter/compressor/light/radio etc.

Does this sound doable or any other simpler suggestions?

 

[wrcsixeight]

If your Current battery is not spanking new, and IF you could fit a larger battery in its place, that would be an easy solution to having more capacity to drive further if the charging system decided to quit.

You can certainly do a Ciggy plug to an small AGM battery you secure properly somewhere else.

Generally doing this, the stock power port on the vehicle is too thin to pass very many amps. The voltage drop incurred by thin wiring and multiple connections would prevent overcharge. If the Battery you chose is very discharged, then it can easily blow the fuse when you first plug it in.

I'd recommend wiring a new ciggy plug receptacle dedicated for the interior/aux battery using 10awg wired to the engine battery and fused there at 35 amps. You can just pull the fuse to disconnect it electrically.

If your only goal is to have a small interior battery to supplement the engine battery, then You can hardwire the interior battery to the engine battery and then it does not need to be hooked to the engine battery with clamps on short thick wires. For jumping a dead battery, you need short thick wires with good clamps.

 

[theksmith]

for simple and small - get this. every few months, plug it in to the cig lighter for a couple days to top it off.

http://www.amazon.com/Jump-N-Carry-JNC300XL-Ultraportable-12-Volt-Starter/dp/B000XQ9MGE

you don't want to leave it plugged in all the time, or whatever ends up draining your main battery might drain this one too, leaving you SOL.

and as the previous post mentions, you don't want to start the vehicle through a cigarette lighter... so this enables you to have a small lightweight package that you can still remove and hook up for a jump start.

i've cranked a v8 with it before, but not in the cold... i'm sure it'll crank over a 6 cyl in the cold.

 

[dwh]

One issue is that if the main battery is drained, and you turn on the ignition to start the truck, then the starter is going to pull as many amps as it can from the aux battery - and could very well toast the wiring to that cig plug in the back.

So you need a way to prevent backflow from the aux battery. In other words - a diode. You could probably 'shadetree engineer' something like this to work:

http://www.amazon.com/Wagan-EL9796-...r=1-1&keywords=lighter+to+lighter+jumpstarter


Or, you could rig an auto resetting breaker to protect the wiring. If the starter tried to draw too much, the breaker would trip. A Bussman Type 1 (auto reset) in 10a size would probably protect the wiring and I doubt the aux will draw more than 10a just sitting there being topped off when you drive:

http://www.wiringproducts.com/contents/en-us/d60_dc_circuit_breakers.html



Another thing to consider: A small battery will probably run the truck for a few hours - during the day. At night, with the lights on...prolly no so much.

 

[Haf-E]

You actually need to fuse or breaker both ends of the wire connecting the two batteries together - and a diode is a good idea also to prevent overcharging the AGM battery - most don't like to be kept at 14+ volts for very long (although some can tolerate it...) The diode would drop about 0.7 volts from the alternator. I've set up breakers "around" or "across" the diode to allow it to be removed from the circuit when the breaker is closed - allowing the aux battery to charge the engine battery when closed.