Secondary battery for the gadgets?

[Szabi]

Hello, I am thinking of connecting my small gadgets (Bttf speedometer, ThinkGeek flux capacitor, GPS, etc.) to a secondary battery, instead of using the main battery and the car`s cigar lighter adapter. I would mostly use it when I am on a veteran car meeting or a comic-con, so the car is just parking for several hours and the engine is switched off.

Can you recommend a battery that would fit in the glove box (or in the compartment behind the driver`s seat)? What would be the min/max technical requirements? I had some universal 6V batteries (4 Ah) at home, connected 2 of that and did some initial testing: it worked well, but after a few hours it killed the 1 Amp fuse of my Garmin GPS. I guess I`d need something more sophisticated here.

[DMCMW Dave]

Avoid using a second full-size battery in this application without a proper cutover box (like in Motor Homes) of some sort. If you have two batteries in parallel, disconnect one and run it down, then start the car and hook the "dead" one up to charge it, you can damage the alternator or the batteries due to the huge current flows involved between the two batteries. You are probably OK with a tiny battery as mentioned.

[jangell]

This is another one of those things I need to do eventually, once I get to the point of addling all my extra electronics. I haven't done it yet, though. I even considered a second alternator to keep everything filled, but I'm not sure I'll need to go that far.

Based on Dave's comment, I googled around and found battery isolators. You should be able to find a lot of information about them and about adding second batteries by googling for information about adding a second battery for audio systems. They do tend to assume you want to add a second full-sized car battery, but you can use a smaller 12v lead acid battery and charge it with the alternator through an isolator just like a car battery. I found a quick intro video on isolators as well, which mentions diode ones (slight voltage drop) vs. relay-style ones (moving parts but cheaper): https://www.youtube.com/watch?v=CGD8...A&spfreload=10 Looks like they can be had for <$50 in some cases.

For my additions, I was thinking of using one or more scooter batteries, hooking them to one side of an isolator (probably diode-style), while the the other side had the starter battery. Scooter batteries are smaller than full-sized car batteries, so I'd have more mounting options, but they're probably far too large for your purposes. I used three 32 Ah scooter-style batteries to run some lights in my rental garage until the landlord upgraded the 100 year old wiring for me.

Another option might be something like this 5 Ah 12v sealed lead acid battery . Being 12v lead acid, you could hook it to your electrical system through a battery isolator, or just leave it in your glove box and hook it up when you need it, possibly with a switch to toggle your electronics from primary power to this battery. If you don't hook it up to your alternator, you can just take it out of the car and recharge it with whatever charger you may already use for your car battery at home. It's about 4"x4"x3", so it'll be small enough that it won't be in the way. A 10 Ah 12v sealed lead acid battery is a bit larger at about 6"x4"x3", but should still comfortably fit in your glove box.

One thing I learned is that you shouldn't let lead acid batteries drop below 10v if you want to be able to recharge them again. For my garage power I tried to keep them at at least 11v, and bought a panel-mount digital readout so I could track it throughout the day (I didn't actually mount it; I just rested it on top of the batteries).

I'm confused about why the Garmin's fuse would blow. That implies a surge from somewhere, but I don't quite know where a that would have come from.

-- Joe

[Bitsyncmaster]

Not sure your glove box could hold the weight of a lead acid battery.

The Garmin may have a switching regulator. That type draws more current when the voltage is reduced.

You need to add up your loads to size your battery for how many hours you want to run.

[Lou and "Boo"]

I've seen twelve volt batteries the size of motorcycle batteries on the Fiero forum fiero.nl

[Flicky]

A UPS battery like a Yuasa NPW36-12 would be a good choice.

Might be even safer to use a gel motorcycle battery from TMS or BikeMaster.

They are both about 7lbs and should be small enough to go in the glove box, but all the bouncing around a car does might stress something. I'd consider giving it a more supportive place to sit.

[Exolis]

So first, you'll need to know the total amount of loads and the time you plan on letting the electronics be turned on. With that info you can figure out what capacity battery you'll need.

For example, if you use a 5 AH battery, with 3 Amp load for 4 hours, your battery will be dead before 4 hours. The max you can do on a 5 AH battery for 4 hours is 1.25 Amps. To continue on the example, if you are using 3 amps for 4 hours, you'll need 12 AH.

Depending on what your loads are, what kind of battery you have in your vehicle and for how long, you might be able to get by just staying with using your vehicle battery. For example, if you have a Group 35 Battery that is rated for 63 Ah and you want to run it for 4 hours and not reach past 50% SOC, you could run loads of 7.8 Amps for 4 hours, and still have plenty charge left to start the vehicle and wouldn't damage the battery or alternator. Something to consider if you don't want to go through the hassle of buying another battery and running wires all over the place.

[Szabi]

Thanks for all advices, at the end I decided to go with two 6V 7.5Ah batteries in the glove box, I also added a car battery protector that switches off the gadgets if the Voltage drops. Total cost was cca. 25 $. Maybe I`ll get some backup batteries, it is cheap and easy to replace. I`ll have to think it over how to hide wires and socket distributor, but otherwise it is working well.

[Farrar]

Previous owner of my car used two UPS batteries for car shows - would disconnect car battery and use the UPS batteries to power courtesy lights.Just mentioning this for what it's worth.

Glad you found a solution.

[Timebender]

I'd put a small 12volt, 7ah battery or two (for 14ah) in the locking cubby behind the driver seat, and as Farrar mentioned, a switch or just manually disconnect the main battery and then hook those up for shows. Or, you could put them on a separate circuit for all your accessories, with a small inverter connected to a charger, so when the car is on, it will recharge the batteries in there.