Is a back up car battery a good idea?

Factual Questions
1

So I’ve seen those ads for those battery/cable combinations that’re supposed to jump your car if the battery dies. There’s even that small stick like thing that you can plug into your cigarette lighter. I’ve heard, though, that they’re not always reliable, so when I’ve been at costco or someplace with car batteries, I always wonder if I should just by a 2nd car battery and leave it in my trunk. (There’s a little area that’s akward to put stuff in, but would be good to store the battery.)

Will the battery maintain the charge for years, sitting in my trunk (NY winters & summers)? How many jump starts should an average battery give without being recharged between times?

PC

2

I really don’t see the advantage of carrying an extra battery around, as it will likely be drained by the time you actually need it. I’d use the space for a good set of jumper cables instead. Plus, most batteries come with an X year warranty, usually labelled on the outer case, meaning if you used the spare battery, say, 3 years later or so, you’ve lost the benefit of that many years of the warranted battery life.

I have no experience with the cigarette lighter plug in thingies, so I can’t comment on them.

As a preventative measure, you can keep your battery current, and replace it before the stated life is up. It’s been my experience that the battery usually dies close to the manufacturers life expectancy. Unfortunately, there’s no way of telling when the diodes in the alternator are gonna go bad, so you’re pretty much SOL on that.

3

Having an extra car battery in your trunk is hazardous and I wouldn’t recommend it. Aside from taking up trunk space, there is a risk you could crack the case accidentally as you put other stuff into the trunk, plus the possibility of accidentally shorting the leads if you drop something metallic onto the battery. Leaving the spare battery in your car year-round (as opposed to your house) exposes it to a broad temperature range that will gradually affect its ability to hold a charge.

This site decribes procedures for long-term battery storage. Nowhere does it suggest just letting the thing bounce around in your trunk. Also, if you leave the thing there for a few years without regular recharging, you’ll probably find it wn’t work when you need it. That would really suck, getting the spare battery out, unhooking the old one, hooking up the new one, and then finding out it’s dead, especially in the middle of winter.

Now, if you wanted to get serious, you could run wiring to your trunk and hook up a second battery. I’ve seen a military van altered in this fashion. The main leads were split and a second set of batteries stored in the cargo area in a big plastic tub were hooked up. They would charge and discharge like the van’s normal battery. The advantages of this were:

-It was virtually impossible to drain all the batteries unless you did so on purpose, meaning the van could always be started, and
-The spare batteries could be easily unhooked and used to power the sound equipment (through a DC-to-AC inverter) we periodically used for outdoor presentations.

This isn’t quite as tidy as just buying a unit at CostCo, mind you.

4

As has been pointed out, carrying an extra battery in the trunk is a bad idea for several reasons. One more reason: Car manufacturers increase MPG by reducing weight and it seems a bit silly to lower your mileage by hauling something you don’t really need in the first place.

5

The nice thing about the gel-battery-with-cables jumper batteries is that they pretty much eliminate the negatives of carrying an unused regular car battery around. They will slowly lose charge over time, but if you remember to recharge them once every month or two (they come with a charger, sometimes built in) it can work out great.

There’s quite a variety of them available, and I question whether the best ones are sold where the general public goes. I would suggest do a little research, and see if you can talk to some tool wagon vendors (e.g. Snap-On or Mac) to see what they carry. The latest one I got is about half the size of the ones most commonly seen, yet has a higher than average amperage rating.

Anything that plugs into the lighter socket is unlikely to help in many situations. They may be convenient, but they’re probably not very effective.

6

The lighter socket typically has a 10-amp limit. This is okay for a “trickle” charge when your just-barely-dead battery needs a small boost to get your car started, but it’s not suitable for a battery that is further gone. For that, you’ll need a charger with the big alligator clips that can carry much higher current.

7

Alright, so spare-in-the-trunk is out and so are the lighter socket types. What type of charger do you have Gary T?

I don’t like the idea of having to remember to charge it once a month though. I’m afraid I’ll forget (probably soon before I’ll need it).

PC

8

If you’re really worried about it you could probably retrofit your car to run dual batteries. The ambulances I drove all had them plus 115 amp alternators to help deal with the extra electrical load from lights/sirens, and AC inverter for various monitoring gear and baby isolettes.

sample wiring diagram

There is a link to the picture of the switch itself near the bottom of the page.

9

That’s not the question I expected. The charger is one of those little black boxes that plugs into the wall, with a cord and special plug that plugs into the jumper battery. It’s the type that typically comes with an answering machine or wireless phone.

The jumper battery is a Snap-On Lightning model. Its case measures about 7x8x4"–very compact. Also expensive–it cost me 270.

All the other jumper batteries I’ve seen are two or three times that big. Also all the others I’ve seen have the charger built into the unit, with a two-prong plug sticking out that takes the socket end of an extension cord. Having the charger inside prevents misplacing it, but of course makes the unit bulkier.

The Lightning has a built-in beeper that lets you know when it’s time to recharge. Any second battery you have, whether it’s a jumper battery or a second convential battery, will need to be recharged occasionally. If you hook up a second conventional battery so that it’s charged by the alternator, you don’t have to worry about charging it, but it then could be run down if the lights are left on or a battery draining short develops. To get around that, you’d have to have a switch that was turned off each time you left the car, or a diode set-up that prevented the second battery from providing power until you engaged a switch to allow that.

Ain’t no perfect solution here.

10

For a while someone (Diehard?) sold a battery which had a second, backup, battery attatched in a case that took up the same amount of space as a normal battery. The idea was if you ran your battery flat you could throw a switch and use that mini-battery to get the car running.

I think they discontinued it because the little battery couldn’t always start the car and it made the main battery weaker.

11

The bigger question is, why are you concerned about your battery going dead? If the car is used properly and well- maintained, there is little reason for the battery to go dead.

Car batteries last somewhere between three and six years, and things are at their worst when temperatures are low. A lousy battery will generally still get you by in the summer, if your car’s engine is in good tune. And then on the first cold day of winter, the car won’t start. Solution: If the car is more than two years old, have a battery dealer perform a free load-test every fall.

If you find yourself frequently leaving domelights, headlights, and other accessories on by accident, there are small devices that attach to the battery, and shut down power to the car if/when the battery gets drained too much. I’m not sure which of these devices are actually good, but I think Consumer Reports checked them out at some point.

If you’re inadvertently leaving heavy-duty items plugged into the cigarette lighter at night, consider having a mechanic rewire thelighter to only work when the car’s key is switched on. (Most import cars are, annoyingly, already wired that way at the factory.)

In my five years’ experience as a former tow truck driver, I’d say the battery is by far most likely to fail at home, in the morning, after the car’s sat in the cold all night. Second most often would be after it’s sitting all day at work. It’s highly unlikely that your battery will fail after you’ve made a ten-minute stop at the corner store.

Bearing that in mind, if you park where there’s electricity nearby, you might buy a $40-50 homeowner-grade battery charger and a long extension cord. A 10-amp model could get the car running after about fifteen minutes to more than two hours, depending on how dead the battery is.

Also, no car should be without good jumper cables. 6-gauge/12 foot at a minimum. (A lower gauge number means thicker wire.) If you can afford it, a 4-gauge/20-foot set would be ideal. The wire is thick enough to get most cars going in even the coldest weather, and the 20-foot length means you can usually connect to the helper car without pushing yours out of its parking spot.

You didn’t ask, but what if your car’s battery goes dead while driving? That generally means that your alternator, the device on the engine that recharges the battery, has failed. (Or, its associated wiring and circuitry have failed.)

If you’re driving along, and your battery light comes on (or your voltmeter is reading unusually low), this means that all that’s keeping you on the road is whatever juice is left in the battery.
You should immediately turn off any non-essential electrical accessories (heater, radio, rear defrost, et cetera), and find a safe, legal parking spot as soon as you reasonably can.

12

Or, more likely, that the belt that drives the alternator has failed (definitely preferable).

13

Yes, Ringo, I should have mentioned that as well. I was trying to keep my post as short and clear as possible (not that I succeeded!). Of course, there are a multitude of things that could cause the car’s alternator to stop charging the battery, a broken belt among them.

14

I have a backup battery thingy. It’s fairly enormous. (maybe 10"W X 15"H X 6"D) But it works great–I’ve used it to jump cars, with success. I also use it for a flashlight (built-in!), and to run our portable stereo when we go camping (it has 2 cigarette lighter plugs on the front). It holds a charge pretty much forever, and I have yet to drain it below 50 % capacity (at least judging by the status LEDs … though those could be lying). I plug it into the wall overnight to recharge once or twice a year.

I endorse them. It’s just one more thing in my road emergency kit, along with regular jumper cables, empty gas can, flares, jack, etc.

Bought it at Costco in 1999 (yet another Y2K purchase I’ve never regretted). Don’t recall the brand offhand.

15

Wow. Thanks for the comments. My battery rarely dies on me. I have jumper cables that I usually use to start other cars, but every couple years, or so, I leave the lights on, or something like that (I’ve even had a door left open so the dome light drained the battery over time). It’s usually not been horribly inconvenient, but it has been pretty annoying occaisionally and it would be nice if I didn’t have to rely on the kindness of strangers.

Recently, my battery went (almost) dead, but I’m not sure why. I’d been driving in stop & go traffic for a long time with the a/c and radio on and after parking for about 45 minutes, the battery juice was low. I was thinking that maybe the battery wasn’t being recharged enough to replace what was being used by the a/c, but I don’t think the a/c drains the battery when the motor runs, does it?

PC

16

This is a little overstated. Granted, you don’t know how much longer you can go, but I once ran for about 4 hours on battery power alone, in a Sentra with the windows up, air off, radio off, not using cruise control, etc.

You see, the tensioner pulley broke in the middle of nowhere, and the only way to get back to civilization was to buy a new battery at the nearest service station and hope it would last. But I just put it in my trunk in case I didn’t need it. Which I didn’t, until parking, no kidding, at the dealer the next day.

17

This also isn’t the question I expected. When I lived in Colorado the first time, I owned a big truck that I used, among other things, for making extra cash in the mountains in wintertime, giving jumpstarts and tows for stranded off-roaders.

I had a pretty big potential current drain, and was often out in very cold weather, so I wired it with a dual-battery system. Essentially, I set it up with a switch that allowed me to isolate the batteries so that one would run the normal truck functions (starter, radio, etc) and the other ran the high-drain items (lightbar, winch, etc)–while each charged from a single high-output alternator. But in a pinch, I could join the batteries to achieve up to 1700 CCA of juice.

I don’t think a setup like this would be needed except in the extreme circumstances i’ve described–heck, it wasn’t even necessary then. But it was mighty effective, if expensive and a bit of a pain to set up. If you do try something like this, I gather there are now kits to make the wiring and switching easier. They also provide startup tips, such as making sure both batteries have the same age and amperage rating.

18

Your first thought is probably right. With the engine running, one expects the alternator to supply power for electrical consumers and top up the battery’s charge. The more electrical consumers engaged (lights, wipers, blower for A/C or heater, etc.), the less available to charge the battery. But a good alternator should still be up to the task. I suspect your alternator may be weak.