Keeping my battery charged through driving alone

Question

I have a 1991 Mazda Miata MX5 as a second car.

My commute is 53 miles, but it can take about 2 hours in the morning, although it is 55 minutes in the evening.

The battery is less than 6 months old. Given that this is a second car, how often do I have to make the commute to keep the battery charged through driving alone?

If you need any more info, please ask.

---

[Update] The car is parked on the street when not in use.

Btw, please note that I do not own a charger and am asking about keeping the battery charged through driving alone

---

[Upperdate] Some nice answers there, all amounting to the length of a piece of string. While I realize that it is somewhat subjective, I would be happy with a definitive, albeit qualified, answer once very X weeks. I currently drive it once per week, but would be happy to do so less frequently.

---

[Yet another update] There are cheap and simplistic devices, like this one, which can read vattery charge from the cigarette lighter. If I get one, how much charge do I need before knowing that I need to drive to charge it - or shoudl that be a separate question?

---

[Final update] I bought such a device. I am currently giving the car 2 @ 50 mile trips once every three weeks. She always starts first time, no problem, and the device always read about 13(!) volts. I might stretch to once every four weeks and see how that goes

Answer 1

It is impossible to tell for the following reasons:

• A battery degrades over time, it will loose capacity. As a rule of thumb you need to replace a battery after 5-7 years, even when charging it every day. A battery needs three things regularly:
  • Charge: Provided by your car while running.
  • Water: Non maintenance free batteries need a regularly top-of with distilled water. Maintenance-free batteries will also need water, but significantly less. It should be possible to extend the lifetime of your battery significantly by topping it of.
  • Movement: The liquid in your battery suffers from an effect called "acid stratification". The acid will flow to the bottom of the battery and the water will accumulate on the top. Moving your car around provides enough shaking to mix those two layers.
• Your car consumes battery charge when not in use. It will consume much more if there is leakage current, caused by conductive dirt accumulated somewhere in your car.

So nobody can give you an exact answer to your question. I would advise you to drive this car on every second journey. Should you decide to "maintain" your battery (charging, topping of) you could extend the lifetime significantly. Should you decide against maintaining your battery you will need to replace the battery more frequently.

Answer 2

Instead of a float charger, get what's called a battery maintainer. A maintainer will monitor the condition of the battery and charge it when it needs charged. It will help reduce what is called sulfation of the lead plates, which reduces the efficiency of the battery. Sulfation mostly occurs when the battery power levels get below peak. The further below peak, the more sulfation occurs. Some of the sulfation goes away upon charging, but not all of it.

Since you stated in your edit the car sits on the curb most of the time, another avenue you could try is a small solar panel recharger. You plug it into your cigarette lighter and leave it on the dash of the car. Here's a representative model:

The other option is to run the vehicle every so often. There's been a lot of posts on Mech.SE about how often you should run your car. There are many different reasons other than to maintain the battery. You need to think about fuel decay as if you are in an area which has ethanol, your fuel will absorb water, which could cause you issues. The easiest way to combat this is to keep a full tank of gas in the car, and then put a product like StaBil into the tank, which in the right quantities can keep it "fresh" for up to two years. Next you need to think about seals. No, not the kind which swim in the ocean, but the pesky rubber kind which keeps fluids in your vehicle at bay. If you aren't running your car around to fully heat everything up (engine, transmission, A/C, etc.), you run the risk of deterioration of the vehicle itself. This seems counterintuitive, but it happens. Driving the vehicle at least once a month would put all of these fears to rest ... driving it once every other week would be an even better alternative. Plus it would maintain your battery without issue.