Fun with Car Batteries

So after I parked my car in the lowest level (as in deep underground) in one of MIT's parking garages this morning I turned on the "dome" light while I search around in one of my bags for something. Unfortunately I forgot to turn it off when I was done and I blissfully headed off to the office.

I had an important errand that I needed to run this afternoon, THAT I COULD NOT BE LATE FOR. I needed to be there at 5:20pm and I knew that it was about a 30 minute drive. So I headed out of the office around 4:30, giving myself plenty of time. Of course, when I get to the garage, you can guess this part, I found a car with little juice.

Fortunately this garage is near the subway, so off I ran to the subway (not really understanding why the car's battery was dead). I got to my appointment just in the nick of time.

But now (as Paul Harvey would say) comes the rest of the story.

Red BoxA number of years ago I bought this "battery box" for about $90 at Home Depot. It has a built in battery, jumper cables, an air pump, a 12 Volt outlet and a 400 Watt inverted to supply 110 VAC. I have only used it as a 12 Volt power source and occasionally I have used the inverter to power my computers when the power is out (such a nerd!). It isn't very heavy, but neither is it particularly lightweight either.

When I get home I realize that the problem is likely that I forgot to turn the dome light off. So after dinner my wife drops me off at the local subway stop with my battery box. Off to MIT and the underground. Hook up the cables and bingo! The car starts and I drive home. As I am writing this I have the battery box recharging inside and I have a 10 AMP battery charger hooked up to the car. I think I'll take the battery box with me tomorrow for good measure. And I probably also need a new car battery. I don't believe a day with the dome lights on should deplete the battery to the point of not starting the car!


From: Bob Kerns

I've killed batteries by taking a nap with my foot on the break. That's connected somehow with being a parent; it's either napping with a kid, or napping while waiting for a kid.

I've long been puzzled why cars let accessories drain the battery that far. We seem to take it as a given that car electrical controls are really really stupid (as in back when we were young), while everything else about the car is high-tech and computer controlled.

The batteries in your laptop have FAR more smarts about protecting themselves RIGHT IN THE BATTERY, than is in the entire car. And your typical high-end motherboard has all kinds of bells and wistles to manage power usage.

I drive a Prius. The traction battery? It is NiMH, carefully managed to maintain it within a certain range of charge, to maintain a 10-year lifetime. And it did last just over 10 years.

But the battery that powers the accessories -- and ALSO the electronics tha are required to turn on the car (the actual starting energy coming from the traction battery)?

Regular stupid Pb/H2S04 battery with mechanical switches everywhere, perfectly happy to run down, destroy itself, and leave you without enough juice to trip the friggin relay to tap all that energy in the traction battery. Which would then both start the engine AND charge the poor accessory battery.

I hope the newer Prius's are smarter about that. But I thought it was silly back in 2000.

From: Jeff Schiller

We have a 2000 Insight. Same batteries (but fewer of them). Pack lasted 9 years. And like the Prius it also has a relgular (motorcycle sized) 12 Volt car battery for the accessories and other electronics. Indeed the "big" battery starts the car (though if the big battery pack is dead, there is a starter motor on the 12 Volt system). Unlike the Prius, the Insight (at least the old 2000) can run without the electric assist working. Btw. It is also a stick shift car. Cannot get cooler then that!

From: Louis Mamakos

I had this weird problem with a loose negative battery cable on my battery.  I thought initially I had a soft battery, and happened to be visiting my sister and bother-in-law at their garage.  How convienient!  He just jump-started me, and I was on my way, figuring the 2 hour drive home would charge the battery..  

But as I begin to drive, the battery clamp apparently started to loosen and increased the voltage drop across the connection to the battery.  It got to the point where when I stepped on the brakes, the brake lamps drew enough current that the voltage drop across the loose terminal was enough to cause the dashboard to "reboot" due to the undervoltage condition.

This was very disconcerting.

Then the ignition would start to miss, and I knew I had a problem.  So now the goal was to get somewhere with minimum use of the brakes to diagnose WTF was going on.  My car (a 1996 BWM Z3) had a manual transmission, so that helped quite a bit. 

I got it back to my brother-in-law's garage and he diagnosed this pretty quickly by measuring the drop between the battery terminal and battery clamp.

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