Miata Charging

The Miata has not been used a whole lot since we’ve moved to the island. I think the last time that I put gas in it was Mind Camp back in April — since then I’ve driven it 5 times, tops. Before that it wasn’t used a whole lot either, so it’s had a long time to sit and not a lot of alternator charging of the battery.

So of course, when I wanted to drive it it was dead. Jump started it, drove to Freeland and back, the long way, probably an hour. Two weeks later, dead again. So it was time for a new approach. I wanted to do a trickle charge, something that I could leave on and keep the battery topped up continously.

There are two problems with that option.

  1. I’m cheap, and I have a power supply that could do the charging, but isn’t exactly trickling.
  2. Attaching leads to the battery is a pain because it’s in the trunk and under the fender. The next best solution: the cigarette lighter.

Leads on the battery are a non-starter for this car, and there’s really no other good place to tie in. The cigarette lighter is the perfect option, but for one catch. The key needs to be at the accessory position for the socket to be active. Thankfully, that’s not too much of a problem in this neck of the woods, but not something that’s really a good idea for long term charging. I cannibalized a cord from a Rat Shack lighter extension, wired to the terminals of the power supply, and did some measuring.

This power supply is a 15V nominal, quite heavy, and was obtained from someone who just wanted it out of their garage. It powered my subwoofer crossover for a while, then got back to taking up space in a garage. It’s not fused, but the lighter cord and the lighter socket are, so I was pretty sure that nothing would get too fried.

The power supply turned out to be a bit high powered for a trickle charge, as it was kicking out a little more than one amp into 11 volts on a pretty much dead battery. That went to about .6A into 13.5 volts at the end. I’m guessing that there was an ohm or two of internal resistance in the system, and the rest of the voltage drop was the internal voltage of the car battery. The Miata has an interesting battery, it’s small, long life, and expensive. It’s a fiberglass mat lead acid cell, without the normal free flowing acid inside.

Since this battery is about 35 amp hours, the initial charge rate works out to about a .03C charge. (C is a measure of battery charge/discharge rate, and is the equivalent of rated capacity in one hour.) That rate is high for a trickle but perfectly reasonable for a slow charge. A fast charge can go up to .2C or so for a lead acid battery, but isn’t terribly good for it. (see http://batteryuniversity.com) So this isn’t a set and forget thing, but it’s something that I can run for a couple of days and end up with a charged battery.

After two days of charging, the car starts up very well and even seems to run a bit better. I’m wondering if the alternator hasn’t been working well over the past year or so, but I’m not inclined to put too much effort into that diagnosis until I see how long the charge lasts. At 150 miles in the last 4 months, it’s not like I’m likely to need it.


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