 Battery Tender on a Budget |
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| Spoke |
 Jan 27 2013, 06:30 PM
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Jerry  Group: Members Posts: 7,185 Joined: 29-October 04 From: Allentown, PA Member No.: 3,031 Region Association: None 
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Over last year the 914 was off the road until I got it inspected in Nov. By that time the Optima battery seemed to take a dump. Voltage was around 10V and would barely turn the car over.
I ended up fast charging it before driving it and it seemed the battery will hold a charge for a while then drain a bit. Not wanting the battery to dump again over the winter, I needed a battery tender to keep the charge up. I figured that as long as the tender (actually trickle charger) charged the battery with more current than the clock (and other current leaks) draw, the tender should keep the battery healthy. So I settled on 50ma as a target current. To get that current, I used an old laptop ac supply with 15V output. I put a 26 ohm resistor (2 13-ohms in series) with a diode to protect against reverse voltage. I used some battery clips and a length of extension cord for distance. System works like a charm. Attached image(s) 
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