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EFIS backup battery charging

Lars

Well Known Member
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I have a Grand Rapids backup battery to supply my GRT Sport SX in case something goes awry. Being condition inspection time, I ran a test on it, powering the EFIS only via the backup battery (as I always do pre-start to avoid a voltage sag and reboot) while monitoring the voltage. It took less than 5 minutes to drop below about 8 volts, despite having been apparently fully charged the day before (full charge being indicated as a value of less than 40% on my EIS, which monitors the battery condition).

OK, new battery time. I ordered a replacement battery from GRT. The existing battery was 5 years and 500 flight hours old. It wasn't a surprise. I'd noticed that simply allowing the backup battery to run the EFIS for more than a minute or two meant that the charging circuit would be in high gear for at least an hour before a charge state of less than 40%, which per GRT documentation implied fully charged, was reached.

When I ordered the replacement lead-acid battery I mentioned to the GRT person (name forgotten, my bad, I remember numbers much better, a weak excuse) that most of my flying is less than 20 minutes at a time. Despite flying nearly 7 days a week, that is apparently not enough to top off the thing after depletion from my pre-start regimen. Given that the point of a backup battery is having a full charge available when you need it, that doesn't seem acceptable to me, and I started thinking about a way to keep said battery topped off.

I have a cigarette lighter (AKA accessory port) in my panel, wired to the always-hot (battery) buss, that among other things allows me to charge the ship's battery by simply plugging the charger into said port. Seems to me that if I install a SPDT switch on the supply side of the backup battery so that in "normal" mode it draws power from the main buss, but in "switched on mode" it draws power from the battery buss, I could charge the backup battery in parallel with my main battery, thus ensuring that my backup battery is truly ready to back things up. Obviously the switch would be turned off any time that the battery wasn't being charged, and yes there is the risk that I could forget, just as one (BTDT) can forget to turn off a master despite all sorts of warning lights and sirens and humiliating laughter from the peanut gallery down the hangar row.

Does this make sense? Feedback desired, thanks!
 
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