Battery load rating for an IO-540?

stringfellow · #1 11-03-2016, 03:25 PM

Looking at buying two EarthX batteries for my EFII setup, and trying to decide between the 680 and 900.

The 900 is recommended by EarthX for engines 540 cubic inches or less. Is this valid? Really the only downside is the extra $140 for the 2, and the weight gain is only 2 lbs.

Anyone running these on their big 6 cylinders?

Mjuckes · #2 11-03-2016, 05:37 PM

I bought the 900 at OSH this year and installed it in my Rocket. 3 months and Multiple starts. Works way better than the Odessy it replaced. So far so good.

Carl Froehlich · #3 11-03-2016, 06:34 PM

Engine cranking amps is only one consideration for required battery capacity. Decided for your plane, panel and mission, your battery reserve for a loss of alternator. In other words, how many minutes of battery only IFR flight do you want to design into your plane?

Carl

stringfellow · #4 11-04-2016, 09:36 PM

Quote:

Originally Posted by Carl Froehlich

Engine cranking amps is only one consideration for required battery capacity. Decided for your plane, panel and mission, your battery reserve for a loss of alternator. In other words, how many minutes of battery only IFR flight do you want to design into your plane?

Carl

That's a very good point. Thanks. I suppose erring on the upper end, all things dimensional being equal, is the way to go. It's less than $200 extra in total.

vic syracuse · #5 11-05-2016, 05:11 AM

I recommend and have installed the 900 and the 1200 in the RV-10's. I am currently using the 1200 and am amazed at the reserve capacity. And jsut the other day I got to see the difference because of a distraction and silly mistake.

I landed for lunch and then taxiid over for fuel. When I restarted for takeoff I either forgot to turn on the alternator or I bunped the switch when turning around to get some stuff in the cockpit. That had happened a couple of times prior when I had the PC925 battery installed, and I always got a volts warning and Alternator light just about the time I was ready to pull onto the runway for departure.

This time, I saw a flicker on the alternator light about 20 minutes into the flight! The battery voltage was still around 12.8 volts. That's with a full up load of all avionics and external lights. Of course it charged at 60+ amps for the next 30 minutes, but I was pretty impressed nonetheless.

I have now configured the amp gauge for a low output alarm as well.

Vic

Mich48041 · #6 11-05-2016, 07:47 AM

The discharge curves for Lithium batteries are different from Lead acid batteries. Lead acid batteries will slowly drop in voltage as they are discharged. Lithium batteries, on the other hand, will maintain their voltage as they are discharged until almost depleted, then the voltage will quickly drop. So the pilot can not predict how much charge is remaining by watching the Lithium battery voltage.
A low voltage alarm set at 13.5 volts will warn if the alternator is not charging.

Carl Froehlich · #7 11-05-2016, 08:40 AM

Quote:

Originally Posted by stringfellow

That's a very good point. Thanks. I suppose erring on the upper end, all things dimensional being equal, is the way to go. It's less than $200 extra in total.

I've used a redundant power distribution design on three RV-8s and one RV-10, all based on having two PC-625 batteries (17ah each). I prefer the 625 over the 680 as I find the form factor easier to work with, each are a pound lighter and they have a little more AH capacity and CCAs. Two fit well in the standard battery spot on an RV-10.

The original design was to have adequate battery reserve and to have multiple options to get power to the panel if a component failed (alternator, master switch, etc.). Before the RV-10, this was at least two hours of IFR flight. On the RV-10 however, the full boat dual screen EFIS panel translated to about 90 min of full panel ops on just battery power. This was below my design target and exceeded my risk tolerance. Remember that a battery's AH rating is very dependent on current draw. The advertised rating is at a very low current. For more typical needs the total AH capacity will be much less.

The options to address this were to add more battery capacity (and weight) or to add a standby alternator. As the 625s have more than adequate cranking capacity for an IO-540, I opted for the B&C 30 amp standby alternator. I have it feed the left and right vital buses (avionics buses) via isolation diodes so that whatever the fault was that took out the primary alternator does not take out the standby alternator as well.

Carl