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Mixed Battery Chemistry, Common Buss?

Bus Manager

More than half our systems go out with a Bus Manager.
It is the simplest solution for all these desired functions.

Robert
 
So a buddy just purchased a small Li-Fe battery to play with that has shifted my thinking around a bit. Since I have an existing PC 925 as my "main" battery in the tail of the Rocket, I'm thinking of ways to use this tiny little 2 pound 14 ah battery as backup.

It's too small to power the engine buss for enough reserve time, but it appears to have the power to crank the engine. Why not reverse roles? The little battery gets mounted on the firewall with a short cable to run the starter, the alternator remains the primary power source for the whole ship, and I get to strip out that heavy length of #4 AWG cable running the length of the airplane.

Since the avionics all have redundant power and are generally "optional" anyway, the little battery should have an easy time as the avionics "backup".

The only question in my mind is if this battery can really crank the engine.

I have not done the math yet, but it looks like the addition of the battery and configuration change might be weight neutral to my current, single battery setup.
 
Why not?

That sounds like a good plan. Give it a shot! You might want to set up so the 925 can also start the engine just in case..

Carry on!
Mark
 
We are going to do a test with my buddy's high compression Rocket and see if the basic battery has enough. To your point however, I'm working on a scenario where the little "main" battery gets a boost from the 925 during the start sequence. One concern with this configuration is the likelyhood of the #12 AWG between the 925 and the buss getting hot with extended cranking. That said, I expect the SDS EFI to minimize the time spent on the starter.

At any rate, testing will happen soon enough.
 
Toobuilder said:
To your point however, I'm working on a scenario where the little "main" battery gets a boost from the 925 during the start sequence. One concern with this configuration is the likelyhood of the #12 AWG between the 925 and the buss getting hot with extended cranking.
At any rate, testing will happen soon enough.
Click to expand...

I share your concerns and advise not to do this. If any battery is going to be used for cranking, use the correct wire size.

As you will have a higher capacity battery (925) it will, assuming normal battery internal resistance, provide the majority of the cranking current if in parallel with a smaller battery as you describe. The 925 current will be limited by your #12 wire as it has too much resistance for the current demand - up to the point it fails.

There are several options for two battery systems that are simpler and more robust.

Carl
 
If you don't need the ballast in the back, the lightest configuration would be the lithium-iron starting battery & a pair of identical alternators. Wiring complexity should be no worse (likely simpler) than dual batteries.

If you keep the PC925 but downsize the wire, don't forget to fuse at the battery to protect the smaller wire. (Both of which mean you can't use it for starting.)
 
I do need the ballast in the back, and Id expect that the little battery would deliver the majority of the muscle to turn the starter. The 925 and its smallish feed wire would merely "help". I see this much like those emergency jump start kits for cars where a whole bunch of D cell batteries backfeeds through the cigarette lighter socket. Its not a jump start per se', but helps to fill in the valleys, so to speak. Yes, if I continue to grind on the starter the fusible link on the 925's feed wire will fail, but I hope the start sequence is fast. We will see.
 
Current photo link

I?m curious is there is a current link for the diagram listed here Dan? I?m mulling over adding a backup battery to my elelectrically dependent engine similar to the original poster.


DanH said:
Mike,
Played around a bit last night. This diagram assumes the VFR airplane is already wired conventionally, and the goal is to add a backup battery system dedicated solely to keeping an EFI/EI working if the primary electrical system goes down...due to failure, or because the pilot found it necessary to kill the master.

It is also simple, requiring little in terms of special knowledge to fly it safely. It's got a master switch; on or off has nothing to do with keeping the engine running, just like a magneto airplane. It's got one or two ignition switches, depending on single (left diagram) or dual EFI/EI controllers (right diagram). It's got a fuel pump switch, which the pilot must turn on or the engine won't start. The pump switch only allows running one pump at a time, important with EFI. A pump failure requires a switch action from the pilot, but any current EFIS/EIS takes care of problem identification. The pilot gets a low fuel pressure warning, and reaches for a pump switch, again very conventional. BTW, that too can be made automatic.

I've drawn in fusible links for the EFI/EI battery feeds, bulletproof when fabricated properly.

The two batteries are isolated, so a lithium aux battery is possible, assuming the answers to other issues are positive.

One of these might work well to feed the single EFI bus:

http://www.periheliondesign.com/powerschottkydiodesfiles/Power_Deuce_Schottky_ Manual.pdf

Click to expand...
 
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