Mixed Battery Chemistry, Common Buss?

[Randy]

I've been going though many electrical schemes also as I am about to convert back to an electrically dependant airplane, after having same for several years of Subaru experimentation. This time it will be with my IO-375 and all of Ross's latest offerings.This time I decided I want to have the confidence to fly right past good landing spots to a place of my choice where I will repair or replace my main alternator when it fails. To get to this comfort level I will use a B&C backup alternator running on the vacuum pad. I like that it will be waiting to see the voltage drop down and automatically come in as needed. This feature is dependant on whether the busses are kept separate. If kept separate, I can adjust the regulator on the backup alternator to a charging voltage the backup battery likes, but keeping the busses separate requires the second battery.

I echo Dan's thoughts in that the system needs to be simple enough that I could send an experienced pilot up in my plane with a simple briefing of the system. He should not have to study for a long time to understand how to react in various failure situations.

Dan, your dual EFI scheme is very close to what I have been coming up with. I keep comparing this simple approach to the use of the Bus Manager that provides the automatic switchover to backup fuel pump, and an emergency power switch that bypasses the "box" and connects both batteries to the ebus.

I do not understand the comment about not running both fuel pumps on an EFI system. Some people would routinely turn on the backup fuel pump when taking off or landing, much like turning one on as a backup to a mechanical pump. As far as I know, the fuel pressure regulator adjusts accordingly. The fuel rail pressure might come up a little and cause a slightly richer mixture but with no associated problems that I am aware of.

I want a system that uses the tried and true AGM PC680 as primary battery with main and backup alternators. Adding a second AGM is a weight penalty I do not want to pay, but adding a 4 pound Lith Iron batt is appealing as it allows a scheme (Bus Manager) that keeps the busses separate except in two instances: If you choose to use both batteries during start operation, and if you need to engage the emergency power switch to keep her running.

Robert says it is acceptable to have the mixed battery chemistries in this situation but i am looking for more reassurance.

I am surprised no one has jumped in here with experience with use of both battery types in a single system. Seems like a good mix of reliability and light weight if it works properly.

For those considering the EarthX, take a look on their web site at the discharge charts. It clearly shows how the voltage stays relatively flat until a sharp drop off where the BMS will kick in and shut it down. This feature rules it out as primary use in my opinion, and in my situation, at least until I get some real world experience with one.

[Marc Bourget]

I have to go back to my notes to refresh my memory but I do recall that there is a problem in this subject area - dealing with mixing batteries - due to a phenomena called "circular currents."

Just on a break from re-organizing the shop, with that being said, I ask assistance from those with more knowledge-experience and smarts! :^)

mjb

[DanH]

Originally Posted by Randy

Dan, your dual EFI scheme is very close to what I have been coming up with. I keep comparing this simple approach to the use of the Bus Manager that provides the automatic switchover to backup fuel pump, and an emergency power switch that bypasses the "box" and connects both batteries to the ebus.

Automating the backup fuel pump engagement is easy. I drew manual switch control for both primary and backup pumps. In reality, your ECU may control the primary pump through a relay; you would switch it only as a test function.

Bus Manager power circuits:



The backup power switch is necessary because of the other architecture; a failure of the primary switch, power relays, or any of the control wiring shuts down the buses. The backup switch is merely a relay bypass.

The BM combines the ship's power requirements with the engine management power requirement. The wiring offered in post 39 puts engine management power on isolated, battery-direct circuits, not shared with any other power requirement. Ships power can be single bus, main and essential, whatever you want. Any or all of the ships power can be shut down, and the big fan keeps turning. For the pilot, it's like flying a Cherokee.

I do not understand the comment about not running both fuel pumps on an EFI system. Some people would routinely turn on the backup fuel pump when taking off or landing, much like turning one on as a backup to a mechanical pump. As far as I know, the fuel pressure regulator adjusts accordingly. The fuel rail pressure might come up a little and cause a slightly richer mixture but with no associated problems that I am aware of.

Rotary vane pumps require flow for cooling. A standard backup pump for constant flow injection has a built-in bypass and pressure regulator, so fuel always circulates through the pump, even if none flows downstream. For example, you can run the pump with the mixture knob in idle cutoff.

The EFI pumps have no ability to recirculate. They're just in and out; pressure regulation is at the far end of the fuel rail. If you plumb two in parallel and one has a higher output pressure than the other, the weak sister will have limited flow, and it will get hot. Or so I'm told; I have not personally conducted that experiment. I do note that engine manufacturers with standard EFI are pretty clear about it. This example is from the ULPower installation manual:

Important !
Provision should be made for a separate switch on the dashboard to interrupt power supply to the second (backup) pump. Experience has shown that leaving both pumps running simultaneously can cause one of the pumps to overheat due to insufficient fuel flow through the pump. This can result in a pump failure, eliminating backup if/when it is needed. Install an “or-or” switch to be sure that both pumps never can run together. Before take-off , make a test to be sure both pumps are working.

[rv7charlie]

That note from Protec[edit: should be UL Power; I thought Dan was referring to the same company as the drawing] says to introduce a single point of failure in the 'redundant' fuel delivery system. Pump switching might be better left to the pilot to manage, unless you're willing to accept the complexity of a pressure sensing auto-on circuit for the backup pump.

BTW, for those who don't know, just about everything being discussed here has previously been hashed out on the Aeroelectric list under the guidance of Bob Nuckolls (one of the brains behind most of B&C's products). You can buy the book, or download the PDF version for free. It's a really valuable source for anyone wiring a homebuilt, whether it's 'conventional' or any variation of electrically dependent. There are diagrams for just about anything we can dream up.

Charlie

[DanH]

Originally Posted by Toobuilder

So have we established that Pb chemistry is the optimum choice for this application?

In my feeble mind it seems like the exotics are great for a high discharge like cranking a starter or minimal discharge like Ross' CPI, but a medium draw like I need is a whole different ballgame?

Mike, maybe the best bet (as usual in our game) is to measure it yourself.

Buy an EarthX ETX680 (12.4 ah rating).

Discharge it with some reasonable load until the internal cell-saver circuitry shuts it down.

Recharge it at the bus voltage you expect to supply when installed in the airplane. I would actually do it in the airplane; strap it in the back seat, run a power and ground, complete with a schottky diode for isolation, as it slightly reduces charging voltage.

Now put it on the bench, rig a 12 amp load, and start a timer.

If you like the results, do a permanent installation. If you don't, the ETX680 will easily resell here. Plenty of builders want one as a conventionally-installed primary battery, and the cell-saver circuits mean you can't hurt it with this test.

[Toobuilder]

Yep, testing is the way to cut through the BS. I like testing. Thanks for being the voice of reason (again).

However:

The EarthX is 4 pounds and $400 bucks

The Odessy is 7 pounds and $100 bucks.

The weight savings are compelling, but at a cost of $100 bucks per pound... I'm going to have to think on that one.

[Randy]

Dan,

Thanks for that information regarding running two electric fuel pumps at the same time. I and I suspect many others was not aware of the potential for the limited flow causing overheating of the weaker pump. I will plan on a switch that only allows one or the other to run. Saves a switch position too!
Randall

[rv7charlie]

If we're talking about the newest turbine style pumps, then the issue of no flow from the weaker pump could be an issue. However, I'm not aware of anyone selling a turbine style pump for an aviation application (turbine style pumps must have 'head' on the input; they won't lift to self-prime). All the models I've seen are using either gerotor or roller-vane pumps. Both styles are positive displacement pumps (a bit like an engine oil pump). If there's fuel at the input, I'm pretty sure there's going to be fuel coming out, unless there's a very strong cap on the output. :-)

RE: switches, see my post #44.

Charlie

[rv6ejguy]

We've seen no issues running two Walbro or Bosch pumps simultaneously. Not sure what UL is doing or what type of pumps they use. That being said, there is no reason to run both pumps in cruise.

[rcpaisley]

The isolation of the Bus Manager allows the mixed battery voltages to work fine together. We have many customers running one Odyssey and one EarthX.
The batteries are not tied together except if and when they are both turning the starter motor. At that operating point the voltage difference doesn't matter.

Robert