Mixed Battery Chemistry, Common Buss?

Toobuilder · #1 10-18-2016, 10:03 AM

I?ve been looking at various schemes to provide backup power to my electrically dependent airplane. I?ve considered everything from airliner style dual busses so I can press on with 100% capability in a failure, to dual alternators with somewhat reduced capability, and considering my mission, have settled on a simple backup battery that handles ONLY my engine needs for an hour or so. I?m not here to debate risk posture, so let?s try keep that in check.

I am exploring the technical drawbacks of mixing battery chemistry on the same buss however. My thinking is that I?m going to run the airplane with a single alternator and battery just like we?ve been doing for decades. However, I?d like to add a smallish Li-Ion (or other suitable chemistry) battery in reserve to power the fuel pumps, ignition and engine CPU if the ship?s main system goes Tango Uniform. Think of this as a scaled up EFIS backup battery. I?d rather not have the various auto isolation methods taking care of tending the battery though ? I want a stone reliable toggle switch that connects the battery to the engine buss through a single circuit. I?m not opposed to charging the battery once a month on the ground and keeping the backup strictly isolated from the ship?s main buss, but I think it would be beneficial to have a ?charge? position on the toggle so that when skies are blue and all is well, I can use the ship to keep the backup topped off. Most of the time the switch would be in a ?NORM? position ? backup battery completely disconnected.

I am only considering the more recent, exotic chemistry because this battery will be 100% dead weight until an emergency so I want it to be as small as possible while meeting the power requirements. I have only scratched the surface of the various battery offerings and know only that some of these batteries have different charging requirements WRT input voltage and amperage.

So long story comes down to a simple, broad question: Is anybody running mixed battery chemistry on a common buss, and what are the cautions/concerns/mitigating actions?

rapid_ascent · #2 10-18-2016, 10:25 AM

Michael,

I've been doing some related work but it's sort of a back burner project right now since I'm trying to focus on building my fuselage.

I designed a small circuit board that includes a battery charger circuit and ORing diodes which have been implemented using MOSFETs for higher efficiency. I had planned to use a gel cell (like a PS-1290) for my backup battery. My plan is to have both batteries isolated by the ORing diodes and the backup battery charged using my charging circuit. My charger design is limited to a few amps of charging current so it will only work for smaller batteries. I wasn't planning on lithium but that may be a possible future extension.

One other benefit of my charging circuit is it is a buck-boost configuration so it will try to supply charging current even if the input voltage is less than the battery voltage. The idea being to use as much power as is available for these emergency backup operations when the power demand has been reduced.

When I'll have my circuit working is TBD but I haven't been working on it lately unfortunately. My charger was getting a little warmer than I liked so I need to track that down. Otherwise it seems to work as expected.

Randy · #3 10-18-2016, 04:19 PM

I posed a similar question to Robert at EFII regarding use of mixed chemistry and the Bus Manager. He told me it was fine to have for instance a PC680 as primary and an EarthX as secondary. They are kept isolated except for those instances when you choose to use both batteries for starting which would not cause a problem as they would be isolated again as soon as the engine starts.

Without the Bus Manager I am sure there is a way to do this with Diodes and I am attracted to keeping it simple also. Interested in hearing from others that have done this...

BobTurner · #4 10-18-2016, 04:26 PM

I think you do need to be careful mixing battery types with a simple system, due to small but important differences. For example, a conventional wet cell lead acid battery works fine in a system with the alternator set at 13.6-13.8 volts. OTOH Odyesse recommends alternator voltage be set above 14.0 for their batteries.

Toobuilder · #5 10-18-2016, 06:21 PM

While I would like the option of adding the backup battery to assist with the engine start duties on occasion, its the charging that has me the most worried. Despite Robert's assurances, my research indicates that there can be significant differences in input voltage and charge rates between chemistry types. That is what I'm trying to nail down.

F1Boss · #6 10-18-2016, 07:17 PM

I have read that the batteries should not be mixed on the same buss - that could extend to same systems too? It might work if you have both hooked up thru their own contactors so they are never on line at the same time.

Changing from one to the other would require that the entire elec system be shut down and then 'rebooted' - but isn't that the situation you want to be able to recognize and rectify with the #2 battery? In other words, you would use #2 only if the main battery/system fails?

I'm setting up a 2 battery system in my next build, but it will use 2ea EarthX units with two contactors. I can use the #2 for checking the AWOS and getting my flight plan set up etc, and then turning on #1 for start-up. The architecture will not allow for the #2 to help with starting...so far...

I am neither an EE nor a person who stayed at a Holiday Inn last night. My ideas are highly suspect!

Carry on!
Mark

DanH · #7 10-18-2016, 07:44 PM

Mike, I've been thinking about the same stuff, as I'll install a second EI this winter. My conclusion is (1) go old school, or (2) go whole hog.

Old school is a main bus PC925 and a 2.5 to 4 AH AGM battery, diode isolated, dedicated to a single ignition. Whole hog is the same scheme, but using blue batteries.

Here's the thing...when discussing main batteries, the blue battery saves a lot of weight. However, when adding a small battery dedicated to a single EI, there isn't a lot of weight to be saved.

I like the 925; zero maintenance, lots of reserve, and at 28 lbs, good for CG with the 390 and a Hartzell. No flashing lights, no special care. I'm inclined to keep it.

A 2.5AH AGM Motobatt MTB4BB is 2.31 lbs, while a 3.8AH MB3U is 2.95 lbs. They're $32 and $38 on Ebay.

Bet I can tell, the smallest EarthX is an ETX12A, rated at 4AH. It weighs 1.3 lbs. They are $169.

If I was willing to pair an AGM with a lithium, I'd only save 1.65 lbs (2.95 - 1.3). If I drop back to a 2.5AH battery the difference is one pound. 2.5AH isn't crazy; the EI draws less than an amp at cruise power. With an alternator out, I'd shut it down and hold it in reserve anyway.

So, I don't see much value in mixing battery chemistry to drive an EI alone. However, I assume you need more AH. What is the combined draw for the EI, EFI, and fuel pump?

rv6ejguy · #8 10-18-2016, 08:02 PM

Total current draw for our EFI/EI driving 8-12 plugs, 4-6 injectors with a single Walbro pump running is around 10-12 amps at 2500 rpm.

I run a PC680 main battery and an 18 amp hour AGM battery aft of the baggage bay, dedicated to backup power for the engine electrics.

The AGM is charged monthly on the ground, voltage tested prior to startup, load tested every 6 months and tied to the essential buss through a 30 amp ATO fuse and 10 gauge wire/ heavy duty toggle switch. I can isolate from the main battery and alternator by turning off the master. This should give me a solid 45-60 minutes if the main battery takes a dump or about double that if just the alternator dies.

Many ways to approach backup power and we find few people agree on the same solution...

Mich48041 · #9 10-18-2016, 08:23 PM

Here is a circuit that will prevent brownout during engine start. A fuel pump and ignition could also be powered by the ebus for an electrically dependent engine. The second fuel pump and ignition could be connected to the main battery.

Toobuilder · #10 10-19-2016, 07:07 AM

I was thinking I needed something along the lines of a PC680 just for the backup. Like Dan, I'm running a 925 for the main.

I was hoping to bring the 925 forward to the spar to shorten up the heavy guage battery cable run, but that is going to only add to my forward CG problem. Maybe a 680 way back in the tail will make use of the extra weight and simplify my charging scheme at the same time.

I guess what I really need is a backup battery that can deliver 12 amps for at least an hour (preferrably two hours), is feather weight, and can tolerate riding on the main buss with a 40 amp and 14.5 v input.