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Dual Batterys of differing size

M

mikelupo

Active Member
Patron
Hi all,
My RV-10 has a dual battery system since there are 0 steam gauges and an E-mag.

There is a primary and back up battery. They are ​an Odyssey 625 and Odyssey 680.
I believe the 625 is the primary but I need to verify that.

My question to the forum, is ...should the batteries be dis-similar sized?
I can't imagine that this is by accident.

Thanks!
 
The short answer is no. Batteries in parallel only care about buss voltage. I’d be more concerned about the system design. I don’t know what “Primary” and “Backup” mean for a two battery ship.

Side note - the 625 has slightly more CCA and Amp-Hr capacity (and a little lighter) than the 680. I run two in all my RV projects.

Carl
 
mikelupo said:
Hi all,
My RV-10 has a dual battery system since there are 0 steam gauges and an E-mag.

There is a primary and back up battery. They are ​an Odyssey 625 and Odyssey 680.
I believe the 625 is the primary but I need to verify that.

My question to the forum, is ...should the batteries be dis-similar sized?
I can't imagine that this is by accident.

Thanks!
Click to expand...
SHOULD they be the different sizes is impossible to answer without knowing the design and design intent of the electrical system.
Most likely it does not make a difference, especially since the 625 and 680 have very similar specs. 170 vs 220 CCA, 16 vs 17 ah @10 hours.
"Typically" a dual battery system has 2 of the same battery or one big battery and one small battery. It is unusual that the batteries are so similar. My guess is that one of the original batteries was replaced and it was replaced with a similar (not identical) battery, based on availability. Or it could be that the slight physical size difference made installation easier. A review of the builders log or the aircraft log may provide some insight.

Since the batteries are the same chemistry, I do not see any reason why they MUST be identical.
 
As mentioned above - different size but same chemistry - no problem.

Do NOT mix lead-acid and lithium on the same buss.
 
mikelupo said:
Hi all,
My RV-10 has a dual battery system since there are 0 steam gauges and an E-mag.

There is a primary and back up battery. They are ​an Odyssey 625 and Odyssey 680.
I believe the 625 is the primary but I need to verify that.

My question to the forum, is ...should the batteries be dis-similar sized?
I can't imagine that this is by accident.

Thanks!
Click to expand...
Hi Mike, If you have an electrically dependent aircraft, you are doing the right thing by trying to understand the design of the system. There are many factors that go into the design of the electrical system, and as others have said, battery size can be different without issue. I strongly recommend digging a bit deeper and either getting design info from the original builder, or reverse engineering it yourself. You will find it rewarding and you will increase the likelihood of your flights ending safely. Can't hurt to scratch down a few notes as you go along.

You mentioned an "e-mag" - there are a few versions of electronic ignition, do you know which one you have? Some have built-in power generation like the surefly and the emagair pmag. Some require power from the ship. Really important to know which you have, how they get power, and what to do if you lose one part of your electrical system in flight.

Good luck with your research, and you will find lots of people that want to help you here on VAF.
 
rv8ch said:
........You mentioned an "e-mag" - there are a few versions of electronic ignition, do you know which one you have? Some have built-in power generation like the surefly and the emagair pmag. Some require power from the ship. Really important to know which you have, how they get power, and what to do if you lose one part of your electrical system in flight.........
Click to expand...

I'm not 100% certain, but I don't think Surefly has an internal power supply. (Could be wrong....)
 
Airguy question a?

Thread drift but useful to the OP.
Volts are determined by the regulator, and batteries take what they need from that line voltage. Charge rate can be an issue for LiFe type but that has nothing to do with the second battery chemistry. A lead acid AGM will fully charge to a lower voltage than LiFe from the buss voltage.
I have a Shottkey diode to isolate the two batteries to have 100% independent ignition systems.
BUT, why can’t two separate types feed off a common line and be happy ? With the master off, the LiFe cannot overcharge the AGM and the LiFe does not want anything from the AGM. What am I missing ?
 
PCHunt said:
I'm not 100% certain, but I don't think Surefly has an internal power supply. (Could be wrong....)
Click to expand...
I think you are right! I'm not sure why i thought that it had an internal generator like the pmag, but I was clearly wrong. Thanks!
 
rv8ch said:
Hi Mike, If you have an electrically dependent aircraft, you are doing the right thing by trying to understand the design of the system. There are many factors that go into the design of the electrical system, and as others have said, battery size can be different without issue. I strongly recommend digging a bit deeper and either getting design info from the original builder, or reverse engineering it yourself. You will find it rewarding and you will increase the likelihood of your flights ending safely. Can't hurt to scratch down a few notes as you go along.

You mentioned an "e-mag" - there are a few versions of electronic ignition, do you know which one you have? Some have built-in power generation like the surefly and the emagair pmag. Some require power from the ship. Really important to know which you have, how they get power, and what to do if you lose one part of your electrical system in flight.

Good luck with your research, and you will find lots of people that want to help you here on VAF.
Click to expand...

Hi Mickey,
My E-Mag is a Surefly SIM6L. I installed it last April after Sun N Fun. If I lose all power in flight, I lose the mag's spark.
 
airguy said:
As mentioned above - different size but same chemistry - no problem.

Do NOT mix lead-acid and lithium on the same buss.
Click to expand...

Thanks. I did read that same comment about not mixing chemistry types on the Spruce FAQs for the EarthX batteries.
 
RV10Pilot said:
SHOULD they be the different sizes is impossible to answer without knowing the design and design intent of the electrical system.
Most likely it does not make a difference, especially since the 625 and 680 have very similar specs. 170 vs 220 CCA, 16 vs 17 ah @10 hours.
"Typically" a dual battery system has 2 of the same battery or one big battery and one small battery. It is unusual that the batteries are so similar. My guess is that one of the original batteries was replaced and it was replaced with a similar (not identical) battery, based on availability. Or it could be that the slight physical size difference made installation easier. A review of the builders log or the aircraft log may provide some insight.

Since the batteries are the same chemistry, I do not see any reason why they MUST be identical.
Click to expand...

Thanks. This is good information. So if I try to start the plane on the primary battery, (primary is the split switch for battery/Alt field). It never seems to have enough charge. I'll try to charge it overnight and see if that helps. Once I turn on the AUX battery switch (engaging the second battery, in parallel) then the starter will spin. Unless the charging is the problem, then I think the primary battery is dying and needs to be replaced.
 
Batteries are typically chosen to handle the starting load with minimal impact on battery life. If you are routinely starting with just one of these batteries - which is smaller than the standard one - I would expect that to impact its lifetime. e.g., I think I’d routinely start on both batteries. What do others say?
 
Larry DeCamp said:
Thread drift but useful to the OP.
Volts are determined by the regulator, and batteries take what they need from that line voltage. Charge rate can be an issue for LiFe type but that has nothing to do with the second battery chemistry. A lead acid AGM will fully charge to a lower voltage than LiFe from the buss voltage.
I have a Shottkey diode to isolate the two batteries to have 100% independent ignition systems.
BUT, why can’t two separate types feed off a common line and be happy ? With the master off, the LiFe cannot overcharge the AGM and the LiFe does not want anything from the AGM. What am I missing ?
Click to expand...

The issue is not storage while sitting idle with the master open, it's charging in flight. The two chemistry's want to see different voltages to be at 100% charge. In order to fully charge the lithium battery you will be overcharging the lead-acid, and if you set the voltage lower to keep the lead-acid battery happy then you won't fully charge the lithium.

https://earthxbatteries.com/faqs
 
I start on both Batteries

I have a Solenoid (marked master two) that brings both batteries together, same 12 volts but combines the cranking amps and spins the Prop at higher speeds as if one BIG battery. This really helps with "Hot Starts". But after warm up and run up and Taxi (Charging both batteries) and before entering runway, I turn off the Master two Battery Solenoid and continue on with just one battery that I refer to as "master one". If the alternator and/or Master battery were to fail in flight I intend to bring the Master two Battery back on and seek the nearest airport to determine the problem. Reason for two Batteries: I do have GRT Glass, and ONE Surefly Mag, both depend on 8.5 Volts or more to operate. So far warm up, run up, and taxi seem to be enough to keep that #2 battery charged up. Others experience and opinions may vary. Ed S.
 
Airguy

My B&C regulator was factory set at 14.2 and produced that. The PC 680 was happpy charged at 12.6ish and the earthx 680 sits charged at 13.3 from same alternator. So what am I missing ?
 
Larry DeCamp said:
My B&C regulator was factory set at 14.2 and produced that. The PC 680 was happpy charged at 12.6ish and the earthx 680 sits charged at 13.3 from same alternator. So what am I missing ?
Click to expand...

I'm not going to argue with you about it, go talk to the battery manufacturers. It's their statement on the product, not mine.
 
Last edited:
mikelupo said:
Thanks. This is good information. So if I try to start the plane on the primary battery, (primary is the split switch for battery/Alt field). It never seems to have enough charge. I'll try to charge it overnight and see if that helps. Once I turn on the AUX battery switch (engaging the second battery, in parallel) then the starter will spin. Unless the charging is the problem, then I think the primary battery is dying and needs to be replaced.
Click to expand...
I believe the system is designed to be started with both batteries.
The batteries in your plane have 220 and 170 cold cranking amps
Van spec battery is a Concord 25RG-XC with 500 cold cranking amps (IRP@-18)
I have a single Odyssey PC925L with 330 CCA and no starting issues
Many RV10s have dual Odyssey PC680 batteries @170 CCA or 340 CCA when combined and when using 2 PC680's the design is to start with both batteries.
 
I too have a dual battery, dual alternator system. My main bus and battery is a PC625 and my Aux is a PC680. My system is independent and designed when starting to use the Aux Bus to power up all the avionics and keep them isolated while starting. After the engine starts both busses are tied together using a Blue Seas ACS Module. Only one alternator is running during normal ops.

If you are having trouble starting on the PC625, you either have a weak battery, wire that has been sized to small, a bad starter, or loose connection. You should not need both batteries to spin the engine over. You might try this procedure before you purchase a new battery. https://www.odysseybattery.com/wp-c...Y_Battery_Reconditioning_Charge_Procedure.pdf

Also from their website " If the battery is used in a starting application, the alternator should provide between 14.2 and 14.5 volts at the battery terminals"
 
Last edited:
Bill.Peyton said:
I too have a dual battery, dual alternator system. My main bus and battery is a PC625 and my Aux is a PC680. My system is independent and designed when starting to use the Aux Bus to power up all the avionics and keep them isolated while starting. After the engine starts both busses are tied together using a Blue Seas ACS Module. Only one alternator is running during normal ops.
SNIP
Click to expand...

I suggest you can get more out of your system components with a difference design, such as:
- Crank with both batteries.
- Have one EFIS come up before crank. I bring up the left side of the panel that includes the left EFIS but does not include the GTN-650. The SkyView is happy to ride the starting voltage dip, I don’t know about the GTN-650 so it stays off.
- If you are using the nice B&C standby alternator there is no reason to not have it on line continuously. The alternate does not produce power until the bus voltage drops below whatever you set it at. If you loose the primary alternator the standby automatically picks up the load - no pilot action required. You know this happened as you will see buss voltage drop to your standby alternator set point and the nice little standby alternator yellow indicator light comes on.

Carl
 
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