Bridging EarthX Batteries

[hoz]

Hi, I intend to install 2x EarthX ETX-900 batteries in my RV-10, and want to have separate contractors to allow the electrical system to be fed by Batt 1, Batt 2, or Both. When selecting Both, Battery 1 and Battery 2 will be effectively operating in parallel.
EarthX notes that 'When connecting two batteries, it is important to make sure the charge levels are similar (voltages are within 0.3 volt) before connection. If there is a large difference in charge level, high current can flow between the batteries.'
Any idea how to ensure that this is complied with?

 

[Richard Connell]

The most effective way of ensuring that would be to make sure that the two independent busses - battery and alternator - are set up at the same charging voltage.
I assume this is what you have in mind.
In practice, my two buses charge at exactly same voltage in flight and I almost never see the two voltages differ by more than 0.1V unless I leave them both on on the ground for some length of time as one bus has some larger loads than the other on the ground.
FWIW I only tie them together for starting. Otherwise they are effectively two separate electrical systems.
I’m running a system that is effectively a Nuckolls z14.

 

[rleffler]

The most effective way of ensuring that would be to make sure that the two independent busses - battery and alternator - are set up at the same charging voltage.
I assume this is what you have in mind.
In practice, my two buses charge at exactly same voltage in flight and I almost never see the two voltages differ by more than 0.1V unless I leave them both on on the ground for some length of time as one bus has some larger loads than the other on the ground.
FWIW I only tie them together for starting. Otherwise they are effectively two separate electrical systems.
I’m running a system that is effectively a Nuckolls z14.

I have a very similar installation, except I also have vpx/pro. I have two independent busses that can be put in parallel, which helps in cold weather starts. The battery circuits were modeled on the z14 It’s been flying for ten years without any issues. Half the time with Oddesey and the last five years with EarthX

 

[Richard Connell]

I have a very similar installation, except I also have vpx/pro. I have two independent busses that can be put in parallel, which helps in cold weather starts. The battery circuits were modeled on the z14 It’s been flying for ten years without any issues. Half the time with Oddesey and the last five years with EarthX

Good to know. I’ve only been flying for 15m and around 200h. No issues to report. Odyssey’s for now but I may consider the EarthX when I come to replace the Odyssey’s. The weight improvement is impressive.

 

[lr172]

supplier said the "charge level" not "charging level." They are referring to the state of charge (SOC) of the batteries (as measured in voltage) and NOT the charging voltage of the alternator. When batteries are strung together on a common bus, balanced SOC's across the batteries is important for a variety of reasons.

As long as the batteries are healthy and always used in tandem, there is not much to get them to different SOC levels, beyond accelerated degradation of one battery. However, there is no real way to test that on each flight with normal instrumentation. I have that type of set up and don't worry about it, but I use AGM.

 

[DanH]

Hi, I intend to install 2x EarthX ETX-900 batteries in my RV-10, and want to have separate contractors to allow the electrical system to be fed by Batt 1, Batt 2, or Both. When selecting Both, Battery 1 and Battery 2 will be effectively operating in parallel.
EarthX notes that 'When connecting two batteries, it is important to make sure the charge levels are similar (voltages are within 0.3 volt) before connection. If there is a large difference in charge level, high current can flow between the batteries.'
Any idea how to ensure that this is complied with?

I contacted EarthX a few months ago regarding this exact issue.

The website cautions about a 0.3V difference. Engineering was kind enough to plot the actual expected amperage vs deltaV. Turns out the caution doesn't realistically apply for two batteries connected with #2 cables and master contactors. For a pair of ETX900's, a full volt difference results in 70 amps for about 10 seconds, falling to 10 amps or less. Please note the caution is entirely valid if the system offers a battery-to-battery current path via small wire, which may well become a fuse.

 

[BillL]

Basics

supplier said the "charge level" not "charging level." They are referring to the state of charge (SOC) of the batteries (as measured in voltage) and NOT the charging voltage of the alternator. When batteries are strung together on a common bus, balanced SOC's across the batteries is important for a variety of reasons.

As long as the batteries are healthy and always used in tandem, there is not much to get them to different SOC levels, beyond accelerated degradation of one battery. However, there is no real way to test that on each flight with normal instrumentation. I have that type of set up and don't worry about it, but I use AGM.

Larry, it is both "charge level" and voltage input i.e. "charging level" - - this is a fundamental issue operating two batteries in parallel with one charging system.

Basically, any connection has resistance, and even a small difference that results in a consistent difference in charging and discharge between the batteries adds up to a different SOC with time.

Each electrical system and connection method should take this into consideration. Tenths of a volt charge difference counts when a charge imbalance is noted.

This result is not unique to aircraft or AGM vs LiFePO4 types, although the failure modes are different. If AGM (PbA) one battery will just die, lose its capacity, LiFePO4 can create high amps and smoke things. (Ref DanH comment)

 

[dmattmul]

Size matters

That is one reason why I went with the ETX 1200 over the 900. The 1200 can take up to 120 amps charge vs the 80 amps a ETX 900 can absorb. Besides greater capacity larger amp charge capabilities.

 

[Walt]

That is one reason why I went with the ETX 1200 over the 900. The 1200 can take up to 120 amps charge vs the 80 amps a ETX 900 can absorb. Besides greater capacity larger amp charge capabilities.

Your alternator loves you (not)...

 

[rocketman1988]

But

That is one reason why I went with the ETX 1200 over the 900. The 1200 can take up to 120 amps charge vs the 80 amps a ETX 900 can absorb. Besides greater capacity larger amp charge capabilities.

Unfortunately, the alternator isn’t going to put out that 120 amps!

 

[dmattmul]

Your alternator loves you (not)...

"Hi, I intend to install 2x EarthX ETX-900 batteries in my RV-10, and want to have separate contractors to allow the electrical system to be fed by Batt 1, Batt 2, or Both. When selecting Both, Battery 1 and Battery 2 will be effectively operating in parallel.
EarthX notes that 'When connecting two batteries, it is important to make sure the charge levels are similar (voltages are within 0.3 volt) before connection. If there is a large difference in charge level, high current can flow between the batteries.'
Any idea how to ensure that this is complied with?"

True, but the Ops question was how you comply with the Earth X mandate to ensure your battery can take the "high current flow between batteries" if required. Do you have a better way? Maybe better to install a 680 with half the capacity and the ability, to deal with half the inter-battery ampacity? I'll take my method for the risk/reward factor. Twice the capacity to get me on the ground only to change (maybe) a rugged B&C alternator later, priceless. (Besides I'm pretty sure my 60-amp alternator will not output more than 70 amps)

 

[Richard Connell]

Maybe this is another use for the G3X logic states.
Or perhaps a voltage mismatch alert.
I’m still struggling to see how you actually end up with a meaningful voltage differential in a normally functioning z14 style dual alt / dual battery system.
The only way I can generate that is on the ground for a meaningful amount of time with lots of things on. Even then I don’t think Ive ever seen more than 0.2V

 

[Walt]

"

True, but the Ops question was how you comply with the Earth X mandate to ensure your battery can take the "high current flow between batteries" if required. Do you have a better way? Maybe better to install a 680 with half the capacity and the ability, to deal with half the inter-battery ampacity? I'll take my method for the risk/reward factor. Twice the capacity to get me on the ground only to change (maybe) a rugged B&C alternator later, priceless. (Besides I'm pretty sure my 60-amp alternator will not output more than 70 amps)

The only point that I was trying to make was that asking your alternator to output the maximum it’s capable of (110%+) on a regular basis will most certainly shorten its life.
One of the “features” I don’t like about lithium batteries is their ultra high charge current.
Excess battery capacity is like carrying extra fuel, serves no useful purpose IMO.
PS: battery voltage differences occur as batteries age, think cell balancing as relates to lithium charging.
Forgive my ignorance but what does a “TB” engine stand for?

 

[Richard Connell]

The only point that I was trying to make was that asking your alternator to output the maximum it’s capable of (110%+) on a regular basis will most certainly shorten its life.
One of the “features” I don’t like about lithium batteries is their ultra high charge current.
Excess battery capacity is like carrying extra fuel, serves no useful purpose IMO.
PS: battery voltage differences occur as batteries age, think cell balancing as relates to lithium charging.
Forgive my ignorance but what does a “TB” engine stand for?

I interpret his larger battery current capacity is to balance the batteries (presumably before starting) rather than to suck 100A out of the alternators.

 

[DanH]

The OP's question had nothing to do with charging or alternators.

The issue is what happens when a fully charged EarthX is connected to a partially discharged EarthX, for example by closing a crossfeed contactor after cranking on one battery. There is an initial rush of current between batteries, but given large cables and a standard contactor, it's not a problem.

 

[hoz]

My setup is intended to be similar to Dan's: 2x ETX900's working in parallel but with separate contractors providing a degree of resilience in case of 1 contactor failure. Normal operation will be to operate both together, thus both will have same/similar charges. The batteries will interconnect with #2 cable which is sufficient to handle up to 1V difference between the batteries (i.e. this will cause a max flow between the batteries of 70A which is ok for the cable and battery). However, there seems to be no way of correcting a larger imbalance if that occurs for whatever unforeseen reason. I have asked EarthX if there is a suggested battery balancing solution that will allow a 'trickle charge' to flow between the 2 batteries to equalise them in a controlled manner.

 

[Richard Connell]

My SOP is to keep the xfeed open. The EIS monitors the voltage and amps for each bus independently. It’s easy to therefore monitor the health of each system.

If you operate 2 alternators and 2 batteries continuously with the xfeed contactor closed, I’m interested as to how you would you determine whether you had any electrical system issues?

I only ever start with both.

 

[dmattmul]

Forgive my ignorance but what does a “TB” engine stand for?

You're certainly not ignorant and I appreciate your input very much on this forum and stands for Thunder Bolt.

 

[DanH]

The batteries will interconnect with #2 cable which is sufficient to handle up to 1V difference between the batteries (i.e. this will cause a max flow between the batteries of 70A which is ok for the cable and battery). However, there seems to be no way of correcting a larger imbalance....

Assume #2 and a standard contactor. It appears we can safely close that contactor to connect two batteries with an SOC difference more than 1V. #2 cable has a 10C rise rating of 100 amps. Both it and the contactor will obviously handle a lot more (cranking can be 200), and the current rush time is short.

Below from a previous conversation with EarthX. The engineer plotted a delta SOC of 0 to 1 volt, but nowhere in our conversation did he suggest a 1V limit. The plot suggests a surge current roughly proportional to delta SOC.

Don't think I would subject a continuous duty contactor (the typical master) to this abuse on a regular basis, as I would expect some contact arcing with the low power coil. IIRC, starter contactors draw more coil amps and have more contact force.