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Replacing lead acid with LiFePo3 in Z-11+

D

davevon

Member
My 6 has been flying with Bob Nuckolls Z-11+ small aux battery for 19 years without issue. I'm wanting to switch over to LiFePo3 batteries, but I'm unsure of how their characteristics will behave with this power system (see attached).

Anyone have experience with LiFePo3 batteries and this architecture?

Thoughts?

I've tried reaching out to Bob without response.

Thanks!
 

Attachments

  • N277T Z-11+.pdf
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Bob checks/answers emails intermittently. For questions like this he prefers them to be posted on the AeroElectric List where he is very active. I just went through this myself.
 
One consideration about going from lead acid to lifepo4 is charging voltage. Check the specifications on the battery you are installing, and make sure that your voltage regulator is set to a high enough voltage.
 
The size of a lithium battery should be determined by output capability of the alternator.
Suppose that the alternator is rated at 60 amps. It could actually put out more than that, perhaps 65 amps.
A discharged lithium battery should be able to accept 65 amps without overheating.
A diode in series between the charging system and the lithium battery will limit the current.
Lithium batteries and lead acid batteries should not be connected in
parallel because the lithium battery will discharge into the lead acid battery.
However, the two types of batteries may be connected together in parallel while the charging system is operating
because each of the two batteries will only see the highest voltage, which is the charging system voltage.
 
The lead acid battery is losing voltage during each start attempts.
This means that the charging current (A) will increase the more the lead acid battery is depleted.
A LiFePO4 battery on the other hand does not loose voltage during start attempts until the battery is depleted to about 20 % power left.
Only then the current will increase because the voltage is lower.
The alternator current (A) is a function of the battery voltage.
As long as the LiFePO4 has a state of charge that is more than about 20 %,
the charging current will be fairly low.
A LiFePO4 battery that has been depleted to a charge below 20 % should be
charged with a battery charger with limited capacity (10 A).

Good luck
 
EarthX batteries FAQ.
-
Question: I just installed my EarthX battery in my aircraft and started the engine and my charging amps was really high, is this normal?
-
EarthX response:
Yes, this is normal. A lithium battery can and will accept a much higher charging current than an equal size lead acid battery. If your battery is not fully charged, your alternator can potentially output a current equal to or slightly above the alternator’s rating for a few minutes to top the battery off. Your fuse, breaker, or current limiting device should be sized approximately 20% above the alternator rating. Example, if you have a 40 amp alternator, use a fuse, breaker, current limiter set to 48 amps (or 50amps).
.
Question: Why do you use the alternator output to size your batteries in cars or airplanes?
.
EarthX response: Starting the engine (cranking amps) is only one part of what the batteries does. Your battery is also part of the charging system and the current from the alternator goes through the battery. You must use a lithium battery that is designed to handle the current from the alternator or you can damage it, cause premature failure or possible even cell rupture. In the lithium world (LiFePo4) the maximum current rate is 5C x true amp hour (not equivalent rating). As an example, the ETX900 is a true 16Ah battery. The max charge rate then is 80A (5C x 16AH = 80A).
 
Mitch 48041 question ?

I have an earthX primary battery in parallel with a small AGM for the second EI as recommended by the manufacturer. The small battery has a high capacity Shottkey diode to isolate it from the main buss . In a worse case scenario, the little AGM is intended to run the second EI completely independent of the electrical system to the nearest “ flat spot”.
Question:
The Shottkey voltage drop is small but finite, maybe 1/2 volt. At 14.2 volt alternator output, I assume the little AGM will still charge to full capacity around 12+ volts. CORRECT ?
 
Larry, yes the AGM battery will charge at 13.7 (14.2 - 0.5) volts given enough time.
If the AGM battery is not discharged very much, then there should not be a problem.
 
Larry DeCamp said:
I have an earthX primary battery in parallel with a small AGM for the second EI as recommended by the manufacturer. The small battery has a high capacity Shottkey diode to isolate it from the main buss . In a worse case scenario, the little AGM is intended to run the second EI completely independent of the electrical system to the nearest “ flat spot”.
SNIP
Click to expand...

Off topic, but….

I know this “small battery to keep the engine running” is in this vendor’s install instructions. I have found such dual EI RVs flying around with a completely dead EI backup battery - and they did not know it.

I suggest two mitigations:
- A better power distribution scheme that incorporates the requirements of an electrically dependent engine (as in not just adding a little battery).
- At bare minimum check this battery several times a year and replace it often (as in no less than every two years). Run that side ignition on this battery and see how long it last.

Carl
 
Carl Froehlich said:
At bare minimum check this battery several times a year and replace it often (as in no less than every two years). Run that side ignition on this battery and see how long it last.

Carl
Click to expand...

If you're running a backup battery to power your EI/EFI after the alternator and main battery take a dump, our recommendation is to check battery voltage prior to EACH flight. Put that item on your pre-start checklist and have a means to check the backup battery voltage independently.

You are electrically dependent so be sure all electron supply sources are functional before launching.

Load test each battery annually as well.
 
Avanza said:
A LiFePO4 battery on the other hand does not loose voltage during start attempts until the battery is depleted to about 20 % power left.
Only then the current will increase because the voltage is lower.
The alternator current (A) is a function of the battery voltage.
k
Click to expand...

As long as the charge voltage is above the batteries resting state voltage, the battery will consume as much current as it can until fully charged. A lithium battery does NOT wait until it is 80% discharged before it starts accepting current, assuming a typical VR voltage setting. Altnerator current is a function of both voltage and the system current load. As load increases, the voltage drops and the VR up's the alt output current to keep it at its target voltage. The VR only sees voltage. When bus voltage seen by the VR drops (even though the batteries voltage doesn't drop, once it is accepting current, the alt output voltage WILL drop), it keeps increasing the alt's current output until the target voltage is met or the Alt reasches max current output. Chargers are different as many can do both constant voltage AND constant current in different charging phases.
 
Last edited:
Voltage reference suggestions

Thanks for the attention guys. Yes, the small AGM has dedicated volt meter on a switch to reference voltage before master switch power up for main buss. A live test of duration for the solo EI requires pulling the field breaker so the only power is the small AGM.

However, a periodic bench test of capacity would be prefered rather flying an hour without any avioncs. Suggestions welcome that dont add more switches to manage .
 
All LiFEPO4 are not made with equal technology.
Battery BMS are made differently to reduce the cost or increase performance.
My Aliant starting battery has a max charging limit of 20 A.
The BMS limits the charging current when battery is discharged.
I have never seen charging currents above 20 A.

Good luck
 
terrye said:
Bob checks/answers emails intermittently. For questions like this he prefers them to be posted on the AeroElectric List where he is very active. I just went through this myself.
Click to expand...

I've been trying on and off to log in. It's been so long since I've been logged in that my account is now inactive, but I can't reregister because my email is already in use... How do contact Matt?
 
davevon said:
I've been trying on and off to log in. It's been so long since I've been logged in that my account is now inactive, but I can't reregister because my email is already in use... How do contact Matt?
Click to expand...

mail.com thought everyone had a few email addresses
 
davevon said:
I've been trying on and off to log in. It's been so long since I've been logged in that my account is now inactive, but I can't reregister because my email is already in use... How do contact Matt?
Click to expand...

Another trick is to use a + to change your address. This does not always work, but here is an example:


[email protected]

and

[email protected]

both of these will go to the same gmail mailbox.


Not all systems will cleanly process this "subaddressing" feature. You can read more about it here: https://en.wikipedia.org/wiki/Email_address#Sub-addressing
 
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