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Earthx vs AGM for backup battery ?

Larry DeCamp

Well Known Member
A 5AH Earthx "backup" battery was recently linked here on VAF. LiFe with a brain seems like overkill for a simple voltage supply.

Earthx has a relatively flat voltage decay while AGM has a more linear voltage drop during discharge. I only need 7V to keep the fan turning to a runway when needed. So educate me, why I would select an expensive, complicated battery for this task.
 
Better question

In my opinion, the question should be “ why would one use a expensive back up battery that will automatically disconnect itself if it senses a problem (Earthx)? I think one would want a dumb battery that would let itself be completely discharged and destroyed in order to save the plane. But then you will have a mix of battery technologies. Your decision.
 
Thanks John

I share your logic, I'm just open to other perspectives.

WRT two battery chemistries, I had a post on this way back. The consensus was:
* 14.2 volts is just that. The batteries don't care.
* A 12.8V AGM is happy with a charge voltage less than 14.2V due to Shottkey slight voltage drop.

My backup is completely isolated from the main buss by a Shottkey and has a dedicated voltmeter for reality check.
 
...and

"...I think one would want a dumb battery that would let itself be completely discharged and destroyed in order to save the plane..."

You do realize that the electronics have a minimum voltage required, right?

Compare the discharge curve of that dumb battery to that of a LiFeO4 battery...
 
"...I think one would want a dumb battery that would let itself be completely discharged and destroyed in order to save the plane..."

You do realize that the electronics have a minimum voltage required, right?

Compare the discharge curve of that dumb battery to that of a LiFeO4 battery...

A traditional lead battery might discharge down to a lower voltage (at least to the point where whatever it's powering cuts off) but being able to "use every drop" doesn't automatically mean you have more usable power.

If a lead battery lasts 30 minutes till I've used every drop of juice... and the LiFePO4 lasts 50 minutes before it cuts off to prevent thermal damage... which one is better?

I'll take the bigger fuel tank with more usable fuel, even if it means there's some small volume of unusable fuel in it.
 
Useable juice ! Good point .

Point taken. The true value is how much energy is available before “not enough”. With that said , the less complicated battery with “tested” capacity is the true value. Thanks.
 
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Agm charge voltage question ?

I have structured my ignition for dual SPS CPI using an Earthx primary battery and Powersonic 7AH backup for ONE ignition. The power draw is about 3A. The goal is KISS so the second ignition is totally isolated from a primary system failure allowing complete shutdown of master bus .

If the alternator maintains a 14.2 bus voltage and a high capacity shottkey drops backup charge voltage to 13.8, the AGM backup should remain properly charged, right ? One option is a switch instead of a shottkey, but that is one more thing to manage 😏
 
I have structured my ignition for dual SPS CPI using an Earthx primary battery and Powersonic 7AH backup for ONE ignition. The power draw is about 3A. The goal is KISS so the second ignition is totally isolated from a primary system failure allowing complete shutdown of master bus .

If the alternator maintains a 14.2 bus voltage and a high capacity shottkey drops backup charge voltage to 13.8, the AGM backup should remain properly charged, right ? One option is a switch instead of a shottkey, but that is one more thing to manage ��

Also using a Powersonic battery as backup (with Oddessey for main battery), but in my case a 18ah backup. If I have to deal with the stress of running on a failing electrical/charging system, I don't also want the added mental strain of running on a degraded EI/EFI system too. My power structure allows for quick disconnect from Main buss, switching the whole SDS buss to the backup battery.
Also I have a volt meter on the left side of my panel that is handy to check condition of each battery as part of my pre-flight walk around. If either battery is down on voltage - no flight that day!
 
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