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-   -   A Simple Question About LiFePO4 Batteries (http://www.aviationbanter.com/showthread.php?t=246357)

[email protected] May 11th 18 08:37 PM

A Simple Question About LiFePO4 Batteries
 
Not to start another chapter in "Battery Wars" but I have a simple question about "drop in" LiFePO4 battery replacements for SLA batteries in gliders.

For years, I've used an SLA battery with the same footprint as the common glider size of 7Ah to 9Ah--i.e., 5.95" x 2.56"--except a little taller--i.e., 4.61" high vs. 3.86" (specs are from the PowerSonic line but other brands are similar). It's an easy swap in my ASW 24, only requiring a different hold-down strap. There's about 20% more internal volume, which seems to equate to at least 20% more claimed capacity. I've used enough of these that I know they do perform longer than the conventional-size SLA batteries most pilots use.

However, I can't find a LiFePO4 equivalent for it. Does such a thing exist? All I've seen discussed here or available online seem to be the "traditional" size. I've been told that this is because there's already excess space inside the enclosure, that it's not packed to the gills with mat/chemicals as SLAs are, and that the extra 3/4" wouldn't allow adding additional cells.. And besides, I don't need it anyway (I'm running PowerFLARM, radio, CNv vario, and PNA; no transponder yet).

Then I see that some LiFePO4 batteries are rated slightly higher for the same enclosure size. Even the K2 battery touted in adverts as "10 Ah" seems to be 9.6 Ah in the specs. And that's ignoring loads, discharge rates, temperatures, etc.

Given the price differential and the necessity for a new charger (although not according to the K2 site), and especially in light of the recent stories of LiFePO4 batteries crapping out long before their promised lifespans, maybe I should just continue to buy a new SLA battery every year or two and wait for this fad to pass. :)

As I said, a simple question.

Chip Bearden

[email protected] May 11th 18 09:28 PM

A Simple Question About LiFePO4 Batteries
 
On Friday, May 11, 2018 at 3:37:03 PM UTC-4, wrote:
Not to start another chapter in "Battery Wars" but I have a simple question about "drop in" LiFePO4 battery replacements for SLA batteries in gliders.

For years, I've used an SLA battery with the same footprint as the common glider size of 7Ah to 9Ah--i.e., 5.95" x 2.56"--except a little taller--i.e., 4.61" high vs. 3.86" (specs are from the PowerSonic line but other brands are similar). It's an easy swap in my ASW 24, only requiring a different hold-down strap. There's about 20% more internal volume, which seems to equate to at least 20% more claimed capacity. I've used enough of these that I know they do perform longer than the conventional-size SLA batteries most pilots use.

However, I can't find a LiFePO4 equivalent for it. Does such a thing exist? All I've seen discussed here or available online seem to be the "traditional" size. I've been told that this is because there's already excess space inside the enclosure, that it's not packed to the gills with mat/chemicals as SLAs are, and that the extra 3/4" wouldn't allow adding additional cells. And besides, I don't need it anyway (I'm running PowerFLARM, radio, CNv vario, and PNA; no transponder yet).

Then I see that some LiFePO4 batteries are rated slightly higher for the same enclosure size. Even the K2 battery touted in adverts as "10 Ah" seems to be 9.6 Ah in the specs. And that's ignoring loads, discharge rates, temperatures, etc.

Given the price differential and the necessity for a new charger (although not according to the K2 site), and especially in light of the recent stories of LiFePO4 batteries crapping out long before their promised lifespans, maybe I should just continue to buy a new SLA battery every year or two and wait for this fad to pass. :)

As I said, a simple question.

Chip Bearden


You don't want to drain an SLA all the way, as that shortens its life considerably. But you can drain a lithium battery "all the way" (to the point where the protection circuitry prevents further discharge) with not much loss of battery life. Thus for example a 7AH lithium battery will offer more usable capacity than a 10AH SLA. Except that you say you replace the SLA "every year or two", so perhaps you do discharge it deeply, or don't care about the longevity. SLAs are still so much cheaper than lithium batteries that so far I've stuck to SLAs, although I use them for 3-4 years. Those extra-large SLAs may be more expensive than the standard sizes, though?

[email protected] May 11th 18 10:00 PM

A Simple Question About LiFePO4 Batteries
 
I don't usually drain them. I switch to the backup if I'm on a long flight or it's cold. Rarely does the main batt go low enough to get the "low battery" warning blinking (11.2v). They don't cost that much more than the standard-size SLA battery. And they probably last 3 years on average, IIRC. I haven't done any testing nor do I write a date on them when installed. I just usually replace them when I have to start going to the backup on long flights.

So I'm surprised that a relatively new battery (2 seasons?) hasn't performed that well in two flights this spring. The last time, I decided to stay with SLA because it had worked well enough in the past and was a lot cheaper. But now that I'm facing replacement again so soon, I'm reconsidering.

Chip Bearden

Tango Eight May 11th 18 10:45 PM

A Simple Question About LiFePO4 Batteries
 
A g a i n ?

The -only- advantage of SLA is lower purchase price. And considering that "7 AH" is really about 3.5 useful, the "economy" here is dubious, at best.

A good battery is $100.

Some problems are hard to solve. Not this one.

best,
Evan Ludeman / T8

kinsell May 12th 18 12:14 AM

A Simple Question About LiFePO4 Batteries
 
On 05/11/2018 02:28 PM, wrote:
But you can drain a lithium battery "all the way" (to the point

where the protection circuitry prevents further discharge) with not much
loss of battery life.


This assumes that all lithium batteries have a full-featured BMS circuit
that protects against battery abuse. That seems to be a bad assumption,
particularly for LiFePO4 batteries. Pfiffner has posted some
interesting failures he's seen there, in the previous incarnation of
this discussion.

[email protected] May 12th 18 12:35 AM

A Simple Question About LiFePO4 Batteries
 
Do it. Im using the Australian made Fusion battery, 10ah, and approved by the GFA.
1500 FULL DISCARGE CYCLES, 2500 80% discharge, will probably outlast my ability to fly.
Discharging an SLA more than 50% damages it, fully flattening them kills them completely.

[email protected] May 12th 18 12:52 AM

A Simple Question About LiFePO4 Batteries
 
On Saturday, May 12, 2018 at 7:00:52 AM UTC+10, wrote:
I don't usually drain them. I switch to the backup if I'm on a long flight or it's cold. Rarely does the main batt go low enough to get the "low battery" warning blinking (11.2v). They don't cost that much more than the standard-size SLA battery. And they probably last 3 years on average, IIRC. I haven't done any testing nor do I write a date on them when installed. I just usually replace them when I have to start going to the backup on long flights.

So I'm surprised that a relatively new battery (2 seasons?) hasn't performed that well in two flights this spring. The last time, I decided to stay with SLA because it had worked well enough in the past and was a lot cheaper. But now that I'm facing replacement again so soon, I'm reconsidering.

Chip Bearden


You may well be discharging your battery more deeply than you think, hence the short life. 11.2v open circuit for a SLA is well below fully discharged at around 11.7 to 11.8V open circuit, but your meter is probably showing you the voltage with a load on so you really don't know how deeply discharged the battery is. If you want to check the actual approximate state of charge, put a multimeter on the battery after it is disconnected from all loads.. The table in the following article shows an approximation of state of charge based on open circuit voltage (from 100% @ 12.7+V to 0% at 11.8V) for various types of LA batteries.


[email protected] May 12th 18 12:54 AM

A Simple Question About LiFePO4 Batteries
 

You may well be discharging your battery more deeply than you think, hence the short life. 11.2v open circuit for a SLA is well below fully discharged at around 11.7 to 11.8V open circuit, but your meter is probably showing you the voltage with a load on so you really don't know how deeply discharged the battery is. If you want to check the actual approximate state of charge, put a multimeter on the battery after it is disconnected from all loads. The table in the following article shows an approximation of state of charge based on open circuit voltage (from 100% @ 12.7+V to 0% at 11.8V) for various types of LA batteries.


Doh. Forgot the link. https://www.energymatters.com.au/com...age-discharge/

JS[_5_] May 12th 18 01:08 AM

A Simple Question About LiFePO4 Batteries
 
Chip, I think that QV (Richard's first ASG29, not Al's) had the same batteries when I bought it.
There were custom made hold-down straps.
I replaced the AGMs with 10Ah LiFePO4 and use the factory hold-down straps.
That larger battery size is unusual, haven't seen LiFePO4s that size.
Jim


On Friday, May 11, 2018 at 12:37:03 PM UTC-7, wrote:
Not to start another chapter in "Battery Wars" but I have a simple question about "drop in" LiFePO4 battery replacements for SLA batteries in gliders.

For years, I've used an SLA battery with the same footprint as the common glider size of 7Ah to 9Ah--i.e., 5.95" x 2.56"--except a little taller--i.e., 4.61" high vs. 3.86" (specs are from the PowerSonic line but other brands are similar). It's an easy swap in my ASW 24, only requiring a different hold-down strap. There's about 20% more internal volume, which seems to equate to at least 20% more claimed capacity. I've used enough of these that I know they do perform longer than the conventional-size SLA batteries most pilots use.

However, I can't find a LiFePO4 equivalent for it. Does such a thing exist? All I've seen discussed here or available online seem to be the "traditional" size. I've been told that this is because there's already excess space inside the enclosure, that it's not packed to the gills with mat/chemicals as SLAs are, and that the extra 3/4" wouldn't allow adding additional cells. And besides, I don't need it anyway (I'm running PowerFLARM, radio, CNv vario, and PNA; no transponder yet).

Then I see that some LiFePO4 batteries are rated slightly higher for the same enclosure size. Even the K2 battery touted in adverts as "10 Ah" seems to be 9.6 Ah in the specs. And that's ignoring loads, discharge rates, temperatures, etc.

Given the price differential and the necessity for a new charger (although not according to the K2 site), and especially in light of the recent stories of LiFePO4 batteries crapping out long before their promised lifespans, maybe I should just continue to buy a new SLA battery every year or two and wait for this fad to pass. :)

As I said, a simple question.

Chip Bearden



[email protected] May 12th 18 02:23 AM

A Simple Question About LiFePO4 Batteries
 
On Friday, May 11, 2018 at 7:35:08 PM UTC-4, wrote:
Do it. Im using the Australian made Fusion battery, 10ah, and approved by the GFA.
1500 FULL DISCARGE CYCLES, 2500 80% discharge, will probably outlast my ability to fly.
Discharging an SLA more than 50% damages it, fully flattening them kills them completely.


Would be nice if you could actually get those 1500 discharge cycles, but I expect the battery will die from "old age" well before then, even with light use. How many years a LiFePO4 battery will really last is still a mystery, although some people "here" can report what, 6 or 7 years and still OK?


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