Lithium battery for ignition back-up?

[Mark Albery]

I'm just about to replace the PS-1221S back-up battery and was thinking about alternatives.

That battery is rated at 2 Ah but at the 1.3A needed for the ignition it would only last about 30 minutes. I believe that has been confirmed by others.

A small lithium motor sport battery like this could fit in the space with little modification and give about 2 hours. That would be more re-assuring if flying in remote areas.

Has anyone tried something similar or can see any snags with the idea?

It does have a built-in BMS so should be reasonably protected against damage causing a thermal runaway.

[ATXCubDriver]

I have customers using the following setup: Microstart XP-3 or variants. Install the corresponding Microstart jumpstart pigtail from the battery terminals and route forward to where it is accessible at the front of the seat right on top of the ELT. This makes the pigtail accessible in flight and your XP-3 unit could be plugged into the electrical system and used as additional capacity for the ignition system in addition to the capacity of the PS-1221-S battery. The XP3 has 8000mAH capacity. Links to both items below. This gives you jumpstart capability and backup battery capability in one package.

https://antigravitybatteries.com/pro...o-starts/xp-3/

https://antigravitybatteries.com/pro...newer-xp1-xp3/

I'm just about to replace the PS-1221S back-up battery and was thinking about alternatives.

That battery is rated at 2 Ah but at the 1.3A needed for the ignition it would only last about 30 minutes. I believe that has been confirmed by others.

A small lithium motor sport battery like this could fit in the space with little modification and give about 2 hours. That would be more re-assuring if flying in remote areas.

Has anyone tried something similar or can see any snags with the idea?

It does have a built-in BMS so should be reasonably protected against damage causing a thermal runaway.

[Mark Albery]

Thanks for the tip Mike.

I already have a Winplus which is functionally identical to the old XP-3, so I've just ordered a pigtail and will try your suggestion.

I see they quote it as 8000 mAh, but that's only at 3.7V so 29.6 Wh. One ignition is about 16W so it should be good for over an hour if only powering one Lightspeed unit.

[ATXCubDriver]

Yeah my thoughts were that with the LS system working down to 5 volts this setup would give you more than ample time to get somewhere safely. With the tremendous reliability of the LS system it is unlikely we would ever need it but its good to have choices over rugged terrain.

Thanks for the tip Mike.

I already have a Winplus which is functionally identical to the old XP-3, so I've just ordered a pigtail and will try your suggestion.

I see they quote it as 8000 mAh, but that's only at 3.7V so 29.6 Wh. One ignition is about 16W so it should be good for over an hour if only powering one Lightspeed unit.

[Andrew Y]

Executive Summary:

• Aircraft systems designed around lead-acid batteries do not adequately protect lithium batteries.
• There are lithium battery systems designed to replace lead-acid batteries by utilizing additional protection.
• Not all intermediate, or built-in, protection systems are equal, or necessarily compatible with the power generation system.
• Choose carefully, implement correctly.

Long Summary:
Lead-acid batteries have different design parameters and limitations than lithium batteries. Lithium chemistries generally have more restrictive operational condition limitations than lead-acid batteries. Given the numerous warnings lithium batteries are less tolerant of “abuse” when operated outside of those given limits. Vehicle systems designed around lead-acid batteries do not have the correct protections to ensure safe operation of lithium batteries without additional system interfaces and modifications. Some lithium technology manufactures have developed built-in intermediate protection systems to adapt the external system designed around lead-acid batteries to work with their lithium based product. In the original post’s example, the built-in “battery management system” provides only cell balancing, but no over-voltage or under-voltage protections. While that protection may be adequate for an open air motorcycle driving along the ground, that is inadequate for aviation applications. TCW Technologies and EarthX have done a great deal of development to create lithium battery systems to replace lead-acid batteries, even to the extent of attaining an FAA TSO for particular models. While some battery systems produced are intended for aviation use, there are still aircraft system modifications necessary for incorporation and operation.

Details to arrive at summaries:
One of the primary concerns is heat. This heat can be a combination of environmental, over-voltage, and over-current. Plain old air temperature is an easy one to define. While lead-acid batteries can operate anywhere from -40°F up to 176°F, lithium chemistries usually have a comparatively reduced temperature range. The example in the link given above prescribes a limited range of +14°F to 110°F (inconsistent upper limit numbers between many different warnings raised yet another flag on the given example). I have flown our aircraft when it was 100°F outside. How warm was it in the cabin sitting on the parking ramp? How warm was it in the space where the battery is mounted? With a lead-acid battery I wasn’t very concerned. With a Lithium battery, it gives pause to consider what operating temperature the battery is limited to. Look for lithium batteries that have a higher operating temperature than the one linked above.

Voltage is another system property related indirectly to heat. Too high voltage will overcome electrical limitations at the component level and create heat. That is why our aircraft employ over-voltage protective devices that will disable and disconnect the source of the over-voltage (alternator) in such a scenario. Existing over-voltage protection is designed for lead-acid batteries that can tolerate up to 16V without causing serious damage, so that is when the protection kicks in. The lithium batteries in the given link will not tolerate over 14.7V. If the alternator is gradually failing and working up to the 16V existing protection, there will be a duration of time when the voltage is damaging to the lithium batteries before the aircraft protection activates, thus building heat and causing damage. This is one of the key differences in system design and reasons additional voltage protection is needed.

Related to voltage and heat generation is the amount of electrical current the charging system can provide to the battery and how much the battery can absorb or reject. To store energy the battery turns electricity into a state of chemicals. That conversion produces heat. To keep from producing too much heat, the electrical current must be limited by the source (alternator) that cannot produce more than the battery can handle. Or the battery must be protected by in intermediate device or circuit that keeps the source from overpowering the battery. Following the link above, I could only find reference of limiting the power source but not the quantitative limit. A similar sized battery was found with a charging current limit of 15A. While that may be adequate for an off-road race motorcycle with a small alternator, that is much less than our 40A alternators are capable of producing.

Unrelated to heat, but important in the use of, lithium batteries operate at different voltages than lead-acid (yes, voltage keeps coming up). Therefore unless reprogrammed, any indication for the pilot associated with that battery will have misleading markings for good, bad, and ugly. Lithium technologies also have an almost flat discharge “curve” and quick drop-off at the end, so monitoring state of charge during utilization is also more challenging.

With regards to the specific example linked in the original post and looking into warnings that discourage the use in passenger cars and watercraft. That battery is not for use in closed spaces. Oh, and don’t get them wet, or store them in Arizona without air conditioning. Off-road racing is a great platform for creative weight saving technologies. Aircraft would be better advanced by design changes at a system level, rather than just a discrete component level. Lithium is a great technology to get more power for less weight, but choose carefully. If an alternate battery is sought, TCW technologies and EarthX offer good possibilities for incorporation.

Also, a friendly reminder that design changes on Standard LSA aircraft require an approval, MRA change request forms can be found on Cub Crafters' Service and Support web page.

[kiwibob]

• There are lithium battery systems designed to replace lead-acid batteries by utilizing additional protection.

Thanks for posting this Andrew. The consequences of sitting on a lithium battery that catches fire are too horrible to imagine. The photos are of a lithium battery that I used for a while in my LSA CC. Luckily it did the swelling after it was taken out.
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