K2 battery endurance

Hi,

I'm hoping someone with experience with K2 batteries can help determine if mine are due for replacement. I have dual K2 batteries wired in parallel to supply avionics power. I have an LX9000, S8 and V8 varios, trig transponder, FLARM core, and becker com radio running off them. I'm not sure how old they are but they're at least 3 years old as this is my 3rd season with the ship. Yesterday, after a little over 5 hours I got a low battery warning with the batteries at 11.3 volts. I had all the above avionics on but did virtually no transmitting on the com, just listening. They were fully charged at takeoff.

I'm not sure if this normal endurance for these batteries or not, any advice would be appreciated.

Thanks,

John S.

John Silverberg wrote on 5/8/2020 12:21 PM:
Hi,

I'm hoping someone with experience with K2 batteries can help determine if mine are due for replacement. I have dual K2 batteries wired in parallel to supply avionics power. I have an LX9000, S8 and V8 varios, trig transponder, FLARM core, and becker com radio running off them. I'm not sure how old they are but they're at least 3 years old as this is my 3rd season with the ship. Yesterday, after a little over 5 hours I got a low battery warning with the batteries at 11.3 volts. I had all the above avionics on but did virtually no transmitting on the com, just listening. They were fully charged at takeoff.

I'm not sure if this normal endurance for these batteries or not, any advice would be appreciated.


I recommend charging each battery separately and completely, then doing a
discharge test on each battery individually at it's 20 hour rate. For example,
discharge the 10 AH battery at 0.5 amps during the test.

While the test is under way, determine the current draw of your instruments. I'd
guess it's high, and over 1.5 amps at 12.8 volts.

Once you know your current draw and the capacity of each battery, you can consider
your options.

If you decide to replace them, try to use a single 20AH battery, instead of
paralleling two 10 AH units, or perhaps a single 15AH if size matters.


--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1

On Friday, May 8, 2020 at 3:21:11 PM UTC-4, John Silverberg wrote:
Hi,

I'm hoping someone with experience with K2 batteries can help determine if mine are due for replacement. I have dual K2 batteries wired in parallel to supply avionics power. I have an LX9000, S8 and V8 varios, trig transponder, FLARM core, and becker com radio running off them. I'm not sure how old they are but they're at least 3 years old as this is my 3rd season with the ship. Yesterday, after a little over 5 hours I got a low battery warning with the batteries at 11.3 volts. I had all the above avionics on but did virtually no transmitting on the com, just listening. They were fully charged at takeoff.

I'm not sure if this normal endurance for these batteries or not, any advice would be appreciated.

Thanks,

John S.

John: At 3 years of age the batteries (I assume they are LiFePO4 type) should hold almost as much charge as new ones, assuming they were stored under reasonable conditions (not in a hot trailer). You know from your experience what is normal endurance with your avionics, so compare with that. If this is way lower than normal then that's bad news. But try recharging them and trying again, perhaps the capacity will return. Was this the first use after the winter lay-over? Some types of batteries need "exercise" after a long rest, although LiFePO4 are less prone to that AFAIK. If there is still an issue, try using one battery at a time, perhaps one is bad but the other still good.

On Friday, May 8, 2020 at 12:21:11 PM UTC-7, John Silverberg wrote:
Hi,

I'm hoping someone with experience with K2 batteries can help determine if mine are due for replacement. I have dual K2 batteries wired in parallel to supply avionics power. I have an LX9000, S8 and V8 varios, trig transponder, FLARM core, and becker com radio running off them. I'm not sure how old they are but they're at least 3 years old as this is my 3rd season with the ship. Yesterday, after a little over 5 hours I got a low battery warning with the batteries at 11.3 volts. I had all the above avionics on but did virtually no transmitting on the com, just listening. They were fully charged at takeoff.

I'm not sure if this normal endurance for these batteries or not, any advice would be appreciated.

Thanks,

John S.

I am not sure which K2 batteries you are using, but they are likely at the end of their life. I have all of the avionics you have, and am drawing 2 - 2.1 A at a higher voltage (meaning you are drawing even more). I put in a Bioenno 20AH lithium battery and it can handle an 8 hr flight. You should only be discharging an SLA battery no more than 50% of its capacity.

Tom

My 8 year old 9 Amp Hour K2 battery, just tested about a month ago, delivered 8.33 Amp Hours at a 1 amp rate. Capacity new was right at 9 Amp Hours.

Danny Brotto

There was a thread about this, K2's mentioned, a rew years ago that Steve Keorner replied too. He asked the K2 engineers about reduced capacity he was getting, IIRC, their answer was to leave them on the charger for weeks at a time so they can balancecout the cells with the BMS on board.

CH

On Friday, May 8, 2020 at 4:55:35 PM UTC-4, wrote:
There was a thread about this, K2's mentioned, a rew years ago that Steve Keorner replied too. He asked the K2 engineers about reduced capacity he was getting, IIRC, their answer was to leave them on the charger for weeks at a time so they can balancecout the cells with the BMS on board.

CH

Question: I don't know about K2 batteries and chargers specifically, but generally I thought the BMS in LiFePO4 batteries disconnects completely from the charger once some maximum battery voltage is reached. How, then, does leaving them wired together for "weeks" going to do anything?

Perhaps over time the battery voltage drops back somewhat and the BMS re-engages with the charger?

On Sunday, May 10, 2020 at 10:06:53 AM UTC-7, wrote:
On Friday, May 8, 2020 at 4:55:35 PM UTC-4, wrote:
There was a thread about this, K2's mentioned, a rew years ago that Steve Keorner replied too. He asked the K2 engineers about reduced capacity he was getting, IIRC, their answer was to leave them on the charger for weeks at a time so they can balancecout the cells with the BMS on board.

CH

Question: I don't know about K2 batteries and chargers specifically, but generally I thought the BMS in LiFePO4 batteries disconnects completely from the charger once some maximum battery voltage is reached. How, then, does leaving them wired together for "weeks" going to do anything?

Perhaps over time the battery voltage drops back somewhat and the BMS re-engages with the charger?

The internal BMS itself has a top balancer. It will detect when one or more of the cells are undercharged and very slowly discharge the others to first equalize them, then top them all off together. This may happen multiple times over a fairly long time until all cells are equalized since the undercharged cells themselves are unequally so. This can happen by passive balancing (resistive) or active (energy transfer from one cell to another).


I have seen my charger cycle when charging my LFP batteries (it's not K2 though).

There is no hard disconnect. Monitoring by the BMS is continuous.

David

On Sunday, May 10, 2020 at 1:40:44 PM UTC-4, wrote:
On Sunday, May 10, 2020 at 10:06:53 AM UTC-7, wrote:
On Friday, May 8, 2020 at 4:55:35 PM UTC-4, wrote:
There was a thread about this, K2's mentioned, a rew years ago that Steve Keorner replied too. He asked the K2 engineers about reduced capacity he was getting, IIRC, their answer was to leave them on the charger for weeks at a time so they can balancecout the cells with the BMS on board.

CH

Question: I don't know about K2 batteries and chargers specifically, but generally I thought the BMS in LiFePO4 batteries disconnects completely from the charger once some maximum battery voltage is reached. How, then, does leaving them wired together for "weeks" going to do anything?

Perhaps over time the battery voltage drops back somewhat and the BMS re-engages with the charger?

The internal BMS itself has a top balancer. It will detect when one or more of the cells are undercharged and very slowly discharge the others to first equalize them, then top them all off together. This may happen multiple times over a fairly long time until all cells are equalized since the undercharged cells themselves are unequally so. This can happen by passive balancing (resistive) or active (energy transfer from one cell to another).


I have seen my charger cycle when charging my LFP batteries (it's not K2 though).

There is no hard disconnect. Monitoring by the BMS is continuous.

David

So you concur with the BMS-reconnects-over-time hypothesis.

I usually charge my battery with my iMax B6 charger because it shows me what's going on, voltage and mAH. But when the battery reaches about 14.6V my battery's BMS disconnects - a bit before that charger would have stopped the charge anyway. And that charger then complains loudly about the connection being lost, and won't restart automatically.

If I want to give the battery BMS time to balance the cells further, I'd need to put it on a different charger. Would connecting the battery at that point to a "dumb" fixed-voltage source of about 14V be a good idea? Would using an SLA charger that aims for a "float" at 13.6V or so be a good idea?

The batteries are most likely not at the end of their life.

There are a few things that you can do.

1. Check the battery type on your LX9000, is it set to LiFe?
2. Check the voltage reported by the LX9000 and compare it to the voltage reported by another instrument. Most radios have a voltage readout and often this is more accurate than the 9000 readout. IF they are different use hte voltage offset function in the 9000 to have it read the same voltage as the radio.
3. If 1 and 2 don't help, put the K2 batteries on an extended 10 day charge cycle. This helps balance the cells and restore capacity in the K2's.
4. If this doesn't work, put them on a longer 20-30 day charge cycle.

The K2 batteries should never be stored for a long duration at full charge. Always store them at about 1/2 charge and then when they come out of storage put them on a 10 day float charge. Do this once a year.