Battery issue

[solentlife]

What is important to note is that LiPo cell IR is a derived value. It is not a direct measurement or resistance, rather it is determined by measuring voltage drop below open load voltage and a known applied external resistance. This makes it very useful as it demonstrates the ability of a cell to deliver power under load, a direct measure of efficiency. And yes, as you say- it is a very useful indicator of cell health. Reduction in capacity is also a useful indicator as is cell balance across a pack. The DJI smart board is able to correctly report actual capacity, it performs IR monitoring of individual cells also during charge and discharge (amongst other things).

Not wanting to start disagreement - but a batterys overall capacity never changes even when dud ... its USEFUL capacity does though.

A mistake some users makes is to measure IR with for example a LiPo charger and then later compare to results via an IR meter OR at different state. IR is the best indicator of cells degrading - but needs to be compared using same apparatus, at same state and temperature.

Anyway - DJI do not allow us to access to measure it properly.

Nigel

 

[N017RW]

Actually Cell IR reduces with increasing temperature.

Yes.

The rate at which a chemical reaction occurs increases exponentially as temperature rises This allows more power from the battery at higher temperatures. At the same time higher temperatures improve electron or ion mobility reducing the cell's internal resistance increasing its capacity.

However high temperatures may also initiate unwanted or irreversible chemical reactions which can cause permanent damage to the battery.

 

[sar104]

Yes.

The rate at which a chemical reaction occurs increases exponentially as temperature rises This allows more power from the battery at higher temperatures. At the same time higher temperatures improve electron or ion mobility reducing the cell's internal resistance increasing its capacity.

However high temperatures may also initiate unwanted or irreversible chemical reactions which can cause permanent damage to the battery.

Given that charge flow in LiPo batteries is not due to actual chemical reaction, it's seems unlikely that it would follow an Arrhenius rate equation. But since it is the physical movement of Li+ ions between the electrodes, and since ionic mobility is likely to rise with temperature, I agree that one would expect decreasing internal resistance with increasing temperature, at least up to temperatures at which undesirable elecdrode and/or electrolyte chemistry kicks in and increases it again.

 

[solentlife]

All good stuff !! Pity that DJI doesn't let us at it with our batterys - unlike 99.9% of other LiPo using items !

But people - please can we be clear that Total capacity is not same as Useful capacity. Total capacity of a cell or cells does not change other than zero or full. It is the capacity that can be used and rate of use that changes.

I have LiPo's that will charge up and give same mAh results of charge as they did from day one ... but put them in the model and it hardly flies or instead of 5 mins flight - gives hardly a minute before ESC LVC kicks in ...
Those same LiPo's can run 12v Hot Melt guns and other gear such as Rx's / servos etc. .... but only when low WATT demand.
People then equate this is reduced capacity ... no it is not. It is greatly increased voltage drop under load pushing up amps to maintain the Watts ... and when batterys really get tired - they cannot supply the amps so flight / work required fails.... because the Watts are not there. Capacity is but work demand cannot be sustained.

Nigel

 

[N017RW]

You seem to be the only on concerned about such nit-picking.
All that matters is what's usable/useful anyways.

Yea, my battery shows full charge... but I only get 12 min flying time!...

...BUT, it still has FULL CAPACITY.

Meaningless.

 

[sar104]

All good stuff !! Pity that DJI doesn't let us at it with our batterys - unlike 99.9% of other LiPo using items !

But people - please can we be clear that Total capacity is not same as Useful capacity. Total capacity of a cell or cells does not change other than zero or full. It is the capacity that can be used and rate of use that changes.

I have LiPo's that will charge up and give same mAh results of charge as they did from day one ... but put them in the model and it hardly flies or instead of 5 mins flight - gives hardly a minute before ESC LVC kicks in ...
Those same LiPo's can run 12v Hot Melt guns and other gear such as Rx's / servos etc. .... but only when low WATT demand.
People then equate this is reduced capacity ... no it is not. It is greatly increased voltage drop under load pushing up amps to maintain the Watts ... and when batterys really get tired - they cannot supply the amps so flight / work required fails.... because the Watts are not there. Capacity is but work demand cannot be sustained.

Nigel

I don't entirely agree on the capacity issue. First of all, with lithium ion batteries there are several distinct changes occurring that limit stored and available energy, and I think that you are really only considering one of them. But, for example, cathode oxidation reduces the number of mobile Li+ ions and that does reduce absolute stored energy capacity.

Additionally, just because you are seeing the same number of coulombs of charge (or mA hrs) pushed in the charging process doesn't mean that it has been stored - some will have been converted to heat. You may well get more back out at a lower discharge rate, but not the full, original capacity. If that were the case then, in the limit as discharge current tends to zero, the full capacity of the battery would still be realized.

 

[solentlife]

You seem to be the only on concerned about such nit-picking.
All that matters is what's usable/useful anyways.

Yea, my battery shows full charge... but I only get 12 min flying time!...

...BUT, it still has FULL CAPACITY.

Meaningless.

It may seem to be nit-picking .... but IMHO its a valid point ... Is it not better for understanding what is going on ?

But I also said as you have as well ..... USEABLE capacity is what matters.

Nigel

 

[solentlife]

I don't entirely agree on the capacity issue. First of all, with lithium ion batteries there are several distinct changes occurring that limit stored and available energy, and I think that you are really only considering one of them. But, for example, cathode oxidation reduces the number of mobile Li+ ions and that does reduce absolute stored energy capacity.

Additionally, just because you are seeing the same number of coulombs of charge (or mA hrs) pushed in the charging process doesn't mean that it has been stored - some will have been converted to heat. You may well get more back out at a lower discharge rate, but not the full, original capacity. If that were the case then, in the limit as discharge current tends to zero, the full capacity of the battery would still be realized.

Seems that 'other battery' form is being applied here in part ...

Capacity that can be used changes with battery age ... fact.

Nigel

 

[With The Birds]

Not wanting to start disagreement - but a batterys overall capacity never changes even when dud ... its USEFUL capacity does though.

A mistake some users makes is to measure IR with for example a LiPo charger and then later compare to results via an IR meter OR at different state. IR is the best indicator of cells degrading - but needs to be compared using same apparatus, at same state and temperature.

Anyway - DJI do not allow us to access to measure it properly.

Nigel

We aren’t in disagreement- my reference to capacity is “usable or available capacity”. With respect to all the current DJI packs using the TI battery management the battery original new cell parameters are known and compared to actual which is measured over time. Given evaluation routines are run in actual use the reported mah capacity figure is accurate and very relevant.

 

[sar104]

Seems that 'other battery' form is being applied here in part ...

Capacity that can be used changes with battery age ... fact.

Nigel

I wasn't disagreeing that usable capacity decreases - I was discussing why it decreased. I don't understand your first comment.