12 s setups?

[Edgeflyer]

I need to run a motor on 12s. Is it a definite no-no to run 2 6s packs in series if they are different brands? Even if ratings similar?

[Philip Lewis 3]

If the true C rating (internal resistance) is different which it is likely to be then almost cerainly you will kill one pack and that is likely to be the better pack with the higher C rating as the lower internal resistance means that it will try and supply more than it's fair share of the current.  

Edited October 7, 2022 by Philip Lewis 3
My awful spelling!

[Chris Walby]

Just a matter of opinion...depends on how different the packs are and how hard you are pushing them. In the end it would be a try it and see if one pack discharges significantly more than the other...Might be able to measure each 6S pack voltage and alarm if one is getting low.

[Dickw]

In series the packs will have the same current flowing through both, so will discharge at the same rate (i.e discharge the same capacity in the same time). should work OK so long as you base the time/capacity flown on the lowest capacity pack.

 

Dick

[Edgeflyer]

I agree that the current has to be the same through both.

Probably safer to use a matched pair.

[Simon Chaddock]

This does raise the question of just how well matched are the cells in the same pack.

As stated how successful using LiPos of the the same specification but of different brands, although they might actually made in the same factory, comes down to how hard you are using them.

The cells with the higher IR will almost certainly give concern first, regardless of which pack they are in. 😉

[Dickw]

1 hour ago, Phil B said:

I agree that the current has to be the same through both.

Probably safer to use a matched pair.

That seems like a sensible idea, but preferably bought at the same time as well.

I have had otherwise identical packs bought nearly a year apart that were noticeably different - the later ones being inferior and the earlier purchases outlasting them.

As Simon says, there is no guarantee that cells in the same pack are all that well matched either and hence the advisability of noting individual cell voltages when charging (or discharging if possible). If a pack goes bad it is often only 1 cell that is the problem.

All part of the daily fun routine.

 

Dick

[PatMc]

3 hours ago, Philip Lewis 3 said:

If the true C rating (internal resistance) is different which it is likely to be then almost cerainly you will kill one pack and that is likely to be the better pack with the higher C rating as the lower internal resistance means that it will try and supply more than it's fair share of the current.  

The pack with the higher IR is most likely to suffer as it will heat up more. 

[Phil Green]

3 hours ago, Phil B said:

... as the lower internal resistance means that it will try and supply more than it's fair share of the current.  

 

Erm....   no.  😃

Edited October 7, 2022 by Phil Green

[PatMc]

You're quoting the wrong Phil, Phil. 🤣

[Phil Green]

Whoops, sorry Phils ! 🙃

 

Edited October 7, 2022 by Phil Green

[Frank Skilbeck]

The problem will mainly occur if the IR and actual capacity of the packs is different, although the current through both packs will be the same the voltage drop across each won't be equal and the ESC LVC will only be watching the overall voltage the higher IR/lower capacity pack could be drawn down to too low a voltage. Not a problem if you land with adequate capacity, but if you are using telemetry to monitor total voltage and use this as a measure of when to land you could accidentally over discharge one pack.

 

The thing to do would be to do a check and look at the voltage drop across each pack under load, to check if they are similar.

[Chris Walby]

Is there any telemetry out there with the capability of measuring individual cell voltages? Ok in this case it would be 12 inputs, but it could be handy on the low cell set-ups to see if one cell is on its way out (like EDF set-ups that have high current draw)?

 

[PeterF]

3 hours ago, Chris Walby said:

Is there any telemetry out there with the capability of measuring individual cell voltages? Ok in this case it would be 12 inputs, but it could be handy on the low cell set-ups to see if one cell is on its way out (like EDF set-ups that have high current draw)?

 

Jeti can do this with their MULI sensor, one sensor measures up to 6S. , They can be connected in series for higher cell packs but as far as I can make out when used for higher than 6S each sensor only does 5 cells. I have not used them for more than 6S.

[Frank Skilbeck]

Multiplex do the Liposaver which does upto a 6s, you could use two to monitor a 12s, it's doesn't actually report each cell, but lets you know if one cell has gone to a low voltage. But it's M Link only.

 

I did see a while ago an arduino based sensor which would monitor 6 cells, so maybe to of those, the one I saw, but I can remember where, was Frsky Open tx.

 

The SM Modelbau Unilog 2 will also record individual cell voltages on a 6s battery and is multi protocol.

 

But you could just plug in a regular cell monitor and do a check on the ground for a weak cell.