quick battery question for the wise and expirienced

[Rolling_Friction]

hi I have a question. I just got a new turnigy lipo 5000mah 4s pack. What I didn't notice was that it is rated 30-40 c, unlike my other 4s lipos, which are just 30C. is it safe to connect the two in series? would they play nice together? or will fire and carnage ensue?

 

[Ptown_Cruiser]

The best way to know if they will play nice together is to measure the resistance across the terminals for both batteries. If the resistances are the same or close then you are good to go. If you are geared low and don't expect to be pulling a lot of amps you are probably good to go either way.

 

[Hwy89]

Please someone correct me if I'm wrong, but is it a good idea to advise someone to try measuring the resistance of a live circuit? Sounds like a good way to smoke an ohm meter.

 

[friendly1uk]

Most meters have a high input resistance, but still the voltage present in the pack would fudge the reading. You would want a loop impedance meter, which has a low internal resistance and momentarily shorts the pack, looks at peak current, and calculates the pack resistance from that.

I wouldn't get involved.

In effect, the greater pack will supply current more readily, it's voltage will drop below that of the others, so the others charge it.

imo it should be fine, but it's a good question I will leave open

 

[Ptown_Cruiser]

In effect, the greater pack will supply current more readily, it's voltage will drop below that of the others, so the others charge it.

If i am not mistaken I think that is the behavior you get with packs in parallel...

Rethinking my previous answer a little bit: With the packs in series the same current has to be going through them so they should discharge at the same rate. The pack that has a lower C rating should have higher internal resistance, which means that you will get more voltage sag across that battery and it will heat up faster. Basically the 30 C rating battery will be your limiting factor, meaning at best you will get 30C discharge rating out of the two packs in series. However, as always, that is 30C hobby li-po style, which really means like 10C or less in real life if you want your batteries to have any kind of descent longevity. Also, it never hurts to throw in a little more fudge factor with something you are not sure about, so maybe only count on like 7C???

 

[Rolling_Friction]

Basically the 30 C rating battery will be your limiting factor, meaning at best you will get 30C discharge rating out of the two packs in series

That doesn't sound bad to me. Until my other VESC arrives, I want to connect my 4 packs of 4s 30C 5000mah in 2 pairs and run the pairs parallel. so 3 of the 4 packs are 30C and one special snowflake is 30-40C. based on the replies (thanks btw) I seems that whatever abnormalities the 30-40C pack brings to party, will be smoothed out by the rest.

this sound safe enough to test out. Now I don't have a proper Ohm meter, so for now I guess I will just take short test drives and compare the the individual voltages of every cell with a battery doctor device. If they all discharge evenly then I guess its fine.

so long as this pack doesn't cause a revolt( get it? :wink: ) in the battery bay I'm happy.