2/4 Turnigy battery packs suddenly died?

I was out riding with a friend, we'd wired up 4x identical high discharge turnigy rapid 140C 4s2p rc packs to power his bike in 16s, I was using my usual 13s3p Samsung pack.

We started with the packs all equal, all charged to 4.1V... I have a photo of this actually since I'd WhatsApp'd him a pic to day "ready". We measured the while pack at 65.5V before setting out... Indicating 4.1V per cell.

Suddenly, half way through the ride, his bike lost 90%of power... Turns out 2/4 packs had dropped to virtually 0 (lowest was 600mV on a cell, typically they were at 1.7V). The other 2 packs had cells at 3.65V only about 1.5km before we'd stopped to measure the pack and had 59V implying 3.68 per cell.

Now I'm presuming they're wrecked... But... Wtf happened? How can that happen when they're all in series, and with such a big difference? Nearly half full on the other packs. And such a sudden crash.

If they'd shorted out there would have been signs like... Fire... But they were only mildly warm. And and cables all A-ok...

Any ideas?

I put them on charge... They almost immediately (15 mins at 1.5A on a 5.2Ah pack) went to 3.5V and have since been slowly slowly creeping up over the next hour and a half.

It's like their "flat" voltage is 3.5V or something.

Yeah about 10-15% of those Hobbyking lipo packs are duds when purchased new.
5yrs ago a member on ES tested a whole bunch of HK Lipos

Did you test those lipo packs when you received them new?
Maybe one pack just conked out and took down the brick.

What capacity are those packs ...?.. 5.2 Ah ?
How far had you ridden , total, before this problem ?
What motor current is the bike drawing ?

How can that happen when they're all in series, and with such a big difference? Nearly half full on the other packs. And such a sudden crash.

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Easy... packs in series do not maintain a common cell voltage. If you did not check/ test each individual pack voltage before using, then those two packs could have less charge than the other two,..hence “bottom out” first
Lipo packs are effectively empty at 3.5 x3.6 volts...and their voltage “falls off the cliff” when they are drained.

mxlemming said:

We measured the while pack at 65.5V before setting out... Indicating 4.1V per cell.

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Strictly speaking, this (pack voltage divided by series = V/cell) is only true if balance charged.

It's possible that dodgy HK LiPo could have been or become imbalanced when bulk charging.

Even at 4.1v , they would not be fully charged, so probably less than 5Ah available.
If only a 10A average load, you would not even get 30 mins run time before the pack expired .

mxlemming said:

The other 2 packs had cells at 3.65V only about 1.5km before we'd stopped to measure the pack and had 59V implying 3.68 per cell.

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Not clear on this. 1.5km before power loss, pack was 59V (= 3.68V/cell average), and the two sub-packs that turned out good were 3.65V/cell? If so, the other two sub-packs that died would have to have been higher at 3.73V/cell in order to average to 59V. Seems more likely that the pack was already imbalanced and that these V/cell averages are not reliable.

ES member Icecube57 did the Hobbyking tests.

For a series, if one is low and you bulk charge then the others will be higher and the bad ones will be lower, then over multiple bulk charges the differences get bigger and bigger.

20V+18V+20V = 58V
20V+17V+21V = 58V
21V+16V+21V = 58V
The weak group being the middle, just getting worse and worse after each use and bulk charge.
Because by the time the total voltage reaches LVC, the middle group is super low and super weak and you stressed it superly. Until the weak group plummets because its to low, you stressed it to much, or maybe the internal resistance is getting worse and worse and it has a snowball affect.

Hi, thanks for the replies.

The packs weren't bulk charged. They were balance charged with a hobby king dc-4s all to exactly 4.10V with splitting leads (all connected in parallel so exactly the same).

They were the rapid 140C 5200mAh packs, 4s version.

We've done about 200km on this pack set in 15-20km bursts. This was the longest by a little bit.

I think the graph posted by markz says it all... They fall off a cliff at 3.6V, whereas my 30Q pack falls off a cliff at about 3V.

I think probably what's happened is I've previously always thought I've had 50% left at 3.7V when actually it was more like 5%... And this time the two weakest packs crashed suddenly.

Assumption the mother of all frock ups etc...

One pack may have recovered, the other is a write off. A50gbp lesson then.

fatty said:

mxlemming said:

The other 2 packs had cells at 3.65V only about 1.5km before we'd stopped to measure the pack and had 59V implying 3.68 per cell.

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Not clear on this. 1.5km before power loss, pack was 59V (= 3.68V/cell average), and the two sub-packs that turned out good were 3.65V/cell? If so, the other two sub-packs that died would have to have been higher at 3.73V/cell in order to average to 59V. Seems more likely that the pack was already imbalanced and that these V/cell averages are not reliable.

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Sorry, not clear...
We measured 59V implying average3.6875V
We rode 1.5 km
The thing died
The cells that weren't broken then measured 3.65V

mxlemming said:

The packs weren't bulk charged. They were balance charged with a hobby king dc-4s all to exactly 4.10V with splitting leads (all connected in parallel so exactly the same).

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:thumb: Good risk mitigation

mxlemming said:

I think the graph posted by markz says it all... They fall off a cliff at 3.6V, whereas my 30Q pack falls off a cliff at about 3V.

I think probably what's happened is I've previously always thought I've had 50% left at 3.7V when actually it was more like 5%... And this time the two weakest packs crashed suddenly.

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:thumb: Agreed, but why no controller LVC?

mxlemming said:

Sorry, not clear...

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:thumb: Thanks for explaining

Here is what I know from using RC packs for many years:
They are the most powerful packs in terms of discharge that you can buy. They are designed for that, usually they are discharged in RC models in runs of 4 to 30 min. (So around 2-15C continuous discharge and up to 40C peaks)
But they are NOT designed for long life, you can not expect more than 100 cycles even at low discharge rates.
In some RC applications they are discarded after just 15-20 cycles.

So if you want burst accelerations and racing use this kind of packs, but do not expect they will last long (Racing is expensive)
If you want normal driving, then use Lion cells designed for EV applications, although you will have way lower power, they will last much longer.

fatty said:

:thumb: Agreed, but why no controller LVC?

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Basically because i haven't programmed that yet. Until now it's been bottom of my list. Also a bit of an irritation switching it between pack voltage and 24V lab PSU voltage. Shit excuse really.

_GonZo_ said:

Here is what I know from using RC packs for many years:
They are the most powerful packs in terms of discharge that you can buy. They are designed for that, usually they are discharged in RC models in runs of 4 to 30 min. (So around 2-15C continuous discharge and up to 40C peaks)
But they are NOT designed for long life, you can not expect more than 100 cycles even at low discharge rates.
In some RC applications they are discarded after just 15-20 cycles.

So if you want burst accelerations and racing use this kind of packs, but do not expect they will last long (Racing is expensive)
If you want normal driving, then use Lion cells designed for EV applications, although you will have way lower power, they will last much longer.

Click to expand...

I bought them for double pulse testing. Perfect for that really. Not so great for riding around.