At what voltage your pack reaches 100%

ESK8 Electronics
81

2.5V under any load is empty. That’s what I meant by absolute minimum. Under no circumstances should your battery ever be under 2.5V regardless of the no load or rest voltage.

The battery cannot have an empty no load voltage as you voltage sag varies with current draw. How would manufacturers define empty voltage? 2.5V at no load 2.3V at 5A 2.0V at 10A 1.5V at 15A You see how fast that gets confusing plus the cell only cares about the actual voltage and not the no-load.

Its easier to determine a voltage which you should stay above under any current draw. This is why the higher your current draw the lower the capacity of the cell is.

E: @Okami 25R at 1A to 2.5V= 2.4Ah 25R at 10A to 2.5v= 2.3Ah

If you slow down the current draw from 10A to 1A keeping the voltage at 2.5 (like what happens when you charge), you would get almost the same capacity (difference would be losses due to heat at higher current draw)

1 Like
82

@PXSS @Okami Not sure if you realize it but you are making a very good argument for using a bms for discharge. Seeing that a bms will monitor individual cell group voltage and shut you down if any cell group dips below a safe level while under load.
I ran my Li-ion battery until the bms shut down and there was no damage to the battery. My low voltage trigger was set at 2.8v

83

Yeh, cutting down power once cell reaches certain ‘‘treshold’’ sounds reasonable…

Though… it pinpoints the ‘‘problem’’ which you are talking about… if load is not big, you can continue to ‘‘sip’’ the power from battery for longer period of time before it gets drained…

I’ve also seen charts where energy count graph is cut short, since load for cell was way higher… and the process was stopped way earlier… than when cell was discharged at very little load, for example…

@Namasaki

I suspect the ‘‘danger’’ might be way higher for these who dont use any other cell monitoring methods or ‘tactics’…

Generally, I think it can be felt quite easly if battery / pack is quite empty or close to depletation… setting some limits or ‘warnings’ of course is a good choice and just provokes / sustains longer battery life… if we assume cells might get a bit more damaged if overdischarged regularly…

On a side note… if talking about BMS’es, I think they can be ‘‘poked around’’ a lot… but the final fact, conclusion i have come to, is that they need to be functioning or of high quality not to cause more problems than they are meant to solve… have heard of too many stories of faulty bms’es which cuts power way too early, stops working, does not balance cells properly or cause some other issues.

So far Raphael Chang’s bms seems to be the most promising… but I believe he is only in early production phase as the first bms’es are yet to be received by the first customers / testers…

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Sorry for taking this reply so long, sometimes it just happens when I want to adress more than one issue / argument to which I want to respond…

Though I still find it interesting that I was able to draw enough power to propel the board forward… but non the less, the last 5-10 mins i had of it riding slowly was not very ‘enjoyable’ in a sense that the lack of power could be felt drastically.

And to those who might not be aware, I did that on purpose to see how low the cells can go and what rest voltage will they sit, since there are still discussions about when power should be cut and when some time ago I came to conclusion that 3.3-3.4v no load voltage is a somewhat good point, some ppl were suggesting to deplete them further since there was still energy left…

84

I agree totally and that is why I cringe whenever I see anyone saying that they just want to buy a cheap bms.

85

How is your power supply holding up?, I’ve read too many DOA stories on the US$ 65 power supply I found on Amazon, I wonder if I should just pull the trigger and buy a better power supply like your Korad

86

The one I got works perfectly. It’s has a fan that runs the whole time its on and it never gets hot or even warm even when charging at 5a. It has good reviews which is partly why I pulled the plug and bought the more expensive model. I think it’s a good idea especially if your gonna be charging on a daily basis. This one also has a really good feature: There is a main power switch that you turn on and set the power settings and then there is another switch that enables the output.

87

I’d like to know if you were able to complete balance you unbalanced cells with only one charge on this power supply

88

Surfing today and tomorrow. We have a swell that peaked today and hopefully some leftovers tomorrow. I’m gonna try to get the test done by Saturday and I’ll post the results as soon as I have them. Both batteries are over 39v now so I want to drain one down at least the nominal and record the voltages before and after charging.

89

I pulled the trigger and got this one, you really convinced me. It’s the same as yours but only rated at 3A instead of 5A, I don’t mind waiting a bit longer to charge a pack, just need to keep them balanced.

Hope it has the same features as yours.

I’ll wait eagerly on your charging results

1 Like
90

It’s exactly the same as mine except for the amperage output. 3a should be fine.

91

This is not a problem. It’s just how batteries work… Less wasted energy by drawing little current.

The process was not stopped short. It was stopped when the cell reached 2.5V. You need to forget about the energy count, the rated capacity is just what the manufacturer guarantees the cell will output at a specified current. This would change if you ran it faster or slower, it’s just how the chemistry works.

@Namasaki, ish… If you set your esc limits correctly it shouldn’t be an issue but if you’re trying to empty the cells constantly and are not properly balancing your cells then as @Eboosted found out, you will kill your cells. Having the battery discharge through the BMS would have saved his pack.

There is nothing wrong with the BMSs, its the combination of BMS and crappy charge. I think I mentioned it in another thread already. The reason @Eboosted BMS stops his brock charger from balancing is because the brick charger has a minimum current output, once the current gets too low in cv mode then the brick stops charging so it doesnt damage the cells. The BMS from batterysupports only consumes like 80mA when balancing, if this is lower than the cutoff current then the brick charger will cutoff current before he gets to the balancing stage. Since he didn’t have a low voltage cutoff on the individual cells then the ESC only stopped working when the whole pack was under a specified voltage. So max voltage per cell is 4.2V and I’m assuming his ESC low voltage cutoff is 30V for a 10S pack. Then he can have 7 cells at 4.2V, 1 at 0.6V and 2 at 0V before his ESC and BMS limits converge. At this point 3 of the cells in the pack are dead.

2 Likes
92

Well, from my own tests I found out the brick charger stops as soon as the first pack reaches 42V, I guess in this stage BMS restricts current draw and brick charger stops, so the charging cycle stops because a combination of the two.

93

Doesnt this suggest that voltage should be higher when using brick charger + bms?

How much over-voltage can bms take?

94

Yeah, the brick charger has a minimum current which is higher than the balancing current. That would be why it stops as soon as the BMS reaches the balancing stage.

The reason lab power supplies work is because they can go down to miliamps

95

If the assumptions from this thread are correct, the charge shouldn’t exceed 42V, it should be lower at e.g. 41V (set with an adjustable CC/CV power supply) but the current shouldn’t stop flowing, this would allow all empty cells to be charged at 41V at minimum current (CC mode on the power supply), after that you could normally charge the remaining 1V untill 42V normally (power supply or brick charger), until this stage BMS shouldn’t have been working at all.

However if the cells are in fact 99% balanced, BMS will work blancing them as soon as the whole pack reaches the balancing voltage, 42.7V for the Bestech BMS, not sure about the Battery Supports one, but I guess somewhere below that.

96

@Eboosted The BMS will start working as soon as ony one cell reaches 4.2V. This could be at 26.7V if all of your other cells are at 2.5V. You don’t need to be afraid of the BMS doing its job. The reason your stuff failed is because you were just using a bad combo.

Seriously, read the following slowly…

Brick Charger, 42V 4A output cutoff at 400mA

BMS Input 42V, 20A Balance current: 120mA

If you use these two, your board will NEVER balance because the balance current is lower than the cutoff of your brick.

Here’s a different example:

Brick Charger, 42V 4A output cutoff at 50mA

BMS Input 42V, 20A Balance current: 120mA

In this case, your BMS will do its job and balance your pack just fine. This is because the cutoff for your brick charger is lower than the balancing current.

It has nothing to do with the voltage of your brick charger, you can have a 60V charger, if the cutoff is higher than the BMS balance current then the BMS will NEVER do its job.

2 Likes
97

Thanks for taking the time to explain, would you share where to find the Lab Power Supply charger that that you use?

From the Battery Supports web, their 42V 60A BMS has 60mA and starts balancing at 3.90V

Technical Parameters: Balanced current: 60mA (VCELL = 3.90V when) Balanced for: 4.20 ± 0.05 V

So the charger should go lower than 60mA

98

I have a brick charger from alibaba through a groupbuy, but yes, you got it now, your brick should charge all the way down to 60mA

99

Damn, it’s really hard to find a brick charger like yours. Sellers do not usually state this info on the spec sheets.

100

I have a 5a brick charger from battery supports and it shuts off befor the pack is full.