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Testing cells EV battery module
#11
Like for instance I can’t power a regular house light bulb directly off the battery.. it’s 60v not 120v
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#12
(10-05-2019, 09:45 PM)Doin it Wrote: I was thinking that maybe the entire module could have low capacity but i thought if all the cells voltages match after resting that all the individual cells are of similar capacity

Given the quality of cells normally used in EVs it's a reasonable assumption to make that the capacity of all cells will be reasonably well matched (but unless you test you'll have no idea what the usable capacity actually is) - discharge testing across the entire pack, while monitoring each cells voltage will illustrate the weaker/stronger cells.

(10-05-2019, 10:21 PM)Doin it Wrote: Like for instance I can’t power a regular house light bulb directly off the battery.. it’s 60v not 120v

Assuming it's not a fluorescent or LED an old fashioned nichrome lamp will have a resistance, so yes you could use it - but you'll have to measure the actual current draw, and derive the actual wattage - a number in parallel might get you close to a realistic load of perhaps 500w to reflect a baseload consumption - remember you'll need to plot voltage and time.

There's plenty of capacity testers available, just not many that can accurately test, within a reasonable timescale the large format cells.
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#13
If the capacity of an individual cell (while there isn’t any charging or discharging and battery has rested) is different than another cells capacity in the same module then will that individual cells voltage be different than the other cells voltage?
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#14
(10-05-2019, 10:41 PM)Doin it Wrote: If the capacity of an individual cell (while there isn’t any charging or discharging and battery has rested) is different than another cells capacity in the same module then will that individuals cell voltage be different than the other cells voltage?

It is impossible under standard load conditions for cells in parallel to have different voltages. Voltages will always balance out with the other cells in parallel.

A lower capacity cell will also not really drain faster than a higher capacity cell, generally. Again, under standard load conditions.

A non-standard load condition would be "heavy" current draw that is pulling the max the cells can output, and the connecting material between cells if it can handle the current as well. But this is usually accomplished with a dead short and you have other issues to worry about then.
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#15
Thx for the explanation.. I think I’m going to just test the capacity while supplying the inverter and just make sure I have the low and high voltage cutoffs set conservatively until I know the actual capacity of the battery

Ok korishan (under standard load) I have 2p so your saying if one cells voltage would be lower or higher than all the other cells than that difference would be shared with the cell the higher or lower cell is paralleled with? So then that set (2) of paralleled cells would have a higher or lower voltage than the rest of the parallel sets?
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#16
(10-05-2019, 10:41 PM)Doin it Wrote: If the capacity of an individual cell (while there isn’t any charging or discharging and battery has rested) is different than another cells capacity in the same module then will that individuals cell voltage be different than the other cells voltage?

That's not a safe assumption - a practical illustration would be Dala's fault finding his single leaf module, all appeared normal at high states of charge (so balanced voltages) but under discharge a significant imbalance was seen.

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#17
Ok I got ya Sean.. so this is why I cell needs to be tested while being discharged and charged to confirm capacity-health of the cell....?
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#18
(10-05-2019, 10:59 PM)Doin it Wrote: Ok I got ya Sean.. so this is why I cell needs to be tested while being discharged and charged to confirm capacity-health of the cell....?

Yes - and despite what was mentioned above, cells arranged to form a parallel pack will only be at the same voltage when there is no charge/discharge current - under any charge/discharge there WILL be a differing voltage on each cell, the voltage delta across the parallel pack will depend on how closely you follow accepted best practice for physically interconnecting your parallel cells, and the series connections.




There is some weirdness going on with the quote button.

Given the obvious fact that every cell will have a voltage delta across them, that will be current dependent (unless you have zero resistance interconnections) the lower capacity cells WILL see a faster voltage drop across their terminals.

(10-05-2019, 10:45 PM)Korishan Wrote: It is impossible under standard load conditions for cells in parallel to have different voltages. Voltages will always balance out with the other cells in parallel.

That's simply not correct - with no charge/discharge there will be an immeasurable (using common DIY test equipment) voltage delta across the parallel cells - under loads typically seen in an ESS (anything between zero and x hundred amps) the voltage delta in a pack of parallel cells is easily measurable with almost any digital voltmeter.

Another pictorial illustration of my point .... 

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#19
1) I completely agree with what @Sean is saying... cell voltage has no relation to cell health, capacity, quality, or even the actual balance levels.  You could have a small pack with uneven capacity cell groups, and balance that battery at one specific voltage.  The battery will be balanced and have the same voltage at that one specific state of charge.  For example a common would be a top balance, or a bottom balance. 

2) Testing (Hobby charger, constant current load and timer, power analyzer / watt hour counter.)  If you cant find a resistor load or something to load up with 60v, you would always get a DC-DC step down and use something else too.

These little inline Watt Meter & Power Analyzer things that cost <$20 are awesome... great for testing random things, but I forgot what its max supported voltage it.  Might stop at 60v


The Victron Battery Monitors are also good options if you need / want something like this anyways.
https://www.victronenergy.com/battery-monitors

If you an break out the cell groups, a RC charger like this would also work well... my preferred method as it also allows you to balance the pack.  Not instructional, but this is how I was testing my EV modules... just to give you an idea of what type of info you might see with a charger. This would could do 20s total, assuming I can break it out into two 10s cell groups.



Electronic DC Load (not a bench power supply) has also been a great tool for battery work.  Here I have an 8 amp load on this PCB for testing. BattGo to monitor cell groups.



Hope that gets the creative juices flowing.  Lots of ways to do this.
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#20
(10-07-2019, 05:32 AM)Crimp Daddy Wrote: 1) I completely agree with what @Sean is saying... cell voltage has no relation to cell health, capacity, quality, or even the actual balance levels.  You could have a small pack with uneven capacity cell groups, and balance that battery at one specific voltage.  The battery will be balanced and have the same voltage at that one specific state of charge.  For example a common would be a top balance, or a bottom balance. 

2) Testing (Hobby charger, constant current load and timer, power analyzer / watt hour counter.)  If you cant find a resistor load or something to load up with 60v, you would always get a DC-DC step down and use something else too.

These little inline Watt Meter & Power Analyzer things that cost <$20 are awesome... great for testing random things, but I forgot what its max supported voltage it.  Might stop at 60v


The Victron Battery Monitors are also good options if you need / want something like this anyways.
https://www.victronenergy.com/battery-monitors

If you an break out the cell groups, a RC charger like this would also work well... my preferred method as it also allows you to balance the pack.  Not instructional, but this is how I was testing my EV modules... just to give you an idea of what type of info you might see with a charger. This would could do 20s total, assuming I can break it out into two 10s cell groups.



Electronic DC Load (not a bench power supply) has also been a great tool for battery work.  Here I have an 8 amp load on this PCB for testing. BattGo to monitor cell groups.



Hope that gets the creative juices flowing.  Lots of ways to do this.


Ive had a couple of those little inline meters, mine were calles WattsUP. They all maxed out at 60V and 40A. They are awesome but I traded up for a WestMountainRadio PWRcheck.
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