stress/load per cell. small battery pack vs large

[carpetpaul2129]

so far, I've done a couple of repairs, replacing cells in power tools, and am slowly working on building a Ebike battery pack, in the meanwhile, I'm still struggling with technical understanding.

I replaced the cells in my Dyson cordless vacuum, it has 6 cells, quoting 21.6 Volts and 16.2 Amps. Volts x Amps is power, and this comes to 350 Watts, which is what it says on the motor.

1, would I be correct in assuming that just knowing the above information is enough to work out what the battery can power, but not for how long?

2, I could time it if the information was not available ?

upon checking further on the battery pack, it says 2200mAh which I think is 2.2 Ah

3, kWh is Ah x Watts /1000, so 0.77kWh

4, If Amps is a measure of instantaneous current, how do I work out how much current this is per cell ?

5, If I were to have a 6S2P or 6S3P pack to power the same device, I would have more capacity and a longer run time, my guess would be that the current is then shared between those parallel cell, how do I work out how many amps per cell then ?



as ever, many thanks, I will look forward to your replies


Paul

[BreakingOhmsLaw]

1, would I be correct in assuming that just knowing the above information is enough to work out what the battery can power, but not for how long?
Yes, you will need the capacity rating in mAh for that.

2, I could time it if the information was not available ?
You could, but it would be somewhat inaccurate because the current may not be linear throughout the entire process. The better way is to use an electronic load, most have a test mode for capacity.

upon checking further on the battery pack, it says 2200mAh which I think is 2.2 Ah
3, kWh is Ah x Watts /1000, so 0.77kWh
If your battery has a nominal voltage of 21,6V and 2200mAh, you theoretically get 47,52 Watt hours out of it. At 350 Watts, that's about 8 minutes runtime. Sounds about right for a portable vacuum.

4, If Amps is a measure of instantaneous current, how do I work out how much current this is per cell ?
If the cells are all in series, the current is equal for all of them. If not, it is the amp rating divided by the number of parallel circuits - in theory. In practice, that will not be the case.
In your case, as the voltage is 21,6V, divided by 6, equals 3,6V, which is the usual range for a lithium cell. So they are in series.

5, If I were to have a 6S2P or 6S3P pack to power the same device, I would have more capacity and a longer run time, my guess would be that the current is then shared between those parallel cell, how do I work out how many amps per cell then ?
Running cells in groups is not trivial. As they have manufacturing tolerances, the capacity is not 100% the same. So they end up with different charge levels. That is a problem when charging them, too.
This is a very deep topic.  A good place to start is:
https://batteryuniversity.com/
If you need more runtime, use a regular VC, really.

[tunk]

1.
Yes, assuming the cells can deliver the amperage.

3.
At high loads the capacity usually is a bit less.
You can find some tests here: https://lygte-info.dk/

[carpetpaul2129]

haha, what a subject this really is, wow. I can see it will keep me busy for some time

sure, the vacuum cleaner was just a repair, the question was to try and understand the theory slightly more, and applies more to the bike battery pack as it's bigger.

I've decided to upgrade it to 3P from 2P, and just wondered if the cells are less stressed when there are more handling the same load ?

I'm going to be using a balancing BMS, but I can see already that the groups are only monitored 'as a group', so one or more cells could be individually out of balance within that group

I wonder how they deal with this with electric cars they must have some serious numbers of cells, and must also be monitored in groups ?

interesting

[tunk]

I assume you have and will use new, high-amp cells?
E.g. used cells from laptop batteries typically have a ~5A max rating.
If you will use used cells for your battery pack, you should test all of
them for capacity and self discharge, and maybe internal resistance.

[rstofer]

I wonder how they deal with this with electric cars they must have some serious numbers of cells, and must also be monitored in groups ?

interesting

The Chevy Bolt has 96 cells in series and 3 parallel strings to give 350 Volts.  The motor is 150 kW so this would require 429 Amps.  FWIW, the battery pack weighs about 960 pounds.  In "Sport Mode" with "Traction Control" off, the car is pretty sporty.  Not as sporty as my previous Chevy Spark EV with 400 ft-lbs of torque (muscle car category) but still a handful.

[carpetpaul2129]

rstofer :

I have a 2019 Tesla Model 3 Performance, 2014 Vauxhall Ampera (Chevy Volt), 2020 Super Soco CPX Scooter, 2018 Brompton electric folding bike, and a 2015 Dyson handheld vacuum 

electric stuff is great, if only I could actually understand it

Love my Ampera by the way, what a car 158,000 miles and going strong. A very rare car in the UK, the Ampera was a failure here, I think they only sold it for 3 or 4 years, and even the Vauxhall garages have never heard of them. There are currently only 11 cars for sale in the whole of the UK, had mine for 3 and a half years now

[rstofer]

There is almost a requirement for retired EEs to drive electric vehicles.  None of this 'hybrid' nonsense, battery all the way!  The software guys must have had a lot of fun designing the UI.

[james_s]

A lot of this understanding you're looking for is just basic electronics stuff, you could pick up a book like The Art of Electronics.

As far as cars go, I've always seen hybrids as a stopgap, they made a lot of sense when the battery tech wasn't there yet and they will continue to fill a niche for people that have range anxiety or lack the ability to charge at home. Pure BEV is obviously the future but this isn't really the place to discuss that. I drive too little for it to make any sense for me to buy another car but if history is any indication somebody will probably give me a broken one at some point. More than once I've been given cars with minor problems, I've had to turn down even more of them because I had nowhere to put them.