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Building a big battery pack for EV (motorcycle)
#1
Good day all, 

My first post here, kinda new to the battery world as well Smile.

I am going to convert a Suzuki GS500 from a combustion engine to an electric motor.
Now I've done the calculations and I would need a 72V battery pack with about 70Ah, totalling to about 5kWh.

To build this battery pack using 1800mAh cells, I'd need to build a pack of 20s40p.
So my question is what would be the best configuration to build this pack?
I've read that building the parallel blocks first would be my best bet, so would I need to build this into packs of 40 (8*5) and build 20 of these in series?

I would also like to use a BMS on these packs, as I'd like it to be as safe as possible.
How would I install the BMS on these packs?
I assume I'd need to install a BMS per parallel block or would this need to be installed in the 20s40p total?


Please let me know if you require any additional information, all help is very welcome!

Regards,

Hans
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#2
Nice bike, almost no electronic inside, and very hard.

I think first you have to check the space available for the batteries, and then make a custom pack, with the shape of the free space, comercial cell holders are 5x4, 6x3 or so, making a full battery putting 20 packs of this kind will make your battery very square, maybe just better to make a big block of cells with the shape  needed. As this video https://www.youtube.com/watch?v=K0lDrhwqPRc

Regards
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#3
Thanks for your reply.
I will see if it works with squared blocks first and if it doesn't Ill look into your idea Smile

I would also like to know about the BMS, is anyone able to help me with that?
I assume I would need a 20S bms, but I haven't been able to find one that is able to hold the 400A of peak current that my bike will pull.

Anyone got any ideas?

Regards,
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#4
Please some one correct me if I'm wrong, but I think in electric cars and bikes the BMS is only for info or balancing while charging not to pass the 400A through the BMS in discharging mode.
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#5
(07-13-2018, 12:53 PM)MrDobby Wrote: I assume I would need a 20S bms, but I haven't been able to find one that is able to hold the 400A of peak current that my bike will pull.

20s does not equate to Amperage. It only equates to Voltage. The cells in parallel is for Amperage. 20s would only be 84V - 60V (4.2 - 3.0V).
If you want 400A, then you will need either high drain cells rated at 5C or more, or go with lots of laptop cells in parallel. To get 400A from laptop cells, you'd need about 300 to 500 (that's 1.3A per cell to .8A per cell respectfully).

Now, if you mean 400 Watts, that's different. 400 W @ 74V (nominal) = 5.4A per parallel set. So, if you have 20s10p, that would be ~500mA per cell.
Proceed with caution. Knowledge is Power! Literally! Cool 
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#6
You don't say if you are planning to use new cells ??? I think new is essential for a bike ...second use cells just have too high internal resistance to work well at high drain ....

I would dispense with black plastic cell holders , it will make the battery too bulky .... spot weld them together close pack hexagonal style as ebike builders do ...many Youtube videos on this ...
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#7
(07-13-2018, 09:38 PM)Korishan Wrote:
(07-13-2018, 12:53 PM)MrDobby Wrote: I assume I would need a 20S bms, but I haven't been able to find one that is able to hold the 400A of peak current that my bike will pull.

20s does not equate to Amperage. It only equates to Voltage. The cells in parallel is for Amperage. 20s would only be 84V - 60V (4.2 - 3.0V).
If you want 400A, then you will need either high drain cells rated at 5C or more, or go with lots of laptop cells in parallel. To get 400A from laptop cells, you'd need about 300 to 500 (that's 1.3A per cell to .8A per cell respectfully).

Now, if you mean 400 Watts, that's different. 400 W @ 74V (nominal) = 5.4A per parallel set. So, if you have 20s10p, that would be ~500mA per cell.

Hi Korishan, 

thank you for your reply.
Sorry if I haven't been clear, what I thought was happening was that the BMS would also need to be able to hold the 400 Amps, but I now understand the 90A on the BMS that I found was for charging only.

I am going to use a LEM D127 motor, which has a peak current of 400A (25,4kW).
I was planning to build a battery pack of approx. 20s40p, which would give me about 5,5 kWh.

As I understand this will give a peak drain of 25.400W @ 74V = 343A per parallel set?
Which would give a peak drain of 8,5 Amps per cell?
Would this be possible to do with old laptop cells or am I asking too much?

Regards,
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#8
25.4kW at 72V is 353A but you need some headroom anyway so let's assume 400A at 72V. In a 40p setup we're talking about 10A per cell. You couldn't do this with laptop cells even if they were new. I suggest you get some Samsung INR18650-25R or INR18650-30Q. Ideally you would just get bigger, prismatic LiFePO cells but I guess you need small cells to be able to build an odd shape?

If you absolutely want to use old laptop cells then your battery has to be five to ten times bigger, 20s200p at least, but ideally more like 20s400p. Needless to say that this is going to be huge and heavy, you'd need a trailer for that. Just the cells would be 360kg without mounting are wiring and any connection.

As for the BMS, I probably wouldn't use one. With new cells the cell drift will be very low for quite a while and later on I would think about bottom balancing. It's correct, if you use a BMS and the load output goes through the BMS it will have to handle the 400A. This is not going to be a small and cheap unit, you'd need to get something serious. A way of balancing the cells while having the load connected directly to the battery could be useful in this case. But I've no idea what sort of BMS the DIY EV builders use if they use one in the first place.
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#9
(07-16-2018, 08:22 AM)DarkRaven Wrote: 25.4kW at 72V is 353A but you need some headroom anyway so let's assume 400A at 72V. In a 40p setup we're talking about 10A per cell. You couldn't do this with laptop cells even if they were new. I suggest you get some Samsung INR18650-25R or INR18650-30Q. Ideally you would just get bigger, prismatic LiFePO cells but I guess you need small cells to be able to build an odd shape?

If you absolutely want to use old laptop cells then your battery has to be five to ten times bigger, 20s200p at least, but ideally more like 20s400p. Needless to say that this is going to be huge and heavy, you'd need a trailer for that. Just the cells would be 360kg without mounting are wiring and any connection.

As for the BMS, I probably wouldn't use one. With new cells the cell drift will be very low for quite a while and later on I would think about bottom balancing. It's correct, if you use a BMS and the load output goes through the BMS it will have to handle the 400A. This is not going to be a small and cheap unit, you'd need to get something serious. A way of balancing the cells while having the load connected directly to the battery could be useful in this case. But I've no idea what sort of BMS the DIY EV builders use if they use one in the first place.

Hello DarkRaven, 

As I understand, old cells are out of the question here?
I have been looking into the INR18650 Samsung cells as well, but I came up to about 800 cells which I needed to have some range on the motorcycle, which was out of my budget.

Furthermore I was wondering about how to charge the 72V system?
Seen some 72V chargers on eBay and AlieExpress, but I don't trust those kind of chargers.
Is a balance charger needed (as you say a BMS is not)?

Regards,
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#10
For charging 72V I used a 2800W Cisco Catalyst power supply to provide 50V and then used a couple of 1500W boost converters running at 1000W each as they are just CC CV boost. Provided me with 2000W charge capacity for $30 for the PSU and around $17 per boost unit.
This is the post of the power supply and the boost units


26650 5000mAh cells $3.20 new from bak in China, sold under LittoKala re-branding (cell test data) - I used 40 of these for an electric skateboard, spot welded pack. Can run over 1C but I would stay at 1C or lower.Cycle life is around 500 cycles at 100% DoD so run them 85% capacity.

There are fakes of these - they either have missing serial number or the serial number does not change between cells...
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