Is higher voltage better for laptop 18650 packs?

[titusmc]

Hi everyone -

I have a seemingly simple question, but I'm curious about the implications for the rest of the e-bike system.

I have probably 800 or so 18650 cells recovered from discarded laptop packs that I have tested for capacity at 1A discharge using 5 Foxnovo 4S units (20 cells can test simultaneously). My current bike setup is a 48V 1000W front hub motor (eBay generic with silver "rings") run by a grinfineon controller (24-48V, 20A - Model C3620-NC) and Cycle Analyst V3. The battery is currently 13S7P (48V) of recycled laptop cells (all above 2 Ah) and has pretty significant voltage sag under load (5-10V). My performance requirements are very modest - slow acceleration doesn't bother me and I typically cruise at 18-20 mph (400W or so).

My question is the following: When using recycled laptop cells, is it better to build packs with higher voltage (say 20S, 72V) because this will lower the current draw per cell from the pack given the same power and number of parallel cells in each group? My thinking is that as long as I limit the power using the cycle analyst I should get much better battery performance in terms of range because of the lower discharge rate and lower voltage sag. Of course, the obvious alternative is simply more parallel cells, but I also like the idea of lower current through the whole system.

I know I will need the 72 volt grinfineon controller for this, but I've seen threads suggesting that the eBay DD hub motors can take higher voltage well assuming the current is kept in check.

What would happen to the motor and controller efficiency (flats and hills up to 10%) when running this setup limited to 1000W and 20 mph? My thinking is that any drops in efficiency there will be overshadowed by the improvement in battery performance.

Thanks,
ES is awesome!

 

[Wheazel]

Thats significant voltagesag, and to fix this with the cells you use you simply need more cells to share the load.
You are on the right track.

Lets make an example:

Two individual batterypacks, one built as 12s8p and the other as 8s12p.
They have the same amount of cells, and the same amopunt of energy (Wh)
But they differ in voltage and capacity.

What you need is simply more cells to share a spefic load. If you went from 13s7p to 20s7p, you have alot more cells that share the load and you get a lower sag for a specific power.
You can achieve similar results by keeping 13s but putting more cells in parallell aswell. (The current wont decrease for the power required, but more cells share the current load)

 

[Punx0r]

For a given load and number of cells in the pack, current per cell is exactly the same, as power remains constant - your series/parallel arrangement just changes. E.g. doubling the voltage will halve the current required by the load, but you also have half as many cells in parallel (since you have twice as many in series). The only thing you can do is increase the battery size by putting more cells in parallel at the same voltage.

 

[titusmc]

I guess my intent wasn't perfectly clear as there are clearly two ways to reduce the sag from laptop cells. I understand that for the same number of cells it makes no difference because the load per cell is identical, but I am also asking what happens to the controller and motor efficiency if you have a higher voltage battery - if it improves motor efficiency (presumably due to lower current for the same power, hence lower I2C losses), then that is a reason to choose a higher voltage pack from laptop cells rather than a higher current pack (where both can reduce the load per cell).

Does that make more sense?

I am trying to plan out a long distance trip (say 100 miles) from Upstate NY into NH where I will be riding across Vermont, so I'm trying to see what e-bike system changes/upgrades make the most sense. My normal work commute is 8 miles one-way and dead flat, so a hub motor is perfect, but the hills in VT may be too much. I understand mid drives are great for hills, but I'm reluctant to give up regen and I'm concerned about reliability and drive-train wear. I was considering a low KV hub motor like the Crystalyte H3525, running it at 72 volts? My thinking is that it would provide lots of low speed torque for climbing, and more than enough top end speed for me (25 mph is my desired max, cruising at 18-20 mph). I have 26" wheels with Maxxis Hookworms.

I'd appreciate thoughts - but I realize now that this is less of a "battery technology" question than I was originally thinking when I posted it here - sorry!

 

[999zip999]

I like higher than but harder to find travel charger. Do you have a bms ? How you monitoring your individual cells ? Or parallel strings ? I say 14s10p of used cells.

 

[titusmc]

I am using a balancing BMS from SuPower Battery (http://www.batterysupports.com/) on my 13S7P pack, and purchased a 16S BMS to use on a future 14S8P pack I am building (their support person told me it would still work for 14S with 2 balance leads left unused). The parallel groups are balanced using software I wrote in Python and are matched to within 18 mAh for a 20.4 Ah pack (pretty good, I think, considering it is likely within the test error).

I originally decided on 14S since it is the maximum voltage compatible with 48V controllers, but I have since been wondering about the benefits (if any, other than top speed) of moving to 72V down the road...

I am also still debating individual cell fusing using 4A PTC fuses like shown here (https://www.youtube.com/watch?v=5EE4TsuzD_Q). My first pack didn't use them and I see no commercial e-bike packs using them, but my intuition says they should be there (Tesla does something similar, after all).

The pack I am building is a 14S8P hybrid pack that has 14S1P of Samsung 20Q from new Ryobi drill packs, and 14S7P of matched recycled laptop cells (all individually better than 2450 mAh). The question then becomes, should the 20Q's have a higher-rated fuse than the others since they are intended to provide current surges? I'm thinking 8A fuses for the 20Q's and 4A fuses for all other cells. This is based on assuming the 20Q's have roughly 1/3 the DCIR, and hence would supply 3 times the current of a laptop cell under load - so for the max of 20A, algebra says 6A from the 20Q's and approximately 2A from each laptop cell (<1C).

Thoughts anyone?

Thanks,
titusmc

 

[Punx0r]

The only thing changing voltage will do is change your top speed, - unless you intend to use a gear reduction, then you can design an appropriate reduction to get the motor spinning at a speed good for efficiency that is different to your wheel speed. However, if you're using a direct-drive motor or one with a fixed reduction then you have no choice. You should select a system voltage that best suits your desired speed and available components.