Battery speed boost

[DumBum]

I already have a 16s8p battery, would it be possible to make a 2s8p with its own bms etc. (same 18650) backpack speed boost battery, and connect it in series? Therefore I could toggle it from being 16s8p to 18s8p. Thx

 

[Wattman]

In theory, it can be done of course but here is the problem. Better to have both packs to be charged to identical voltage when throwing the switch. How feasible is that given the premise of your question which is an on/off switch presumably out on your ride? You want to use the small additional battery as a nitrous bottle to give you a burst of speed. Nice idea but the practice is a bit more complicated.
If you leave the house running on the big pack, the big pack will deplete a bit and not be voltage matched to the small pack when you switch it on. What happens then? There is current transference from small pack to big pack because they aren't at the same voltage. All cells with the same parallel amp-hr's want to be at the same voltage. Depending on what the voltage drop is, that won't bode well for cell life over time.
I personally wouldn't do it but of course your call and my thoughts.

 

[Sturmey]

Your suggestion reminds me of the 'End Cell Switch' that was used on old 50 volt strowger telephone switch power supplies to boost the battery under load during power failures. Anyhow, it could be implemented something like below in your case but I'm not sure if its a good idea. It would be important that the 2S 'end cells' would be protected from possible total discharge.

 

[BVH]

Far better to decide before you begin the ride if you're going to ride with 2 paks ON and in parallel for the entire ride or start out on the small pack, deplete it, turn it off, turn on the big pack and deplete it. (or vice-versa) I have two paks and run them in parallel every time I ride. You're being nicer to you packs when running them in Parallel, both are sharing the load.

 

[DumBum]

In theory, it can be done of course but here is the problem. Better to have both packs to be charged to identical voltage when throwing the switch. How feasible is that given the premise of your question which is an on/off switch presumably out on your ride? You want to use the small additional battery as a nitrous bottle to give you a burst of speed. Nice idea but the practice is a bit more complicated.
If you leave the house running on the big pack, the big pack will deplete a bit and not be voltage matched to the small pack when you switch it on. What happens then? There is current transference from small pack to big pack because they aren't at the same voltage. All cells with the same parallel amp-hr's want to be at the same voltage. Depending on what the voltage drop is, that won't bode well for cell life over time.
I personally wouldn't do it but of course your call and my thoughts.

That indeed was my thought at the beginning, but I came to same conclusion as you, that's why second pack would be turned on all the time, it would be build on same cells as the big one and would have same Ah.

 

[Wattman]

Far better to decide before you begin the ride if you're going to ride with 2 paks ON and in parallel for the entire ride or start out on the small pack, deplete it, turn it off, turn on the big pack and deplete it. (or vice-versa) I have two paks and run them in parallel every time I ride. You're being nicer to you packs when running them in Parallel, both are sharing the load.

Indeed you are correct about being nicer to the packs connnected in parallel because of additive capacity but of course this doesn't not serve the OP's objective of more speed. I am quite sure you know that and quite right, if both batteries are charged to the same voltage and the switch in thrown at the beginning of the ride, there should not be an issue. But, takes this discipline of course.
PS. I have switching hard drives on my computer builds. I am very careful not to turn on a hard drive or two with the computer on. Some care is required but really comes in handy for keeping a back up operating system and files you want to isolate from the computer when running.

 

[Wattman]

That indeed was my thought at the beginning, but I came to same conclusion as you, that's why second pack would be turned on all the time, it would be build on same cells as the big one and would have same Ah.

I believe you should be ok with that strategy if both batteries are charged to the same voltage. A far more common practice of course isn't a speed bump but a capacity increase by connecting batteries in parallel.
What I do personally is just build a fast battery, controller and motor combo and rarely use the upper speed level. But when I do when having fun with homies, its there. Also, more batteries in series in theory does add more watt-hrs because of the equation W = A X V so if you don't use the amps which translate to speed, you should have more capacity with a higher voltage in aggregate battery aka two batteries combined in series.

 

[From-A-To-B]

Just wanting to point out that both packs being charged to the same voltage, for fear of a rapid equalization between the two, is irrelevant if we’re talking about connecting these packs in SERIES to provide more voltage for a “speed boost.” Packs connected in series don’t equalize, they add voltage. Packs connected in parallel do equalize and need to have the same voltage when connected.

However, two batteries at different SOCs will finish their discharges at different times, so a main pack that’s half depleted when the “boost pack” is plugged in will hit LVC before the boost pack, if the two batteries have the same capacity (p-groups).

I agree that the simplest option is to run the two packs together all the time, or rather, just get one larger battery in the first place.

Make sure your controller and accessories (throttle, display) will tolerate the higher voltage lest you let out the magic smoke.

Or use a higher voltage battery/system with a controller that has a speed switch so you can electronically turn down your performance as desired.

 

[Wattman]

Just so I understand A to B and thanks for sharing your wisdom on this, therefore other than SoC of let's say the bigger battery after a few miles into the ride and throwing the switch to the fresh and higher voltage battery for a 'speed bump' is this perfectly fine with no downside??...because that is what the OP is asking. I am curious as well. Sounds as though other than the bigger battery depleting faster than the smaller battery with same number of parallel cells in each battery..just no. of series difference, there is no other concern, is this correct?
Thanks again.

 

[DumBum]

Just so I understand A to B and thanks for sharing your wisdom on this, therefore other than SoC of let's say the bigger battery after a few miles into the ride and throwing the switch to the fresh and higher voltage battery for a 'speed bump' is this perfectly fine with no downside??...because that is what the OP is asking. I am curious as well. Sounds as though other than the bigger battery depleting faster than the smaller battery with same number of parallel cells in each battery..just no. of series difference, there is no other concern, is this correct?
Thanks again.

Not really, from what I understand your idea is to switch to higher voltage battery. My idea orginalny was to boost main battery voltage by adding a battery in series.

Main problems with this idea
1.If the boost battery is smaller capacity it could run out/deplete completely and shut down the circuit, before the main one.
2.If it is used, while the main battery is low, main battery might not supply enough amperage as the fresh one.

Modified idea. Run two packs 16s and 4s to create 20s all the time. Main problem: charging it ig? Correct me if I'm wrong, but ig it should work.

If I could I would just build battery with higher voltage as some of you were saying, but I already have a 16s battery and some spare cells at home to build the small one. Also there is literally not a centimeter left for more cells in a "triangle of my bike", that's why I am planning to either make a backpack battery or to attach it to down tube.

 

[DumBum]

Just wanting to point out that both packs being charged to the same voltage, for fear of a rapid equalization between the two, is irrelevant if we’re talking about connecting these packs in SERIES to provide more voltage for a “speed boost.” Packs connected in series don’t equalize, they add voltage. Packs connected in parallel do equalize and need to have the same voltage when connected.

However, two batteries at different SOCs will finish their discharges at different times, so a main pack that’s half depleted when the “boost pack” is plugged in will hit LVC before the boost pack, if the two batteries have the same capacity (p-groups).

I agree that the simplest option is to run the two packs together all the time, or rather, just get one larger battery in the first place.

Make sure your controller and accessories (throttle, display) will tolerate the higher voltage lest you let out the magic smoke.

Or use a higher voltage battery/system with a controller that has a speed switch so you can electronically turn down your performance as desired.

Exactly. Everything is already able to work up to 20s. Can you point out, whether I missed anything in my last reply? Thx

 

[From-A-To-B]

Exactly. Everything is already able to work up to 20s. Can you point out, whether I missed anything in my last reply? Thx

Building a new 4s pack, of ideally the same cells, to same capacity to connect to your existing 16s pack for entire discharge cycles makes more sense to me than trying to do something tricky with extra batteries that are connected only some of the time. The two batteries will discharge more in sync, and you’ll spend less time hammering a battery at its BMS-enforced LVC.

You can either buy a 4s li-ion battery to charge the pack separately, or you might be able to buy a 20s li-ion charger and charge the two while they are connected in series. In the latter case, there may be an issue with the BMS’s seeing up to 84V, given they were designed for lower voltage use. Other ES members would have more to say about this than I do.

So the safest thing to do is to charge them separately. A hobby charger might suffice for the smaller pack.

Keep an eye on balance in both batteries throughout their service lives. You’ll now have two batteries to monitor instead of one. And as always, don’t use cheap BMSs and expect them to do their jobs.

 

[From-A-To-B]

Just so I understand A to B and thanks for sharing your wisdom on this, therefore other than SoC of let's say the bigger battery after a few miles into the ride and throwing the switch to the fresh and higher voltage battery for a 'speed bump' is this perfectly fine with no downside??...because that is what the OP is asking. I am curious as well. Sounds as though other than the bigger battery depleting faster than the smaller battery with same number of parallel cells in each battery..just no. of series difference, there is no other concern, is this correct?
Thanks again.

In theory— as long as every component in your system can handle the extra voltage, there is no trouble with “throwing a switch” to add more cells in series in the middle of a ride.

There are implementation issues that crop up, though, that make this idea seem like a headache to me, personally. For instance, switches that can handle 84V+ at 40amps ware large and expensive. You could utilize a relay to do your switching, which adds unneeded complexity compared to having one larger battery in the first place. The two batteries won’t discharge together, so now you have to monitor both of their voltages to make sure one of them doesn’t bottom out before you get home. You now have two batteries to charge for every ride. Because you’re changing the series count of the battery mid-ride, you cannot count on the controller to throw the LVC in the case you run the pack down, so you’ll mange that yourself. If you forget to keep up with some of these items, you may damage a battery (if the BMS doesn’t save you.)

All of that is less simple and has more failure points than having one big battery with a high-quality BMS. In this case, my advice is to build a bike that will reach the top speed you want to go, and then use moderation and go slower than that speed. A controller with a three speed switch helps if one’s throttle wrist can’t be restrained (this is true for many of us.)

I want my own ebike to be reliable and safe as possible. To me, this means keeping things as simple as possible and using electronics well under their rated spec. I already have to think about my single battery and check my axle bolts and brake pads and chain lube, etc. Making things more complex is a big negative for me, personally. Other people have different goals for their bikes, and that’s great. I hope everyone has fun and stays safe while building and riding.

 

[Wattman]

You bring up many great points A-to-B.
Thank you. What you wrote makes a lot of sense.
Its always those pesky details that stand in the way.
Totally agree about your last paragraph and how I roll as well. Technical complexity all said has a reliability cost.
Also greater risk when playing with electricity stored in capacity.
Thanks again.