2S 2P Booster Battery in series with 50% drained 14S 20ah

[Popstar]

To start off, this is purely hypothetical. I don't care enough to risk damaging my main battery.

At 14S full 58V charge my MAC 10T delivers an almost perfect 30mph experience. But when I go out on long rides and reach ~50% charge around 50V, the top speed of ~25mph somehow is not as satisfying.

Now what if had a switch that would put a fully charged 2S2P pack in series on top of main battery to restore total voltage to ~58V?

What are the real dangers here?
Obviously the LVC on controller would no longer work, but would the main battery BMS LVC save it from over draining?
It would put a lot of strain on the 2P booster pack, but replacing that is only ~$20.
I guess would have to ensure that if it gets flipped while main battery is fully charged that the combined 16S doesn't hurt the controller.

Is this stupid just because complexity of it for minimum gain, or are risks to damage real?

 

[harrisonpatm]

Obviously the LVC on controller would no longer work, but would the main battery BMS LVC save it from over draining?
It would put a lot of strain on the 2P booster pack, but replacing that is only ~$20.
I guess would have to ensure that if it gets flipped while main battery is fully charged that the combined 16S doesn't hurt the controller.

Is this stupid just because complexity of it for minimum gain, or are risks to damage real?

It's an interesting idea, for sure, but in the above quoted paragraph, you're correctly anticipating the potential downsides, and in my opinion, those downsides are potentially serious enough to warrant not doing it.

If the LVC of the controller is now essentially bypassed/tricked, do you really want to rely on your main battery BMS to cut power? Have you tested it, you know it works?

You're also risking overdischarging both the main pack, as well as the 2s pack you're proposing. I would suggest if you really want this slightly increased performance without stressing the battery, perhaps consider a 15s or 16s main pack, and just don't charge it up all the way. If you can afford the weight and volume of the slightly larger pack.

I like out of the box ideas. 95% of the time they aren't worth it, but they do make you think and consider new things, and there's always the 5% that could be clever.

 

[eMark]

At 14S full 58V charge ...

14s FULL charge should be 58.8V suggesting that you have an ageing battery that's only capable of 58.0V at FULL Charge OR your battery charger's voltage trim adjustment is set to stop charging at 58.0V.

Now what if had a switch that would put a fully charged 2S2P pack in series on top of main battery to restore total voltage to ~58V?

Total voltage to 58.8V (14S x 4.2V = 58.8V)
2P is a mismatch as your existing (older battery?) is likely at least 5P. You might consider trying a 1S(5P?) with same number of parallel cells that's in your current battery. Even so will you gain that much extended power performance even if you charge your 15S (combined) battery to 15.8V. Then there's the task of possibly replacing the current BMS with a 15S BMS

... perhaps consider a 15s or 16s main pack, and just don't charge it up all the way.

58.8V divided by 15s is 3.92V per cell with 16s only 3.675V per cell. If you were to charge your 15s combined battery to 61V you would get a 2-3 extra miles before reaching 25mph, but you are also shortening the cycle life of the battery.

The top ebike speed for Class 3 ebikes is from 25-28mph depending on the state, Oregon is 28mph.

 

[docw009]

The weak link in a series scheme is circuit failure of the BMS, The BMS circuits in all the batteries have to have a breakdown voltage higher than the total voltage of the packs. If a BMS shuts down, its open circuit transistors will see that voltage. A 2S or 3S BMS probably only uses 30V transistors. so they are likely to blow with 50 or more volts.

Not knowing the above fact, I have put a 3S-4P and 10S-4P in series to get 48V. I never ran it to LVC, so the BMS never shut down. I found it too unwieldy and I quit using it when I got a 48V battery.

ANyway, ignoring the prospect of the BMS blowing up, those little blue batteries probably cannot sustain much more than 6-8 amps. They'll sag and give you poor performance..

 

[eMark]

2S 2P Booster Battery in series with 50% drained 14S 20ah

A FULLY charged 14S 20Ah battery is at least 4P (that's assuming 21700 cells are each 5000mAh)​

Assuming there is a safe workable way(?) to add a Booster Battery shouldn't the Booster Battery also be at least 4P instead of only 2P (having similar 21700 A-grade cells)? Would a 1S5P or 1S6P be even a better Booster for his particular 15S application for maintaining 35mph speed (IF such a Booster Battery was a safe workable solution).

If such a Booster Battery was practical you'd think there would be at least one youtube; however none seems to exist.

 

[floydr]

There is no such thing as a 4000-7000 mAh 18650 last i looked. Plus typically battery packs give the nominal voltage. Buyer beware false advertising.
Later floyd

 

[E-HP]

What are the real dangers here?

Charging and discharging, and your willingness to rely on BMSs and cheap batteries. You need to monitor both pack voltages while discharging and be your own LVC, and charge the packs separately (disconnected) and a safe routine that you always follow.
The problem is, once you go to 16s, it seems like you will like it and always want 16s or maybe more, if the difference between a fully charged vs 50% is a big deal. I tried voltages from 14S to 22S using packs in series with my 14s pack to determine what voltage worked best for me.
Alternatively you could get a bigger more capable 14s pack, since a lot of the loss in performance comes from voltage sag.

 

[eMark]

FWIW (copy/paste from older Booster Battery ES thread) ... might help thumb up this thread -

I like the idea of the booster pack and I'm contemplating it. But then I'm not seriously...

The idea of capably going 35 mph and conquer those slight hills at 25 mph kind of excites me, but I can imagine I'd "abuse" it. I seem to get enough attention by fellow cyclists when I pass them up at 30 mph pedaling(especially the "lycra" folk), and they're usually going 15-22 mph; I don't think I want to find out what attention I'd get from police at 35. I already "speed" in neighborhood sections. :lol:

I also noticed how my 36V usage of 15 wh/mi. slowly evolved to 25 wh/mi when I upgraded to 48V(I'm a bit more conservative on "long" trips though. Something like 20 wh/mi). I think I'm becoming accustomed to the power.

“ebikes are the sweet spot between a leisurely ride and a brisk race.”
(if only my battery would travel me more miles at 35mph instead of trailing off to only 30mph)
​

 

[leffex]

To start off, this is purely hypothetical. I don't care enough to risk damaging my main battery.

At 14S full 58V charge my MAC 10T delivers an almost perfect 30mph experience. But when I go out on long rides and reach ~50% charge around 50V, the top speed of ~25mph somehow is not as satisfying.

Now what if had a switch that would put a fully charged 2S2P pack in series on top of main battery to restore total voltage to ~58V?

What are the real dangers here?
Obviously the LVC on controller would no longer work, but would the main battery BMS LVC save it from over draining?
It would put a lot of strain on the 2P booster pack, but replacing that is only ~$20.
I guess would have to ensure that if it gets flipped while main battery is fully charged that the combined 16S doesn't hurt the controller.

Is this stupid just because complexity of it for minimum gain, or are risks to damage real?

View attachment 355423

View attachment 355424

True it will work but you are adding failure points and safety issues.

Get quality cells first. Second. Only use it experimentally and after that an evaluation. Maybe you get the point that safety and health is a concern that should be prioritized. A dead you ain't a world I would like to live. It is better with you in it.