4S2P to 8S1P with the flick of a switch?

Batteries, Chargers, and Battery Management Systems.
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rastamax   10 mW

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4S2P to 8S1P with the flick of a switch?

Post by rastamax » Dec 07 2019 8:36pm

Hello everyone,

I just had an idea that would help me in my particular application and I'd like to know if it's possible to implement cause I've never come across such a setup.

Basically, on my Electric Motion trials bike, most of the riding is done on a 48V/24AH battery. This is working fine but on occasions I'd rather have a 96V/12AH battery for top speed when I have to move from one spot to the other.

The Question is: Can I wire a 4S2P LIPO battery in a way that it can go to 8S1P without fiddling with battery wires (i.e with a handlebar mounted switch).

I'm pretty sure it's possible but since I've never seen it there must be something I'm missing.
Please help.

Thanks.

john61ct   100 MW

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Re: 4S2P to 8S1P with the flick of a switch?

Post by john61ct » Dec 07 2019 8:46pm

Anything is possible, but that's a heck of a challenge.

These days most would say program an Arduino to flip a whole bunch of SS relays or MOSFET switches or something, but I'd be very wary of the reliability even fire safety issues.

I've seen those old-school mechanical cylindrical multi switches, but think they weigh as much as a motor and the guys that know them aren't likely around much - hey @methods?

Personally what I'd do is run all the cell-group lead pairs out, via waterproof AMP/TE Deutsch connectors, and have two junction boxes with the xPyS intra-pack wiring done inside, then just swap the plugs from one box to the other.

But be **damn** sure to remove all plugs from one box before plugging any into the other, be a very interesting accident having a lithium explosion going off between your legs.

Best to do the switching stationary by the side of the road.

I'd be **very** interested if you find a better way, please do post updates back here.

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Re: 4S2P to 8S1P with the flick of a switch?

Post by john61ct » Dec 07 2019 8:49pm


rastamax wrote:most of the riding is done on a 48V/24AH battery. This is working fine but on occasions I'd rather have a 96V/12AH

4S2P LIPO battery in a way that it can go to 8S1P
Those xPyS numbers are way way off BTW, check your math


rastamax   10 mW

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Re: 4S2P to 8S1P with the flick of a switch?

Post by rastamax » Dec 07 2019 9:29pm

Yeah my math is off but you still get the general idea. Like 6x 4S 10Ah lipo packs to get to 12S 20Ah or 24S10Ah for example. What about using a couple solenoids instead of switching wires around?

john61ct   100 MW

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Re: 4S2P to 8S1P with the flick of a switch?

Post by john61ct » Dec 07 2019 11:11pm

Solenoid = relay = switch

just different words.

Do a pair of schematics and look how many points need switching, either A/B or on/off.

To keep it simple, bring each cell / group into a set of busses for each layout.

Do not try wrapping your head around connecting sub-pack xPyS modules in series, whole 'nother can of worms there.

Then think about LVC, or dog forbid BMSs.

I'm not saying plug/unplug is that practical, but IMO it's as close to it you're gonna get.

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Re: 4S2P to 8S1P with the flick of a switch?

Post by amberwolf » Dec 07 2019 11:35pm

If you use any automated system, relays, etc, or even switches, you can get an instant unstoppable fire if anything goes wrong during switching.

For instance, if one of the relays' contacts welds shut, and your system attempts to open it and then it closes the next relay, it's now shorting across whatever the stuck relay was supposed to have disconnected, pouring full battery current directly thru that connection.

You can add fuses to ensure that when this kind of failure happens, it at least doesnt' take out more than the relay itself. But now the pack doesn't work in either mode, until you replace the fuse after troulbeshooting and fixing the problem. (you don't want to replace the fuse until *after* you fix it).

Another problem is that relays have mechanical contacts, and if they are not springy enough, then sufficient bumps or vibration can cause them to either disconnect or even (less likely) connect. Either case could cause arcing across the contacts (because it would happen under load, unlike normal switching), and arcing can eitehr burn contacts to make them no longer connect, or it can weld them togehter so they can no longer disconnect.


If I were going to do this at all (whcih I wouldn't), I'd use keyed connectors as a plug that must be completely removed from one configuration, and replaced with a different plug for the other configuration. My personal preference is for Anderson SB50s (and larger), because tehy are desigend with mounting points for handles, etc., making them easy to use for this purpose. PP45/75/etc wold also work since they can be keyed in many ways, so the configuration plugs CANNOT be plugged in the wrong way.


What I would *really* do, to have speed available, and *also* range, is just leave the pack wired as the high voltage configuration, because it has exactly the same Wh available in either configuration, so ti has exactly the same available range.

If I simply couldn't control my throttle usage, I'd either make a switch for the throttle itself preventing me from using the speed, or more liekly I'd simply setup a speed limiter of some sort (myself, I'd use the Cycle Analyst for that, since I'd already have one in the system for power monitoring).


But switching the packs around is asking for a dramatic failure, eventually. :/


Something else to think about is that the pack wont' have a BMS that can be switched around like taht, so the pack is at risk from overdischarge (at the cell level, at least). YOu *could* build two totally separate packs, each with it's own BMS, paralleling or seriesing them at need...but unless the BMSes both have FETs that can handle the full pack voltage at 96v, then if one shuts off when seriesed, it could blow the FETs in that BMS (which may either make the pack no longer work at all, or may prevent the BMS from being able to shut the pack off to prevent overdischarge, and the cells would be damaged).


There also won't be a controller LVC, because if you have it set for the 48v setup, to protect the pack that way, it can't protect the 96v version. If you set it to protect the 96v version, the 48v version won't even work at all. If your controller is programmable, you can of course set it every time you change it, but that would get old real fast. ;)


If instead you just use a 96v controller with the proper LVC for a 96v pack, and use a BMS for a 96v pack, and leave it as 96v all the time, and charge it as a 96v pack, everything is much simpler, and probably safer.

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spinningmagnets   100 GW

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Re: 4S2P to 8S1P with the flick of a switch?

Post by spinningmagnets » Dec 08 2019 12:01am

I don't see any technical reason that this couldn't be done, but a working system would be very complex. The first hoop you'd have to jump through is to accept that nobody here knows about any controller that will work with both 48V, and can then switch over to 96V. Sooo...you' have to have two controllers as a starting point. Plus a crap-ton of switching FETs or semi-conductor relays.

I am by no means an electronics expert, but it would definitely be a Frankenstein.

rastamax   10 mW

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Re: 4S2P to 8S1P with the flick of a switch?

Post by rastamax » Dec 08 2019 9:08am

Thanks guys for the feedback. I'll move on to other crazy ideas cause this one is too complicated and/or dangerous. I just thought I'd ask people with a lot more experience than me.

Moving on now!

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