Dual switched battery setup for BBSxx unit?

Hi Folks,

I've searched the forum and haven't been able to find a specific solution to this so here's my situation. I have my cargo bike set up with an EM3ev 36v BBS drive and frame battery (with low current on-off switch built into the case). I also have a 36v/10ah unswitched Ping battery that I'd like to carry as an alternative backup for longer trips. I know that I can simply connect both batteries with a common ground and use a single pole, double throw switch on the other side, a bit like the 'multiple battery' schematic that Ping has here: http://www.pingbattery.com/content/wiringguide.pdf

However I'm aware that the controller on the BBS is in the motor housing so I'm guessing that any power-bus dual (i.e.; 'A/B')battery switch is going to have to handle what I think is called a 'capacitive load' - that is, potential for massive sparks. So will I need additional 'anti-spark' switches for each battery to protect the double throw switch?

As an alternative to trying to switch from one battery to another, could I simply unplug one battery from the motor's power cable and then plug the other in? Will I still need individual 'on/off' switches for each battery in this arrangement as well (the Ping comes without such a switch, the EM3ev frame battery has a low current 'on/off' - not quite sure how it works)?

Thinking about a switched arrangement is all getting a bit complicated - is there a simple solution? Should I just buy a second EM3ev frame battery and use the 'swapped connector' approach? A switch sounds like a much 'cleaner' solution but...

I'd appreciate any ideas, links and experiences using such a set-up. A schematic would be great!

Thanks,

Sam.

Just a thought but what about a marine battery selector switch such as: https://www.amazon.com/dp/B000NI3EXY

If battery #1 is your primary then you have the soft switch for it to avoid the sparking and arcing. Switching though "both" to the #2 auxiliary ping battery would maintain the circuit (i.e. the capacitors would remain charged) and avoid the problem for the ping.

Thanks LewTwo,

That sounds very logical! I forgot to add that I was anticipating unplugging the Ping to charge it. However I now see that if I can build in an 'on#1-off-on#2' switch such as the one you linked to and always start my trip with the frame battery (with it's 'soft start' switch) I should be able to:
- avoid sparking
- charge both batteries on the bike (without disconnecting anything.

I'd just have to figure out an appropriate switch/junction box somewhere.

Does this sound right?

The switch you linked to looks a bit big for my purpose - I may try to find a smaller high-amp SPDT toggle switch that might perform the same function. Does anyone know of something suitable?

Sam

If both packs have the same full-charge voltage, you'll get better performance and avoid the whole switch and spark problem if you simply connect them in parallel while fully charged, whenever you will be on a trip that needs the extra pack.


If you parallel them, then there will be less voltage sag, and it won't be as hard on either pack.


I did this with an EIG 14s NMC pack and an A123 16s LiFePO4 pack for both CrazyBike2 and SB Cruiser.

samsavvas said:

The switch you linked to looks a bit big for my purpose - I may try to find a smaller high-amp SPDT toggle switch that might perform the same function.

Click to expand...

I think that you will find that is a universal problem for switches that are rated to handle large DC currents. There are some audio fuse blocks which incorporate a switch that may be a bit smaller (but not much). Another problem is that most DC switches are rated for 12, 24 or 36 volts. Those rated for 48 volts are few and far between.

There are some DC toggle switches here: http://www.texasindustrialelectric.com/switches.asp
Note that the switch currents are rated at 12 Volts. As the voltage goes up the current capacity is derated.
example: 40 Amps @ 12 volts, 20 Amps @ 24 Volts, 10 Amps @ 36 Volts

amberwolf said:

If both packs have the same full-charge voltage, you'll get better performance and avoid the whole switch and spark problem if you simply connect them in parallel while fully charged, whenever you will be on a trip that needs the extra pack.


If you parallel them, then there will be less voltage sag, and it won't be as hard on either pack.


I did this with an EIG 14s NMC pack and an A123 16s LiFePO4 pack for both CrazyBike2 and SB Cruiser.

Click to expand...

+1 , just parallel the extra battery. Forget the switch. If you use heavy gauge wire and sufficiently high current connectors for your charge port, it can double as the parallel connection of the extra battery. Don't forget to make a male/female adapter for that extra battery for charging, and depending on the connectors used, short circuit protection when not plugged together with the main pack.

Agreed on if you carry two packs, may as well parallel them unless there's a mounting issue or something. It's the rare time 1+1=a little more than 2 with the lower losses. Re a switch, it really won't be sparking... It will almost always be switched with no current thru it because the system will be turned off, unlike say a circuit breaker. So as long as it's sized for the amps it's not a huge issue, as long as you always turn off the system before switching.

If you're not using top cells which I would assume to be the case here paralleling the packs also has the benefit of greatly increased cycle life and capacity if you're taxing the cells. And if not, you will still get less voltage sag under load so more speed power and crisper acceleration.