DIY Lipo Pack - Running different voltages in parallel?

Batteries, Chargers, and Battery Management Systems.
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Psychophoria   1 µW

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DIY Lipo Pack - Running different voltages in parallel?

Post by Psychophoria » Nov 03 2018 6:06am

Hello all,

Just got my 48v, 1000w hub motor & speed controller kit.
I ordered an ebike battery pack as well, but I need this bike to be on the road by Monday.

I'm planning on combining (x2) 18.5v 3000mAh and (x1) 11.1v 5200mAh to create 48.1v, which is damn near spot on to what the speed controller wants. (I was also considering running 3 of the 18.5v lipos, but am worried about over-volting the speed controller...)
I'm not going to be charging these batteries in parallel, but only plan to use them in parallel.

Ultimately, I'm wondering if the voltage different will be a problem.
I know that batteries of the same voltage, mAh, age, etc would be preferred but I'm only planning on using this setup for 2-3 days until my ebike specific battery pack comes in.
Another thought, is a BMS required for this type of setup? Or can I connect the battery leads directly to the speed controller.

If anyone has any input on this, I'd really appreciate it.
Have a great weekend everyone.

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dogman dan   100 GW

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Re: DIY Lipo Pack - Running different voltages in parallel?

Post by dogman dan » Nov 03 2018 6:53am

To parallel two batteries, they MUST be the same voltage. Same number of cells, and close to the same state of charge. Best way to do this is never make the connection till both are full.

So your two packs will have to run one, then unplug, and plug in the other to discharge it. It might blow your controller to use the higher voltage one, but maybe not. Judge the risk by how much you paid for the controller. Risk a 40 buck controller, sure, it may work fine on the higher voltage. ( but not too high, like 72v) If its a pricy unit, another battery makes a lot more sense.

Besides, buy that one more battery, and you can have two packs you can parallel.

Re the size difference in a series connection, your pack will have only 3 ah, and will need monitoring of the smaller cell packs separately so you know when to stop. One of those inexpensive 6s voltage alarms will work fine for that.

Yes, you absolutely need a bms, always. But the good news is you and your brain can be the bms. You will need a voltage monitor though, to be a human bms. For a few days running naked, just keep those rides damn short, like 3 miles or so just to test it out.

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Alan B   100 GW

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Re: DIY Lipo Pack - Running different voltages in parallel?

Post by Alan B » Nov 03 2018 7:06am

Parallel or Series?

To sum voltages batteries are connected in series. Parallel is only for equal voltage batteries, to increase current (amps and amp hours).

48V controllers are usually used with 13S or 14S Lipo packs (and are usually ok to 59V max). (13 or 14 cells in series). 18V is 5S, 11V is 3S. 5+5+3 = 13.

48V normally means 4 x 12V lead batteries in series. Fully charged they each might be around 14.6V for a brief moment, hence they generally use 60V or higher components in these controllers.

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