Parallel connecting two different packs

[Timpo]

What do you think about parallel connecting two different packs?
I'm talking about the same voltage, but different packs.

For example,

I know it is easy to connect them physically, because if I search for ebike parallel connectors, they're all over AliExpress.
www.aliexpress.com/item/32917889344.html?spm=a2g0o.productlist.0.0.3c1330eeVhtiqz&algo_pvid=35e9ee25-1684-4d1c-8e7d-3273870489e7&algo_expid=35e9ee25-1684-4d1c-8e7d-3273870489e7-35&btsid=0ab6f83915902869187914926e7106&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_

I am talking about rear rack battery like this.

499.0US $ |Electric Bike Battery 48v14ah 1203 Black Rear Rack Battery For E Bicycle - Electric Bicycle Battery - AliExpress

Smarter Shopping, Better Living! Aliexpress.com

www.aliexpress.com

Many ebikes come with 48V (54.6V) battery packs, Amego, Eunorau, Rad Power, Surface 604, Volt, etc.
They're all mid mounted. (I mean, they're usually right in the middle of the frame)

To extend the range, there's room in the rear rack. So you can get a double battery set up.

1) What would happen, for example, if I were to parallel connect Eunorau 48V rear rack battery on a 48V ebike like Rad Power?
2) What would happen if cells were different? For example, Rad power has Samsung 35E cells, and what if I connect them with 30Q or 26F rear rack battery?
3) What if I connect 35E frame pack and 35E rear rack packs? But potentially comes with different BMS? For example, Rad Power and Eunorau might use different BMS. (Eunorau and Rad Power packs are both Samsung 35E, 48V, 14Ah)
4) I know that different capacity does not matter (look at dual battery bikes like DOST) but how would they drain?
5) If it's safe to connect the two, what would happen if one is 100% and the other is 70% charged? Would it cause any damage?

 

[harryS]

You can only connect them when they are at the same voltage. If you connect them when freshly charged, that's best.

If there is a voltage difference, the higher voltage battery will try to charge the lesser voltage. If the difference is large, then high currents can flow which is not good. When both packs are at the full charge, then this won't happen. You do have to break them apart for charging.

I have been running some smaller packs in parallel like this for several years. Since they're small and have less current capabilty, running in parallel lets them run at lower current, which allows them to last longer. I also don't have to run them down as low as I would, if I only used one pack. The motor is happier because the volts are higher.

In my application, my packs are the same number of cells, voltage, and construction. I would not assume that all packs can be paralleled. The BMS safety circuits may shut a pack off and let the other one do all the work. In an extreme case, if one pack was extremely weak and unable to run the bike on its own, the larger AH pack might be doing all the work, running the motor and also charging the low AH pack.

So I will leave you with the suggestion that if you have two good packs, just run them separately unless you need to do it or if the thrill of scientific discovery makes it worth the trouble.

 

[teskow]

I too have been thinking about this setup but have concerns of the same issues mentioned here. It seems to me that the stronger battery charging the lower voltage battery could be eliminated by placing a diode in the
auxiliary battery feed line.
Would or would this not work?

 

[harryS]

Power diodes, of course. But they do get hot and waste some heat.

 

[Sierratim]

yea I know about the voltage, so that's why I specified the same voltage.

But I was not sure if it's a big deal if battery cells were different.. for example, battery pack with Samsung 30Q and 26F have drastically different drain performance.

Not only that, even though Eunorau and Rad Power both use Samsung 35E, 14Ah, 48V.. what if they have different BMS? Would that be a problem?

The lower capacity pack/cells will try to discharge to the higher capacity pack as soon as you start to draw any power without some balancing circuitry. The lower capacity pack may be may be protected from damage by its BMS but it will reduce the apparent capacity of the two packs.

 

[harryS]

Two ongoing discussions you can read in another forum about paralleling packs. I've no concerns about my packs. I have been paralleling them for several years.

In the Dost case, they are likely similar cells in each battery. So it works. As long as you aren't drawing huge currents relative to what the battery supplies, these worst case scenarios where the cells unbalance probably don't happen. Paralleling should reduce the current per battery, which improves its loa performabce.

Parallel yes or no

This forum is a wealth of knowledge and I have studied it at length. Many things would be easy for me to answer but my problem is one of maybe's and possible range bias. I would to know the answer to paralleling two batteries. The reason is for voltage sag and ultimately pulling a bit more...

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parallel different packs

I have a 4p of 25r so 80amps and 10ah equals an 8c pack I have a 3p of 18650pd so 30amps and 8.55ah equals a 3.5c pack I run the 25r upto 10amps/4c BUT if I do the math of my max settings 2200 watts, dead battery is 51v so 43 amps, fresh battery at 70v is 30 amps so at WOT 4p is 7.5a-11amp per...

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[Scott Adams]

Here's another good thread from ES -

Max safe Voltage Difference when connecting two Batteries in parallel

What would you consider the maximum safe voltage difference when connecting two 14S10P 18650 batteries in parallel? Is it possible to calculate the maximum equalizing current? ( maybe U(difference)/(Ri1+Ri2) )

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