Parallel Battery Set Up’s

[E-StreetBikes]

Hey guys i want to know more about parallel battery set ups. When running two batteries parallel what requirements need to be met? Do both batteries need to be identical in voltage, current and capacity? I think depending on the conteoller the current of the batteries can differ. Like if one is 52v 25bms and another is 52v 30bms and their both hooked up parallel to a 22a controller that should be fine right? Also can the capacity if the batteries be different?

 

[E-HP]

Do both batteries need to be identical in voltage, current and capacity?

Yes, no, no.

Like if one is 52v 25bms and another is 52v 30bms and their both hooked up parallel to a 22a controller that should be fine right?

Yes. But if you were pulling a lot of amps, then you want the packs to have similar internal resistances.

Also can the capacity if the batteries be different?

See first response.

 

[john61ct]

Within 0.1V per cell-group count before connecting

So 20S batteries within 2V

 

[E-StreetBikes]

Thanks for the info man. My battery supplier was making it sound like the batteries had to be identical to run them parallel so i wanted to find out the truth. I have one more question if you dont mind. In what case is it ok to run 2 batteries in series? I know this doubles the voltage. if i did this on my 48v 1000w bike would that blow my controller?

 

[JackFlorey]

In what case is it ok to run 2 batteries in series? I know this doubles the voltage.

If they are:
1) The same batteries
2) The same age
3) They are charged independently to the same level (or are charged with a single charger in parallel)
4) They both have good BMSes, or an external BMS that takes into account each battery voltage

Then it might work.

The problem is during LVD. They won't both hit it at the same time. So one battery will shut down (go open circuit) and the second battery will remain connected. Depending on the design of the LVD and what it's attached to, you may get some current "backwards" through the battery that tripped its BMS. With a good BMS that opens the circuit completely (i.e. that has back to back LVD FETs on the power output path) or an LVD on the controller that stops the current draw that won't happen.

if i did this on my 48v 1000w bike would that blow my controller?

Absolutely. Controllers have margins, but if you have a controller rated for 48 volts and you apply 108 volts (which is two fully charged 48V packs in series) you will blow it.

 

[E-HP]

Thanks for the info man. My battery supplier was making it sound like the batteries had to be identical to run them parallel so i wanted to find out the truth.

You described a specific scenario. If your batteries are capable to providing up to the BMS ratings, then you have 55A of discharge capacity and only 22A of load, so the batteries will never break a sweat. If you were trying to pull closer to 55A, then things like the differing internal resistances create more issues, since one battery is working harder.

 

[E-StreetBikes]

I have another question. I see a lot of arguments about running batteries parallel. What is this .01 i hear about? If the voltage is off by .01 it will over volt the battery or something i dont really get it? plus id like to know what are the requirements to successfully hook up two batteries parallel safely?

 

[E-HP]

I have another question. I see a lot of arguments about running batteries parallel. What is this .01 i hear about? If the voltage is off by .01 it will over volt the battery or something i dont really get it? plus id like to know what are the requirements to successfully hook up two batteries parallel safely?

The batteries have to be at the same voltage BEFORE you connect them together in parallel, otherwise the higher voltage battery will try to dump a bunch of energy into the lower voltage battery when you connect them together, which could be bad news.

 

[E-StreetBikes]

How do you ensure that? I wana make sure i do it right.

 

[E-StreetBikes]

Wiuld you mind explaining how to safely use two batteries parallel?

 

[E-HP]

How do you ensure that? I wana make sure i do it right.

Voltmeter.

 

[ZeroEm]

Your running two similar threads. https://endless-sphere.com/forums/viewtopic.php?f=2&t=112615

Would refer to the other link and follow directions.