How do I know if I need a battery isolator?

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jimbo69ny   1 W

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How do I know if I need a battery isolator?

Post by jimbo69ny » Feb 13 2018 10:46am

Hey guys!

I am working on a project to add a battery pack to my 2014 Rav4 EV to extend range. The OEM battery is made by Tesla and uses 2600 mah 18650 cells in a 92s48p configuration.

My plan is to add 3400 mah 18650 cells in a second battery pack. My pack will be 92s31p.

I thought I could simply run them in parallel but someone told me that I would have issues because the amp hour rating is different. He told me I should use an isolator. Is there truth to this? I was worried about the difference in amp hour ratings but I havent read anywhere that it would matter.

I've never used or researched isolators. Do I need one and do they make them that can handle 386 volts and 325 amps?

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parajared   10 kW

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Re: How do I know if I need a battery isolator?

Post by parajared » May 22 2018 4:22pm

In parallel no, in series yes (sorta).

Alternately you can ride around, deplete one pack, plug in the next one.

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flippy   10 kW

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Re: How do I know if I need a battery isolator?

Post by flippy » May 25 2018 3:53pm

no, you dont need to separate them. as long as you have the same number of cells in series the packs will balance out naturally.
its also a LOT better for the cells as you spread the load.
just make sure they have a EXACTLY the same voltage when connecting them together. with a pack like that you will have some serious current flowing between the packs even with the slightest imbalance.
Lithium beats liquid dinosaurs.

jimbo69ny   1 W

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Re: How do I know if I need a battery isolator?

Post by jimbo69ny » May 25 2018 6:21pm

They actually dont have to be the same. This has been tested by Jack Ricard from EVTV. My numbers might not be exact but roughly, he connected 1 Tesla 85kwh pack that was 90% charged to another Tesla 85kwh pack that was 30%. The packs shut down. It was too much current.

So he got them a little closer, one was 90% and the other was around 40% and it worked! The peak current was only 255 amps (nothing for a Tesla pack). They balanced out after about an hour.

So it can be done.

Since posting this I completed my range extending battery pack. I have 2 Tesla Rav4 EV packs running in parallel. Here is my youtube page. https://www.youtube.com/user/jimbo69ny

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