Do I need a battery management system (BMS), or not?

by Peter Schmidt

My dad and I are in the process of converting his 77 VW Rabbit to electric. We're getting very close to ordering the new parts, but there's one in particular that seems very "ambiguous" to me - the BMS. I understand what it's for and what it does, but I can't get a clear answer as to whether we actually need one.

We were hoping to get 4x Tesla batteries in series (to get 96v), a pre-programmed Curtis 1238E-7621, and the AC-20 motor. We're keeping the original transmission.

Of all the sites we looked at providing kits and EV parts, they either have a BMS that's tremendously overkill (and expensive) for our needs, only suitable for 48v vehicles, or, doesn't have a BMS to offer at all. I don't know where to go to find the right one.

The thing is, I don't know if we actually need one at all. We plan on mounting the batteries either in the engine bay, or, where the gas tank used to be, so they'll get ample airflow for cooling. If this makes a difference: the car is not going to be a daily driver, we don't live in a hot climate, and the original transmission ought to help reduce some of the load of the motor.

Any advice would be much appreciated.

Hi, Peter -
I took your question to Steve Clunn at Greenshed Conversions in Florida.

He says:

"There are two schools of thought on this BMS stuff.
Some say they are not necessary, and some do real well keeping them balanced without a BMS.

We here at Greenshed Conversions, with 25 years of experience and seeing way too many fires from no BMS, tend to go with the BMS.

After all, you spend $5,000 on batteries and don't want to spend another, maybe $150.00 for a BMS? Doesn't make sense.

Which BMS, though?

Now if you had 10 or 12 or 14 of the Tesla trays, I would suggest the ORION; but for 4 trays, our BMS will work.

We sell the boards you'll need. They are $100.00 + shipping, and that includes 15 minutes of consult time on the phone.

Check out my video above for the details. These boards connect to your iPhone, so you can SEE how balanced and charged (or, unbalanced and needing a charge) your batteries are right on your phone. It's great!"

Hope this helps, Peter, and good luck with your project!

- Lynne

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