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Dimension the LiPo Equivalent of Your Lead Acid Pack

A project log for Portable Battery Pack for Light Electric Vehicle

Versatility. Just enough energy for 1 day's commute.

Samir DupontSamir Dupont 06/05/2017 at 05:160 Comments

No matter what you had before, we need to figure out how to power the controller. Here's how :

This is not exact business as batteries have a significant higher voltage when charged than when they're discharged. This is more subtle (and way less linear) in LiPos than in Lead Acid, so we need to be careful here.

In my case, I had 5 x 12v Lead Acids. That gave me 66v charged and 57v discharged. Look at the Lead Acid discharge curve at ~2C for reference.

While trying to provide a similar range, I went for a no matter how, 16S pack. That'd give me 67.2 charged and 59.2 discharged.

I know, it's not exactly the same, but we can only work at increments of one cell. So that's the closest possible. Also, most circuits don't have a mandatory input voltage, is more of a range, as they generally have internal regulators and protections.

If it was real electronics, we'd have all the specs from the manufacturer and be sure. As we're talking about cheap, brandless circuits we have to hope for the best and buy a 50 bucks replacement if something goes wrong.

FWIW In the beginning of my LiPo journey, I thought that voltmeter readings would be close to useless or misleading. In practice they're pretty helpful due to the LiPo discharge curve. You just need to be very careful, as after it reaches 3.7v per cell, the discharge to damage will be pretty fast. Which is why most BMSs have a cutoff close to that point.

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