Understanding Battery Current and the Balanced System

This is really a very simple concept to get your head around.  The problem is that you have to have the basic concepts down before it all makes sense.  Ah, Watts, current, C rates, all that stuff…   I want to start by thanking Noah Podolefsky for his help distilling this down to a “eureka!” moment.  This is about understanding how your total battery pack capacity determines the current delivered to the motor, and how to balance your battery type, it’s discharge rate, and the total capacity of the pack.

This starts with the fact that battery discharge ability, measured in amps, is cumulative when you wire batteries or cells in parallel.  So, you have a cell that can discharge a continuous 1amp, and parallel it with 99 others like it, you get 100amps.  Here’s why that’s important.

When you’re planning your build, you need to make sure you have adequate current delivery from your batteries to match (balance) the rest of your system, in particular, your motor.  If you have a motor that can handle 150amps continuous, but your pack can only feed it 50amps, then you’re underpowering the motor.  When you’re making the decisions about the size and type of your battery pack, you have to consider the discharge rate.  When you put your packs together in a parallel/series configuration, usually you’re thinking about voltage and Ah – amp-hours –  which will give you an idea of what your range will be.  You have to keep in mind the total discharge rate, or amps, you’re putting together too.  To do that, it’s better to think in terms of Watt-hours rather than Ah.

Here’s the reasoning behind that.  A typical motorcycle uses, say, 100Wh/mile, regardless of the voltage.  Now…  let’s decide our range.  We want a range of say, 50 miles, you need 4.3 kWh.  4.3kWh figures out to 60Ah at 72V.  If you want that range, with a 72V system, you’re going to need 60Ah…  simple enough.  When you change the voltage around, you’re going to change the Ah around too…  that’s why it’s better to talk about the “work” done – or Watts – than the capacity (or Ah).

How ‘m I doin?

Here’s where it gets interesting, from a current standpoint.  When you start putting your batteries together, you’re adding the voltage together when you put them in series.  One cell that can do 3.2V at 10 amps with 9 others can now do 32V at ten amps. Now parallel them with nine other modules.  You now get 32V at 100amps.

Secret of the Universe: You can only get a grip on the current the pack can deliver once you decide how you want to configure them.

Let’s break it down and throw out some configurations, based on a 72V system and a target of a 50 mile range.  We can do it a few ways.

GBS 60Ah batteries will give me 3.2V and 60Ah, so I need 24 of them to get 4.3 kWh.  This will give me a total current of 180amps continuous.

Headway 8Ah cells will give me (at 24s 8p) my 4.3 kWh, and because they have a higher discharge rate they will deliver, what, 600amps?  Crazy talk.

Running my RC lipo that are rated at 20C, 60Ah of this stuff will give you 1200amps continuous.

These rates are far more than my motor can handle. There are, of course, other decisions that go into picking a battery, like, for instance, my lipo packs are half the size and weight of those Headways and GBS.  Then, you have to consider cycle life- if you’re hitting the pack for all it’s worth, all the time, it’s going to wear out faster.  If you’re tapping it for a fraction of what it can do, it will last longer.

So, to sum up – the procedure would be, pick your desired range.  This would determine your kwh for your pack.  Once you have that number, compare your battery choices at that kwh for current, cost, size and weight.

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2 responses to “Understanding Battery Current and the Balanced System

  1. Great post. I’m still trying to figure this stuff out. All I know is at this point I want my build to do 100mile range and hit 100mph.
    I’m guessing I’ll need about 72 LifePO4 cells but I need to do the math you explained here.

  2. Ok. Simplifying and rounding off are fine. Arranging to arrive at a convenient battery pack size I understand. But as I understand math, saying that 100wh per mile x 50 miles =4.3kwh is just confusing. Am I missing something?

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