Confused About AH Ratings

[The Journey Guy]

I have been reading and studying this forum the best I can, for a couple of weeks now. I am a newbie when it comes to battery building, but I would very much like to learn. I believe I have a basic grasp of what's going on, but I could use some help understanding AH ratings. I have read the definition that says 10ah equals 1a for 10 hours, or 2a for 5 hours, and so on. But beyond that definition, is what I'm trying to wrap my mind around.

Here is what I believe to be true, and I'd like to confirm that I understand all correctly.

Let's say that for each and every ride, the amount of throttle remains the same, and the terrain remains the same. At a very basic level, would AH ratings then generally indicate how far one can travel? For example, in a controlled test run, if you use a 24v 10ah pack, you go 10 miles. If you then use a 15ah pack, you could then go 15 miles, with a 20ah pack you could go 20 miles, and so on......

Is that basically how it works?

Thanks!

 

[Anonymous]

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[dak664]

Watt-hour ratings are more useful for range estimations, since the voltage drops ~20% during battery discharge with corresponding drop in miles-per-amp-hour. Lower voltage means higher currents and less motor efficiency which can have a multiplying effect that also reduces miles-per-watt-hour; for example a ride that is downhill the first half and uphill the last half may not be doable in the reverse direction where the hill is climbed at a lower voltage. And of course the heavier the battery pack the less distance per watt-hour; doubling a 36 volt, 12AH lead-acid pack (12kg each) gives me only about a 75% range increase on my bike.

On the other hand if the internal resistance of the battery pack is significant then doubling the pack will halve the current and losses which could more than double the range.

Amp-hours are useful as a measure of charge level (the "coulomb gauge") and for estimating charge time when using a charger with constant current (e.g. PV panels inherently, most others by design) which however masks the fact that the charging watts per amp is increasing as the voltage rises.

 

[Anonymous]

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[The Journey Guy]

That's right but wind is the factor that is not predictable or repeatable.
Wind, your throttle level, and pedaling force determine how far you can go.

Understood. I realize that a headwind will reduce distance capability quite a lot. That's why I said that, if it were possible, using a controlled environment and all settings remaining the same (weight, wind, level ground, pedaling force, etc), that a general yardstick for the distance one can travel would be the ah rating.

Thanks for your answer!

 

[The Journey Guy]

Watt-hour ratings are more useful for range estimations

You offered many comments, which I thank you for, but I would like to address the above comment first.

When I look at battery packs offered for sale, or even read about the ones that people are building here, what is generally offered as information is just the voltage and amp hours of that pack. How would I determine which battery pack has the higher watt-hour rating, if no one refers to the packs using that information?

 

[Anonymous]

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[adrian_sm]

How would I determine which battery pack has the higher watt-hour rating, if no one refers to the packs using that information?

Roughly speaking, Watt-hours = V x Ah.

My battery is a 48V 10Ah, so roughly 480Whrs.

My bike averages 11 Whrs/km, on my usual commute (35km/hr average, 40km/hr on the flat, ~30 km/hr most hills)

So at these speeds I should be able to do 480/11 = 44 kms.

If I go slower I can go further.

Hope this helps.

 

[dak664]

When I look at battery packs offered for sale, or even read about the ones that people are building here, what is generally offered as information is just the voltage and amp hours of that pack. How would I determine which battery pack has the higher watt-hour rating, if no one refers to the packs using that information?

The graph of pack voltage for several discharge rates will show what watt-hours to expect for the current profiles used in a particular application. Electric submarines alternate between high and low currents to get the maximum range, based on a detailed computer program that analyzes all the variables. Maybe the next version of the cycle analyst can include that feature

 

[The Journey Guy]

Roughly speaking, Watt-hours = V x Ah.
Hope this helps.

Yes it does, thanks much for that!

 

[The Journey Guy]

So, to sum this all up for me, would it be safe to say the following:

It does not matter if I use a Headway, A123, DeWalt, or any other name brand LifePo4 type cell, as far as distance is concerned. What matters in the big picture is the watt-hours of the given battery. For example, no matter what name brand cell is used, a 24v 10ah battery is still only going to have aprox. 240watt hours of power in it. It is still only going to go so far, no matter what the name brand. Would that be correct?



Note: I understand that one brand name will be able to give more amps on demand, better than another. I believe I have learned that is the C rating of the battery..

 

[adrian_sm]

What matters in the big picture is the watt-hours of the given battery. For example, no matter what name brand cell is used, a 24v 10ah battery is still only going to have aprox. 240watt hours of power in it. It is still only going to go so far, no matter what the name brand. Would that be correct?



Note: I understand that one brand name will be able to give more amps on demand, better than another. I believe I have learned that is the C rating of the battery..

Roughly speaking, Yes. But some batteries voltage sags more than others as they discharge, giving you effectively lower Whrs, since V x A still holds under load.

The batteries that can deliver more amps, are ones with lower internal resistance. So when the current draw is high, the voltage sags less, and less energy is wasted to heat due to the resistance.

If you only have relatively low current demands, this isn't an issue and the Ping type packs will be fine.
For me my controller can draw 35Amps, on a 10Ah battery, which is called 3.5C (Amps/Ah=C), which is probably too much for a Ping pack but okay for Headways.

So the high discharge batteries will be a bit more efficient, since they don't waste as much energy to heat, but personally I wouldn't let that decide which battery I buy.

This would be how I decide:
1) Select motor
2) Select max speed, gives you the battery voltage you need.
3) Pick a battery chemistry (SLA,LiFePo4, LiPo)
4) Pick a desired max. range, gives you the desired battery capacity (Whrs = Range * Wh/km)
5) Pick the max current you will be drawing ( keep it bellow the recommended max discharge rate)

Then weigh up the options, based on how long the battery will last, and how much cash you have.

Of course you can make batteries last longer if you don't use there full Whrs capacity, or full discharge rate all the time.

Probably a longer answer than you wanted, but thought it might be helpful.

 

[The Journey Guy]

It was a very good answer, as were all in this thread. I have written down the formulas you have provided, and I think I have learned a bit more by studying the answers given today.

Many thanks to you all!

 

[morph999]

That's right but from what I understand, a 48v10ah Lifepo4 is like a 48v 3ah SLA. Lifepo4 is like 2x -3x more powerful than SLA.

 

[spinningmagnets]

As you have stated its common sense that a hilly path, a heavier rider, higher top speed, plus a strong headwind will reduce miles from a given aH pack size.

Batteries can be expensive, and anyone new to e-bikes want to know the smallest (cheapest) pack that can satisfy their needs. I believe a big factor in the range results is also the efficiency of the motor, and its seems very difficult to get accurate info.

Direct-drive hub motors are so simple I'm certain they will remain the most affordable entry-level option. But since they spin at the wheel RPM's they are inefficient concerning range per aH of battery. The large Crystalyte hot-rod 5XXX series are the worst. They are watt-hogs, even when just cruising along (or so I've read). The smaller 4XX series is slightly better.

Internally-geared hubs (Bafang, Ezee, BMC) allow the motor to spin from 5 to 9 times per wheel revolution (depending on model) so they have more torque, less heat, and more range efficiency per aH of battery.

If money were no object, the new RC builds are very light, have great heat dissipation, and due to their ability to spin (with a gear-down) up to 3,000 RPM's, they would be my first choice for optimum range from a given aH size of battery.