Batteries AH watt the?

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Gregski
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Joined: 04/12/2009
Points: 91

OK, so I started learning about batteries, and some basics, standard car batteries are about Cranking Amps (bad) we need ones that are about Amp Hours so we need Deep Cycle Batteries, (good). Lead Acid Batteries is what I can afford, so let's talk about those. I plan on using six 12 volt 100 amp hour AH batteries and a 72 volt 100 amp motor.

First of all who came up with this crazy amp hour rate system based on 20 hours, I mean what's the significance of 20 hours. It's not even a day's worth. Then some provide amp hours in 25 hour increments, other's in 5 hour. Now somewhere I read you take the 100 AH divide it by 20 hours and you get 5 amps per hour. Though some battery specs show a different 1 hour rate then if you divide it by the 20 hour rate, what gives?

Now since I will be connecting these batteries in series (plus to negative, plus to negative, plus to negative, etc) the volts will add up 12 + 12 + 12... = 72 volts. However from what I gathered, in a series the amps stay the same so they will not add up, so I will still have a 100 amp system at the 20 hour rating.

So does this mean if I plan to ride the bike for 1 hour the motor will be sucking in 100 amps and therefor running at it's theoretical peak performance, aka providing 100% of its horse power? Assuming somehow I gauge it / throttle it in such a way.

To follow up on this question, if I ride the bike for 2 hours will it be using 50 amps per hour, and running at 50% horse power? Assuming somehow I gauge it / throttle it in such a way.

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antiscab
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Joined: 07/07/2007
Points: 1599
Re: Batteries AH watt the?

before lead acid batteries were used in EVs, their application was in solar power setups, where the 20Hr rate was more important to know.

in EVs, we are interested more in the 1Hr and 0.5Hr rates.
the usuable capacity is usually half that of the 20hr rate

If the particular battery you are planning on using isnt rated at 1Hr, then id suggest not buying it.
unless you can carry a very heavy battery, id suggest going lithium.

the actual current draw from your controller will depend largely on the load placed on the motor.
tell me more about your planned setup, and i can help you determine wh/km.
from wh/km, you can work out how much range you can get from a particular battery pack.

halving the power going to the motor will reduce your top speed. It will increase run time, and might increase range (depends on a few things).

an amp is defined as a coulomb per second. so there is no such thing as an amp per hour.
AH is an easier measure of how many coulombs of charge a battery can store. one AH = 3600 coulomb

Matt

__________________

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst replacement TC Charger
conversion
Spent so far: $5800 + $7000 + $1720 + $960 + $320 + $720 + $140 + $600
Cost to do it again: $2500 + $5600 + $1720 + $960 + $320 + $720 + $140 + $600
Cost for a Petrol bike:$6000 + + $1440 + $6000 + $800 + $1400 + $3200 servicing
Total spent: $17260
Total to do again: $12560
Total to have used a petrol bike: $18840
Total distance travelled so far: 79'120km

garygid
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Joined: 12/19/2008
Points: 441
Re: Batteries AH watt the?

The term would be N amps (to or from the battery) "for an" hour (rather than "per" hour) would be N amp-hours, or N Ah of "charge".

So, using 15 amps for 2 hours would remove 30 Ah from the battery.

Specsmanship:
For some battery chemistries, like Lead-Acid (LA), the battery will provide significantly more Ah of useful output if the battery is used (drained) slowly, not quickly. Thus, a LA battery might provide 100 Ah when used at a 5-amp rate (over 20 hours), but only 50 Ah when used at a 25-amp rate. Yes, only half the energy. So, to make the battery "appear" better, they will publish the 20-hour specification.

Sealed LA cells (SLA) are much safer, since they would tend to not spill acid all over you if/when you tip over, crash, or just lay the bike on its side. So, the safety advice on 2 wheels, is do not use LA, use SLA if using the Lead-Acid chemistry at all.

Cost:
60 Ah from a 12-volt battery for perhaps $120 for LA or $180 for SLA (both are guesses) vs. the "same" 60 Ah from three 3-volt (really more like 3.3 volts) cells of LiFePO4 in series for $360. Yes, it can be 2x or 3x the cost, but much less weight, perhaps something like 45 lbs (a guess) vs. 3 x 5 lbs = 15 lbs.

Note: the LiFePO4 (Lithium-Iron-Phosphate) chemistry is significantly different from, and SAFER than, the older "laptop" chemistry called "Lithium-Ion" (really a Lithium-Cobalt chemistry).

Charging LiFePO4 is SIGNIFICANTLY different, and more "difficult/complex" than, the rather simple LA/SLA charging. However, the lifetime (useful charge cycles) of the well-cared-for LiFePO4 (maybe 1000 to 2000 cycles) can be perhaps 10 times that of the LA (maybe 100 to 200 cycles?). So, perhaps the LiFePO4 batteries would cost less in the long run.

__________________

Cheers, Gary
XM-5000Li, wired for cell voltage measuring and logging.

jdh2550_1
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Joined: 07/17/2007
Points: 2338
Re: Batteries AH watt the?

Gregski,

The short answer to your two questions is "No" and "No". Here's why...

Welcome to the wonderful world of battery capacity! You're in for a "fun" time :)

First off check out this wikipedia article on something called Peukert's Effect: http://en.wikipedia.org/wiki/Peukert%27s_law

Next, you can look at this handy-dandy Excel spreadsheet to look at some real world numbers for the amount of energy you can expect to get out of batteries depending on your amp draw: http://www.smartgauge.co.uk/calcs/peukert.xls Also make sure you check out the technical articles at http://www.smartgauge.co.uk He's one smart cookie!

The bottom line is that because of the internal resistance of a battery it means the faster you try and draw amps out of a battery the less usable capacity it has. This is because as you draw out more amps the amount of heat generated goes up - and heat is wasted energy. This is why you can't divide the 20 hour rate by 20 to get the 1 hour rate. But you are right the 1 hour rate is the most sensible rate to consider for EVs. However, if you can find out the "Peukert constant" for your chosen batteries (often available on the manufacturers website) then you can find the theoretical capacity at any given rate of draw.

One of the BIG advantages of the lithium technologies is that the Peukert constant is close to 1 something like 1.08 is a typical value. Whereas for various different lead configurations the numbers are more like 1.2 (good SLAs) to 1.5 (older flooded cells). What this means is that Lithium batteries give very close to the same capacity if you draw low amps or high amps (as long as you stay within the specs of the cell!)

Are you having fun yet?

Now you need to bear in mind a couple of other aspects when choosing batteries: how do they respond to temperature? and how do they respond to abuse?

First temperature: short version is lead acid sucks at colder temperature (apart from the very new versions of lead acid like silicone lead acid which can work down to -40C with (I think) about 80% of capacity). Lithium tends to be able to work down to -40C with about 85% capacity. Anyone who rides a motorcycle at -40C deserves either a medal or a warm weather vacation!

Next, how well do they handle abuse: Let's face it - before long you're going to figure out that although electric motors are efficient and that they can provide gobs of torque at low RPM they do need to gulp down a fair amount of power to do that. Let's face it - you can't break the laws of physics (well if YOU can then please tell US how). It takes a certain amount of power to move that big hunk of metal up to speed. Power = volts * amps. Amps is what we're talking about here. So, you'll end up drawing a high peak amp rate from your batteries - even if your average draw is less. What's going to happen to your batteries when you do that? This is where good old flooded lead acids actually shine - they can take a lot of short-term abuse as long as you keep them watered. Now that's a real PITA and most folks don't do it which is why SLAs are popular. Basically when you abuse a flooded battery you "boil off" the electrolyte - but you can easily replace it. When you abuse an SLA you "boil off" the electrolyte (it vents as explosive hydrogen) but you can't replace it - thus you permanently damage your SLA in terms of reduced capacity. Once again Li doesn't suffer this type of problem - however, you can abuse Li. Draw too much power from a Li-Po cell and it tends to go into "thermal runaway" (i.e. it burns). Draw way too much power from a LiFe cell and it will get hot and smolder but not "runaway". Draw a bit too much power from LiFe cells over a period of time and the expansion and contraction of the electrodes will cause it an early death (i.e. lasting hundreds of cycles instead of thousands).

See, I just know you're having fun reading all this. You are reading this or are you already asleep?

Bottom line when choosing batteries and battery technology think in terms of the following characteristics (in no particular order):

- peak and constant discharge rates
- the Peukert effect (i.e. how much usable power based on your high average amp draws)
- how well the battery works in hot and cold
- how heavy and big the battery is
- how long it will last
- how safe it is
- how much it costs over it's lifetime
- how well it handles abuse when you push it to its limits

There is no "perfect battery" - it's a compromise between all those variables.

If at the end of the day you can only afford SLA then don't despair - lots of folks have built successful bikes with as little as 48V of SLAs. My friend Terry who works at EVMFG did just that - I might persuade him to chime in here with his experience. Or drop me a line at john@evmfg.com and I'll "introduce" the two of you.

However, for just a little extra money* up front you can go with LiFe cells - which over the long run will cost you less AND give you a better bike at the end of the day. * to help define "a little extra" compare the price of good SLAs (say Optimas) with entry level LiFe (say Thundersky).

Same basic arguments hold true for different chemistries - it's just that I have most experience with lead and lithium.

Good luck - I hope this rambling helps answer your questions - at least indirectly! Battery technology is complex stuff - and as folks who deal with EVs like to say "It's all about the batteries"...

__________________

John H. Founder of Current Motor Company - opinions on this site belong to me; not to my employer
Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas. No Worries." - JDH, CuMoCo || "Make Volts Not War" - anon.

garygid
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Joined: 12/19/2008
Points: 441
Re: Batteries AH watt the?

Of course, battery lifetime, size, and weight are important factors, as are component reliability, suitability for the application, and design "margin" and robustness.

There is one other, often-overlooked dimension:
the "marriage" of the charger and the BMS, the batteries, the controller, and the motor.
Often the adventurous ignore the fact that changing one component of even a well-designed system can over-stress the others.

Let's add a couple more cells, or change the motor (and see if it goes past the limits of something). Ever hear that?

__________________

Cheers, Gary
XM-5000Li, wired for cell voltage measuring and logging.

Juiced
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Joined: 04/28/2009
Points: 31
Re: Batteries AH watt the?

To add a bit more to this equation and to the cost of the LifePo4 batteries is the BMS and the charger.

I have been having fun with the SLA batteries, a Kelly $220 72 volt charger and Alltrax controller.
The 72 volt system is getting me 50 MPH and about 15-20 mile range if I "coast" correctly. Just fine for me at this time. Since the bike is already built, I can upgrade as money permits.

The LifePo4 batteries are great, but for many who have just started their first project, may be overwhelming.
I calculate that by the time these SLA batteries die out, the LifePo4 batteries will be even cheaper!

Whatever you choose, make it a fun project, make it safe and what you can afford.

Ed

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