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Old 01-23-2012, 11:01 AM   #18
J117
Battery Tech
 
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Joined: Nov 2010
Location: Southern Oregon
Oddometer: 22
There are a couple of things to clear up here. You guys may know all of this already, but then again you may not.

When you are dealing with a lead acid battery of any chemistry, you have to take into account Peukert's law.

The basic idea here is that the realized capacity of a lead acid battery goes down proportionally related to the amperage of the load placed on the battery. Example, look at any deep cycle battery's 20Hr rate, vs. 10Hr rate. A perfect example is that BMW 51913 batteries used to be rated on a 20 hr rate to give the 19AH rating. They are now rated at a 10 hr rate to give the 18AH rating. Ampere Hour ratings mean that a load of (X) was placed on the battery for a certain number of hours, standard rating is 20 hours, and that is lasted that full 20 hours before flat lining. For example if a battery is rated at 100AH @ a 20Hr rate, then they placed a 5 amp load on that battery, and it lasted for 20 hours. The problem comes when you try to then mentally translate that to a 10 Hr rate. So if you say, want to run a 10 amp load on the battery that is rated at 100AH @ 20 Hrs, you think it will last for 10 hours, but that is not true. As your amperage increases, the realized capacity of the battery will decrease in proportion to the amperage. If you have two ratings, eg. a 20Hr and a 10Hr, then you can use Peukert's law to figure out what the exponent that determines the proportional change is. That is far more complicated that I need to go into here, but what you need to know is that different types of lead acid batteries of differing qualities will have different rates of discharge.
When you then translate the AH rating to Lithium you are faced with some challenges. The standard for so long has been lead acid, rated at a 20hr/then 10 hr rate. To further complicate things, when you are dealing with starting engines, the construction of the standard standard starting lead acid battery is heavily in favor of the short term discharge rate, and not so much on the deep cycle side. This leads to ratings like CCA, taken at freezing temp. for 30 seconds at 1/2 voltage. That sort of situation is absolutely impossible to put on a lithium based battery, because they cannot put out power at 1/2 voltage. So the CCA ratings on lithium batteries are essentially the short circuit current possible for the cells they are constructed with through their BMS systems.



The problem with amp hours though, continues. Some manufacturers of Lithium starting batteries, Shorai, Braile battery, and others, have chosen to differentiate between AH, and AH PBEQ. If Shorai or Braile ever makes a deep cycle application battery, then they will have to rate that battery in actual AH @ 20 Hr rate. Which, btw does not change for Lithium. There is no Peukert's law for lithium batteries. A lithium 100AH would discharge in 20 hrs at 5 amps, and 10 hours at 10 amps. But I digress. For starting motorcycles, Shorai and Braile know there are all of us persnickety BMW guys out here who want an 18 AH battery Darn it! So, they use AH PBEQ to compare their batteries to the Lead Acids we all have been using forever. The issue is that there are people who don't read the warnings, like (DONT USE ON ELECTRIC VEHICLES!!) because they figure that they have just figured out a way to power their electric tike off of a 2 pound battery. A Lithium battery that had an actual 18AH rating would weigh a lot more than 2 pounds. But, if you are just trying to start engines, and run lights while your bike is running and producing a charge, then you don't need a true 18AH. You need the equivalent to your old lead acid 18AH battery.


Moral of the story boys and girls is to use these batteries for what they were designed to do. (And when you disregard their purpose and try to run your laptop/coffeemaker/fridge/freight train off of them and it doesn't work , don't yell at their makers, go look in the mirror and have your anger management session.) A Shorai/Braile battery that is rated at 18AH PBEQ will only have an actual 6 AH inside of it in flat prismatic cells, not A123's, I have taken them apart. Granted, you can actually use 96% of that 6AH, but it will not run a 2 amp electric motor for 9 hours, it will run it for 3. And for the record, your BMW 18AH @ 10Hr rate starting battery will only run it for 3 hrs as well.
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J117- Batterystuff tech


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J117 screwed with this post 01-23-2012 at 11:18 AM Reason: spelling,grammar..
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