# lifepo4 batteries ?? im confused !



## crackerjackz (Jun 26, 2009)

Im confused . I am an electician and am very able to calculate voltage and currents but batteries seem like they are hard to calculate.


I know a bms is a battery managemnt system but does this system allow for lifpo4 batteries to be bulk charged with only one charger ??

also if you get a 200 aha thundersky battery and hook 44 of these in series obviosly you get 150 nominal volts . since they are in series your whole pack is limited in current to the max current output equivalent of one battery ... right ? what is this max current output for a battery ?? is it calculable using the amp hour rating ??

all my parts are ready to be order by my boss but I cant seem to be sure of my calculations concerning the batteries ... I simply dont understand the charging process of the damn lifepo4 batteries and there discharge capacities without damaging them ...  ... please help if you can


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## AmpEater (Mar 10, 2008)

It might to help to not look at the pack as 44 batteries, but instead one battery with 44 cells. Just like a typical lead acid is actually 6 cells stuffed into one case. 

The way a BMS functions is to monitor each cell individually and make sure none go over the max voltage. When one hits the cutoff voltage, say 3.6v (thundersky lists 4.2v max but it is recommended to stay a little lower), the BMS signals the charger to cut back its current to a low amount, like 2 amps. At the same time it turns on a shunt resistor accross that cell so the others can keep charging without pushing that one even higher. It "burns off" the extra while the others catch up. 

It is also possible to use an individual charger per cell but that gets expensive, complex, and you still need a BMS to make sure none fail. If a charger does fail it would render the whole pack useless until its replaced.

The discharge is limited to the current of a single cell. It can be calculated based on the amphour rating, you will see "C" or "CA" referenced, that is basically how many times the amphour rating it can produce. Thundersky are limited to about 3C continuous, so for a 200ah cell you can draw up to 600 amps. You can get a bit more in short bursts, maybe 4 or 5C. (The limit doesn't seem to be well established, I've yet to see one tested to destruction). They list up to 10C for a fraction of a second. Generally battery life is better if you keep the amps in the lower C range. 

44 would be more like 141 nominal. (3.2v/cell)
141v * 200ah = 28200 watt-hours, or 28.2kwh

A normal conversion gets around 250wh/mi, so that pack should be good for 112 miles down to 100% DOD (depth of discharge)


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## AmpEater (Mar 10, 2008)

Its important to note that BMS has two meanings;
Battery Management System
Battery Monitoring System

The latter only measures voltages, alerting you if one goes too high on charge or too low on discharge, but doesn't necessarily do anything about it. It can't balance cells, or keep you from murdering them. 

You want a BMS with shunt regulators as part of the package.


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## Bowser330 (Jun 15, 2008)

AmpEater said:


> It might to help to not look at the pack as 44 batteries, but instead one battery with 44 cells. Just like a typical lead acid is actually 6 cells stuffed into one case.
> 
> The way a BMS functions is to monitor each cell individually and make sure none go over the max voltage. When one hits the cutoff voltage, say 3.6v (thundersky lists 4.2v max but it is recommended to stay a little lower), the BMS signals the charger to cut back its current to a low amount, like 2 amps. At the same time it turns on a shunt resistor accross that cell so the others can keep charging without pushing that one even higher. It "burns off" the extra while the others catch up.
> 
> ...


Good write-up!


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## crackerjackz (Jun 26, 2009)

amp eater thank you very much for the inlightening !


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## CroDriver (Jan 8, 2009)

I'm a little confused too. 

The TS batteries are 3,2V nominal but in real life they are 3,2-3,7V when fully charged. (3,2 when almost completely discharged)

How to calculate Wh? Using the real voltage or theoretical voltage?


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## crackerjackz (Jun 26, 2009)

CroDriver said:


> I'm a little confused too.
> 
> The TS batteries are 3,2V nominal but in real life they are 3,2-3,7V when fully charged. (3,2 when almost completely discharged)
> 
> How to calculate Wh? Using the real voltage or theoretical voltage?


 
3.2 is nominal meaning good operating middle voltage ... when fully charged if Im correct they can reach 4.25 and the safety cutoff if they discharge to low is near 2.75 ! please correct me if im wrong . 

when you calculate you must use the nominal voltage . but must make sure the peak voltage will not blow any of your appliances . example if your motor cannot take more than a 170 volts putting youtr battery pack to 165 nominal volts is a pretty bad idea ! as when fully charged you will be going much higher than 170 volts. 

this is my understanding to these damn lithium batteries . please feel free to correct me  !


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## Tesseract (Sep 27, 2008)

CroDriver said:


> I'm a little confused too.
> 
> The TS batteries are 3,2V nominal but in real life they are 3,2-3,7V when fully charged. (3,2 when almost completely discharged)
> 
> How to calculate Wh? Using the real voltage or theoretical voltage?


Voltage when fully charged is a tricky thing, but pretty much all of the LFP cells, so far, seem to state that 2.5V indicates fully discharged.

Others here have reported that fully charge is more or less reached at 3.8V and that the cell voltage rises very quickly from that point to the real maximum of 4.2V, which means very little additional energy is stored between 3.8 and 4.2V.

The nominal voltage is 3.2V, which is the average voltage for a cell over it's "normal" discharge range, so you calculate Wh from that voltage times the 1C Ah rating. A 160Ah LFP cell will deliver 512Wh, then.


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## CroDriver (Jan 8, 2009)

crackerjackz said:


> 3.2 is nominal meaning good operating middle voltage ... when fully charged if Im correct they can reach 4.25 and the safety cutoff if they discharge to low is near 2.75 ! please correct me if im wrong .
> 
> when you calculate you must use the nominal voltage . but must make sure the peak voltage will not blow any of your appliances . example if your motor cannot take more than a 170 volts putting youtr battery pack to 165 nominal volts is a pretty bad idea ! as when fully charged you will be going much higher than 170 volts.
> 
> this is my understanding to these damn lithium batteries . please feel free to correct me  !


As far as I know 3,2-3,7V is the useful voltage. Below 3,2 the voltage drops very quickly to 2,75 so there's no point to go below 3,2V. There is not much capacity difference between 2,5V and 3,2V. Same at the other end at 3,7V. You can charge up to 4,25 but the difference in stored energy is a few %


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## Bowser330 (Jun 15, 2008)

If a controller says its maximum fully charged pack voltage rating is.... 300V (for example)...

Then does that means you take 300V and divide it by 3.8 (the voltage you set in your BMS)....which equals 79 cells...??

...And once the cells get back to nominal voltage you really only have a 253V pack....(3.2V * 79)  ??


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## Tesseract (Sep 27, 2008)

Bowser330 said:


> If a controller says its maximum fully charged pack voltage rating is.... 300V (for example)...
> 
> Then does that means you take 300V and divide it by 3.8 (the voltage you set in your BMS)....which equals 79 cells...??
> 
> ...And once the cells get back to nominal voltage you really only have a 253V pack....(3.2V * 79)  ??


Yep. It sure does.

Even though the controller is only briefly exposed to 3.8V per cell, it's still exposed. Same problem with any other battery chemisty, of course... For example, you may charge at l-a batteries at 14.4V, they might have a resting voltage of 13.2V, which then drops down to 12.6V after the first decent pull of current, declining to a fully-discharged voltage of 11.5V... A pack with 24 batteries might start off at 317V, immediately decline to 302V then drop steadily down to 276V.

This is not a new problem, you know, it just gets worse (in absolute terms) as you go higher in _nominal_ pack voltage.


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## dimitri (May 16, 2008)

CroDriver said:


> As far as I know 3,2-3,7V is the useful voltage. Below 3,2 the voltage drops very quickly to 2,75 so there's no point to go below 3,2V. There is not much capacity difference between 2,5V and 3,2V. Same at the other end at 3,7V. You can charge up to 4,25 but the difference in stored energy is a few %


All of this depends on when you measure the voltage. Under load voltage sags quite a bit, expecially at 3C rates, but quickly jumps back to resting voltage. Useful range of TS cells at resting voltage is 3.0V-3.4V , below 3.0V resting means you better be very close to home since you only have few minutes left to drive. Although I fully charge my cells to 3.8V ( higher than 3.8V just makes no sense ) , the moment your EV starts moving they settle at 3.4V and after 5-10 min of driving they get to 3.2V-3.3V resting voltage.

Bowser, for high voltage pack you can trade off and charge to 3.6V ( this is where main charging phase seems to end and trickle charge begins, so you'd only lose a little useful capacity ). This way you can use 84 cells for 300V pack and be at 269V nominal.


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## milo0105 (Sep 25, 2008)

CroDriver said:


> As far as I know 3,2-3,7V is the useful voltage. Below 3,2 the voltage drops very quickly to 2,75 so there's no point to go below 3,2V. There is not much capacity difference between 2,5V and 3,2V. Same at the other end at 3,7V. You can charge up to 4,25 but the difference in stored energy is a few %


 
Usually lifepo4 battery is stable between 2.7v-3.2v, Above 90% of it's capacity will be discharged in this range.


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## Bowser330 (Jun 15, 2008)

dimitri said:


> Bowser, for high voltage pack you can trade off and charge to 3.6V ( this is where main charging phase seems to end and trickle charge begins, so you'd only lose a little useful capacity ). This way you can use 84 cells for 300V pack and be at 269V nominal.


Excellent point...Thanks for that


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