# Controller for 24 LiFEPO4 cells



## calvin2 (Apr 26, 2009)

I am planning on using 24 LiFeP04 Thundersky cells 60Ah and understand the max voltage is 4.25V/cell. So right after charging, the battery pack can be as high as 4.25 x 24 = 102V. I understand this spike levels out shortly to 3.4V/cell which is 3.4 x 24 = 81.6V for the pack. I am planning on using the Alltrax 7234 for my build. The spec on the Alltrax controller says the high current limit is 90V. It seems the 72V Alltrax controller cannot handle handle this 102V voltage spike initially? I always thought one uses 24 LiFePO4 cells for 72V battery systems. Does anyone have any experience or knowledgeable using Alltrax AXE 72V controller with 24 cells?


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## Tesseract (Sep 27, 2008)

calvin2 said:


> ..The spec on the Alltrax controller says the high current limit is 90V. It seems the 72V Alltrax controller cannot handle handle this 102V voltage spike initially?...


I searched the forum (hint, hint) and came across lots of posts with Alltrax 7234 in them... Might want to see what other people have done.

Semiconductors can usually tolerate a bit of overcurrent (with how much being inversely proportional to time) but not even the slightest amount of overvoltage. If Alltrax says their 7243 controller is rated for "90VDC Max" then I would bet dollars to doughnuts that it has 100V MOSFETs inside. Put 102V on it, even if it is just briefly during startup and it is only a matter of time before the MOSFETs blow. Really, it has been my experience that most motor controller manufacturers are way too optimistic with their ratings in the first place. By all accounts Alltrax makes a pretty good controller, but if they say 72V nominal is the max input then I would only use it with 6 x 12V batteries (or the equivalent in LFP). Maybe 20-22 ThunderSky cells, then, though 22 would be living dangerously to me.

How close you can run a controller to the max rating of the MOSFETs inside depends quite a bit on factors you can't evaluate externally: namely, how much stray inductance there is between the FWDs and MOSFETs, the ESR of the input capacitance, and the switching transistion speed. All of these factor together to create a spike every time the MOSFETs turn off.


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## dimitri (May 16, 2008)

calvin2 said:


> I am planning on using 24 LiFeP04 Thundersky cells 60Ah and understand the max voltage is 4.25V/cell. So right after charging, the battery pack can be as high as 4.25 x 24 = 102V. I understand this spike levels out shortly to 3.4V/cell which is 3.4 x 24 = 81.6V for the pack. I am planning on using the Alltrax 7234 for my build. The spec on the Alltrax controller says the high current limit is 90V. It seems the 72V Alltrax controller cannot handle handle this 102V voltage spike initially? I always thought one uses 24 LiFePO4 cells for 72V battery systems. Does anyone have any experience or knowledgeable using Alltrax AXE 72V controller with 24 cells?


24 cells is correct config for 72V nominal controller. You never charge them to 4.25V/cell. Normally people charge them to 3.6-3.8V per cell, which is right at the Alltrax 90V limit, so you are in the sweet spot  , go for it....


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## calvin2 (Apr 26, 2009)

Thanks for the feedback. Those were great responses. I will charge battery to less than 3.65V/cell x 24 = 87.6V. Now to find a suitable charger that charges at this voltage at around 8-10 amps....


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## kittydog42 (Sep 18, 2007)

I used an Alltrax 7245 with 23 cells with no problems. At 3.2V per cell, that is 73.6V. I saw over 90V while charging.


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