FLEXnet DC and LFP (lithium batteries)

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FLEXnet DC and LFP (lithium batteries)

Postby inMichigan on Tue Apr 21, 2015 2:27 pm

Hello,

I have a what-if question...
I was thinking using a FLEXnet to monitor the voltage across 'weakest' cell in the string. Let's say it's voltage is ~ 2.7V. It appears that 8V is the minimum setting. Anybody way have a way around that? Best I could think of is to arrange the cells so that the 3 weakest are in a sequence and tap the voltage across those.

2nd, Can more than one FLEXnet DC be hooked into the same Hub network? That is, if one unit is spanning the full 48V bank, and the 2nd is monitoring the weakest link; can they co-exist? Yes, it's an extra piece of hardware, but then the cost of lithium LFP batteries makes it seem reasonable.

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Re: FLEXnet DC and LFP (lithium batteries)

Postby klundquist on Wed Apr 22, 2015 9:55 am

Using the FNDC as you suggest will not work at all for many different reasons. Believe me, it will be a waste of your money to even attempt it.

You need a quality Battery Managment System (BMS) when using Lithium batteries. The BMS will take care of the per cell voltage monitoring. Ask your Lithium battery manufacturer if they offer a BMS or have recommendations for one that is compatible.

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Re: FLEXnet DC and LFP (lithium batteries)

Postby inMichigan on Wed Aug 19, 2015 2:51 pm

While on the phone with OB support for some other topic, I did confirm that you can not have more than one FN-DC per system.
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Re: FLEXnet DC and LFP (lithium batteries)

Postby Ampster on Fri Aug 21, 2015 1:44 pm

klundquist wrote: .....
You need a quality Battery Managment System (BMS) when using Lithium batteries. The BMS will take care of the per cell voltage monitoring. Ask your Lithium battery manufacturer if they offer a BMS or have recommendations for one that is compatible.


I am using an Orion Jr BMS. It is less expensive that the larger Orion BMSs but is limited to 16 cells. As I have discussed in another thread, the Outback Radian generates too much EMF for the shunt to be accurate in the BMS. I bought the FlexNet DC and rely on that device for keeping track of SOC of my pack. I occasionally check the BMS through the PC interface to see how my individual cells are doing.

There are cheaper BMS's out there but I also wanted one I could configure the voltage at which balancing took place. I am only going to charge my pack to a voltage that would equate to 3.40 per cell so I wanted to have balancing start at 3.45 per cell to catch any cell that was going past that. As I mentioned in other threads I wanted more control over the charge parameters so I am using a separate charger that begins tapering current when it gets to 54.4v (3.40v per cell). That way my BMS is only having to shunt away small currents as it is balancing the high cells. If you are considering adding more capacity and a BMS I would recomend buddy pairing the cells and then series connecting the parallel pairs. I have 3 cells paired together and I did it incrementally. I started with 16, added another 16 and finished with the last 16. As that process unfolded I noticed the pack became more stable and needed less balancing.

Once the pack was balanced it hasn't drifted and I have turned off the BMS and monitor the pack via a Lee Hart bridge, a homemade device which will detect a voltage difference between either half of my pack. That may be impractical since you have 17 cells.
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