Who do you buy LiFePo4 from

[maker102]

I have been dragging my feet for 6 months now and enough is enough. I am planning on buying Headway cells (BM's choice, I think) this week and was wondering who you have bought from? Did you have any problems with your orders. Any bad cells? In short are you satisfied with your purchase? I am in the USA but doubt this matters. Thanks in advance for your recommendations.

[Burgerman]

No bad cells yet. Built 3 packs so far - one for me, 2 for local scooter/chair users. Most cells from evassemble, some from a uk electric bike shop. In the US there are probably better options.

[Williamclark77]

I've ordered all of my LifePo4 Headway cells from their US dealer. http://www.headway-headquarters.com/

They did fine by me. No real complaints. No dead cells. They do send you a receipt with the serial number for each cell.

[Burgerman]

You don't often get a "bad" cell. Of course it depends on your definition.

You can make them bad fast!

1. Discharge too low. At 2.7v there's no useful power left. Maybe a few yards... No point going lower. They drop fast right at the end. 2.5V is very low and as low as you should ever allow them to go at a push. 2.0v is really, really low! They may not recover fully and will go "bad" instantly.

2. Charge too high voltage. 3.60Volts MAX if you want a long service life and say 2k to 5k cycles. 3.65v means a considerably shorter lifespan, but easier to balance as this is well above the natural "full" state. (So they naturally want to drop down, allowing you to push more power in - making low cells to catch up as you charge. 3.70v and they don't much like it one bit. Almost every battery BMS setup repeatedly allows them to exceed 3.8v and often considerably more over and over in cyclic fashion for many hours... So avoid BMS like the plague.

3. High C rate discharge. If you take 1C from a 10C continuous cell, it will last pretty much forever. Take 5C from a 10C cell and it really wont... Take 10C from a 10C cell, and it may last a few hundred cycles at very best, usually much less. So you want HIGH C rate cells, and low C discharge rates if you want to get a lot of cycles from your pack. For this reason Headway (10C) are preferred to Thundersky, Calb, Winston etc Prismatic cells in a powerchair. We use small batteries due to space. We may need up to 240A from a pack with a powerful control system such as R-net 120.

What does bad mean? Higher resistance or low capacity, hard to balance or a bit of all three etc.

All lithium ion phosphate cells seem to live a very long life if you don't:
Go over3.60v
Go below 2.70v
DO keep as far away from the max C rate as possible.

So fit big capacity, high C rate packs, charge via a proper charge system (not a BMS!) and don't run them flat!

[maker102]

Thanks for the advise. What do you use to keep track of the packs to prevent/stop them from over discharging? Thanks

[Burgerman]

My bum. I tested this a while ago. I am not using the lithium chair much yet - still messing around with it. But I tested how it felt when discharged. Just like my model helis, planes, you hear, feel, rpm drop away and power loss. And stop/land.

With a 3.2kwh battery that wont happen in a powerchair, I will wear out first. But, the roboteq is programmed to power an alarm at the correct voltage. Or use a hobby warning buzzer.

[Sully]

BM,

Is the hobby buzzer further explained elsewhere or could you do it here? Since In any of my current efforts in beach/hunting chairs are using older controllers (24vdc 80 - 100 amp) I "assume" there would be probably a very slight possibility I would ever exceed the desired discarge "C" rate in almost any situation. Wouldn't the controller itself restrict my ability to exceed the "C" rate of a say a setup of 24vdc 80 +/- AH Headways ?

Am I asking questions that the answers may more easily found elsewhere on the site?

[Burgerman]

Headways are safe enough. If you fitted say 100Ah of the blue headways there's no chance at all of exceeding the C rate. C rate isn't a "fixed" amount like a fuse. Its just the max you should ever let happen. The further you stay away from this the better. If you never go anywhere near it, like 10x less, your battery will last a very long time... Headways 2000 cycles are at 1C. CALB 2000 cycles are at 0.3C. At 3x both those figures you would get around half the cycles.

EG 100Ah battery at 10C is 1000Amps. If your chairs controller can do 120 Amps per channel then you could theoretically reach 240A from the battery intermittently. Or about 2 to 3C.

With a 50Ah battery that same 240A of current would be about 5C. A 5C battery could "cope". A 10C battery could cope better. And will last longer. But either a higher rate battery or a bigger Ah battery will cope better. The bigger Ah one will of course also benefit from lower discharge levels, or less frequent cycling.

The voltage level buzzers are plentiful, and all over ebay, hobbyking etc. Many types and styles. I just have the Roboteq controller set up to start a buzzer at 2.8v per cell, as thats a couple of miles to find a charger point. And to stop the chair (safety stop) at 2.4v per cell. Because that's about 200 yard before the batteries are 100 percent dead... Before damage at 2v and below. But I am only looking at total voltage, div by cell count. Because I know this is safe on MY pack as I can see they all pretty much drop off a cliff at the same time, by watching on the hyperion graphs.