Lithium cell tests

A small point though:

Before we focus too much on A123s in particular (which is obvious bacause they are in effect market leader) this is about allowing LiFePo4 for heavies, not A123s alone. There already are more manufacturers out there, some with better specs some with worse, and just like with Nicad and Nimh some will emerge as the best and some will fall short. But none of this is safety related only performance related. The current rules were written in such a way there was no possibility for any Lithium battery to be used including the ones that are not balls of fire waiting to happen. That is why I am in favor of allowing the use of LiFePo4 in heavies.

In the time to come LiFePo4 batteries will get better and better as they are already becoming more and more commonplace in electric vehicle use and DIY equipment. This has been true of all battery types and so will LiFePo4.

Cut & paste from fma direct

An A123 cell would be conservatively rated at 17C, and
would tolerate 33C in bursts. Keep in mind that peak and continuous ratings
depend on acceptable cycle life. At 16.5C, A123 cells deliver 230 cycles

Alan, Their website also quotes:

# 70A continuous discharge
# 120A, 10 sec pulse discharge

70A being 30.5C but I was basing it on the 2Ah capacity hence 35C. Either way it remains far from what is possible in a pack in realistic conditions.

I have just found out some of my results and at 26 to 27A will not quite last 270 seconds and ended up being just under 60 Deg C. 20C lasted about 180 seconds and 68 Deg C. Now put them inside a robot with a number of packs bunched together, with hot motors and then inside an outdoors arena and you would have to derate the packs accordingly.

One interesting point about the A123s under a 20C discharge is that their internal resistance initially rises but then falls as their temperature rises. Just before the cells collapse, they are delivering their max current.

A Bot-Pack at 30A will last just about 300 seconds.

Hi Paul,
I wasnt trying to poo poo your results ...I was backing them.

The link is here.
http://www.fmadirect.com/support_docs/item_1229.pdfhttp://www.fmadirect.com/support_docs/item_1229.pdf


Another extract ...copy & paste.

A123 cells can withstand short bursts at discharge rates up to 33C (33 times
capacity). Despite outlandish marketing claims of 20C-30C capability, LiPo cells
struggle to deliver much greater than 15C, and that is accompanied by very short
life in the range of 50 cycles. Capacity for A123 cells at this time is limited to a
nominal 2AH.

2ah 16.5C or 33 Amps not that far away from your test results.

(Message edited by woody on February 22, 2008)

Hi Alan,

I didnt think for a moment that you were trying to poo poo my results :)

I hadnt seen that document before and I will read it in detail later but I was interested to notice that the multi C graph also shows an increase in terminal voltage (decrease in internal resistance) at then end the 16.5C and especially 20.5C curves.

But still... there is a deafening silence from the FRA ?

Hardly, the topic has been actively discussed since it was opened!?

Well lets summarise as best we can ....

A123 Type batteries dont appear to have any safety issues apart from overheating if the pack is sized wrongly for its given application and abused.

Yes?

(Message edited by woody on February 23, 2008)

That is my view, there appear to be no safety concerns with the a123 LiFePo4. In fact the technology seems very good even if current discharge is not what some would hope for. Weight for weight the technology seems as good as NiCds even if a little pricy and short lived when abused.

I for one will be moving to have the battery rules rewritten soon. Especially as the UK championships are looming.

I think we can say that weight for weight, they are considerably better than NiCads.

We used to run three 24V packs of Sanyo 3Ah NiCads of THz, total weight around 5.7 kg. I dont think many people would consider running a heavy on just two packs.

We now run four packs of 34V A123s, total weight around 3.8 kg. So we have increased the voltage by almost 50% AND saved almost 2kg, with a similar capacity. That is a substantial improvement.

Even after a long hard fight, the A123s only get mildly warm - nowhere near 60 degrees. If your robot has a higher current draw, then you should use more packs (which you would need to do to have a safety margin on capacity) - theyll still be much lighter than NiCads.

Regarding cost, DeWalt battery packs see to be going for around £70 on ebay, so theyre cheaper than the equivalent amount of good NiCads too.

I didnt find them any more difficult to solder than NiCads, but adding the balance wires certainly is more work.

I believe they have now proven themselves and that the FRA should approve them for use ASAP.

Im not sure about approving all LifePO4 cells. I would imagine that there are some pretty ropey ones out there. The A123s are available at a reasonable price and perform well.

John