A sanyo 1.2V 3000mAh cell is 85 gram according to http://www.batt-man.dewww.batt-man.de
20cells x 85 g =1700 gram.
with leads and schrinkwrap youd be looking at 2 kg for 1 pack.
Printable View
A sanyo 1.2V 3000mAh cell is 85 gram according to http://www.batt-man.dewww.batt-man.de
20cells x 85 g =1700 gram.
with leads and schrinkwrap youd be looking at 2 kg for 1 pack.
ok cool.
Thanks for info.
John
it also saves space, 24cells are a lot easier to work away than 40.
John,
In practice a 20 cell NiCd pack is just under 2kg, a 10 cell A123 is 1kg so you should save over 1kg for 8 cell packs. A123 are far more difficult to solder, then you have the added complexity of balancing wires. The voltage drop under load is about the same as NiCds but watch out for the A123 temperature rise as they can very quickly reach their max working temp.
Thats why i went for 3 packs instead of 2.
Sorry Leo but you lost me there?
ok, so best to do what i was going to do, waiti and see how they perform in the test robots before considering buying them.
Thanks for the info Paul. Am i right in thinking eventually when they are ready technobots will be selling them pre built?
John
What I meant was:
By using 3 packs of 8 cells instead of 2 of 12, the packs dont have to deliver as much current per pack and there is more surface area to keep the temperature down.
John, it is highly unlikely that Technobots will be selling them for high current applications like heavies, maybe for feathers and other applications but that depends on their balancing requirements.
Leo, I see, I did assume 3 packs of A123s and my concern over temperature rise remains. Basically, in my opinion based on my own bench tests from what I can remember as I dont have the results to hand, A123s can just about manage a 5 minute discharge in a freely vented area before exceeding 60 deg C. Their Ah efficiency does not appear to be as good as Bot-Packs so a 5 minute constant discharge is between 10-15C (15-33A). Forget the A123 claims of 35C constant. Bosch machines with a heavy nose drag like M2 need an average 90A for 5 minutes so three A123 packs is border line at best, 4 would be most likely but could be 5 packs if the temp rise in hot conditions was a problem.
I see. Doing my numbers on our feather Ive come to the same conclusion. 35C is not reachable outside lab conditions, let alone in a robot.
We run 4 A123s (so 13.2V) in our feather Hannibalito 3 (also a flipper).
With 2 speed 900s on the scorpion XXL I can do 2 fights back to back with it. The temperature is lukewarm at best so thats not much of a problem but the batteries are bone dry afterwards. I need to balance and charge them as soon as possible after that. So for 1 fight its fine, for 2 fights i would go with 2 packs for a feather like mine. I draw pretty much a constant of 20-25A.
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
P.S. ThunderPower do a neat little 10 cell stand-alone balancer - the TP-210V http://www.westlondonmodels.com/product1098/ThunderPower-TP-210V/http://www.westlondonmodels.com/prod...Power-TP-210V/
The technique I would recommend for achieving a good result with such a balancer is:
1) If the pack is badly out-of balance, connect the balancer to the pack (without charger) and leave to balance for a few hours.
2) Charge normally (1 to 5 amps) with balancer still connected. Cut-off at 3.6 to 3.7V per cell.
3) Switch to trickle charge of around 30 mAmps and leave to balance for as long as necessary, but check regularly that the pack voltage does not exceed 3.7 Volts per cell. Switch off the trickle charge if it does.
This ensures that the balancing occurs when they are fully charged - the only place where it matters. You cannot accurately tell if a pack is balanced by looking at the votages when the pack is only partly charged.
The most common out-of-balance situation with these cells is one or two low cells in a pack. This quite often happens if you leave the packs for a few weeks. This takes quite a long time to rectify with most balancers, because they have to disharge all the other cells down to the voltage of the lowest one.
John
Hah! Just read Jonnos post about using only two NiCad pack in Ripper. Trust you to push it...
Ill bet theyre quite warm after a long fight.
I would agree that three packs of 8 A123 cells should be sufficent, if you needed to save max weight by staying at 24V. I wouldnt recommend less than four packs at 36V.
John
@ Little John Lear
The reason it has been suggested that you dont leave a machine powered by A123s in for a second fight is that running them flat damages the cells. It would be OK if you had some sort of low-voltage cut out to protect the cells.
Not a big problem going flat with NiCads, but with A123s you really need to make sure youre fully charged before every fight.
John
who me.... pushing things nah !!!!
Surely I am not the only one who only uses 2 packs?
They dont appear to be too warm, and I can get one and a half fights out of them.
When I used to run at 36v they were a lot warmer, Hot even!
So for me, i dont think it would be worth it as i have a relatively light version in comparison and i am happy with the performance!
Plus I never wait for them to be fully charged at an event as im always raring to get back in the arena. Especially when ive driven down the pit after 12 seconds in a fight !! Grrrrrr
I will see how others perfrom though, as the upgrade to 30v is appealing.
John
Thanks john
answers what i wanted to know
think we would only get these if it was back on TV as thats when we wont have as many fights close together like we do atm
would save abit of weight and i could put the moving wings that i want for our winning pose :P
quote:
I for one will be moving to have the battery rules rewritten soon. Especially as the UK championships are looming.Thank you Kane, much appriciated. When can we expect a final decision on this?
And also a big thank you to John for the great work he has done testing these cells.
I think really, the wording needs to be written, agreed and put into the rules. This needs to happen by the end of March really.
the thunder power balancer john shows is the one ive been using for the last 2 years. Also wherei get them from (only a few miles from me). They can balance upto 10cells in series, but will also balance 2 packs at the same time to be of identical voltages so it is safe to run them in parrallel.
John you are not the only one to only use two packs, we do as well. Would prefer to use a third but we havent got any spare weight.
yes Mr Reid...... I am not the only one so there!!! :)
From our Australian colleages:
http://farm3.static.flickr.com/2041/2088496840_e3f0c250f2.jpghttp://farm3.static.flickr.com/2041/...e3f0c250f2.jpg
http://farm3.static.flickr.com/2159/2088497690_a3dc838800.jpghttp://farm3.static.flickr.com/2159/...a3dc838800.jpg
And yes, I know it is April 1st, but its no joke. :)
I though Id chime in with some of my experiences Ive had with A123s in my featherweight.
I currently run 2 x 6cell packs in my featherweight (1 pack at a time) I dont run balancers in them, as I have found I do not need them. I balance the pack initially by matching the cells with voltages within 0.25v of each other. I then cycle them 3-5 times at 200mah. This is low enough that if one of the cells reaches peak before the others, it wont cook. After that it gets 2-3 charges @ 5amps, if any single cell gets hot compared to the others during this, I pull the pack apart (5min job with the http://www.modelelectronicscorp.com/...uct.php?pid=29
Interesting, I found the paste starts to boil at about 35A in the MEC packs so considered them as being unsuitable for applications above this current. Shame really as it appeared quite a nice solution to the problem.
Heres a link to the pack build...
http://www.robowars.org/forum/viewtopic.php?t=944&postdays=0&postorder=asc&start =120http://www.robowars.org/forum/viewto...rder=asc&start =120
Note that he used Arctic Silver 5 thermal paste ...which I queried.
Scroll down the page for the complete discourse.
Seems to have worked for him ...goodness knows why!
I actually use http://www.pccasegear.com/index.php?..._id=2173Arctic
I think that you may have misunderstood the blurb.
The silver used in the compound is 99.9% pure silver ..the ammount of actual silver present in the paste isnt clear.
I thought Arctic Silver doesnt conduct electricity very well. But I suppose youve tried it at high current, so apperantly it gets displaced enough to make a good connection.
I used the silver paste from MEC and it was tested at various loads on a 10 cell packs and it sounded like a kettle boiling at 45A from memory.
Double post
(Message edited by m2xt on April 10, 2008)
Seems to have worked for him ...goodness knows why!
Possibly just helping move heat around, people have made battery packs just with the cells held together in compression. One disadvantage of this is that the vents are probably compromised.