2008 Off road, Lipo

[mole2k]

Originally Posted by Mark Christopher

you can buy seperate voltage cut offs, novak, trakpower,jperkins and others all do em. when 6 volts is reached they cut the radio, i have used trakpowers and for a few seconds it stutters the motor then shuts the speedo down via the reciever link, they are relativly cheap, small and light

I actually have a lipo cut off in my speedo but I still think from a safety point of view it should be included in the packs themselves, not for racers but for bashers and juniors who tend to run cars out in carparks untill the batterys run flat.

[Chris Doughty]

if you want 'safe' (along the lines of hard case etc...) why not go the whole hog and have min-discharge and max-charge cut-off's for the pack.

if you want cheep, just go shrink wrap and solder tabs.

it would be great if manufacturers had some proposed construction rules for LiPo cells, that might help LiPO evolve and become mainstream?

[DCM]

I agree Chris, at the moment, they are going their merry own route, what will happen, is one will become popular and construction rules will stem from them and everyone plays catch-up.

[mark christopher]

Originally Posted by mole2k

I actually have a lipo cut off in my speedo but I still think from a safety point of view it should be included in the packs themselves, not for racers but for bashers and juniors who tend to run cars out in carparks untill the batterys run flat.

in theory yes, but you would need some heavy elrctronics to do it as a cutr off in the pack and it will have to be able to carry the max current of the pack, as the only way it can stop the pack going flat is cut the main output cables.
this would then reguire a curent to operate which would still draw power when the cell was below 6 volt or it would not come back to life

[MarkWesterfield]

Originally Posted by Cockerill

I'm sorry, but if you are running the same tires surely you have the same contact patch, but less pressure. However, as there is less weight to move around you don't need as much pressure.

As batteries have got heavier I have not seen a harder compound released.

If the tires were solid with no flex, then there would be no change in contact patch, just pressure as you said. But tires are made of flexible rubber compounds that flex and conform to the track surface, especially during weight transfer while corning, braking or accelerating. less battery weight = less contact patch and less weight to transfer (for an increase in contact patch) = less traction. Ask a tire manufacturer. You will still need just as much pressure for the same given tire for the same given traction conditions to go through a corner at the same speeds. You will end up needing softer tires, trust me, thats why everybody adds the weight back to their car with lead. Another bit of physics...by reducing the weight of the heaviest part of an r/c (the battery) that is located in the lowest part of the car, you effectivly raise the center of gravity of the car. Thats why is not just less damping and softer springs to make the cars handle right. Alot of things change when you reduce the weight of our little precision toys by 200 grams.

The minimum weights allowed for the cars haven't changed, so over the years as the batteries got heavier, the car designers designed lighter chassis to bring the cars to minimum legal weight...tires did not need to be harder because the net weight of the car and batteries were still the same.

[Chris Doughty]

but you are also reducing the amount of weight the tire has to 'hold' in the turn.

running the same set of tires I would favour a super light car at pulling lateral G's versus a big fat mama of a car. (even if running hard tires)

[Chris Doughty]

one thing I have noticed with regard to LiPo Vs Nimh...

the EB legalise the use of a 'cell' we then make them up as our own packs of batteries etc...

but it seems with LiPo, people have completely forgot about the 'cell' and are just bothered about the 'pack'

with no 'rules' or legality for the 'cell' part of a LiPo pack it could mean a big can of worms? as long as the 'pack' is LiPo and 7.4v is it 'OK'?

this seems to be a(nother) big differance between LiPo and Nimh mentality

I just through I would throw in this curve ball, I have not seen this element of 'LiPo vs Nimh' debate yet so off you go...

[mark christopher]

hmmm good point chris, i supose it would be fair to say you carnt publicly buy a loose lipo cell to make a pack maybe it needs to be a pack dimension? must be only so much you can get into a box

[Chris Doughty]

I was thinking more along the line of, you buy IB cell's in a 'pack' via LRP (as VTEC) or Orion/Peak etc... you don't normally buy them from IB.

now I know LiPo packs SHOULD come pre-assembled but I believe it would benefit us all if they were made (by the RC company) from a certain 'type' of LiPo cell. for example a 2500mah 3.7v ***x*** size cell. they would proberbly end up in a 2s2p format inside the 'pack' in the same way that our IB cells end up in a 6s1p format.

in the same way that LRP or Orion buy in from IB and they make up their packs from this.

that way there could be a lipo battery list for the 'cell' type and homologated 'packs' are tested and proven to be made up of these cell types.

it just bring about a more 'standardized' LiPo field. I think thats why people like nimh's, we pretty much 'know' what we are getting. LiPo is so 'new' that you get some many crazy combos of batteries.

[mark christopher]

how do you mean crazy combo's?

[Chris Doughty]

some batteries at 4900mah, 3.7v in 2s1p
some 2500mah 3.7v in 2s2p
possible combo of 1000mah in 2s4p etc...

right now for nimh we pretty much have 6s1p and thats it, only the cell 'technology' which is only approved once a year changes to give us more capacity or voltage.

you don't see us finding some 2500 duracel AA's and using 12 cells to make a 6s2p 5000mah 7.2v pack

[mark christopher]

Originally Posted by DoughtyUK.net

some batteries at 4900mah, 3.7v in 2s1p
some 2500mah 3.7v in 2s2p
possible combo of 1000mah in 2s4p etc...

right now for nimh we pretty much have 6s1p and thats it, only the cell 'technology' which is only approved once a year changes to give us more capacity or voltage.

you don't see us finding some 2500 duracel AA's and using 12 cells to make a 6s2p 5000mah 7.2v pack

are they sold as car packs? got to admit only looked at the main manufacures

[Chris Doughty]

if you talking about the 'duracel' thing I just made that up as an example of how we 'could' make a 7.2v nimh 'pack' of cells but the 'rules' that we have (homologated cells of a certain type (subC) mean that it rules out this combo.

[mark christopher]

meant the combo of lipo

think it needs to be 2 cell in series but then a saddle pack would need to be 2x 2 cell so four cells in total

[Chris Doughty]

obviously 2 cells in series to get the voltage.

but the LRP pack has 2x 2500mah cells in parallel too (2s2p)

it is theoretically possible to get 10 500mah lipo cells in paralell and then 2 of those 'sets' in series do make a 2s10p 5000mah 7.4v lipo pack.

see what I mean, the combo's are almost endless.

I like the fact that nimh have a nominated cell 'size' (SubC)

nothing like this exists for LiPo. from what I understand Kokam seem to be like an IB equiv for LiPo, I am pretty sure Orion/Peak use Kokam for their cells.

is there a 'size' chart for LiPo like there is for nimh (AA, C, SubC, AAA and so on)

[mark christopher]

no idea chris about size chart.

[neiloliver]

if you want to see charts, then look at
http://www.kokam.com/english/product/battery_main.html

Kokam make the cells for the Orion packs. i have not examined the list in detail to see which cell they use, but as you can see, the range is wide. Polymer cell tooling is far more flexible than for cylinrical cells so the cost for tooling a special size is thousands of dollars rather than hundreds of thousands. I suspect Kokam use two cells or maybe four in the orion packs, depending how thick they can go with their lamination process. Jimmy and I did discuss me taking one of his packs apart for testing but we never got around to it!

I set x/y/z dimension for the cells would be a good start when talking about setting a standard.

Neil

[Chris Doughty]

would anyone know how (from waaaaay back in the day) we decided that using SubC size ni-cad and nimh cells was going to be the 'standard' for electric racing?

not just 'how' but maybe 'why' we choose this size too?

[neiloliver]

Cs is the industry standard for cordless powertool (23dia43 height) and powertools were to only products with cells capable of delivering high enough currents. NiCd cells started at around 1300mAh (When I started at Ever Ready in 1989 we were still making 1300mAh NiCd cells!) and then steadily progressed to 1900mAh, then to 2400mAh. NiMH for powertool started at 2.0Ah in 1997 (our company lauched the first cell with Makita in that year) and are now between 2600mAh and 4200mAh.

Cs is very versatile when making packs because its diameter is 2x its height.. this makes is tessolate very well in packs when you want to make them into cluster packs for drills, saws etc..

This is why RC cars use Cs.. because of Cordless Powertool. The RC car market is an itsy bitsy % on the end of a pin compared with powertool globally..

Personally, i think it went pear shaped around three years ago... when the Chinese started to enter the market. you see up until then, we used the best of the powertool cells (N-1900SCR, N-2400SCR) from the likes of Sanyo and Panasonic. Being japanese the performance was good and the reliability was also good. The chinese have up'd the capacity and voltage but at the expense of reliability.. the japanese are not following the chinese into this niche market because it does not play to their global goals... but it does mean that to be competititve we have to use hand built chinese cells that dont last so long..

dont get me onto batteries.. i have had enough of them now!!!

[mark christopher]

Originally Posted by neiloliver

if you want to see charts, then look at
http://www.kokam.com/english/product/battery_main.html

Kokam make the cells for the Orion packs. i have not examined the list in detail to see which cell they use, but as you can see, the range is wide. Polymer cell tooling is far more flexible than for cylinrical cells so the cost for tooling a special size is thousands of dollars rather than hundreds of thousands. I suspect Kokam use two cells or maybe four in the orion packs, depending how thick they can go with their lamination process. Jimmy and I did discuss me taking one of his packs apart for testing but we never got around to it!

I set x/y/z dimension for the cells would be a good start when talking about setting a standard.

Neil

track power are defo two cells as they let me take one apart, orion are too as they only have one balance conector and the two main outputs