Lithium cell tests

[maddox]

Lithium cell tests

What drive does Ripper Use? Gear ratio, speedo and motors.
What Ah are you replacing?

[robotwars]

Lithium cell tests

2 bosch 750, 10.5:1 ratio, roboteQ

Replacing 2 x 24v 3amp nicads

JOhn

[leorcc]

Lithium cell tests

I normally calcualte the A123 as 3.3V 2Ah (even though they are a bit more). They weigh 70 grams each cell.

so 24V/3.3V = 7.2 cells, so 8 cells would do 26.4V nominal, 28.8V peak (3.6V per cell).

8*70 grams=560 grams, with shrinkwrap and balancing leads and 4mm2 wire say 650 grams.

3 of these packs will easily get you through a fight, so 650 * 3 = 1950 grams or 2 kg.

With 8 cells you have the advantage that you can use the cheap unitedhobby charger with integrated balancer.

[maddox]

Lithium cell tests

For the same Ah and aboutish V rating you need 3 8 cell packs A123s parallel. Each cell is aboutish £7 and weights about 80 grams.

24*80=1.92kg (remember a 12V 7Ah SLA is 2.65 kg or more)
Total charged voltage is 26.4V and youll have 6.6 Ah at your disposal.

[robotwars]

Lithium cell tests

how much does a 24v nicad pack weigh?

Thanks for the info.

John

[leorcc]

Lithium cell tests

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.

[robotwars]

Lithium cell tests

ok cool.

Thanks for info.

John

[leorcc]

Lithium cell tests

it also saves space, 24cells are a lot easier to work away than 40.

[m2xt]

Lithium cell tests

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.

[leorcc]

Lithium cell tests

Thats why i went for 3 packs instead of 2.

[m2xt]

Lithium cell tests

Sorry Leo but you lost me there?

[robotwars]

Lithium cell tests

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

[leorcc]

Lithium cell tests

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.

[m2xt]

Lithium cell tests

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.

[leorcc]

Lithium cell tests

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.