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Father and Son Antweight Battle Bot Project

#1

James and I are building an Antweight Battle Bot for the upcoming CIRC competition. It’s a high speed vertical saw blade weapon on a Inertia Labs 4WD chassis. We have pretty much everything going on the chassis and are working on the weaponry. My question is we bought some ElectriFly LiPo 7.4V 15C discharge 910mA packs. They have a charge and a discharge cable/plug. To save weight and space can the charging plug and assumed protective circuitry be removed? Also should I charge with the charge cable even if I’m using a good quality charger? I’m just not sure what the charge cable/circuitry does to my charging operation. I.E. limit the current, or adversly effect the charger.

Images soon. :smiley:

#2

The “Charge” plug is simply a wire coming attached between each cell. You can charge through the discharge leads (thats what they did in the past) but that is much more … risk prone. If you do charge through the discharge leads, you will want to keep some wire there so that you can “balance” (discharge to the same level) the two cells in a 7.4V pack. (The reason you balance the cells is because if you don’t, you risk the “big boom”)

Really, I would leave them in, as they really are for your own good, and wire can’t weigh that much!

However, I would worry about your battery choice… only a 15C? That means that at a discharge rate of over 13.5A, you risk exploding the battery… that seems pretty easy to hit, especially as a weapon motor stalls from contact with something!

#3

Interesting. If the charge cable is really a connection between the two packs then you will need to charge it using both the charge and the discharge. Off to test it…
Sorry the facts don’t support your theory. There is 7.2vdc on both the charge and discharge cables, so it can’t be as you said.

I believe the boom is simply if you over charge the pack. It’s nothing to do with balancing the two cells.

The 15C was the most available in the smallest package. It operates the bot and weapon motor without a problem. The motor draws 2 amps no load, 11 amps at peak efficiency. So it’s right in there.

One other thing, Antweight bots weigh only 1lb. We are down to a few grams to spare! :smiling_imp:

#4

You telling me the charge leads only have 2 wires? There should be three wires for a 2 cell pack. One is ground, the other is +3.6V, the other +7.2.

For example, see this:
fmadirect.com/support_docs/item_1254.pdf

The boom is most likely when you overcharge the pack. However, an unbalanced pack has much more potential to boom!

And I shouldn’t be wrong, I spend a lot of time researching LiPos. :stuck_out_tongue:

#5

Not to mention, when LiPos go boom, it’s not pretty. I wouldn’t want to be anywhere near that

#6

Sometimes those little charge wires are attached to a small PCB in the wrapping. I never remove them, and I always use them to charge (given the volatile nature of lipo).

#7

They are balance taps. They are for plugging into a balancer so you can balance each cell while charging.

So such a small pack, I wouldn’t worry about it. R/c guys used lipos for years without balance taps. The plug should contain a common ground and a positive off each cell.

You can cut them off, just be sure they don’t short out on each other. And be sure to watch your discharge voltage.

Overcharging is bad but so it over draining. If you drain the pack below 3.0V per cell, you start to risk making them swell and vent. And they should always be stored at half capacity, never drained.

#8

Thanks for the information. Yes these packs have only two wires on the charge plug. Black and red, ground and +7.4vdc. We will leave them as-is if at all possible. Hope to get the mechanical stuff wrapped up this week so we can see exactly where we are on the weight. We’re making progress. 8)

#9

I would just use the wall adapter that came with the battery. The three wire operation of a LiPo is usually the third wire is sensing the temperature of the pack. A good charger such as the Triton or the Duratrax ICE uses three wire charging scheme.

LiPos are VERY sensitive to an instantaneous shock. Just make sure that it’s in a hard case and you should be fine. If you’re using it for a battle bot application, sticking it inside an aluminum cavity with lid might be the answer. Or better yet, why not make a box out of Lexan? I found the Lexan that you use on your various chassis component are VERY strong :smiley:

Many LiPo packs that are used in RC Car applications are hardcased because these cars bang around alot (I break about an front A-Arm almost every three runs in off-road. The shock gets worse with on-road with the speed that these little monsters are capable of).

Good luck to you and son in the CIRC competition. We’ll all be rooting for you two!

#10

The packs have only two wires for charging, black and red. I only ordered the batteries, they didn’t come with a wall adaptor. I do have the Triton charger, but I don’t have any adaptors for a three wire pack.

Thanks for your support! :smiley:

#11

It’s not recommended that you bypass the third wire, but I’ve see people do this. You just gotta be careful with monitoring the temperature on it. I don’t know exactly what is a “safe” temperature for charging. I would just follow electrifly’s wall charger for this battery…

#12

I use Lipos for rc helicopters and other rc/robot applications.
Just use the right charger and there is no need to really worry about balancing the lipos unless you build the packs your self.

But I would reccomend to leave the circuit on the packs as like evolution said its a safty to prevent over discharge based on cell voltage. If you go below 2.7v at load then the pack is dead and wont charge anymore.

I have killed some packs like this and no matter how long you let them charge the wont hold a charge.

I havent bought a new pack in a while but I assume not much has changed except for pack sizes.

Just as a precaution a would put a fuse in so that if something happens
pop goes the fuse. 18 amp fuse should be good. I assume even thou rated at 15c they should be able to handle 18 amps for a couple of seconds with out damage.

#13

Well I am sharing the build with James. So imagine my surprise when I was poking around his work bench and found no less than two informative papers that were included with the batteries. 8)

So the packs we have do not have the balancing plug. The charge plug goes to a small circuit board to monitor the charging process. It says do not charge with more than 1.5 amps current. I already know about the overcharge and over-discharge conditions that can explode the back. We’re using a Sabertooth 2x5RC so we will take advantage of the LiPo mode so we don’t over-discharge the packs.

#14

WTB Pics!

#15

Soon, I’m swamped for the moment. :frowning:

#16

Umm… In my defense, he didn’t ask for them…

#17

Ewww… what a weird and non standard battery pack :confused:

#18

Yep. My “research” was basically to find the required current delivery and physical size for our requirements. Because this is my first real experience with LiPo I though it WAS the standard. LOL

#19

That sounds familiar. :smiley:

Do you have any pictures of your antbot yet?

#20

yep, we finally have a picture: