Hello, (this is my first post here )
I am in the process of upgrading a 4WD3 rover kit from using 7.2V Gear head motors to 12V gear heads. I could make a 12V NiMH pack out of tabbed sub "c"s, (heavy and less mA) but would rather use 2 - 7.2 6600mAh Li-poly packs (total 14.4v) for lighter weight and more power. My question is can I safely drive the 12V gear head (GHM-13) motors with 14.4Vs instead of 12V? Am I pushing my luck or will they handle the extra voltage for the long haul?
That’s a relatively small percentage of overvolting. The voltage is only present when you are moving at full speed, which is not usually 100% of the time anyway. I would say it’s not a problem.
Thanks Jim, I assumed it would be ok, just wanted to make sure. You’re right, I won’t be driving them (using a scorpion board) at 100% full speed anyway.
Most of the peeps here either can’t get passed the hazards/price of LiPo’s.
If you’re willing to pioneer that front, I’m sure that we’d be all ears as to how you work everything out.
Myself, I’ll be going with LiPos the next time I revise my biped, since I’m currently wasting quite a bit of weight on 6 2800mAh NiMH cells.
I’ve gotten stuck, though, since I can’t seem to find any inexpensive control boards, nor any in-depth tutorials on making one.
So, I’m stuck waiting until my EE knowledge catches up with my wishes.
Far as I know the main pitfall for LiPo is over charging, and most importantly over discharging. I say more importantly because the charger handles charging, you really don’t have any control over that. But discharging is where you can damage the pack. However all you need to do is monitor the pack and don’t let it fall below the limit (not sure what it is). A simple A to D on the robot controller can do this.
Well… I had been hoping to leap the expense of a charger by building my own.
I realise that’s a pretty big EE venture, but I’m not exactly planning on revising my current biped design in the near future, anyhow.
Perhaps I’ll cough up the dough, though, if monitoring is as simple as you say.
Is voltage an accurate-enough way to protect from overdrainage?
And, if so, is there a way to directly monitor the voltage?
Or, should I just do something like placing two large value resistors in parallel with the LiPo and sampling across one?
I’m betting that my poor little micro’s ADC shouldn’t be subjected to 7.4V.
Hi Nick!
Oh, yes charging is a little more tricky. I’m not even going to try and advise on this as I’m sure there are other who know much more about it than I. I will say that the procedure is pretty straight forward. I think manufacturers talk it up a lot to make it sound more complicated than it really is. How else can they justify the steep prices for chargers. I’m not sure about over current protection either. But, a fuse is an easy protection device to implement. But to answer the question about monitoring the battery voltage, yes of course you need a voltage divider. Then it’s a simple matter of preventing the pack from being dischardge beyond the recommended level. That’s what kills LiPo’s.
The packs I will be using (from All-Battery) have built-in over charging and over discharging circuitry. I was testing a smaller pack today and it just quit supplying power to my 4WD when I tried to draw too much mA from it. It protected itself from being overly discharged. I feel pretty safe that the larger packs I ordered should do fine in the charging/discharging department…
A very short summary, over discharge is what damages LiPo cells and over charging them is what causes the fire hazzard. The irony of that is when you over discharge them you decrease their capacity which in turn makes it easier to overcharge them and have a problem.
Before you jump on the r/c LiPo pack for your robot train be certain to look at how the r/c aircraft people are handling them. Ask yourself if it will be practical for you to remove the battery EVERY time you need to charge it, and then charge it in a fireproof location with a charger designed for LiPo packs. There are many pictures on the web of folks who failed to follow this common sense rule and lost planes, equipment, and in some cases fair sized portions of their homes.
On a more positive note, r/c planes are generally fighting for weight and therefore they generally don’t include the protective circuits integrated into the packs as all commercial Lithium packs used for phones, laptops, etc. With the robots the weight fight may not be quite so severe and the weight advantage gained by using LiPo packs AND a protection circuit may make it worth it in the end.
From a recent experience at work, we found a camera battery pack is a pretty tight way to get this combination. Granted it will be 18650 cells and not polymer packs (and therefore rated about 3C) but from a safety and convenience standpoint we found two 7.2V 4000mA packs in parallel (about 8oz) worked great for a head mounted infrared vision system we developed. For the prototypes we hacked a sunpak dual charger for it’s frame and to make the electrical cnnections.
I just realised the packs I alluded to in my first post are indeed Li-Ons not Li-polys. Under the shrink wrap are small circuit boards for over-discharging and charging issues.
There is nothing wrong with Li-ion cells. LiPo is short for Lithium Ion Polymer. They both have very high energy density which is what sets them apart from NiMH and NiCd. The LiPo though have the advantage of higher current density which is what allows the 8, 15, and if you believe it 20C discharge rates. They all charge at 1C however and all of them get damaged by deep discharge. The Li-ion packs we used were not even $40 a pop, were like 4000mAH, and you can’t kill them because they have the protection circuits built in. I am not sure if you can overdraw them though… it might depend on the pack. We were going for a long run time so that is why we used two in parallel, plus the weight helped offset the camera and hud.
In terms of safty, your going to have to be more specific on what kind of batteries you are running. For example what is there C rating, are they balanced, do they have a balancing lead and board, do you have a link to them? Also what motors are you driving with them. Just to be safe you should assume the max current draw is with locked motors, so 3.8 amps a motor. So for 4 motors thats over 15 amps + what ever current your logic and sensors are drawing. Unless that pack is rated atleast 10 C I would probably not use it. Especially if you dont have a balancer for the batteries. It might be ok in practice, so if you do use it just make sure you keep a close eye on it. Lipo cells do not like to be abused, I had one almost rupture in my wing the otherday because 1 cell was failing.
And EddieB, the Lipos I use in my flying machines are rated up to 30C, they go even higher then that but also get more expensive. If you guys are looking to use nice batts check out www.vampowerpro.com. Probably the best batts you can get for the money. Also check out ebay for some of the V28 LiMoly packs. Its a pretty good price for some pretty good batteries.
Hmm, I did not see any 30C rated packs on their site but I only browsed it for a few minutes. If you are using 2p or 3p configurations the 30C is quite easily done of course. 30C cells though… heh… when you see it verified by more than 1 or 2 independent testers and they can duplicate the performance more than 2 or 3 times then you have something. I will say that I’ve been out of r/c planes for about a year though so maybe 30C is the new sales benchmark, where 20C used to be the sales hype-mark.
I’ve lost track, are we still on topic for this thread by talking about LiPo batteries?
Going back to his rryerson’s original question and his following discovery of their being Li-ion based 7.2V 6600mAh packs, of which he plans to use 2 in series. The most common Li-ion cells are 18650 (18mmx65.0mm) and 3C is a fairly typical rating for those cells. So 6600 x 3C = 19800 mA, 19.8 amperes max continuous current draw. There are 4 motors so a max safe/conservative stall current for the motors would really be about 75 to 80% of 19.8/4, around 3.8 ampere.
Topher, did you back calculate 3.8 amperes or did you just happen to pick a motor with a 3.8 ampere stall current? That is just waaay too odd.
)
But to answer the question about monitoring the battery voltage, yes of course you need a voltage divider. Then it’s a simple matter of preventing the pack from being dischardge beyond the recommended level. That’s what kills LiPo’s.
Topher, did you back calculate 3.8 amperes or did you just happen to pick a motor with a 3.8 ampere stall current? That is just waaay too odd.