Limited Amps on Input - RC-Monster Forums

Batfish · **Limited Amps on Input -** 06.29.2005, 02:36 PM

Here's a question to you electronics folks:

If your batteries were physically limited to less amps than your speed control and motor wanted to pull, what happens?

I ask this because I've learned that the new FMA lipos I bought have a note with them that specifically says that their wiring limits them to 60 amps, even though they are capable of more. I assume this means that the pack will only offer up 60 amps regardless of what is being demanded of it. If this is true, that's a great way to help ensure lipo safety since my packs should be physically capable of 128 amps, but are limited to 60. That should mean they would never overheat because they never get pushed too hard. However, I'm curious...

With that in mind, what happens when the speed control and motor want 100 amps during heavy accelleration or other spikes? Do they just operate on the 60 amps and not run as hard?

I'm expecting to get some first-hand experience this weekend, but I was hoping that someone else might have a good idea before I blow something up! :)

squeeforever · 06.29.2005, 04:27 PM

actually i have been told that doing so might harm the packs. if you are using 2 in series then the discharge rate will double to 120 from what i have been told, but im not 100 percent sure since i dont have any lipos. YET.

SpEEdyBL · 06.29.2005, 05:13 PM

Putting the packs in a series will make the motor want to draw more amps due to the higher voltage, but if the packs will only allow 60 amps, they will only provide 60 amps. However, putting the packs in parallel will allow 120 amps (60 from each pack) but the voltage will stay the same and the motor will not ask for any more than it did with just one pack.

My guess is that it's not a good Idea to continuously draw max amps from the pack.

Serum · 06.29.2005, 05:32 PM

the bottleneck of your packs are the wires. They are not meant for currents larger than 60A if you do ask more than 60A from them, those wires will melt. The battery itself is not limited, they simple can't deliver >60A without damaging. the wires will melt. (can be the little flat wire that connects the lipoly with each other or the power leads, but i fear it are the connections that will fail.)

It is a very slow fuse, it won't burn up when you ask 100A for .5 seconds. but they are not meant to deliver that.

That note is probably the result of a bad experience they had. (in a test or from a customer)

@ squee, the packs will rise in voltage, not in max current when put in serie. It takes parallel to double the discharge and the capacity.

RC-Monster Mike · 06.29.2005, 05:42 PM

The controller won't act funny with a 60 amp limit, either. It will simply deliver the power available to it.

captain harlock · 06.29.2005, 07:33 PM

Okay, How can I know the discharge rate of a certain type of battery pack say nimh or lipoly and: If a motor produces 700 watts@55A (the 5300), then how many cells of nimh(3300) do I need to make such rate possible? AAANNNDDD is there any nimh capable of putting out 40-50 amps using 6 cells only?

RC-Monster Mike · 06.29.2005, 09:11 PM

Simple formula, really. amps time voltage = watts. You can find the missing number if you have any two. 700 watts/55 amps = 12.727 volts, so about 12 or 13 cells under a load. The gp or IBC cells will put out 50 amps, but the voltage will drop off at that ouput.

Batfish · 06.29.2005, 10:00 PM

Quote:

Originally posted by Serum
the bottleneck of your packs are the wires. They are not meant for currents larger than 60A if you do ask more than 60A from them, those wires will melt. The battery itself is not limited, they simple can't deliver >60A without damaging. the wires will melt. (can be the little flat wire that connects the lipoly with each other or the power leads, but i fear it are the connections that will fail.)

It is a very slow fuse, it won't burn up when you ask 100A for .5 seconds. but they are not meant to deliver that.

That note is probably the result of a bad experience they had. (in a test or from a customer)

@ squee, the packs will rise in voltage, not in max current when put in serie. It takes parallel to double the discharge and the capacity.

Serum - how do you know that the wires are the bottleneck? I don't know what the connectors are like inside, but the battery leads are 12 gauge with Deans connectors. The manual indicates that it's limited to 60 amps. It does not indicate that the packs can go above that, and there is no warning about trying to draw more from them. My assumption was that there was some circuit in them that just doesn't allow more than 60 amps to be drawn from them. As I mentioned above, I think it would also be a great safety measure to limit the pack amp draw to well below the max, since it would mean safer temps for the cells. I would think they would have a specific warning about drawing more than 60 amps if it means that the wiring will melt.
I guess I'll either email or call FMA and ask them what the scoop is on these packs. I may get stuck using them in my XXX-T and TC4 if they can't handle the amp draw from using them in my MGT or 1/8th buggy.

Serum · 06.30.2005, 03:56 AM

No, there is no current limiter inside that packs. a 60A limiter would be large and expensive to make. like they say; it are the wires that are the limit.

Paul · 06.30.2005, 07:49 AM

Hey Guys,

Those packs will deliver what ever amp draw the motor asks for. I'm not saying it's a good idea to pull more than 60 amps from the pack if that's what their rated for but it can be done. If you expect to pull 120 amps run 2 packs in parallel to divide the amp draw between both packs.

Paul.

supermaxx4190 · 06.30.2005, 08:32 AM

well if everyone is using gps with deans connectors and 12 gauge wire...how is this any different?

Batfish · 06.30.2005, 09:10 AM

As far as running them in parallel; I had planned on running two packs in series so I'd have 14.8V. If I run them in parallel, I'll only have 7.4V, which is probably not enough to be competitive in the 1/8th buggy or MGT.
On an interesting note, though - if I could be competitive with 7.4V and keep the motor and speed control in safe temperatures, I could run quite a long time at 7.4V and 12800mAh. Hour-long race, anyone? :D

I'm waiting on a reply from FMA at this point, and I'll share whatever they return for an explanation/description.

MetalMan · 06.30.2005, 09:50 AM

You mean 6400mah, but 120amps capable, right? But then you have to figure the formula for watts. watts=volts x amps. If a motor wants to use 700watts for a certain setup, then you would *in theory* get the same runtime as if you put it in series, because of the formula for figuring out watts. But, this doesn't work since at lower voltages and faster motors, the efficiency goes down and and you would get less runtime than the higher voltage/lower mah setup.

Batfish · 06.30.2005, 10:22 AM

Quote:

Originally posted by MetalMan
You mean 6400mah, but 120amps capable, right? But then you have to figure the formula for watts. watts=volts x amps. If a motor wants to use 700watts for a certain setup, then you would *in theory* get the same runtime as if you put it in series, because of the formula for figuring out watts. But, this doesn't work since at lower voltages and faster motors, the efficiency goes down and and you would get less runtime than the higher voltage/lower mah setup.

What I meant by my previous post is:
I bought two of the 2s2p packs, which are each 7.4V 6400 mAh 20C (128A) packs. Wired in series, that's 14.8V 6400 mAh 20C (128A). Wired in parallel, that's 7.4V 12800 mAh 20C (256A). Of course, the amp numbers ignore the whole point of this post, which is the 60 amp limit described in the manual. At this point, we can say 60 amps in series, 120 amps in parallel, until I hear otherwise from FMA.
If I could keep ESC and motor temps safe for the entire 12800 mAh AND keep the vehicle (1/8th buggy or MGT) moving about 30 MPH for the entire 12800 mAh, I'd be one happy guy. So far my experience tells me that I won't find an ESC and motor combo that can run on 7.4V, push out that kind of speed, and stay in safe temp range for more than about 10-15 min. In a 1/10th touring car, buggy, or stadium truck - maybe - but in a MT or 1/8th buggy?
Please, someone prove me wrong :)

MetalMan · 06.30.2005, 03:44 PM

That wouldn't be a good idea either. That is, unless you feel like spending $4-500 on a controller! In a smaller 1/10, 7.4v can get you over 40mph easily, with a low amp draw (~50amps). The best thing to do will always be to put batteries in series since with higher voltage efficiency also rises. Efficiency factors into the motor output formula: amps x volts x efficiency = watts.

Back to the original subject. The wires are meant to handle 60amps continuously, but much more than that, and the resistance of the wire will cause undesirable voltage drop, and at too high of an amp, the resistance can cause the wire to heat up too much. But you definately be fine for bursts to 120amps, but not for extended periods.