Battery question
 

hey all-
I'm looking into getting a battery for an idea of mine, only problem is I don't know a thing about them. Does anyone know what would be a good one to use for this application?
-I'm running 2 globes, and a cim constantly, under load, for 2 minutes
-and a vex electronics system

I've already considered the FIRST battery, but it's much too large for the application.

What kind of battery might be best for this use?

Re: Battery question
 

They make smaller version 12v batteries...we had one last year (came out of softball thrower)

I will see if I can find it tomorow and get some more information

Re: Battery question
 

If you are looking for small, lightweight, batteries I would suggest Lithium-Ion Batteries. The only downfall is they require special chargers. Here is a link:
http://www.all-battery.com/index.asp...S&Category=240

Re: Battery question
 

As you discharge the battery at higher rates compare to its 1C, discharged in one hour, rate, the internal resistance of the battery becomes key. The commonly available technology with the lowest internal resistance is NiCd. Your best bet is to measure your current load and plan for a 10 cell NiCd stack that will keep it running for the desired amount of time.

Re: Battery question
 

Check out http://www.bb-battery.com/

Re: Battery question
 

Craig,
This is one of those 'not enough info' questions. The amount of load on the motors is important in determining the battery requirements. The Globe motors will draw very little current compared to the CIM under load. Since all motors will be running full out for two minutes, and under load, it is likely a gell cell in the 7 to 10 amp/hour variety should work. Why not build the device using the robot battery and see what your results are under test. You may find out that the demand is such that a higher capacity battery is just what you need in terms of recharge and length of usage. A lower capacity battery might work but will reguire a recharge after only two or three, two minute runs. The robot battery might give you more runs and a higher reserve in case of random high loads.

Re: Battery question
 

And price...and very sensitive to over-current situations.

Unless you need the greater energy density of a Li-Ion - this is an application for a smaller SLA or NiCad, maybe NiMH.

Re: Battery question
 

Good call. When i first read the post i thought this was for a vex sized project, that was why i recommended the Li-Ion, otherwise SLA type batteries have always done me justice in my applictions :D

Re: Battery question
 

I'd like to use the batteries we use in FRC, as it could run the project easily, however size doesn't allow.

As far as more info, the globes are direct linked to 3 inch wheels, which will be driving around about 50 pounds of weight. The CIM is chain linked on a 2:1 ratio to a 9 pound steel blade. Is this a little more clear, or am I still confusing something? I really have no idea what to go with...

Re: Battery question
 

I'm imagining your device to be a chassis with two powered wheels, carrying a nine pound saw blade that spins at 2500 RPM. :eek: I don't want to be in it's way.

Assuming I've understood you correctly, let's see if we can predict your current draw from the information you provided.

Start with the Globes that drive the wheels. Each of the 3" diameter wheels supports 25 lb. Let's assume they are really good grippy wheels (e.g. rough top tread) with coefficient of friction = 1.3, so each wheel can develop 1.3*25 = 32.5 lb of traction force before it starts to slip. This force will require 32.5 lb*3"/2 = 48.8 in-lb torque from the Globe, which is about 25% of its stall torque, so each Globe will draw about 25% of its 22 Ampere stall current, or about 5.5 Amperes. So together the Globes can never draw more than 11 Amperes, because any more than that will cause the wheels to slip. [BTW, at this wheel size the top speed will be about one foot per second; kinda slow compared with an FRC robot. This is good; it will be easier for me to dodge if it comes toward me.]

Now the CIM that drives the blade. If the chain is well aligned and nothing rubs, the current will not be much more than the CIM's free current; call that 3 Amperes. Bump that up to 4 Amperes to account for imperfect chain alignment.

So when your machine is accelerating as hard as it can with its blade spinning freely, the current draw will be 15 Amperes. Standing still with your blade still spinning, the draw is about 4 Amperes. When the blade hits something, the current will surge to whatever peak value your battery can support, maybe up to half the CIM's stall current; call it 65 Amperes or so, for the split second that the blade is actually loaded by whatever it hits.

Now divide the 2 minute exercise (match?) into some number of seconds for idling at 4 Amperes, some number for charging toward an objective at 15 Amperes, and some number of seconds for striking obstacles with the blade at 65 Amperes. You'll have to decide how much time to allocate to each current level. For example, it your machine spends 60 seconds idling at 4 Amperes, 50 seconds charging at 15 Amperes, and hits twenty obstacles at 0.5 seconds each, then the average current draw is (60*4 + 50*15 + 20*0.5*65) / 120 = (240 + 75 + 650) / 120 = 1065 / 120 = 8.9 Amperes.

Looking at the Exide 18-12 datasheet, we find that the 2 minute capacity of that battery is about 60 amperes, so it is overrated for the requirement calculated above by a factor of about seven, if you want to recharge the battery after every 2 minute exercise. Let's say you want the battery to last three rounds instead. Now you are looking for a sealed lead-acid battery rated for about eight Ampere-hours. That shouldn't be to hard to find, and it should be a little less than half the size of the FRC battery.

So I'd guess the 7 to 10 Ah gel cell that Al recommended is probably about right.