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Originally Posted by 114ManualLabor
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...
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I'm imagining your device to be a chassis with two powered wheels, carrying a nine pound saw blade that spins at 2500 RPM.
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.