(Series 5 - 2001)
DO NOT BUY BATTERIES FOR ROBOT WARS UNTIL YOU HAVE READ THIS!
Team Tornado Battery Test 2001: Method
Terminal voltage was measured across the battery terminals, while a current sensor fitted to one of the battery leads gave a calibrated measure of battery current. These two values were measured 5 times a second with a Picolog A-D datalogging device connected to a PC.
A single 12V battery was tested at once. A large (ex-UPS) inductor placed in a bucket of water provided a convenient 0.13ohm load (100 amps at 13V), while the battery current was switched on and off (on a 50% duty cycle at 0.33Hz) with a well heatsinked 180 amp FET.
Using these two variables, and having access to the on load and off load voltages, enabled calculation of internal resistance, capacity and energy delivered against time. A battery was considered dead when its off load voltage had dropped to below 10V, i.e. one cell was completely dead. Good batteries lasted over 10 minutes with this method, thus when considering the 50% duty cycle of the load, the test is fairly representative of a continuous 80-100 amp load for the duration of a 5 minute battle.
We were able to test three different types of batteries - 7 surplus Hawker SBS15's as bought from Display Electronics, 12 Kiel 18AH as bought from Bull Electrical, and 2 Hawker Odyssey batteries belonging to the Storm team. The test showed that several of the Kiels we used at Extreme had greatly reduced capacity, and also one of the Hawker SBS15's failed completely during the test.
Team Tornado Battery Test 2001: Results
To save confusion and data overload, a typical but good example of each battery type has been shown on the graphs below. Those batteries that we consider damaged or worn out were ignored.
Date of manufacture
Team Tornado Battery Test: Conclusions
It can be seen that in terms of capacity and energy delivered to the load, there is little to choose between the Kiels and the surplus SBS15s. However the Kiels are much heavier, and have a higher internal resistance, so they are a less efficient battery both in terms of waste heat produced and weight useage. The higher internal resistance also means that the Kiels will not deliver as high a maximum output current as the SBS15s, and this is probably what caused the difference in performance we observed at Extreme 1.
In all respects the Odyssey PC680's come out on top. They maintain their off load voltage for longer, their on load voltage doesn't dip as far as the others, they have a lower internal resistance, and their capacity and energy delivered is significantly better. (Update 10/12/01 - Note that the Odyssey's tested above were rather old and abused. More recent tests of our new Odyssey batteries show them to be even better than we thought - see our 2002 Recommendations.)
Note that the capacities we have measured are way below the rated capacity of the batteries. This is because as you increase the load on a lead-acid battery, its efficiency and thus its capacity goes down. It's interesting to note that, very approximately, we have recorded about 50% rated capacity for each battery in this test. It's also worth noting that the results should be scalable for different size batteries of the same ranges tested.
Team Tornado Battery Test: Recommendations
If cost is important, and you're not likely to draw a lot of current (e.g. you have a 2WD robot with a non-electric weapon), then the Kiels (or Steatites) should do the job. You won't have quite the best performance, but you'll save a lot of money.
If weight is important, the original recommendation was to get hold of the below capacity Hawker SBS15's being offered by Knight Batteries. These are unfortunately no longer available! A good substitute would be the 13Ah Odyssey.
If you want the ultimate, then go for the Odyssey batteries from Hawker. You might consider the 13Ah instead of the 16Ah, both perform very well and are available surprisingly cheaply from Knight Batteries. We found Knight Batteries gave us by far the best quote for the batteries we bought.
See our Suggested Suppliers list for details of where you can buy the Hawker, Kiel and Steatite batteries.
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