Archive of the technobots.co.uk support forum
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How long will a 17Ah battery last in a heavyweight robot?
Post Number: 12
This naturally depends on how quickly it is discharged but is also a bit more complicated than that. Firstly it depends on the robot, its motors, the amount of torque needed to drive it and how you drive it. A common set up in heavyweights is to use a pair of Bosch 750W motors at 24V. These motors have a current consumption of 40A at their nominal 750W output power. Let's assume an average consumption of 30A per motor for a 5 minute fight which adds up to 60A for 5 minutes or 60/12 = 5Ah.
You would naturally assume that a pair of 17Ah batteries would be far too big but no. The 17Ah is based on a 20hr discharge, as you discharge the battery more quickly, the effective Ah capacity falls. Our tests on Yuasa batteries give an effective capacity of 33% of their 20hr rating on a 5 minute discharge. Europa give 38% and Hawker Odyssey 43%. Thus, for the worse case Yuasa a 17Ah battery would only give 5.6Ah which happens to be enough for our example above.
In practice, most teams do use 17Ah packs although some do use smaller but largely depends on gearbox ratio, 2 or 4 wheel drive etc. It is worth mentioning that the NiCD Bot-Packs have an effective capacity of 82% on a 5 minute discharge which is why 3 x 3Ah packs in parallel (9Ah) replaces 17Ah of SLA. In fact many users report a longer running time than with SLA's.