Itās 2s2p 25ohm 100w resistors. Based on the calculations total drain should be 1.8a, and each resistor should be burning off a little under 20w or so; well under the 100w they are rated for. Interestingly at this low wattage they are still too hot to touch.
Running them in a 1s4p configuration would result in about an 8a current, and each resistor would be pushed to 80w. Given how hot they are now Iām not too keen on testing this. Might look around to see what else offers a suitable resistance for higher currents (and is able to sustain them) without costing too much.
I use the watt meter for charging, but itās handy here. Looking to see if the capacity just before it (hopefully) gets shut down by the BMS matches what it should be. Speaking of which, the DieBieMS spits out everything the watt meter does but also includes individual cell voltages so itās kind of a little redundant in this case.
Those high power resistors are meant to be mounted to a larger metal heatsink for cooling. Your carbon cellulose slab is not an ideal heatsinking material (You probably just wanted to attach them to something, fair enough)
But for reference, something like this. Might not even need anything huge size wise, if you add some fans for forced convection cooling.
Thatās basically a HiPOT, which is something i need. Theyāre used for load testing and discharging. Most people use lightbulbs now because theyāre cheaper than these huge resistors that also require some kind of framing and have to be kept away from everything.
maybe thereās a fancy way to make a hipot out of lightbulbs. Maybe some kind of hilarious sign full of bulbs.
Just hang a bunch of em up all around your work shop. When it goes dark plug em into your skateboard and poof your shed is lit. Oh and make sure to get multi coloured ones
LHB -Best bet is to get an 8 ohm 100W wirewound resistor (Got mine from Aliexpress) and do a full discharge test from 42V to 30V (for 10S) with regular voltage readings of each cell pack. This will show up any dud cells.See my blog at https://evolveforums.com/threads/bamboo-gt-at-50km-range-project.1169/page-7
for how I did it .Check voltage and AH against cell specs -use exel for graphing and ah calculation -I can send file to you if you want.
Note fan for cooling of resistor . 8 ohm at 40V will give 5 amp and 200W -but with a fan 100W resistor will be fine
Liked your idea so much @SkaterBoy58 I, errrr, stole it
Wrote some python code to periodically poll the DieBieMS and write the data out to a CSV file I could create a chart from fairly easy. Iāll give a few more details on this over in the other thread.
a123 LiFePO3 based pack, so if the voltages seem a little oddā¦
Discharge was no more than 1.6a, so I really wasnāt expecting to see much drift across the cells. Having a less well balanced pack probably would have made for a more interesting chart though.
One of the nice things about the DieBieMS is how it tracks state of charge, and doesnāt just base percentage remaining off the current pack voltage. Itād be tough to get a reliable charge % based on the voltage change shown above. Red line seems right as the pack was drained using a constant resistance.
Will be interesting to run the same tests after the pack has been used for a while.
Actually it is the typical consumption of power from your battery pack. That can vary greatly depending on hills, rider weight and quality of the cells
Other people have come up with results showing that around 4000 watts is required to reach 40mph (64km/h). According to what you have said it would only take about 650 watts.
Load test setup v2. Added some bullet connectors so I can switch between 2s2p and 1s4p. 4p drops the resistance and ups the amps to about 6, power works out to be approx 250w.
It was fairly obvious something was going to catch fire if I didnāt change anything, so hereās the new setup. As suggested I dug the fan out, but also g-clamped a meter long strip of aluminium I had lying around across the top of the resistors.
Kept an eye on temps throughout the test, and it was cooler than when I was testing at 1.6a. Nothing too surprising about the discharge curves, which is a good thing. Kept recording data after the BMS shut down discharge to see how the cell voltage bounced back.
And hereās the charge cycle after this test. The DieBieMS cell mismatch is set to 0.01v, which makes sense when you look at the chart.
Pat
Think you getting mixed up with instantaneous power required for a particular speed and friction overcomming requirement with energy (with a time element) over a range of riding conditions