Hello, I'm a student at Augusta Prep and our Electrathon car is not able to race for 1 hour straight. We've done 4 races and the longest we've ever gone is 42 minutes. Our car is roughly 250 lbs total, and we're using 4 12v 20 ah lead acid batteries, and a 3kw motor from Golden Motors. Our controller is the Kelly KLS7240D Sine Wave Brushless Motor Controller. I've linked the Cycle Analyst log from our most recent test run and linked the trip anaylzer below.
IMO your 3kW motor is a bit on the large size to be powered by 20AH batteries.
You probably already know but, I'm going to go into it anyways
20AH batteries are rated as such because they are designed to supply ~20A for an hour ...or (theoretically) ~40A for 1/2 hour ...or sometimes even ~60A for ~15min
A 3,000W motor ran at 48V, has a rated draw of ~62A (3,000W/48V=62.5A) ...so, you guys running for ~40 min is probably draining those little batteries down pretty low
(What is the cut-off voltage of your speed controller set at?)
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That is another issue. When Lead Acid batteries get "drained" below ~10.5V (so, ~42V for 4 in series) they start getting internally damaged. ...& the more often it happens the worse they get.
Here is a racer (Polaris) powered by (4) 12V SLA's & a 48V 1,000W MY-1020 brushed motor ...notice the power draw
-- Edited by Functional Artist on Saturday 9th of March 2024 05:07:05 AM
Peukert's law was proposed by German scientist Wilhelm Peukert in 1897. It states that the capacity of rechargeable lead-acid batteries changes at different discharge rates. As the discharge rate increases, the available capacity of the battery will decrease.
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Racing is real competition and everything else is just a game.
I am confused by the .txt file. It seems to say that it is a 48 volt pack that delivered 26 Amp Hours. If your batteries were four 12 volt 20 amp hour, you will have gotten more Amp Hours than the batteries are rated for. That is improbable.
To make 48 volts, the batteries will have to be in series. When batteries are in series, the Amp Hour rating will be 20 amp hours. If they wired in parallel, then it would be a 12 volt pack rated at 80 amp hours. Either way, it is a 960 Watt Hour rated pack at the 1/20C rate (20*48 or 40*12).
Lead Acid batteries are normally rated at the 1/20 C rate. This means that they are discharge at a constant current over 20 hours. So a 20 Amp hour battery will deliver 1 amp for 20 hours before it is fully discharged. If the pack is discharged at a higher rate, it will deliver less energy. For Electrathon, we try and average a full discharge in 1 hour. This is called the 1 C rate.
More information here: https://en.wikipedia.org/wiki/Peukert's_law
If your batteries deliver the full 20 amp hours, then regen would have to provide 6 amp hours. That is 30% extra capacity which is possible, but pretty high. I suspect your batteries are rated at the 1/20 C rate and should realistically deliver much less which puts the extra capacity above 30%. I suspect your meter might be optimistic.
The best way to check is to do a discharge test. Set up a load to discharge one of your batteries in an hour. I would suggest using a load that pulls around 17 amps at the start. I use a bunch of old extension cords but light bulbs can work too. Record the voltage and current every 5 minutes. When the battery hits your minimum voltage, stop the test and calculate the Watt Hour capacity of the pack. If the pack discharges in under an hour, you know you need to use a lower current. Then repeat the test with each battery to make sure you do not have a bad battery. They should be pretty close. Your pack capacity will be that of the weakest battery times 4 since the pack will die when the weakest battery is empty.
Hey Cliff, so you were right about how many amp hours. I double checked our capacity and we have 26 ah of capacity at 1/20 C rate. The manufacturer of the battery lists the 1C rate capacity to be 16 a/h, which seems to be where we started losing power. We plan on doing a battery test, and are currently thinking about purchasing 3 interstate DCM0035 12v 35ah batteries. Steve Archer uses this battery setup, and his car roughly weighs the same as ours. The manufacturer of the interstate batteries lists the 1C rate to be almost 19 ah, so we're hoping that the extra capacity will help us reach the 1 hour mark.
It looks like the Ah column starts at 6.7822, like maybe the cycleAnalyst (CA) had not been zero'd?
Excluding all the .05A readings, I get the net total energy used in the log as 19.3 Ah (798.7 Wh) (Effectively the same as just subtracting off 6.7822 from the Ah column)
That breaks down as: used= 20.98 Ah (888.85 Wh) regen = 1.67 ah (89.8 Wh)
I think around 10% regen sounds reasonable?
avg power consumption for each of the 5x sessions 1st = avg 381 W 2nd = avg 883 W 3rd = avg 1311 W 4th = avg 1318 W 5th = avg 438 W
So, regardless of the exact batteries, y'all aren't going to last a full hour driving like in the 3rd and 4th session...
Do you have plans to get the CA hooked up so it can log speed or motor RPM? How about GPS and some accelerometers too?
