On my car we run a 36v system, using 3 batteries each 12v and weighing 23 pounds. Doing that math we run a little over 69lbs of batteries. So that means we have about 3 lbs of battery weight we can still use. But we can't add just a small battery in series or the pack with die quickly. So I was thinking having a smaller 12v battery hooked up so I can turn a switch and it will connect the battery so the motor will have 48v, then after 2-3 minutes when the battery is dead switch the switch and go back to the normal 36v and the dead small 12v battery is out of the system and not being charged/discharged. Attached is an image of my plan right now. Would this be possible so the battery will boost the voltage but when not in use not be in the circuit? Would this even be worth my time? Has anyone tried this before?
Just keep the weight savings. Even a little weight shaved off may save you a few inches for every mile you travel. With less weight you can accelerate (At the same rate.) with less power usage and you can go further with less rolling resistance. Of course keeping your design as light weight as possible comes with plenty of challenge!
There's a few things I'd suggest on this;
- You'll reduce the recharge life of your batteries considerably by doing this.
- 3lbs is a very minimal weight; less than 5% of your energy storage capability, so as long as you've got a good battery density of energy storage vs. weight, you're already on a winner with your current packs.
- If you were to do something to increase overall capacity, you'd need to use the same battery type to comply with the rules, so you'll have to find a sub-3lb battery of the same current draw and voltage.
- If you had a bigger mass to make up, it be worth setting up a secondary system to boost the capacity, but would need a second entire battery circuit of the same voltage & current draw, or a flip-switch to change to the other system.
(Don't forget that you need to keep under the 1kWh limit also...)
I know that in the NW there have been a couple of teams that have used large capacitors for this "boost". They have to start with the capacitor drained but can charge it from the battery pack. They then kick it in on a switch to boost power much like an AC motor on a grinder of lathe that the capacitor kicks in to help it get up to speed. I think in Washington State University but am not sure. They seemed to do OK with it but their cars were not very good so whatever help this gave could not balance out the lack of efficiency of the total car. They said the numbers looked good though.
(Don't forget that you need to keep under the 1kWh limit also...)
What 1kwh limit?
Whoops, sorry about that; I thought it was in all classes but it's only Advanced Battery class. Rule 28.2, but if you're running SLAs, cram in as much power as you can as long as you're under 73lb.
Regardless, 3lb from the maximum weight is a really good result. Better to spend time finding the best charging methods to add energy to the pack in my opinion.
-- Edited by Brendan_Smith on Tuesday 15th of March 2016 05:01:11 AM
I know Cloud would have a "helper battery" hooked up when they were running the Optima batteries to boost the voltage a little bit. How they wired it up, I haven't a clue. But it seemed to work for them since one of the two batteries would usually have a lower voltage compared to the other one.
Ideally, you want Optimas that have the exact same capacity. That way you could use their full capacity. It is hard to find an exact matched pair.
So, if you have one battery with 400 Watt hour capacity and another with 350 watt hours, your race will end after you use 700 Watt hours, 350 from each battery. The weaker battery will be dead and start acting as a resistor, so you can not use the remaing 50 Watt hours in the stronger battery.
But if you add in a 50 Watt hour helper battery to the weaker battery, the car gets to use that 50 Watt hours and the 50 Watt hours trapped in the stronger battery, giving you 800 Watt hours.
I do not know how to make this work best but I would guess wire the 12 volt helper battery in parallel with the weaker battery with a switch that you throw on when the weaker battery gets low.
I hadn't originally planned to wire them like that, I was thinking to add a small booster but that wouldn't help with that idea of having 1 slightly inferior battery. I really like the idea of just making that inferior battery better.
So if I understand your idea I have 3 batteries, lets say 2 have a 300 watt hour capacity, and my 3rd has a 275 watt hour capacity. So when near the end of the race when the two better batteries are low, and the 3rd not as good battery is nearly dead turn I on the back-up battery. Then that battery would have power drawn out of it increasing the capacity of the low battery. This I think would require some testing, mostly on the track with full hour practices. But what if I just had had the low battery and the backup little battery always in parallel, would that just kill the whole battery system?
I also have never done battery testing, I would like to know how. All my team does is check the voltage of the batteries after the race. How would I go through testing their capacity? I believe one team, the Electrocutioners use amp hours. Would one be easier to test watt hour/amp hours?
There area couple of ways to test your batteries out.
Option 1. Drive the car for an hour to get a rough estimate for how much battery power your pack can do. You'll be using up tires this way is the only draw back.
Option 2. Have a dummy load hooked up to the car instead of the motor. This could be a very large nichrome resistor (think heating elements for commercial use). I know Kirk Swaney in Albany, Oregon has a similar set up but would have it wired into his car then set up a different throttle to control it. You'd need a fan to blow over the resistor since it would get pretty hot for an hour of testing.
In terms of how to measure what your batteries are doing are two ways of going about it. The cheaper option would be to have a separate voltage meter and a clamp on amp meter to figure out what the batteries are pulling every minute or two minutes depending on how accurate you want the data to be.
Or if you have a in car power meter (say like a Cycle Analyst for example) it will tell you what you're pulling too.
I know Cloud uses amp hours, personally I use watt hours. Once you've done a test or two to check numbers against one another to see how consistent you are, take that number and either divide by four or six. If dividing by four, this will tell you how much power you should be using for every fifteen minutes of a race. If dividing by six, this will tell you how much power you should use for every ten minutes of a race.
Wiring the booster battery in parellel with the weaker battery from the start might work. They might share the load proportionally. Might be worth testing.
It is possible to build a nice battery capacity tester with an Arduino and about 10 dollars of parts but that is probably an off season project. Here is our quick and reasonably repeatable method of battery capacity testing.
Please keep in mind that there is the danger of shocks, fire and general mayhem. Be careful. Think through what you are doing and do not count on my explanation. Work with adult supervision and walk through with them, what you are trying to do.
You will need pen and paper. A watch. A multimeter. A clip on amp meter, 3 or 4 outdoor extension cords of various length. A short piece of wire 12 Guage or thicker. A set of Jumper cables.
The clip amp meter is not critical but makes life easier and is a good tool to have. You can track current using a shunt and a voltmeter. A shunt is just a conductor with a known resistance, so using the multimeter to measure the resistance of the Jumper cable could give you your shunt.
First step is to record which battery you are testing, how it was charged, how long it has been since it was charged, battery temperature, resting voltage.
Take one of your extension cords. Make sure it is not plugged in to anything! Take the short piece of wire and short the outlet end by sticking an end of the wire in each hole. You now have one wire twice the length of your extension cord. Plug it into another extension cord for an even longer wire.
The next step is to check the resistance of the double extension cord. Put your multimeter to read ohms and hold the leads together. It should be around 1 Ohm. Now connect to the two plug prongs and read the Ohms. Let's say it shows 3 ohms. Subtract the resistance of the multimeter leads and you have the resistance of the extension cord. 3 -1 = 2 Ohms.
Guessing that your battery might have a capacity of around 240 Watt hours at the one hour rate, we calculate that the battery should supply 20 amps for one hour. (240 Watt hours ÷ 12 volts = 20 amps for one hour ).
Ohm's law says current = voltage ÷ resistance. So for a 20 amp current from a 12 volt battery, we need a resistance of 1.67 Ohms. (20 = 12x so x = 1.67).
Connect the Jumper cables to the prongs of the extension cord. Make sure that the Jumper cable clips do not touch each other where they attach to the extension cord prongs. It is easiest if you have an old bad extension cord that you can cut up. Plug the extension cord into it and cut and strip and seperate the two wires and clip the Jumper cable to each wire.
Now check the resistance at the unconnected Jumper cable clips. Add or subtract extension cords until your Ohms reading is reasonable close to your target.
Make sure your extension cords are spread out and not looped over themselves, preferable on a non conductive fireproof floor (outside on a driveway is good). The wire will get warm and could melt.
Connect one clip of the Jumper cable to the positive terminal. Set your multimeter to read voltage and connect it to the positive and negative of the battery. Put your clamp on amp meter around a single wire anywhere on the Jumper cable set up.
Start the watch and connect the other clip of the Jumper cable to the negative terminal. There will be a spark. The voltage and current change quickly, so I usually wait for 15 seconds before taking the first reading. Read voltage and current at the same time. Write them down. Then every 5 minutes, read voltage and current and write them down.
Do not worry if current is a little higher than your target at the start. It will go lower as the voltage drops. Do not worry about getting it exact.
When battery voltage approaches your cut off voltage, be ready. Record time and current just before you disconnect one of the Jumper clips.
Do not forget to remove the shorting wire from the extension cord when you done with it!
The math goes like this:
Amps * volts = Watts the battery is delivering at that instant.
(Watts at the start of the first five minutes + Watts at the end of the first five minutes ) ÷ 2 = average Watts during those five minutes.
Five minutes is 5/60 of an hour so Average Watts ÷ (5 ÷ 60 ) = Watt hours discharged during that 5 minutes.
Add all the time periods together to get Watt hours capacity when discharged at how ever long it took total.
Some useful tests that you can do:
How much does capacity increase between a battery that was charge 24 hours ago and one that was just discharged and recharged?
What charge method works best?
How does capacity change between a one hour discharge and a faster discharge?
-- Edited by ProEV on Monday 21st of March 2016 06:43:53 PM
Just hook up your electrical system to a bunch or light bulbs in parallel to set the amperage to your normal load. Drain the battery at your normal motor voltage and simply graph the voltages every 5 minutes, adjusting the motor voltage as you go. You will get a battery curve from this. Drain all your batteries and match up the two batteries, or however many you use, that have the closest curves to find the best match between them. Also do not use amp hours. As a battery drains its voltage goes down obviously, so at "half capacity" or about 11.5 volts, you don't actually have half charge left. You need to up the amperage throughout the race to keep the power output high. Also controllers get more efficient the more they are on the high side during the PWM. So, as the battery gets lower and the amperage goes up, the controller will go from ~90% efficient to ~98%. Amp hours in the battery is a set number and is not proportional to the amount of power left in the batteries.
Also controllers get more efficient the more they are on the high side during the PWM. So, as the battery gets lower and the amperage goes up, the controller will go from ~90% efficient to ~98%
This is an interesting point. On the other hand, as you increase the current, Puekert''s law says the usable capacity of the battery will go down. I guess which effect is more important will depend on what controller and what battery one uses.
Does this imply that the controller should be sized to provide a maximum continuous power just above the 1C rate of the battery (so that the controller is always "on the high side during the PWM") for maximum efficiency? As opposed to running a more powerful controller for that extra oomph when needed.
A different controller will only make a difference in peak efficiency. With pwm, any time the transistors switch state, they use up energy. This implies that it will be more efficient the less the transistor switch, which is true. Thus, it is better to have a motor voltage as close as you can to your battery voltage level. However, when the battery starts out at ~24 volts under load (or whatever car voltage you are running) a high motor voltage will simply use too much energy to last the entire race, so you keep it low and constant. In the beginning of the race, a high voltage and low amps will produce the same amount of power as low voltage and high amperage. Keep the motor voltage low and as the race goes on, the car will get faster and more efficient, since the battery voltage approaches the motor voltage until the controller is wide open. At that point a nos switch might be worth it to avoid the voltage drop from the controller by bypassing it entirely.
-- Edited by Lutzedarknut on Thursday 31st of March 2016 02:28:32 AM
