Making custom battery charge circuits for your electric vehicle - DIY Electric Car Forums

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#1
02-17-2017, 03:37 AM
 electric_driver Junior Member Join Date: Feb 2017 Posts: 23
Making custom battery charge circuits for your electric vehicle

The alternators of the hybrid drive train I'm making will be using a voltage and amperage that is not very common (48V @ 145-155 A). As a result, it is difficult to find a charger which I can use in this system (since most chargers are meant to be powered from the electricity grid (so 230V or 110 V). Also, most chargers do not allow the batteries to be charged at the highest C-rate they can sustain. To resolve this, I hereby put up a post to let us solve this (and hopefully generate a solution that will benefit, not just me, but anyoone building an electric vehicle that uses unconventional voltages/amperages or batteries).

An obvious solution is that we just describe the procedure on how to create (a very simple) electric charging circuit, from scratch.

First off, here are the details of my electric vehicle system, which can serve as an example:

* the batteries I use are 4 NiFePo4 batteries of 12V, with 2,5Ah which I wired in series to make one 48V battery. I use two of these intermittently, but that's not important here. The C-rate of the batteries is 4C

* the input power is (as allready mentioned above), 48V @ 145 to 155 A

We know that:

* They're NiFePo4 batteries. The nominal voltage of NiFePo4 cells is 3,2V. Since we know the nominal voltage, we also know how much cells there are in the 12V battery. This is 4, since 3,2 x 4 = 12,8 V. So, it's a "4S pack". We don't know how many P's it has but that's not important. We also know the maximum charge rate with this, since the maximum charge voltage at which to charge LiFePo4 batteries is 3,6V per cell; since we have 4 cells, the maximum charge voltage is 14,4V for these 12V batteries.

* We know the C-rate of the 12V batteries is 4C, and the capacity is 2,5 Ah. So 1C= 2,5 and we have not 1 but 4 C, so maximum discharge rate is 10A (2,5A x 4). Charge rate would be a little less, but near that amount.

The charge curves for NiFePo4 tell us that we best charge them:
- from 10 to 90% of State of Charge (SOC) at the full C-rate the batteries can handle (in our case 4C or hence 10A)
- from 90% to 100% of State of Charge (SOC), at 0,5C (or hence 1,25 A)

So ideally, in my example, I'd need to make a charging circuit that is able to charge 4 NiFePo4 batteries of 12,8V from a 145A @ 48V input stream. The rate at which to charge the batteries would be 10 A @ 14,4 V for 10-90% of SOC, and 1,25 A @ 14,4 V for 90%-100% of SOC

How would I make such a circuit in practice ?

Also, I'll only be using a small part of the power (more precisely, the surplus power will be: 145 A @ 48 V - 10 A @ 14,4 V = 6960 watt - 144 watt = 6816 watt ). So where does this power go in such a charge circuit ? Is it normally just grounded ?

The above calculations can obviously be done for any type of battery (LiPo, NiCd, ...) -regardless of their C-rate- and for any other input stream. We just need to specify now how one can make the charge circuit in practice.

#2
02-17-2017, 03:51 AM
 dcb Senior Member Join Date: Dec 2009 Posts: 1,930
Re: Making custom battery charge circuits for your electric vehicle

Quote:
 Originally Posted by electric_driver So where does this power go in such a charge circuit ? Is it normally just grounded ?
Lol .
#3
02-17-2017, 05:19 PM
 Sunking Senior Member Join Date: Aug 2009 Location: DFW Posts: 1,684
Re: Making custom battery charge circuits for your electric vehicle

I hope this is a thought only exercise as a learning experience because you have a lot to learn.

To start I do not know of any LFP battery that can handle a 4C charge current. Even if one existed you would not want to do that as there is just to many things that can go wrong and all end in fire.

Lastly what do you mean ground? Ground has nothing to do with anything or any function. If you are speaking of a Traction Battery Pack, there is no Ground as that would get you killed.

Charing LFP or really any lithium battery is the easiest battery in the world to charge. All it takes is a CC/CV power source that you can control the voltage to within 1% which is extremely easy to do. For LFP any Float Charger with an adjustable output can be used. Set the voltage anywhere from 13.6 to 14.4 volts and you have a charger fo rlithium batteries. They are drop in replacements for lead acid.

The only real trick is to decide if you want to fully charge them up to 14.4 volts which requires some sort of BMS to terminate the charge. Or use no BMS and just charge to 13.6 to 13.8 volts and let them Float.

What you are thinking is quite simply a bomb.
__________________
Dereck BC, PE MSEE

Last edited by Sunking; 02-17-2017 at 05:21 PM.

#4
02-18-2017, 01:43 AM
 electric_driver Junior Member Join Date: Feb 2017 Posts: 23
Re: Making custom battery charge circuits for your electric vehicle

Quote:
 Originally Posted by dcb Lol
Quote:
 Originally Posted by Sunking Lastly what do you mean ground? there is no Ground as that would get you killed. What you are thinking is quite simply a bomb.
Ok. I think you're having the wrong idea of my intentions here. My project intented to build a demonstrator to showcase my hybrid drivetrain (see my other posts). During construction however, it became clear the kart wasn't able to hold the extra weight of the alternators and the internal combustion engine I wanted to put on it. So, I changed my tactic to focus on building the generator (internal combustion engine + alternators) and the electric kart seperately. The generator will hence be a stationary charger. So it's placed on the ground and it would be able to ground excess power.

The fact that I seperated the generator and the kart is also the reason why I mention to possibly ground the excess power generated. In my initial design, I just intented to reroute the excess power to the electric motor, but now that both are seperated, that's not an option any more.

The current design (seperated generator and kart) will generate huge amount of power that is wasted, and so it's uneconomic and unecologic. That said, the generator too is just a device to help me demonstrate my drivetrain, so efficiency/ecology is of no importance here (it won't be the finished product anyway and it will hence never be replicated in exactly this way by others).

I guess that it might be best to nonetheless look for different batteries (especially since I didn't buy them yet), but I don't know of any brand/model that would be
* able to charge at a rate of 145 to 155 A or at a rate that is at least far greater than 10A
* are cheap
* are available off-the-shelf and easily obtainable

The batteries I inititially intented to acquire (which could indeed handle a 4C charge rate) were these: http://cs-batteries.de/LiFePo4-Motor...25Ah-150A-420g
I selected these mainly because of their other specifications:
* high discharge rate (140 A) so able to drive the 10 kW electric motor of the kart (note that although the charge/discharge rate is often about the same in batteries -both being specified by the C-rate -, the example of the cs-shop batteries show that this isn't always the case and the charge and discharge rate can vary considerably; here: 1400%
* low weight
* small size
* very small capacity
That said, since the kart will be seperated off (or even no longer done at all), these requirements all fall away, and only the charge rate and the cost are still important factors. Although some of these are of importance to my patent (of my patented drivetrain), the vehicle such a generator could be intented for can be of any size (even lightweight delivery vehicles -as UTV's, light commercial vehicles, ...-) so the whole can still be considered low capacity/small/lightweight.

If someone can tell me some suitable brands/models, that would be appreciated. I am mainly thinking of AGM batteries currently (since these seem to be used with starter motors too, so have a high output, which could mean that they can be charged quickly too (high C-rate) ?
The capacity of these AGM batteries are generally around 15 to 30 Ah I believe, but as said, that's not a problem any more, and actually, the higher the capacity the better (for the C-rate at least). A 30Ah battery has, even with a 1C rate, then still 30A. I would preferably need to get to 145-155 A, so AGM batteries would then still need to allow for a C-rate of 5. I'm not sure that's possible; if not perhaps someone of you can tell me what battery type does allow such high C-rates, or I can just increase the capacity even more (using 2 batteries in parallel, which yields 60 Ah). One problem that does occur with very high capacity numbers is that it would then no longer comply with the "small capacity" claim of my patent, so I rather not make it too high neither. I mentioned that I consider 1 km of range (which is about 1 minute of driving time I guess) to be a "low capacity" for a vehicle, but a bit more (up to say 10 km or 10 minutes of driving time) can still also be considered low capacity. A 30 Ah battery still falls under this reasoning then and can be used without problem (1 minute of driving at say 127 to 145 Amps per hour makes 2,11 Ah to 2,41 Ah; 10 minutes makes 21,1 Ah to 24,1 Ah). When standardly, the motor is spun at double that amperage (254 to 290 Amps), even 42,2 Ah to 48,2 Ah is needed, so the battery can even be have a capacity of up to 48 Ah, and still fall within the definition. Also, if the torque is higher, it could be of great benefit to the light commercial vehicles/UTV's I'm aiming my system to be used in.

Aside from the batteries -which is uniquely my problem-, the question still holds I think: how is excess power dissipated in a regular charge circuit ? I have an enormous amount of power I would dissipate, but even in regular chargers (say a 5A, 6V charger working off the mains electricity grid -which still means an input of 3680 watt- (230V @ 16 Amps)), there is also surplus energy that is dissipated (in this case 3650 watt).

Last edited by electric_driver; 02-19-2017 at 04:01 AM.
#5
02-18-2017, 02:14 AM
 Duncan Administrator Join Date: Dec 2008 Location: Southland New Zealand Posts: 4,512
Re: Making custom battery charge circuits for your electric vehicle

Hi Electric - driver
You need to sit down with some physics textbooks as you really don't seem to understand the real world

I suggest you have a look at this Thread which is where we put the threads that don't seem to understand the real world

http://www.diyelectriccar.com/forums...ver-13449.html

As far as the "excess power" in a normal battery charger the mains supply is not 230v and 16 amps - it is 230v and - UP TO 16 amps - if the charger only needs 0.13 amps to feed the battery (as in your 5A -6v example) then that is all it takes - there are losses to heat in all energy transformation but in a well designed charger these should be about 5%

Last edited by Duncan; 02-18-2017 at 02:16 AM. Reason: clarification
#6
02-18-2017, 02:34 AM
 electric_driver Junior Member Join Date: Feb 2017 Posts: 23
Re: Making custom battery charge circuits for your electric vehicle

Quote:
 Originally Posted by Duncan As far as the "excess power" in a normal battery charger the mains supply is not 230v and 16 amps - it is 230v and - UP TO 16 amps - if the charger only needs 0.13 amps to feed the battery (as in your 5A -6v example) then that is all it takes - there are losses to heat in all energy transformation but in a well designed charger these should be about 5%
Yes, I thought this would be the case (that it just doesn't draw that much power at all). But then, the question arised to me that if this isn't used, the "excess power" still stays on the grid, and supply and demand of the power can never be equal I assume (ie the power is produced in a power plant, but I doubt the power plant owners calculate out the power that will be drawn by the users). So again, there is surplus power on the grid somewhere that isn't being used, so how is that dissipated ?
#7
02-18-2017, 02:49 AM
 Duncan Administrator Join Date: Dec 2008 Location: Southland New Zealand Posts: 4,512
Re: Making custom battery charge circuits for your electric vehicle

When you add lots of small variable uses together you get a smoother function
The water in you glass is lots of individual molecules but together they act as a liquid

The tens of thousands of consumers acting individually act as a much less variable whole and the power companies can cope with that - famously in the UK when popular programs were on the power companies had to have extra capacity for the advert breaks when half of the country would rush to the kitchen and put the kettle on
#8
02-18-2017, 06:20 AM
 piotrsko Senior Member Join Date: Dec 2007 Location: RENO eNVy Posts: 1,348
Re: Making custom battery charge circuits for your electric vehicle

It is not "excess" power. It is excess potential. It does not function until it is used. Like a battery: no current flow from chemical change occurs until a load circuit is connected.
#9
02-18-2017, 06:29 AM
 ken wont Member Join Date: Jul 2016 Location: New England Posts: 42
Re: Making custom battery charge circuits for your electric vehicle

Quote:
 Originally Posted by electric_driver Yes, I thought this would be the case (that it just doesn't draw that much power at all). But then, the question arised to me that if this isn't used, the "excess power" still stays on the grid, and supply and demand of the power can never be equal I assume (ie the power is produced in a power plant, but I doubt the power plant owners calculate out the power that will be drawn by the users). So again, there is surplus power on the grid somewhere that isn't being used, so how is that dissipated ?
First, I believe you are Hyper-Focusing on a detail that you don't need to understand or worry about. Take a deep breath, relax and think instead about what you really want your project to accomplish.

Is the Kart for short range high power, short range low power, long range low power, long range high power? How quickly do you actually need to charge the batteries? .... etc. Get a good idea of the big picture, then, think about each stage to accomplish that.

Right now you are very enthusiastic,(which we admire), but in this state it is very hard not to get caught in the swirl of Hyper-Focusing! When you find yourself getting frustrated and spending too much time on one detail, do something to distract yourself and look at your goal from a different direction.
#10
02-18-2017, 07:17 AM
 electric_driver Junior Member Join Date: Feb 2017 Posts: 23
Re: Making custom battery charge circuits for your electric vehicle

For the AGM batteries; here's a setup that would hence work (I think, not sure on the C-rates yet since I can't find any data on that):

I can wire 4 AGM batteries of 12V, 30Ah (or 40Ah or 50Ah) in series, yielding a 48V, 30-50Ah battery. I can drop the second battery pack alltogether so I only need 4 batteries, not 12 (else, it would become very heavy/big indeed). An added advantage will be that the electronics system will be much simplified this way. Downside is that charging and discharging will occur from the same 48V battery, so C-rate may be negatively affected a bit, but perhaps it's still sufficient. I'll also be able to run the electric pump for my watercooling on one of the AGM batteries, and be able to run the starter motor from yet another one of the AGM batteries (so I don't need to have a seperate battery just for that, and all batteries will be constantly recharged, so they will never run out of power neither).

One more thought I'm having is whether or not I really need a motor controller ? For my motor, the Sine-wave VEC500 would be needed as the motor controller. The price of that is almost as high as my electric motor (825€) and I rather avoid buying it. Can't I just run the motor by connecting the motor directly to the battery somehow ? I could do without engine speeds (so just use a single speed) and just propel the vehicle by activating and deactivating the motor as needed.

Oh right, and for the problem that the kart and the generator will be seperate (meaning they will need to be connected to a wire and the generator might not be able to move), I have one more idea:
can't I just put wheels under the generator frame and use the kart to tow it ? I guess it will look silly, but the whole is just for demonstrations anyway, so it would be workable, and it will be safer, since if the batteries catch fire, they will be in the towed section, which is further away from the driver.
The alternative is that I keep both stationary and just spin the kart's wheels (off the ground) with it, but that doesn't make for a good impression I think.

Last edited by electric_driver; 02-19-2017 at 05:31 AM.

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