Constant Voltage and Constant Current - How to. - DIY Electric Car Forums

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#1
05-11-2010, 06:40 AM
 illuminateddan Senior Member Join Date: Dec 2009 Location: Brisbane, Austrlai Posts: 118
Constant Voltage and Constant Current - How to.

Hi,

I know this is a really silly question but hey, I'm a silly question kinda guy and my electronics knowledge peeters out when it comes to battery charging....
So... If I wanted to charge a battery by constant voltage, I just set my psu to always supply that voltage, but if I wwanted to supply a constant current how would I do that.

If I charged a LifePo battery in constant current mode at say 10A, what would the voltage do? Would this need to be set at 4.2v or would it just slowly rise up to a point where I then switch to constant voltage of 4.2v?

Thanks
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#2
05-11-2010, 08:07 PM
 Roy Von Rogers Senior Member Join Date: Mar 2009 Location: Florida Panhandle Posts: 506
Re: Constant Voltage and Constant Current - How to.

Quote:
 Originally Posted by illuminateddan Hi, I know this is a really silly question but hey, I'm a silly question kinda guy and my electronics knowledge peeters out when it comes to battery charging.... So... If I wanted to charge a battery by constant voltage, I just set my psu to always supply that voltage, but if I wwanted to supply a constant current how would I do that. If I charged a LifePo battery in constant current mode at say 10A, what would the voltage do? Would this need to be set at 4.2v or would it just slowly rise up to a point where I then switch to constant voltage of 4.2v? Thanks

Let say you have a Lithium cell with a max charge voltage of 3.65v and the max amps is 30 amps on the cell.

You have a 30 amp power supply that can vary from 3 volts to 20 volts.

In constant voltage mode you would set the power supply to 3.65 volts, if the cell is near empty the power supply will charge at 30 amps and as the battery gets charged, the amps will be reduced as voltage goes up on the cell, til it gets to the max voltage you set for..3.65v. At that point the cell is full.

If you want to charge constant current, you then crank the power supply all the way up, to get max amperage, untill the cell hits 3.65v, and then you have to either stop, or reset your power supply to 3.65v, constant voltage, to finish the charging.

Battery chargers have certain curves programmed in to them, to do the CC/CV automaticly.

Roy
#3
05-11-2010, 08:12 PM
 illuminateddan Senior Member Join Date: Dec 2009 Location: Brisbane, Austrlai Posts: 118
Re: Constant Voltage and Constant Current - How to.

Thanks Roy,

So if I wanted to do a first charge (CC then CV) on some 90AH lifepo (TS) up to 4.2v at 0.1-0.3C (9-27A), is this easy to do on a standard lab psu with only voltage control or would I need to sit and watch it constantly altering the voltage to acheive a constant current?

Dan
__________________
One step at a time. Righty tighty, lefty loosey...

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#4
05-11-2010, 08:35 PM
 Roy Von Rogers Senior Member Join Date: Mar 2009 Location: Florida Panhandle Posts: 506
Re: Constant Voltage and Constant Current - How to.

Quote:
 Originally Posted by illuminateddan Thanks Roy, So if I wanted to do a first charge (CC then CV) on some 90AH lifepo (TS) up to 4.2v at 0.1-0.3C (9-27A), is this easy to do on a standard lab psu with only voltage control or would I need to sit and watch it constantly altering the voltage to acheive a constant current? Dan

Yes, you would have to watch the batteries to make sure you dont exeed the max voltage. The only other thing you could do is put a voltage alarm on it, to notify you when it reaches the voltage, but considering the price on those batteries, I would watch them..lol

Actually you could go past that voltage, but the problem is, you would not know when the overcharge occured.

Let me give you an example....if you had a 5 gallon bucket, and you wanted to fill it all the way up without care how fast, you would take another 5 gallon bucket, fill it up and dump it in to the other bucket, and you would know it will be full, no matter how fast you pour it in.

But in your cell there is no good indicator of how empty the cell is, unless you count amp houres in and out somehow. But if your battery was charged to full potential, and you counted the AH you took out, in theory you could then put in max amps, untill you reach the time limit of amps you took out, give and take some of the losses.

Roy
#5
05-11-2010, 09:14 PM
 TigerNut Senior Member Join Date: Dec 2009 Location: Calgary Posts: 277
Re: Constant Voltage and Constant Current - How to.

Quote:
 Originally Posted by illuminateddan Thanks Roy, So if I wanted to do a first charge (CC then CV) on some 90AH lifepo (TS) up to 4.2v at 0.1-0.3C (9-27A), is this easy to do on a standard lab psu with only voltage control or would I need to sit and watch it constantly altering the voltage to acheive a constant current? Dan
If your lab power supply can do constant current (i.e. current limiting) then you can do the following:
1. Set your open-circuit power supply to 4.2 volts. Measure that voltage with a meter you trust.
2. Set the current limit way down, then connect the cell to the power supply. The current-limit light on the PSU will come on, and then you adjust the current limit upwards to the charge range (9 to 27 A) you want. Again, double check your amps with known good meter, if your power supply readouts are at all uncertain.

The supply will rail at the current limit, with a slowly rising voltage until the terminal voltage reaches your OC setpoint at 4.2 volts, then the current will progressively drop off while the voltage holds at 4.2 V.

I wouldn't do this on the first cell without periodically checking up on it, both for the sake of your battery and the PSU.

Last edited by TigerNut; 05-11-2010 at 09:17 PM.
#6
05-11-2010, 10:14 PM
 illuminateddan Senior Member Join Date: Dec 2009 Location: Brisbane, Austrlai Posts: 118
Re: Constant Voltage and Constant Current - How to.

So.... if I had a bench PSU that had a controllable voltage but not a controllable current, but had a max current out I could set the supply to 4.2v and just let the supply crank out its max current until it hits its voltage limit?
i.e. 25A PSU whacks out 25A until cell V = 4.2v then the psu runs at constant voltage for a while.
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One step at a time. Righty tighty, lefty loosey...

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#7
05-12-2010, 01:52 AM
 MN Driver Senior Member Join Date: Sep 2009 Posts: 870
Re: Constant Voltage and Constant Current - How to.

Just be sure that it is a current limited supply. There are quite a few supplies that will blow a fuse, burn up, or just shut down as they try to provide the amperage needed to match the capacity but the amperage is more than it can take and it isn't limited. I have a set of Meanwell supplies that behave this way by shutting down when it is well beyond its rated capacity(I think 180% or something) and it comes back after a few seconds from the hiccup. Meanwell makes a current-limited constant voltage supply, I just don't know the model number or which category it is on in the site as I haven't looked into it enough to know. I'm charging NiMh at high voltage so I take three Meanwells in series and one constant current supply capable of a big voltage swing and use that. My strategy isn't appropriate for your application but just be sure that the power supply you are using won't pop when you connect it and power it up.
#8
05-12-2010, 04:02 AM
 illuminateddan Senior Member Join Date: Dec 2009 Location: Brisbane, Austrlai Posts: 118
Re: Constant Voltage and Constant Current - How to.

Hmm, I'm looking to buy a good PSU without blowing out my already blown out budget.
The TS manual specifies an initial charge at 0.1-0.3C to 4.2v. Would I be able to get away with an initial charge current of 0.0333C. i.e. considerably less than specified? I can see no reason this would cause issues other than taking more time. This means I can buy a cheaper bench PSU to do the charging.

Thoughts?
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One step at a time. Righty tighty, lefty loosey...

http://www.evbeetle.blogspot.com

#9
05-12-2010, 04:39 AM
 Amberwolf Senior Member Join Date: May 2009 Location: Phoenix, AZ Posts: 552
Re: Constant Voltage and Constant Current - How to.

Well, high-current lab PSUs tend to be expensive. I have a Sorensen DCS55-55 here on indefinite loan, and it'll do up to 55V at up to 55A. But it also costs like \$1000 *used* or over twice that new, most places I see them in a google search.

But sometimes, like me, you get lucky, and someone has one that they don't need right now that you can borrow, rent, or buy cheap.

Sometimes you find them in surplus sales or auctions really cheap.

Good lab supplies like Sorenson can also be paralleled for higher current, so you can buy a bunch of smaller much cheaper ones and set them up per the manual so that one of them controls the others, essentially.

I don't see any reason you can't charge them at a much lower rate than usual, since the initial charges are at such lower than normal-use rates already. But I'm not a TS expert (don't have mine yet) and you should wait till someone else chimes in.

Last edited by Amberwolf; 01-18-2013 at 07:42 PM.
#10
05-12-2010, 04:41 AM
 Amberwolf Senior Member Join Date: May 2009 Location: Phoenix, AZ Posts: 552
Re: Constant Voltage and Constant Current - How to.

Regarding the MeanWell supplies, there is a thread on ES specifically about how to mod certain ones to current-limit them. Some can't be done this way, and the model numbers to check for are in that thread.
http://www.endless-sphere.com/forums...hp?f=14&t=4125

Last edited by Amberwolf; 01-18-2013 at 07:42 PM.

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