# Constant Voltage and Constant Current - How to.



## illuminateddan (Dec 19, 2009)

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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## Roy Von Rogers (Mar 21, 2009)

illuminateddan said:


> 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.
> ...


 
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


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## illuminateddan (Dec 19, 2009)

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


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## Roy Von Rogers (Mar 21, 2009)

illuminateddan said:


> 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


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## TigerNut (Dec 18, 2009)

illuminateddan said:


> 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.


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## illuminateddan (Dec 19, 2009)

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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## MN Driver (Sep 29, 2009)

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.


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## illuminateddan (Dec 19, 2009)

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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## Amberwolf (May 29, 2009)

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.


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## Amberwolf (May 29, 2009)

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/viewtopic.php?f=14&t=4125


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## illuminateddan (Dec 19, 2009)

Great, thanks Amberwolf. I may have to chat up some electrical engineers.....
My local electronics store does a constant current one that can do 2x3A so 6A in parrel with current limiting so I may just bite the bullet and get that once someone else chimes in with how TS cells like their first charge!

Anyone?


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## MN Driver (Sep 29, 2009)

Amberwolf, thanks for the link, I had lost it, I was looking for that thread again.


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## TigerNut (Dec 18, 2009)

Amberwolf said:


> 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.
> 
> ...


I've bought used Hewlett-Packard supplies on eBay, in the 100 to 400 watt range. The older ones really give up nothing to the new models in terms of the quality of their output or regulation, you just don't get the digital display or computer control interface, and they are available in various output power ratings and with multiple independent outputs. Shipping can cost $$$ because they're heavy, though.


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## Amberwolf (May 29, 2009)

TigerNut said:


> I've bought used Hewlett-Packard supplies on eBay, in the 100 to 400 watt range. The older ones really give up nothing to the new models in terms of the quality of their output or regulation, you just don't get the digital display or computer control interface, and they are available in various output power ratings and with multiple independent outputs. Shipping can cost $$$ because they're heavy, though.


Yeah; I have several older small Sorensons with analog panel gauges, from the mid-late 70s, acquired from a friend who moved and couldn't keep everything. They all have something wrong with them, from being dead to having wildly inaccurate gauges, but the ones that do work are nice, as long as I don't have to move them anywhere. 

Been looking for the service manuals for them, but so far most places want enough for one to be out of my budget (enough for a week's worth of groceries, maybe 2+; keeping in mind I am very frugal by necessity).


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## illuminateddan (Dec 19, 2009)

I did manage to find this cheap PSU in a electronics chain. 0-30v, 0-5A current limit. cheap enough to buy two and parallel them up to get a happy 10A without destroying by budget.... time to do some overtime....

I even had the guy in the store tell me the best way to get 10A out of two of them was to connect them in series . I said he should give it a go.....


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## Amberwolf (May 29, 2009)

Bwahaha.... I love hearing these guys try to "talk tech"; I stopped trying to teach them many years ago after too many "you're an idiot, of COURSE I know what I'm talking about" responses. I just feel sorry for the customers that don't know and believe them. 

FWIW, since I don't see a brand on there that you can check anything with, I think you should use a volt and current meter that you really trust the readings of to verify that the PSU gauges actually read correctly at various points on them. In my experience, equipment that does not have a prominent brand name embossed upon it and which is not prominently displayed in the ad copy is not trustworthy until I have actually used it successfully for a while. 

Back in my ham days there were not that many companies making "cheap stuff" in this kind of thing and less worry about it, but nowadays there is more of that than new good stuff. Personally I'd rather find a used good brand than buy a new cheap one, since typically they can be had for the same price.  But I also have a personal principle of "recycle something else first" before I buy a new item, since we already have so much waste around us.


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## Roy Von Rogers (Mar 21, 2009)

Back in my Ham day's..lol....Did you let your license laps, or are you still good. I dont do whole lot anymore either, seems everytime I go to 2 meters, no body is talking any more.

Since computers and cell phones, the airwaves are hardly used anymore.

Roy


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## Amberwolf (May 29, 2009)

yeah, since the intarweb became commonly used with aol and the like, ham dropped off quickly. still in use a little, but not much. 

I was KA5TWP, but I let mine lapse; traded my equipment to a friend for other stuff over a decade ago since I hadn't used it for a decade before that, except for turning on the old cast-face Hammarlund receiver now and then. 

For me most of the fascination was talking with people I knew I could never travel to meet--it was like alien contact in scifi, especially when bounce'd be right and I got somebody in Russia or New Zealand or something. Once the internet got big and popular, most of what I wanted to do was more easily done via newsgroups and email, and later forums like this one.  Especially since it didn't have to be realtime; I could talk to a lot more people that way.


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## illuminateddan (Dec 19, 2009)

ah... you hammers.... I just had the joys of little FM transmitters from radioshack for my own 300m pirate station when I was 13... 

Maybe I should look at designing/ building an opensource single lifepo charger that people can make on a shoe string budget.

To isolate or not to isolate, that is the question.....


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## illuminateddan (Dec 19, 2009)

Hoorah! Ebay rules supreme! I manage to find a powerten P62 rackmount lab psu. Its CC, CV, 0-100v, 0-20A. It's big and noisy but it also has voltage feedback and remote current and voltage control via a 25way dsub and voltage scaling.

I'm so happy. I've been discovering the joys of what 20A does to things!


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## Amberwolf (May 29, 2009)

illuminateddan said:


> I'm so happy. I've been discovering the joys of what 20A does to things!


Yeah, 2KW is a lot of power. KFF is not uncommon with learning in situations like that, so be careful.


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## illuminateddan (Dec 19, 2009)

KFF? Killed for fun? I work in stage electrics so am well aware of how lethal a big current is! Thanks for the concern though. Most people forget that there's more danger in an electric car than there is sticking your fingers in a three phase socket. I even stopped wearing my wedding ring. Most of my favorite war stories start "I was tailing in to a live bus bar...." And end "as I hit the far wall....".


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## zwmaster (Nov 23, 2009)

illuminateddan said:


> KFF? Killed for fun? I work in stage electrics so am well aware of how lethal a big current is! Thanks for the concern though. Most people forget that there's more danger in an electric car than there is sticking your fingers in a three phase socket. I even stopped wearing my wedding ring. Most of my favorite war stories start "I was tailing in to a live bus bar...." And end "as I hit the far wall....".


Finally a good excuse why leave the wedding ring at home .
Metal watch, bracelet and necklace are even more fun


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## illuminateddan (Dec 19, 2009)

Krispy Freaky Farads?


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## Amberwolf (May 29, 2009)

Kentucky Fried Finger. 

I've gotten my fair share even just with my crazy ebike experiments. And that doesn't even count using the welder. 

Oh, and yeah, working with battery powered or DC stuff it ain't even gonna let go. At least with AC you got a slight chance of a muscle spasm jerking you away at a zero crossing or something.


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