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Bench PSU to charge batteries?

ApexProxy

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Does anyone have any experience using a bench PSU to charge their batteries? I am looking for a way to charge them using my PSU but I don't know exactly how I should hook up the batteries and if I will need additional components. I would like to use two charging speeds, one at 250mA(slow) and one at 700mA(fast). The batteries I would be charging would only be protected cells to avoid overcharging. My main concern is that I would need something additional(a diode maybe?resisters?) to hook them up and not just drop the power right to the leads on the holder.

Also, I was thinking I could make a large quantity charger so I could maybe charge 4 batteries at a time by placing them in parallel or series but I don't know if that's possible and if I would need to adjust my voltage and/or current.

I could also just be nuts and this could require a lot more circuitry than I think :D

BTW, the PSU is a 30V 5A with 2 separate rails that can be put in series or parallel for 60V@5A or 30V@10A. I love this thing.
 



MadEye

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If you are talking about Li-Ion battery it would be possible if you limit the current to 250mA or 700mA and the voltage to exactly 4.2V, this would charge your battery first with constant current and then with constant voltage (CC / CV), like the good chargers do.

The problem, why I think its not really possible is you are not able to set the Voltage to exactly 4.2V +/- I think 20-30mv. If you get less the batterys will loose to much mah, if you go to high you will destroy your batterys.

Maybe you should look for a cheap hobby charger...
 
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flare09

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Yeah batteries are actually way complex. For the rc stuff, the cells in each pack are balanced, some charged, some discharged, then they are charged equally, the internal resistance is checked and cells are matched into every pack for their resistance and impurities at factory.

So taking a few 18650s and parallel charging them would cause one to charge more then another or faster or slower, due to internal resistance on ecach battery

I have really considered this, and with the lipos I wouldn't risk it. They are dangerous charging with a charger specific for them as is. They sell bags for rc battery charging. LiFes are more stable but as madeye stated, most battery data sheets give on average a ±5 mV charge rate...and I know my psu isn't fancy, but that is hard to get.
 

MadEye

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2-3% less voltage (4.1V instead of 4.2V) will result in at least 10% less capacity. Its indeed not linear but not the good way, even if there is a spike at the end.

The cheap bench power supplys we use are not made to be set in the 100mV range. There is a reason why real lab power supplys can be as much as some small cars... They can be easily off by a few 0.1V and even if not, "4.1V" on display still can be as low as 4.05V.

Also the output voltage can slightly change with temerature changes or simply with time.

While it may be usable, any 5$ charger will do a better job...
 
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ApexProxy

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You guys made some really good points. The only thing I can think of is, would a protection circuit be able to help with the charge? If I set the PSU to even a couple hundredths of a volt over 4.2v wouldn't the protection circuit kick in as soon as the battery capped off at the perfect charge level? Wouldn't my only issue then be having too little voltage instead of too much and end up with less charge or am I putting too much faith in the protection circuit?
 

Blord

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Usually 0.1V above 4.2V shouldn't be a problem. It isn't an ideal situation and your cells will degrade much faster than normal. All the excess voltage will dump into heat and the cell don't like heat at all.

Real danger will occur if the voltage is much higher than 4.2V. Even explosion is possible.

Alway keep the charging process in control. Never left it charge overnight or so.
And sometime touch the batteries for any temperature change. Hot batteries are never good.
 

MadEye

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If you want to add a protection circuit, you could also just add a full charging circuit...

I got some of THESE for a futurue Charger project Im thinking about. They are really tiny, 2.21$ each and all the testings Ive done was great. True CC CV charging and you can set the current with a resistor. I also got some of THESE since they are really tiny and its not that easy to make a matching PCB. For higher currents you have to glue an aluminum or copper heatsink to the chip, or it will fail with the CC at the beginning (But it will still charge safe, just slower).

You could use them to easily build a charging Circuit with 5V in and use jumpers to select current. Also you can get a few of them and make an Multi battery charger...
 
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BShanahan14rulz

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There are cheap charging boards that run off of USB voltage (5V) based on the tp4054 or 4056 (I can't remember right now) on ebay. I hear great things about them, adn they'd be easy to implement with a psu by making a custom USB cable, or soldering cables onto the correct parts on the board.
 

Cyparagon

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2-3% less voltage (4.1V instead of 4.2V) will result in at least 10% less capacity.
Source? Math?

If it were perfectly linear, with 4.2V being full and 2.8V being empty, that would be 100%/1.4V, 10%/.14V, or 7% per 0.1V.

But it's NOT linear, so it will be even lower than 7%.
 

ApexProxy

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So it seems like its a rather complicated can of worms that if done wrong could leave me with exploding batteries. I think I will just stick to using a normal charger for now until I can get charger circuitry set up like what MadEye linked to. Thanks for the links btw.

The protection circuitry I was talking about is the kind thats attached to a protected cell such as this TrustFire.
 

MadEye

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Sry to discuss this on your thread ApexProxy, but it has at least something to do with it.

My source doesnt use any math since as you all said its NOT linear.

Here is a charge graph of the Trustfire 3000mAh (which is actually one of the best cheap batterys you can get and discharge with up to 4-5A)

!The Graph is from lygte-info.dk, not from me. He tested almost any 18650 and other Li-Ion batterys as well as chargers and flashlights, nice site to check out if you are interested in Li-Ion batterys!



If you hit 4.1V and would start the CV charging, the battery would take only a few mAh from then.
Even if you stop charging at 4.2V the battery only has 90-95%, and 4.1V will be missing over 500mAh from 2500...

Charging and discharging are not the same, you cant just calculate some linear graphs. I made a lot of tests with charging circuits and Ive noticed how fast you will hit 4.1V and how long it will take until the Cell is full..
 
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Cyparagon

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I've interpolated (in MSpaint :crackup:) what those values would be at 4.1V, and it looks almost the same. It is already up to 2200 by the time it switches to CV, so we can estimate it gains another 15% from the CV portion. at 4.1V, it's up to 1900 and another 15% is 2200mAh. Meh, still way better than you typical ultrafire.
 

ApexProxy

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I find it interesting how all that works. Now I really have to give a charger build a try. After I get my 638nm finished that is :p
 




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