# Initial paralell charge using ATX computer supply



## Ziggythewiz (May 16, 2010)

If it's really just 3.5 V it will not bring them up high enough (in any reasonable amount of time) to balance and safely charge in series.

How many watts is your power supply? You probably won't pull all 24A with such a small voltage difference.


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## sabahtom (Mar 1, 2011)

Thanks Ziggy._

If it's really just 3.5 V it will not bring them up high enough (in any reasonable amount of time) to balance and safely charge in series._

How high should they be before I can safely charge in series?

It also has 5v and 12v output. The Hipower spec sheet says to charge at 3.65. I could put a resistor in there. Any suggestion how much ohm I need? I guess the supply could handle 10amps so a .2 ohm or so might be right.

_
How many watts is your power supply? You probably won't pull all 24A with such a small voltage difference._

500w supply.


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## Ziggythewiz (May 16, 2010)

Yes, 3.65V is the standard charge voltage. Most chargers will raise the voltage to 3.65 vpc and hold it there as the current drops off to 1/20 C. I understand the desire to not get a special charger for something that could be a 1 time use.

I'm balancing some cells myself right now using a 12V charger. I have 4 in series and let them charge to 3.65 and just cut it there. Then I'll tie em all in parallel (not in parallel yet because I'm still drilling). For me this is good enough as in real use I plan to charge to around 90%, so they will never get this high again and should hold the balance well. If you plan to fully charge in use you'll need a tighter balance.

With a 500W charger I would expect this to take at least 50-100 hours. Do you have a good way to measure what you're putting in? You could try at just the 3.5V and see what kind of current you get.

Don't mess with a resistor unless you really know what you're doing. The resistor would have to be able to soak up a good portion of what you're sending into the cells, which means tons of heat. Also the resistance needed would be determined by the apparent IR of the cells which will vary by temp and SOC.


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## sabahtom (Mar 1, 2011)

_I'm balancing some cells myself right now using a 12V charger. I have 4 in series and let them charge to 3.65 and just cut it there. Then I'll tie em all in parallel (not in parallel yet because I'm still drilling). For me this is good enough as in real use I plan to charge to around 90%, so they will never get this high again and should hold the balance well. If you plan to fully charge in use you'll need a tighter balance._

90% will be plenty for me. I only need 30km out of the pack on a charge, 40km on odd days. But I don't know that my charger allows me to set it at 90%, I can't see any such controls on the face.

4x3.65 = 14.6v : I think car lead acid battery chargers charge that high, but my pc power supply is only 12.4v as I measure it. So 4 in series would give me 3.10v.

Could you fill me in, why would you charge to 3.65 and then connect in parallel? Are you using paralell cells in your traction pack?

Cheers
Thom


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## piotrsko (Dec 9, 2007)

go down to your favorite place of hobbies and get a LIPO field charger. some can be had for $20 USD. They auto shut off, but run on 12vdc.


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## sabahtom (Mar 1, 2011)

_I'm balancing some cells myself right now using a 12V charger. I have 4 in series and let them charge to 3.65 and just cut it there. Then I'll tie em all in parallel (not in parallel yet because I'm still drilling). For me this is good enough as in real use I plan to charge to around 90%, so they will never get this high again and should hold the balance well. If you plan to fully charge in use you'll need a tighter balance._

90% will be plenty for me. I only need 40% DOD out of the pack on a charge, 50% on odd days. But I don't know that my charger allows me to set it at 90%, I can't see any such controls on the face.

4x3.65 = 14.6v : I think car lead acid battery chargers charge that high, but my pc power supply is only 12.4v as I measure it. So 4 in series would give me 3.10v.

Could you fill me in, why would you charge to 3.65 and then connect in parallel? Are you using paralell cells in your traction pack?

Cheers
Thom


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## mora (Nov 11, 2009)

I've bought few rc lipo chargers and all of them have gone bad when charging for long periods. Smoke and flames which was cool, heheh. They seemed to work nicely first though. I guess those are not designed to operate many hours in a row at full power.

I suppose Powerlab 6 or 8 is different animal though. Is it too expensive? Would provide customizable output and data logging capability using computer. I wish I had bought one for top balancing purposes right at the start. Many fellow EVers are using it.

EV-power.eu sells single cell chargers (10A & 20A) but those look suspiciosly same as Chennic units. Not true CC/CV and advertised 10A is peak current.


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## sabahtom (Mar 1, 2011)

*Re: Initial parallel charge using ATX computer supply*

I've connected a single cell with 3.2v resting to the ATX supply at 3.55v. The draw is 1.4 amps according to my clamp meter. I guess that will take about 50 hours to charge up.

My power supply is rated at 24amps for 3.5v.

So I guess I can have 15 cells in parallel on my ATX supply at 3.55v. The ATX should have the standard safety features so I guess if the cells reach 3.55v it will just shut off? 

Cheers
Thomas


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## sabahtom (Mar 1, 2011)

I've connected a single cell with 3.2v resting to the ATX supply at 3.55v. The draw is 1.4 amps according to my clamp meter. I guess that will take about 50 hours to charge up.

My power supply is rated at 24amps for 3.5v.

So I guess I can have 15 cells in parallel on my ATX supply at 3.55v. The ATX should have the standard safety features so I guess if the cells reach 3.55v it will just shut off? 

Cheers
Thomas


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## mora (Nov 11, 2009)

Remember it is not a charger but power supply. It won't shut off when cells reach 3.5V. Safety features mean "output shorted", overheat and maybe overcurrent kind of protection.


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## Ziggythewiz (May 16, 2010)

sabahtom said:


> Could you fill me in, why would you charge to 3.65 and then connect in parallel?


After charging to 3.65 vpc they should have a resting voltage of nearly 3.4 V. I'll then tie them all in parallel and let them sit a while to make sure they are all the same voltage. Right now charging in batches of 4 there's always one cell that ends up higher than the others.

After they've all balanced, I'll reconfigure in series to stick in the car.

Go ahead and stick a dozen cells on the charger in parallel and see if the amp draw changes at all. If not you're in for a long wait. It's safe to leave them on the power supply at 3.4V, but not 3.55, so you'll have to take them off to check the resting voltage. Maybe you could do them in groups of 10-15 for a day each and check voltages after they've been off a few hours. 

Or ya know...you could get a charger  Even a lead (12v) one would work with proper care. What are you using to count AHs?


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## sabahtom (Mar 1, 2011)

_Or ya know...you could get a charger  Even a lead (12v) one would work with proper care. What are you using to count AHs?

_With 6 in parallel its at 1.3amps. Maybe my meter isn't that great, with a single cell it read 1.4. I'm using the clamp at the + terminal of the ATX supply. I should hope to see 1.4*6=8.4 amps with 6 cells?

Guess I'll have to get a battery charger if its that slow.


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## Ziggythewiz (May 16, 2010)

The amps would probably only go up by raising the voltage. 

To run lithium in the car you'll need not just an ammeter, but an AH counter. What do you have planned for that? 

I found my charger actually cuts off at 14.2 (3.55 vpc) instead of the 14.6 I was shooting for. I think it'll go higher on the 2A setting though, so I'll try that after I finish making busbars for the parallel grouping.


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## sabahtom (Mar 1, 2011)

_The amps would probably only go up by raising the voltage._

If I'm looking for 2 amps, I guess I need to double the v. The next setting is 5v, but I thought that was too high for the cells.

If charging v is supposed to be 3.65, is that 3.65 - 3.2 (present v) or is it supply voltage - cell voltage: 5v-3.2 = 2.8v charging v.


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## Ziggythewiz (May 16, 2010)

3.65 is the supply voltage. The cells shouldn't really even get that high (3.4V is full) but they appear to from the surface charge at the end.

You could try it at 5V and see what it does, if the voltage you measure comes out as 3 something then you're okay (with proper monitoring).


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## sabahtom (Mar 1, 2011)

Ziggythewiz said:


> 3.65 is the supply voltage. The cells shouldn't really even get that high (3.4V is full) but they appear to from the surface charge at the end.
> 
> You could try it at 5V and see what it does, if the voltage you measure comes out as 3 something then you're okay (with proper monitoring).


Thanks for the quick replies Ziggy - the other bits of the car are coming along so its great that the battery is also progressing.

The 5v output gives 5.33v, not useable.

I'll end up buying a smaller charger to charge in packs of 4. 

Could you fill me in: I have a 10amp 312v charger for the pack in series. whats the reason for not charging all the cells to 3.29v in packs of 4 (doable with my ATX supply), then connect the whole 100 cells in parallel, let them balance out, then charge up in series all together in 8 hours or so?

Cheers
Thomas


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## Ziggythewiz (May 16, 2010)

sabahtom said:


> The 5v output gives 5.33v, not useable.
> 
> 
> Could you fill me in: I have a 10amp 312v charger for the pack in series. whats the reason for not charging all the cells to 3.29v in packs of 4 (doable with my ATX supply), then connect the whole 100 cells in parallel, let them balance out, then charge up in series all together in 8 hours or so?


Is that 5.33v with a cell connected, or just open circuit?

You have to charge them up into the knee so they can balance. 3.29v is too flat to balance them at (see pic). Also, didn't you figure 50 hours to charge one cell? So you'd need 500 hours to charge them all that way?


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## Elithion (Oct 6, 2009)

sabahtom said:


> I've converted a PC ATX power supply. It gives 3.5v and 24 amps max.
> Is it a good idea to use this to charge 100 100ah LifePO in paralell?


A difference between a power supply and a charger is how each behaves at voltages lower than their settings (i.e. when connected to a cell that is less than 100 % full).


A charger is current limited, and happily charges the cell
A typical power supply sees a low voltage and either blows a fuse, or shuts down, or limits the current but soon overheats and shuts down (the exception is a "laboratory supply" which is really a CCCV source)
I am pretty sure that, when connected to a cell, a computer supply will keep on cycling off and on (too low a voltage, shut down, restart, retry, shutdown, and so on).


Other than that, the idea of charging all the cells in parallel before building a pack is excellent.


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## sabahtom (Mar 1, 2011)

Elithion said:


> I am pretty sure that, when connected to a cell, a computer supply will keep on cycling off and on (too low a voltage, shut down, restart, retry, shutdown, and so on).


Hi Davide

Thanks for explanation re chargers/power supplies. I had the ATX connected for 5 hours yesterday. It gave 1.4amp at 3.35v to a pack of 6 LiFePO in paralell. It seemed to stay on the whole time. It is a cheap one, maybe it is missing some safety features. It only got a little warm, not bad for 28 degrees ambient.


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## sabahtom (Mar 1, 2011)

Ziggythewiz said:


> Is that 5.33v with a cell connected, or just open circuit?


It drops to 5v with a 1ohm resistor, and stays at 5v with 1.0 + another 1.5v resistor. I guess that means if I connect a cell, the cell will also see 5v.
_
You have to charge them up into the knee so they can balance. 3.29v is too flat to balance them at (see pic)._ 

That makes total sense, thanks for the graph.

_Also, didn't you figure 50 hours to charge one cell? So you'd need 500 hours to charge them all that way?_

I hoped to use the 5v rail (with more amps) to charge a combination of paralell and series. I can't see a useful way to do that without using a resistor, but as previously advised I won't try that. Time to get a battery charger.

So the battery charger will be current limited to 7 amps (the one I saw, $50) and it has 6v and 12v choices. To do use it in 2s 4p with 6v:

*Charging-time [h] = (1 - initial SOC [%] / 100 %) * number of cells * cell capacity [Ah] / supply current [A] (thanks elithium)

=(1-50%/100%) * 4 * 100/3.5
=7 hours or so per set, and 100 hours for the pack of 100.
*


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## Beemer (Jun 2, 2011)

It's an old tale with charging voltages. Taking a cell to [email protected]/20C cut-off is considered the norm. Providing it is the highest voltage cell you are looking at.

Also for the Solar power guys, a supply can give a float voltage of, (as stated by CALB) a *maximum* of 3.4V

These voltages are OPTIONAL MAXIMUMS. Safety is always lower.


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## Ziggythewiz (May 16, 2010)

Do you know for sure what voltages that charger can do? My cheapo 12V car battery charger does great, but I've heard from others that their 12V chargers cut off at only 3.3 vpc.


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## sabahtom (Mar 1, 2011)

The shop kindly upgraded my charger. The first one had OCV of 12v and 18v for the 6v and 12v channels respectively, and it cut out at less than 3.3vpc. The new one did as in the graph - how does it look to you? Seems like it has a fairly gentle approach. The spike at 16 minutes was probably due to me pushing the terminal on more firmly.


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