# Using switching power supplies as charger wiring options



## baerfoot (Jan 2, 2011)

I have been using 2 48 volt switching charger supplies (Meanwell style) in series to charge my 72 volt lifepo4 pack. When I was first setting it up I had a big spark when making the connection to the pack. So I wired it up with 2 switches. The first one went through a resistor which I would turn on first, then I would switch on the circuit that went straight through. Not very elegant but seemed to work fine. 
I was concerned that if I just left the power supplies connected it would drain the battery. Does anyone have experience with this, or have another solution?
One thought I had was to use a relay connected to the power supply, so that when I turned on the power supplies it would close the cucuit to the pack. I'm not sure though if I can find a relay that takes 72 volts.


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## major (Apr 4, 2008)

baerfoot said:


> I have been using 2 48 volt switching charger supplies (Meanwell style) in series to charge my 72 volt lifepo4 pack. When I was first setting it up I had a big spark when making the connection to the pack. So I wired it up with 2 switches. The first one went through a resistor which I would turn on first, then I would switch on the circuit that went straight through. Not very elegant but seemed to work fine.
> I was concerned that if I just left the power supplies connected it would drain the battery. Does anyone have experience with this, or have another solution?
> One thought I had was to use a relay connected to the power supply, so that when I turned on the power supplies it would close the cucuit to the pack. I'm not sure though if I can find a relay that takes 72 volts.


Hi foot,

Your situation is similar to that of connecting the controller to the battery. The device has internal capacitors which cause the arc. So controllers use a "precharge" circuit. This is what you accomplished with your two switches. You might find a more elegant solution by looking at articles about precharging 

major


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## rwaudio (May 22, 2008)

baerfoot said:


> I have been using 2 48 volt switching charger supplies (Meanwell style) in series to charge my 72 volt lifepo4 pack. When I was first setting it up I had a big spark when making the connection to the pack. So I wired it up with 2 switches. The first one went through a resistor which I would turn on first, then I would switch on the circuit that went straight through. Not very elegant but seemed to work fine.
> I was concerned that if I just left the power supplies connected it would drain the battery. Does anyone have experience with this, or have another solution?
> One thought I had was to use a relay connected to the power supply, so that when I turned on the power supplies it would close the cucuit to the pack. I'm not sure though if I can find a relay that takes 72 volts.


Just put an appropriately sized diode on the output of the charger, this will bring any leakage current down to negligible levels (then you leave the charger permanently connected). I have a similar configuration set up in a friends car using four mean wells in series to charge a 56 cell pack of CALB 60's. He recently rolled past 2500 miles with this configuration and it works like a charm.


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## Elithion (Oct 6, 2009)

baerfoot said:


> I have been using 2 48 volt switching charger supplies (Meanwell style).


One of I have been using 2 48 volt switching charger supplies (Meanwell style) in series to charge my 72 volt lifepo4 pack. When I was first setting it up I had a big spark when making the connection to the pack. So I wired it up with 2 switches. The first one went through a resistor which I would turn on first, then I would switch on the circuit that went straight through. Not very elegant but seemed to work fine. I was concerned that if I just left the power supplies connected it would drain the battery. Does anyone have experience with this, or have another solution? One thought I had was to use a relay connected to the power supply, so that when I turned on the power supplies it would close the cucuit to the pack. I'm not sure though if I can find a relay that takes 72 volts.[/QUOTE]"]these? Which one?



baerfoot said:


> I was concerned that if I just left the power supplies connected it would drain the battery.


If it's one of these chargers, then, no, they are designed as chargers, so I don't expect that they would drain your battery. Just to makes sure, just measure the reverse current when the charger is off.



baerfoot said:


> I'm not sure though if I can find a relay that takes 72 volts.


It's not as much the 72 V, as the DC rating, that makes it a bit harder to find.

Check the Digikey relays; in the "Switching Voltage" column, select 100 VDC or above. Here's a nice one.


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## baerfoot (Jan 2, 2011)

rwaudio said:


> Just put an appropriately sized diode on the output of the charger, this will bring any leakage current down to negligible levels (then you leave the charger permanently connected). I have a similar configuration set up in a friends car using four mean wells in series to charge a 56 cell pack of CALB 60's. He recently rolled past 2500 miles with this configuration and it works like a charm.


Do you know if the diode would affect the output voltage? I am charging 25 cells, so I adjust the output to 91.25 volts.


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## baerfoot (Jan 2, 2011)

Elithion said:


> One of these? Which one?
> 
> 
> 
> ...


The model # I am using is "S-350-48" I'll check what the drain is when the charger is off.


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## Elithion (Oct 6, 2009)

baerfoot said:


> The model # I am using is "S-350-48"


I can't find it in their website. Can you help me?


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## rwaudio (May 22, 2008)

baerfoot said:


> Do you know if the diode would affect the output voltage? I am charging 25 cells, so I adjust the output to 91.25 volts.


Yes, the diode will affect the voltage slightly. Between around 0.4 and 0.7v depending on what type of diode. You would need to install the diode and reset the voltage.


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## baerfoot (Jan 2, 2011)

Elithion said:


> I can't find it in their website. Can you help me?


I think might be the same as this:
http://www.meanwell.com/search/sd-350/default.htm

But I bought it from China on Ebay, so it could just be a knock off.


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## Elithion (Oct 6, 2009)

Thanks.
It's not a charger, so it's quite possible that it will slowly drain your battery. Use a diode in series, as others have said.


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## baerfoot (Jan 2, 2011)

rwaudio said:


> Yes, the diode will affect the voltage slightly. Between around 0.4 and 0.7v depending on what type of diode. You would need to install the diode and reset the voltage.


I bought a diode rated at 100 volts and 60 amps. It lowers the voltage 0.4 volts. I was able to adjust the voltage back up to my desired level.
Thanks for everyone's input on this.


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