EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: mscreations on August 29, 2014, 08:57:25 pm
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I need some help with a truck I'm working on at work. It is a 48v electric golf cart. Originally, they were having issues with constantly destroying two batteries by running them down much further than the other 6 batteries. This is because there was a 12v tap on the last two batteries (with B+ being common between the 12v and 48v systems) and they were using it to charge cellphones and what not all the time.
So, I got a DC/DC converter, isolated the 12v system from the 48v system and wired it up. I noticed when I plugged the battery back in that there was a large spark when I connected it up. Gave the truck back to the operators. Short time later I get it back with them complaining it's not lasting all day (or two). I checked for current draw and found that I have approximately 100 to 200 mA draw all the time (only from the DC/DC converter).
No problem. Rewire the truck so the converter stays off until the key is turned on. Use 48v at keyswitch to switch a relay (which uses the 12v tap. Minimal draw so it shouldn't affect much.) After checking it out, the truck works mostly normally (with the exception that they can't charge phones or anything while the keyswitch is off). I give it back to them.
Week later I get the truck back AGAIN. No 12v system at all. No lights. No horn. No chargers. Only the 48v traction system is working. Check and find the relay is toast. It's a 40A relay!!! Cut it open and find the contact tips are ALL melted up. Replacing the relay fixes it. I cut open the new one and check what happens when the key is on. Can we say ARCING!?! It appears that the DC/DC has one hell of an inrush current that is destroying the relay. The only thing I can think of to try would be to use a 48v contactor to apply power for the converter, but that seems excessive.
So... is there any way that I can limit the inrush current on this converter? The converter itself supplies 25-27 A at 13.8VDC from a 48V supply. Not positive how efficient it is, but with a 15A output draw it pulls approx. 7A from the supply. Also, it arcs regardless of whether or not the output is connected to a load.
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How much input filter capacitance does the DC-DC converter have? High capacitance at relay closing would cause a huge current spike and vaporize contact material. High inductance in the wiring loop will also cause additional arcing when the relay breaks contact.
You might be able to work around those issues by increasing the soft-start delay on the converter, putting a current-limiting resistor between the converter's input and capacitors, and adding a relay to short that resistor out once the converter comes out of under-voltage lock-out.
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A NTC thermistor should do the job. Use it in series with the DC-DC converter input. One with a 10 A steady state current rating should work because the maximum output wattage of the converter is 27*13.8 = 373 W. On the input side, that would be 373/48 = 7.7 A, but the converter won't be 100% efficient, so 10 A would be the minimum; 15 A to be really safe. You need to be aware of the limitations of NTCs. They are affected by ambient temperature. They also require a cool-down time, so may not work that well if the load is switched on and off frequently.
I'm somewhat surprised that a 100-200 mA constant current draw is a problem. Golf-cart batteries have a lot of amp*hours in them when fully charged. Your clients must not plug it into the wall outlet very often. I have a similar vehicle with a DC-DC converter for the accessories. We plug it into the wall outlet whenever it's not on the go and have had no problem with the batteries being drained by
the converter.
Mike in California
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+1 for the NTC thermistor, but use an additional relay to bypass the thermistor a few moments after switch on. This has two benefits. 1/ You can use a higher resistance thermistor than otherwise because it only dissipates for a short time. 2/ Thermistor is able to cool down so that if power is turned off then straight back on again it will be ready to limit the current.
Edit -> I see DanielS mentioned the bypass relay.
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I'm somewhat surprised that a 100-200 mA constant current draw is a problem. Golf-cart batteries have a lot of amp*hours in them when fully charged. Your clients must not plug it into the wall outlet very often. I have a similar vehicle with a DC-DC converter for the accessories. We plug it into the wall outlet whenever it's not on the go and have had no problem with the batteries being drained by
the converter.
You are assuming that they don't have 3 or 4 phones plugged in and charging which they probably did...
+1 for the NTC thermistor, but use an additional relay to bypass the thermistor a few moments after switch on. This has two benefits. 1/ You can use a higher resistance thermistor than otherwise because it only dissipates for a short time. 2/ Thermistor is able to cool down so that if power is turned off then straight back on again it will be ready to limit the current.
Edit -> I see DanielS mentioned the bypass relay.
I'm going to be looking into the NTC thermistor as that was one of my first thoughts as well (just have no experience using one). How should I activate the bypass relay? RC circuit feeding the coil of the relay?
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Relays require a finite time to act, so the inherent delay in the closing action of the second relay may be enough. Have the NO contacts bridge the NTC thermistor. Connect one coil lead to the converter side of the NTC thermistor and the other coil lead to the opposite polarity rail. When the first relay closes, the NTC thermistor will limit the inrush surge. Very shortly thereafter, the second relay will close bypassing the NTC thermistor. The second relay will remain closed until the first relay opens again. For this to work, the surge has to be pretty much over by the time the second relay closes. The reason for connecting the coil lead of the second relay on the converter side of the NTC thermistor is to have the NTC thermistor enhance the time for the second relay to close.
Or you can also spend a more and use a time-delay relay.
Using an ordinary relay to generate a delay is an old trick. Quick and dirty and not very sophisticated. Was used a lot more in the caveman days of electronics.
Mike in California
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You could use the accessory DC voltage to close a relay with 12V coil: this way, the resistor or NTC does not get switched out until the DC-DC converter has started.
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Great idea; better than mine. In addition, relays with 12 VDC coils are much easier to find and cheaper than relays with 48 VDC coils.
Mike in California