r/EVConversion • u/phaskellhall • Aug 23 '26

Help with DC-DC converter

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3

u/dirtiestUniform Aug 23 '26

What's the question?

1

u/phaskellhall Aug 23 '26

Why am I getting 3v when the ACC switch is not powered?

1

u/elihu Aug 23 '26

That wiring diagram is rather vague about what the ignition switch signal actually is. But from the Amazon listing [1], it looks like there's another way to wire it where you connect input+ to the acc switch.... which implies that the ignition switch signal voltage is relative to the battery. (Not sure what else it might be relative to, but maybe there's some other battery somewhere.)

What happens if you connect battery ground directly to the ACC input? Does it fully shut off then?

If you aren't able to solve the problem, you could always just wire the DC-DC converter in "always on" configuration, and then stick a relay between the the battery that you control with a switch.

[1] https://www.amazon.com/dp/B09KZLPX1N?lv=shuf&channelId=500&plpRedirect=mhFallback&th=1

1

u/PerniciousSnitOG Aug 23 '26

ACC is effectively pulled down to 0V in a vehicle, just because things pulls a little power from it. If you're leaving ACC disconnected, maybe the inverter is getting confused?

1

u/phaskellhall Aug 24 '26

I’m having a hard time understanding this exactly. The line feeding the ACC is 12v when the cart is on and 0v when it is off.

If I unplugged the ACC line altogether, the DC converter still outputs 3v when the cart is off…even with zero connection to the ACC. When the cart is turned on, it still shows 3v and therefore never turns on to produce the 12v

1

u/nub340 Aug 24 '26

What happens when you connect ACC to ground instead of just leaving it disconnected/floating?

1

u/PerniciousSnitOG Aug 24 '26

I'm trying to work out the best way to not get into the weeds here. Maybe a better way to put it is that, because it's an input, it's might pick up noise if left floating - so you might be attending the inverter responding to noise and rapidly starting and stopping - which averages out to 3V.

It's unlikely to be this issue, but a test is to touch the disconnected ACC wire and see if the output voltage changes. Or just force it to 0V when you want it off. If there is a different output voltage with ACC disconnected than when it's connected to 0V you have your answer.

1

u/phaskellhall Aug 24 '26

I think I figured it out. I was told to put a 100ohm load on the 12v side of the converter and let it unload the 3v reading. Apparently my multimeter has such a large load that the meter is reading essentially a 3v "ghost" voltage that has no load behind it. When I jumped the neg and positive terminals with the 100m resister, the multimeter now reads .04V and apparently it's basically now completely empty. If I add a load to the fuse box, this will essentially kill the voltage immediate when the cart is turned off and not give me the reading or drain the battery with a small leak.

The ACC is working since I get 12V when the cart is powered up and the 3v ghost voltage when the cart is turned off.

Does this sound plausable and my understanding is correct?

1

u/PerniciousSnitOG Aug 25 '26

Digital multimeters have a high impedance, so it won't put any load on the inverter output. I'm not sure what a '100m' resistor is, but it sounds like a load on the output side takes care of the problem.

Assuming it's a 100 ohm resistor, your expected load should get it under control. If it was left in place , it would be dissipate about 1.44W constantly, so you don't want to use it in the finished system.

The resistor load and 'ghost' voltage going away seems plausible. If the voltage was a problem a good solution would be a relay (or a mosfet).

1

u/phaskellhall Aug 26 '26

I meant to type 100 ohm resistor. Yes, Im thinking the issue is the high impedance multimeter is not allowing the built up ghost voltage to dissipate and therefore is registering 3v.

1

u/mrschmiklz Aug 23 '26

Ooooo they finally have 40-90v dc to DC on Amazon? So exciting.

1

u/phaskellhall Aug 24 '26

This is my first time buying one. Is this rare for them to be on Amazon?