Can't tell which is - and which is + on charging port

Hello, so I decided to upgrade to a BMS system instead of taking out the batteries and charging them every time. I got these sockets:

And they just arrived. This might be a dumb question but how do I know which of the sides is + and which is -? There’s another piece of metal there but it looks different, I’m assuming that’s ground and I don’t use it? Thanks Edit: 2018-06-15_13h58_57 Here is a closer look, two identical and then the flat one.

Id use a multimeter in continuous mode to check which pin connects where. You could also hook up your charger to the plug and measure the voltage accross the different ports, then you’ll know where your + and - are. These plugs dont care about which side is positive and which is negativ. Its up to you to make sure your charger can charge the batteries correctly.

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I don’t have any voltage meters or anything like that, so I cant check it like that. If it doesn’t matter, can’t I just connect the + to one of them and - to th other and it will work?

You should get a voltage meter truly. Much needed tool in this hobby

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What other uses will I have for building an Esk8 with a voltage meter?

Anything and everythimg battery related. It is an amazing diagnostic tool. Downright necessary…

Boi, a Voltmeter costs below 10$. If you want to build anything that uses power yourself you need one!!


Alright, I’ll get one. Can anyone answer my question though?

Edit: Will that do for basic electronics?

Harbor freight even sometimes has a coupon for a free multi meter. If you are doing a lot of the work your self I would definitely get one. It definitely matters which one is + and which is - if you do it wrong you can probably cause a fire. I would google at least the connection because you should be able to find what the pin out should be so then you can cross reference that with the multi meter.

It can but it won’t be accurate in the levels. I would go with a better one then that I found a amprobe model on amazon for like 30 bucks and it has auto ranging. My work had bought me a cheap multimeter and the continuity feature didn’t work out of the box and that is very useful feature.

Yes, that one would do, but I would recommend investing about 40$ for a much higher quality one. If you’re dealing with low voltages like we are you’ll most likely be fine, but I wouldnt trust a 5$ multimeter to be safe around medium voltages / household AC voltage. On the other hand, if you don’t know what you’re doing you shouldn’t touch that anyway.

I can recommend the UNI-T 139c for 41$ on aliexpress:

A review of it here:

By auto ranging you mean something like this? My main concern right now is knowing which is the negative and which is the positive, it’s the final part before I can reassemble my board.

The connector that is bent in an l shape will be positive. The other ones is anyones guess. You could find out via trial and error but why would you

Trail and Error, aka how to start a fire 101. ;D

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Is there a way to figure it out from this picture?

I barely even know how to use a voltmeter, other than checking a voltage, and the 40$ one seems like it would be a waste because I won’t use all of its features. To be honest, after a year of messing with parts, building and upgrading my board, this is the first time I’ve needed a voltmeter. I have a battery cell reader to make sure everything is balanced and a battery meter to see how full it is but other than that I never really needed any meter.

Id learn to use it then. Honestly, that’s the fun in DIY, at least for me.You get to build something, be proud of it, and learn new stuff in the process. As a “hack”/ workaround, you could take your battery meter (which is probably a voltmeter in itself, plus some added features) to check the polarity. It might not be protected against wrong polarity though, so it might break. Also the difference between a cheap and more expensive voltmeter may be not only features but also safety, accuracy and build quality.

I agree 100% and that’s the reason I keep doing DIY projects, and I did learn a lot about electronics, but I just don’t get why it’s such a must-have, never needed it until now.

Have you ever had to troubleshoot a dead battery / cell? Check if a VESC is still working? Find out how much the battery is charged without hooking it up to your battery meter thing? Those are all real use cases for a multimeter when building eboards. Someone else can probably thing of a lot more.

By all means, get a multimeter. For starters, the 5$ one will do.

I had a similar problem.


I have a multimeter arriving today though because it’s stupid not to be safe. Don’t want to ruin a £200 battery for sake of a £8 multimeter.