Electric go kart battery wiring

Thread Starter

enthusiast122

Joined Apr 1, 2024
15
Hello, would greatly appreciate help with my circuit that I have in mind. The objective of the circuit is to have two 72v batteries that are connected in series, both of which have a bms on them. Both bms will not be used for discharge and only for charging. Rather for discharge safety I have a dc contactor that I would like to be turned off if either of the bms triggers it to turn off. With this in mind I have attached a drawing of my current circuit. As mentioned there is two 72v batteries connected in series, then from each battery I take the negative from the bms and the positive and run it to a step down converter on both sides. From there I connected the step down converters in series so that i an left with one positive and one negative wire. The step down converter is set to 6v on each output. Therefore two of them in series makes 12v to turn on the contactor. If one bms turns off its not enough voltage to turn on the contactor. However the issue is, when I connected the 72v batteries in series, the step down converter capacitor blew up on the output side. I assume there's a short and for what ever reason I'm having a little trouble figuring out out. Sorry for the long write up. I really appreciate the help.

Link for the step down converter below.
https://a.aliexpress.com/_mK37MYi
 

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LesJones

Joined Jan 8, 2017
4,216
I think you will find that there is no isolation between input and output of your step down converters. (The negative input is connected directly to the negative output.) To do what you are trying to do would require converters with total isolation between input and output. If you draw in the link between the negative input and output of the right hand converter you will see that you are applying 144 volts betweem the positive input and positive output of the left hand converter. (Positive of the 144 volts to the positive input and the negative of the 144 volts to it's positive output.) You have probably destroyed the left hand converter.

Les.
 

Thread Starter

enthusiast122

Joined Apr 1, 2024
15
I think you will find that there is no isolation between input and output of your step down converters. (The negative input is connected directly to the negative output.) To do what you are trying to do would require converters with total isolation between input and output. If you draw in the link between the negative input and output of the right hand converter you will see that you are applying 144 volts betweem the positive input and positive output of the left hand converter. (Positive of the 144 volts to the positive input and the negative of the 144 volts to it's positive output.) You have probably destroyed the left hand converter.

Les.
Hello Les,

Thank you for the reply. You very possibly might be correct that's a good point. I think I can easily test this with a multimeter to see if you are correct.
 

Irving

Joined Jan 30, 2016
3,997
Your buck converters are not isolated, -in is connected to -out so there is 72v between them... this will never work the way you intended. A very different solution is needed...

Check to see if your BMS has a 'battery good' or 'low battery' signal output, many do. Or post a link to your BMS.
 

Thread Starter

enthusiast122

Joined Apr 1, 2024
15
Your buck converters are not isolated, -in is connected to -out so there is 72v between them... this will never work the way you intended. A very different solution is needed...
Thank you for that, are there buck converters out there that would be isolated?
 

Irving

Joined Jan 30, 2016
3,997
A 110-220v AC in, 12v 50W DC out supply as suggested by @LesJones is one option - it might work ok, but you'll have to try it as the inrush current of that contactor is quite high and its hold current very low so it might not work as you expect.
 

LesJones

Joined Jan 8, 2017
4,216
I found this link to the datasheet on the contactor. For the 9 to 36 volt coil the initial coil current is 3.8 amps max. If it is just a simple coil then it will probably require about 1.3 amps with 12 volts. (If it achieves the wide voltage range by having a built in buck regulator before the coil then it would be about 3 amps with 12 volts.) It also says that this model requires an requires external coil economizer.
Do you already have the contactor ?

Les
 

Thread Starter

enthusiast122

Joined Apr 1, 2024
15
I found this link to the datasheet on the contactor. For the 9 to 36 volt coil the initial coil current is 3.8 amps max. If it is just a simple coil then it will probably require about 1.3 amps with 12 volts. (If it achieves the wide voltage range by having a built in buck regulator before the coil then it would be about 3 amps with 12 volts.) It also says that this model requires an requires external coil economizer.
Do you already have the contactor ?

Les
Yes I already have this contactor and tested it with both bms, and it worked fine. I also think I read somewhere it has a economize in it already. And I found a step down relay that might work

https://www.tracopower.com/model/tdn-1-4821wi

https://diysolarforum.com/threads/coil-economiser-help.18423/
 

LesJones

Joined Jan 8, 2017
4,216
I think the power supplies in your post #16 are far too small. They are only 1 watt (That is only 83 mA at 12 volts) You need to be able to supply the initial pull in current whic is going to be about 3 amps. As you have the contactor can you measure this current and the amout of time until it drops down to the holding value.
would an inline power supply such as this one meet your physical size requirements ?
The other link in post#16 explains the part number encoding, in particular the seventh character. I thought it could only be A, D, J, R ( I did not understand the significance of the digits 1, 2, 3) If the 7th character had been a 1 instead of A then it would require an external economiser.

Les.
 

Irving

Joined Jan 30, 2016
3,997
A 1watt converter isn't sufficient. Assuming @LesJones is right about the current its still going to need about 20-30Watts; the one I suggested is a 40watt unit.
 

Thread Starter

enthusiast122

Joined Apr 1, 2024
15
Someone suggested this to me but honestly I'm having a bit of trouble understanding. I'm more of a visual learner.

"How about just using a contactor or relay on each BMS, with the contactors' NO contacts wired in series with each other and the main contactor coil, so that:

As long as the BMS is enabled, then each BMS's charge output powers on it's own contactor coil. When either one shuts off charge for any reason, it opens it's NO contacts and disconnects the main contactor coil from any power, opening the charger connection."
 
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