DC to DC Solid State Relay (SSR)

harrisonpatm said:

Just thought of one more potential downside:

3. Since you're bypassing the bms to get higher current draw than what the bms is rated for, you're also bypassing the BMS's overcurrent protection that would shutoff battery if too much current is being drawn. Hopefully that's obvious to anyone trying this mod, but I wanted to list it in case someone reading the thread hadn't thought of it.

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(Also, to protect against external overcurrent/shorts a current detection device to shut off the current thru the coil would work, or if the BMS has a shunt and can do overcurrent detection, you can remove it's shunt off the BMS and install a new one into the main (negative) current path and wire thin wires from that to the point on the BMS where the shunt used to be so the BMS can detect the current and trigger correctly as needed. The new shunt would have to be able to handle the high current, with a resistance proportional to the original shunt such that the current it detects would be proportional for the BMS to correctly respond. Meaning, if the BMS can only deal with up to 10A, but your system has to deal with 100A, then the new shunt needs to be 1/10th the resistance of the original shunt so the voltage across it is within the limits of the BMS to react to.

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harrisonpatm said:

Ah yes. Does your paragraph describe how to modify the original bms's shunt to respond to overcurrent? So that you don't have to install a secondary device, right?

It went a bit over my head the first time I quickly read it, now I think I get what you mean.

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amberwolf said:

That is a heckuva deal, looks like they can do USPS flat rate shipping (about $8 to me here for a pair of them...very tempting) so it's pretty cheap. (if I didn't already have some other old used contactors out of a powerchair lift and someone's scrapped scooter build that will probably work to build into the next iteration of my power control system(s) I'd splurge).

Tyco Kilovac contactors are a "standard" in various power control applications; I've seen them in all sorts of stuff, not just battery controls, including powerchair (and other even heavier-duty) lifts built into vehicles, used to control the motor direction / engagement, or to turn on/off hydraulic pump motors for the really big lifts.


What I find even more interesting about that page is the info on how to use one "in place of" the BMS FETs to control the battery current, without any hacking of the BMS; I find it clever and never thought of it and feel dumb for not having done so.

I quoted their instructions below, edited for layout and relevance, and notes added where necessary), and if I remember to do it I'll make a simple drawing to show what they mean:

Make a cheap 30a BMS into a 500a BMS.
This will work for batteries in the 9-36v range so perfect for Lifepo4 and Lithium Ion 4s-8s. (higher voltage systems would need some way to limit current thru the relay coil if it's just a coil, or limit voltage to the coil if it has built-in drive electronics that have voltage limits, but they can be used the same way if that is added)

Wire your BMS like normal except the C- that goes to your charge and discharge will go to the black wire on this relay. The red wire on this relay will go to the positive of the battery.

Now your main positive wire for charge and discharge will go right to the battery positive (which is how most packs are already wired) and the 500a relay will go in line with your main negative wire.

When the BMS is on it will trigger this 500a relay on. When it detects a problem the BMS will shut off and trigger the 500a relay to shut off.

You will want to add a fuse in line as well to protect from an external short or fault that is not battery related. Let's say a charge controller breaks and causes a short, you need a fuse to protect your battery from that. Do not use this with a BMS without adding a fuse because you never know what equipment could fail. Chargers, charge controllers, and inverters can fail and you need to protect the battery against that.

(Also, to protect against external overcurrent/shorts a current detection device to shut off the current thru the coil would work, or if the BMS has a shunt and can do overcurrent detection, you can remove it's shunt off the BMS and install a new one into the main (negative) current path and wire thin wires from that to the point on the BMS where the shunt used to be so the BMS can detect the current and trigger correctly as needed. The new shunt would have to be able to handle the high current, with a resistance proportional to the original shunt such that the current it detects would be proportional for the BMS to correctly respond. Meaning, if the BMS can only deal with up to 10A, but your system has to deal with 100A, then the new shunt needs to be 1/10th the resistance of the original shunt so the voltage across it is within the limits of the BMS to react to.

The relay can handle up to 500 amps and works up to 900vdc but the trigger wires work between 9-36v which is perfect for all 12v and 24v systems.

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harrisonpatm said:

He didn't draw a diagram for it, but jehugarcia did a 12v system and contactor, and if you pause at 5:25 you can eyeball what he did. He put the contactor on the main battery (+) cable. Plus his is easier in that he used a 12v system, so his full battery voltage was fine for the coil voltage.
https://www.youtube.com/watch?v=J2KH21Fwi9o

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harrisonpatm said:

He didn't draw a diagram for it, but jehugarcia did a 12v system and contactor, and if you pause at 5:25 you can eyeball what he did. He put the contactor on the main battery (+) cable. Plus his is easier in that he used a 12v system, so his full battery voltage was fine for the coil voltage.
https://www.youtube.com/watch?v=J2KH21Fwi9o

On that note, the contactor I used for my build was the MZJ 400 amp.
https://www.amazon.com/Geofrey-72V-Compatible-Replacement-MZJ-400A/dp/B0B4BBQ2WS/ref=sr_1_2?crid=3ML0MWCQ1P8WX&keywords=400amp%2B72v%2Bcontactor&qid=1665150917&s=instant-video&sprefix=400amp%2B72v%2Bcontactor%2Cinstant-video%2C151&sr=1-2&ufe=app_do%3Aamzn1.fos.d977788f-1483-4f76-90a3-786e4cdc8f10&th=1
The difference with this one is the coil uses the same voltage as the rating. I.E., if you get the 72v contactor for your 72v pack, you can wire the bms to activate the coil directly with full pack voltage. Yes, it's more expensive, but you don't need to work out a way to trip a contactor's 12v coil off a higher pack voltage. Just an option, if that's the way you want to go.
Edit: you said you wanted a 600amp pull, so you'd need the MZJ 600 or 800 amp version, $125-135. Yikes.

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Mattbastard said:

harrisonpatm said:

He didn't draw a diagram for it, but jehugarcia did a 12v system and contactor, and if you pause at 5:25 you can eyeball what he did. He put the contactor on the main battery (+) cable. Plus his is easier in that he used a 12v system, so his full battery voltage was fine for the coil voltage.
https://www.youtube.com/watch?v=J2KH21Fwi9o

On that note, the contactor I used for my build was the MZJ 400 amp.
https://www.amazon.com/Geofrey-72V-Compatible-Replacement-MZJ-400A/dp/B0B4BBQ2WS/ref=sr_1_2?crid=3ML0MWCQ1P8WX&keywords=400amp%2B72v%2Bcontactor&qid=1665150917&s=instant-video&sprefix=400amp%2B72v%2Bcontactor%2Cinstant-video%2C151&sr=1-2&ufe=app_do%3Aamzn1.fos.d977788f-1483-4f76-90a3-786e4cdc8f10&th=1
The difference with this one is the coil uses the same voltage as the rating. I.E., if you get the 72v contactor for your 72v pack, you can wire the bms to activate the coil directly with full pack voltage. Yes, it's more expensive, but you don't need to work out a way to trip a contactor's 12v coil off a higher pack voltage. Just an option, if that's the way you want to go.
Edit: you said you wanted a 600amp pull, so you'd need the MZJ 600 or 800 amp version, $125-135. Yikes.

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Thanks for the link, I see what's up now. I had a feeling it was something to that effect, this confirms it.

I will be using a 12 VDC converter for the accessories on my project so I'll probably go with a 12 VDC contactor, but I see your point with going to pack voltage. I may still consider it. Your way seems cleaner, and less load on a converter.

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harrisonpatm said:

How much of a 12v load are you planning?

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