DC to DC Solid State Relay (SSR)

[amberwolf]

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.

that's what this note is for :

(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.

 

[harrisonpatm]

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.

 

[LewTwo]

I want something for a programmable charger/tester so my needs are a bit more modest. I ordered a couple of these:
https://batteryhookup.com/collections/accessories/products/te-connectivity-2138622-1-135a-0-450vdc-tyco-12v

Manufacturer's web page has spec sheets and cad files:
https://www.te.com/usa-en/product-2138622-1.html

They should work nicely with Drok VAC9005H: https://www.amazon.com/dp/B09P65M2YF

 

[harrisonpatm]

I got two of the big ones. I like oversized components.

 

[amberwolf]

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.

Well, the original shunt is unlikley to be able to handle the amount of current required, and anything you could do to fix that is likely to mean it's not calibrated to a known value; you'd basically be replacing it at that point anyway.

And you have to move it out of the BMS to the new current path, regardless, so you might as well use a new one that's the right value. (it can't measure the current thru the contactor if it's still in the BMS since that current is no longer flowing thru the BMS)

 

[Mattbastard]

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.

Hello. Not trying to sound pushy here but did you ever whip up a simple drawing of this wiring? This solution sounds perfect for my high-amp pack I'm putting together for my EV Motorcycle. I'm using 5 Headway 38120's in parallel (28 in series) for an easy 600A instantaneous discharge. Thanks.

 

[harrisonpatm]

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.

 

[john61ct]

Cheap for high current AND reliability

 

[LewTwo]

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

Notice that the voltage on the DROCK meter is at nearly 10.5 volts. Another thing to notice is that one cell string was at 2.9 volts and another at 2.1 volts which means he was not using cells that had been tested and selected for matching capacities.

I have some of the same used cells he is using. I had to get 12 cells to find four that were close enough in capacity to be used in a single battery.

 

[Mattbastard]

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.

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.

 

[harrisonpatm]

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.

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.

To be fair, it's not my way yet, I just know that this particular brand of contactor uses pack voltage.

I wouldn't worry about load on the converter. The coil for a 12vdc contactor shouldn't be using more than 5 watts. The one's we've been discussing from batteryhookup use 2-3 watts ideally. How much of a 12v load are you planning?

 

[Mattbastard]

How much of a 12v load are you planning?

Not much at all. This bike is basically gonna be a streetfighter with very minimal electrical. Headlight, taillight, signals, and maybe a horn. All LED should put me around 100W max for everything.

Also, FYI, Heltec sells a BMS that's basically exactly what has been proposed:

https://heltec-bms.com/product/350a-relay-bms-3s-4s-32s-3000a-peak-lipo-lifepo4-battery-protection-board-7s-8s-10s-12s-13s-16s-17s-20s-24s-28s-32s-35s/

Only this is 350A rated.

 

[amberwolf]

Easy to upgrade--replace the contactor with the appropriately-rated unit.

 

[Triketech]

Luna used to carry an SSR but I can't find it anymore. I have 3 of them.

The first one blew when the 52V charger spiked > 60V once. Bought a second and they sent me two (warrantee). So at least I have a spare.

Always disconnect battery ahead of SSR before charging; it seems that even very brief over voltages can fry some SSR's.

 

[john61ct]

Even better, only buy devices that are reliably rated, and for 20+% higher than any voltage it will ever encounter

without the operator ever having to remember anything