Overvoltage protection relay - high voltage, high current

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harrisonpatm

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I'm assembling a higher-current charger for my bike for faster charging. Based around an 1800w boost converter, which I will only be taking to 1400ish watts, for headroom. Protections already in place:
-I've reliably and frequently tested the overvoltage protection on my battery's BMS, it cuts off charging at the first sign of cell-level overvoltage.
-Fused at both the input and and output of the boost converter.
-I've tested the boost converter, and it stops delivering current once the battery comes up to its set voltage

Despite all that, I would still like one more level of protection. So, does anyone have a recommendation for an overvoltage relay switch? I would like it to be in line with the boost converter input, and reading the output voltage. Therefore, if output voltage goes above the set parameters, it will switch off the input side.

My system is 24s LFP, and I will be setting the charging voltage at 84v, so I would like to be able to set an overvoltage relay to 85v. My searches so far have only shown me protection relays in the sub-30v range. I'm planning on less than 20 amps of current. Thanks in advance.
 
OffGridInTheCity

OffGridInTheCity

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I'm not aware of equipment that does what you're asking but I have a different thought.

Could you do a full charge / monitor the max voltage the charge attempts? For example, the iCharger X8 let's one export metrics thru it's USB port and you can put them up on a graph to see what it was doing during a charge cycle.

If you could somehow track the voltage behavior of the charger - it might give you confidence it never goes above 84v and then you wouldn't need this extra level of protection. Then in the future, if you get a new charger, you can use the same method to verify it.
 
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harrisonpatm

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To be honest, I have a fair amount of confidence already that the BMS will prevent overvoltage. If you're saying that I wouldn't need the extra level of protection in place, that's fair. I probably don't. I am just being intentionally overcautious, seeing as this is a high-power charger on a high-power battery.
 
OffGridInTheCity

OffGridInTheCity

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I get it as I don't depend on BMSs alone but rely on voltage reporting and operational techniques as my primary defense to avoid max charging.

For example, I don't charge above 4.0v (li-ion) on the powerwall or 4.1v (li-ion) on the trailer battery and I'm sure about it because I trust Batrium longmon pack level voltage reporting along with DIY software to send me alerts which I monitor constantly on my desktop in my office.

For my winch battery with headways, I don't even have a BMS as this is a 'use/charge twice a year' kind of thing and I have a BattGO to show me voltage/cell and can use the iCharger X8 as the charger in which I have good confidence from much use (and voltage graphing) - e.g. this is what prompted my comment above.

Recently swapped out 4 x 12v AGMSs in my E-Wheel scooter with LifePo4 12v and it does bother me a bit that I don't have a way to monitor what's happening beyond the BMSs in each 12v LifePo4 battery. However, this scooter has only been out twice in 3 years - so again, it's not a charge often situation. Its more about LifePo4 will be able to sit for years and not die like AGMs.

I'll be interested to follow what you settle on :)
 
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Korishan

Korishan

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harrisonpatm said:
does anyone have a recommendation for an overvoltage relay switch?
Click to expand...
I don't know of any device as such, as OffGridInTheCity states. However, you could technically build one.

You would need a beefy relay as it needs to be able to handle high current during disconnection. Then you'd use some low level logic to determine voltage. Some resistors, a diode or two, and a Mosfet (recommended, but a regular transistor probably would work too) and when the voltage reaches a threshold, the diode allows reverse flow which then triggers the FET which throws the relay.

As for the relay, I would recommend either one that requires power to stay engaged, or (actually better imho) a latching relay. So that it can be easier to be triggered. Altho, that would require a little more circuitry as it would need to detect which state the relay is in already, as you don't want it triggering it "on" when it's already been thrown "off"

If you want one a little more smarter, you could program an arduino to monitor and then trigger the relay through a FET. This would allow easy overriding of the logic if needed
 
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harrisonpatm

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Actually, this is pretty great. Seems like I should be able to measure the charger output (ignoring any of the battery capacity settings or functions), and set it to cut off the relay at my specified charging voltage. Also, the bike is outside, and I bring the monitor inside, connected wirelessly. Should be able to not only monitor the state of charge, but also start and stop charging. Might purchase the meter for that purpose alone. Thanks!
 
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