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Mine is a 4s (12v) model: JBD-SP04S020-L4S-120A-B-U (v 1.0) The owner's manual they sent me says operating voltage is 14.6v max. I don't know about the 8s (24v) one.

I checked my JBD 120A rated BMS. Its FETs are NCEP40T17A rated at 40vdc (25*C) absolute max limit for drain-source voltage.

There's an Australian fellow on Youtube, "Offgrid garage", that has newer cells. He found running an active balancer all the time was causing more imbalance, as compared to only running one at the upper knee. I think it could also depend on how low the battery is utilized before recharging...

I ended up taking @crossy 's proposed solution a bit further. I had some PC boards made to make an easy build of it and added a couple more features. A miniature SSR can provide control to an active cell balancer at the upper knee of the cell charging graph. ON at (~3.40v/cell) and OFF at...

That's awesome! I only saw the specs on battery hookup. What's the power draw for the combined units together (e.g. the controller relay and contactor together)?

I did find DC-DC buck converters on eBay @ 2 for ~ $1.00 and plan to build the initial circuit with one of those, rather than a zener supply. I don't intend to have battery supply voltage drop below 12v, but will test it down to 10v. Input 3.2v~40v Output 1.25~30v Current 3A (max)

The contactor specs show this as a normally open contactor with relay coil resistance of 3.14 ohms. If the battery voltage is 13.8v for example, wouldn't this be drawing about 4.4 amps continuously to keep the contactor closed during normal battery charging/usage?

You might have a look at Crossy's solution to battery protection at post #191.

Thank you for the offer of assistance. I'll let you know if I need help. I agree it would be redundant and unnecessary to add this to any system if devices never failed. The sad truth is devices fail, and the results can be catastrophic.

I have a JBD BMS and was concerned that it would be subject to the same SCC failure. The specs on the JBD are pretty limited, and my goal is the same as the subject matter of this thread... a failure in the MPPT solar charge controller causing a BMS failure followed by battery failure. The...

What I'm finding on the crappy simulator is that for the Schmitt-trigger resistor thresholds to work at all, the op amp supply and its non-inverting input voltage divider both need to be sourced from the same voltage (e.g 9.1v). If I supply the op amp with battery voltage (14v) and connect the...

Thank you for the advice. I really appreciate it.

That'll be my next step. I don't have a lot of resistor values, etc., so the sim helps me narrow down what I need to acquire. Is it likely that the 7.5v 3.9mA output of the op amp is sufficient to turn on the SSR and allow decent PV panel current to flow? I've never used an SSR before. I...

I have a much smaller solar system on my camper. It has a single 4s LiFePO4 battery and two PV panels, with each panel controlled by its own SCC. I took a stab at modifying Crossy's schematic in the Falstad simulator for the lower voltages in my system. I'm just learning about electronics, so...

Thanks!! The relay in the picture has the name spelled "FOETK"... LOL

What Opto DC SSR 20A would you suggest? I'm looking for one as shown in your schematic above.

I think your on the right track with zinc plated copper buss bars. Zinc will help protect the aluminum battery terminals from galvanic corrosion. Selecting aluminum, or a material that is as galvanically close to aluminum would be best. Zinc is perfect for that. For battery post set screws...

I watched a demo on a Juntek VAT-4300 and the voltage didn't seem very accurate as the current varied (fast forward to 13:37): Maybe it does better when not measuring current?

It seems as though I've hijacked the OP's thread with a fail safe design discussion. I apologize and will stop further comments about it.

One concern I have with the above "Fail Safe" circuit is that once it energizes the relays, it puts a brief load on the battery which brings the battery voltage back in line, and the relay reconnects the PV panel. It will continue to cycle like this, keeping the battery from overcharging, but I...

Does anyone see problems or issues with the proposed "fail safe" solution below to protect the LiFePO4 battery from overcharge? A normally open continuous duty relay is placed across the PV panel terminals, so when energized, it shorts the output of the PV panel A 15A fuse is placed on one leg...

Sounds like the 15A input fuse would blow "IF" the battery was still connected when the shorting relay energized. I'm a newbie, so please bear with me... If the BMS was doing its job, the battery shouldn't be at 3.6v per cell. (or whatever trip voltage selected above normal charge voltage...

Thank you Hedges! One problem I see with my last variation on codfish's idea is in the case of the shorted SCC, the battery will be charged at full PV panel voltage. To fix that I'll propose a mod to the circuit design. I hope you guys don't mind a newbie's ramblings.... A normally open...

How would the "Ideal Diode" be utilized in, or as a "fail safe" circuit? Is it to replace the relay, or ? Could you elaborate a little further?

If at first you don't succeed... LoL Okay, back to codfish's idea...with a little modification: Normally closed (continuous duty) relay in series connection between SCC and the PV panel. Trigger the normally closed relay from a voltage comparator (e.g. via a transistor with enough current...

Okay, I get it. Thanks mrzed001. I thought it disconnected altogether, but I see that's not the case. Still learning...LoL

My take.... SCC fails shorted (this initial failure may have been avoided with the N.O. relay in place, if the cause was a battery disconnect) SCC is shorted and battery continues to charge until BMS disconnects. When BMS disconnects, relay opens, causing the PV panel to disconnect from the SCC...

A more simple approach...no Arduino...no crowbar: Connect a normally open continuous duty relay in series between one leg of the PV and the SCC . Connect one leg of the relay coil to the + battery terminal. Connect the other leg of the relay coil to the BMS P- terminal. As long as the BMS has...

I'm new to solar stuff. Is this common that the BMS would fry upon reconnecting the PV in bright sunlight, or is it the SCC that would fry? I'm assuming the issue is overvoltage? Edit: Never mind... I see in Miles' scenario that the SCC's FETs are already shorted closed, which would then...

Building on Codfish's suggestion... I think an Arduino computer could be utilized to sample voltages and trigger a contactor/relay (instead of a crowbar circuit) for the disconnect. I don't think they draw a lot of power and are certainly cheap. I'm using one to monitor the voltages of the HV...

I think the goal here would be to design, test, and make sure a failsafe works, rather than just "hope it works". On the other hand, I look forward to seeing your "better" BMS working. I certainly hope it works well, and you don't burn up and die..

I'm new to the solar panel stuff, but it does blow my mind about how solar devices aren't better self protected. I would think most everyone would have a BMS on their battery and they generally disconnect the battery to prevent overcharging or under temp or fill in the blank for whatever...

I like codfish's idea...the relay could trigger an alert as well.

One thought would be to connect the crowbar negative directly to battery negative, rather than the BMS negative, avoiding the battery disconnect to the crowbar circuit to allow the battery fuse to blow. I think this would yield the "best case" scenario you described...."hoping the fuse blows...

I see what you mean Miles. Thanks for clarifying. I'm not a EE for sure... LOL