BMS: MOSFET vs RELAY and SoC for parallel strings

Hi. I am considering building a 48V Victron-based system (Multiplus-ii charger/inverter and DC Solar-charger) with DIY lifepo4 battery bank - 3 strings of 16S in parallel - and have a few questions which I cannot find the answer for:

1) Assuming it is recommended to use a BMS that can disconnect the battery in case of an alarm (e.g. and not just rely on the inverter and charger to stop) - some BMS use MODFETs for switching and others use RELAYs ( https://www.alibaba.com/product-detail/JBD-High-Quality-BMS-20S-200A_1600459528836.html ). What is recommeneded? The MOSFETs in BMS dissipate power as current passes through the MOSFET or is this negligible?

2) The Victron system wants to know the total SoC so that it can do its thing. Is it prefered to track the SoC seperately per 16S-string (via BMS) or is it sufficient to track the SoC for all 3 strings combined (using a single shunt)?


Thanks!

sebdehne said:

The MOSFETs in BMS dissipate power as current passes through the MOSFET or is this negligible?

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I built a switch using mosfets to turn my solar array on and off with a digital timer. The power loss is not enough to be detectable with my equipment. A mechanical relay consumes a noticeable amount of power and a solid state relay is generally made from mosfets. Mosfets can short out and stay on and relay’s can fuse closed and stay on under similar conditions. I would be more concerned with the quality of the bms rather than the switching method used. Find out what others recommend based on experience rather than price alone. The best designed electronic device is lousy if it is poorly manufactured and uses inferior quality parts conversely the highest standards of manufacturing and best quality parts will be completely destroyed by faulty design.

Bobert said:

A mechanical relay consumes a noticeable amount of power and a solid state relay is generally made from mosfets. Mosfets can short out and stay on and relay’s can fuse closed and stay on under similar conditions.

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Relay's/contactors that have economizers don't use very much power, probably comparable to the power loss in mosfets. The ones I use have a 1.7w currrent draw, can switch 900v, and 500a.
BMS's that use relays use this type, and some use latching relays that require no holding power.

The benefit to relays is if your charge controller decides to murder your batteries by shorting the PV directly to the battery, the relays can handle shutting this down, where the mosfets in BMSs probably cant with high PV voltage.

The down side to relay BMS is it can't control charge/discharge independently (well I suppose there are some with separate charge, and discharge relays)

Good MOSFet switches are robust

But some of the best MOSFETs have an ON resistance of 0.7 milliohm this leads to appreciable heating at say 100A. Secondly the cheaper mosfets will have max voltage of 40 v. Higher cost units can survive 80-100v

The trouble is then inductive spikes. Coupled in lightening spikes etc can take out mosfets switches whereas relays can be much more rugged. Mosfet heat buildup must be carried away so that the junction temperature never gets crutical even under all extremes of ambients.

The upside is high interrupt ability, extremely low power switching etc.

It’s a “ call”

You can easily have charge and discharge relays. But I never see the point of separate buses.

I agree with @BradCagle. But the BMS OP listed will charge after opening from LVD. It has a tiny circuit, mosfet connected that goes around the relay that detects higher voltage input voltage and closes the relay. Off grid garage has a vid demonstration of this.