Why not to use Daly BMS with MPPT controllers

[BradCagle]

What you guys think about using this attached over/under voltage relay connected between charger controller and battery, that command an ssr installed between pv and charger? If the charger goes caput and let pv voltage goes past it, this overvoltage relay senses it and close the circuit of the ssr that close the connection from the pv. No more tension past the ssr. Hope it make sense (I have a 48v system but there is also for 12 and 24)

I think that will work.

I'm not sure I'm a fan of the SSR over a Contactor w/economizer.
My thoughts are voltage drop of 1-1.6v across SSR. 1v drop @ 30a = 30w even worse 1.6v drop @ = 48w.
The contactor with economizer is a constant 1.7w

 

[Hedges]

Second relay, contact current at 250VAC, no good for DC.

First one, 24 to 220VDC sounds good, for Voc no higher than that.

Oh, the second one is the over-voltage detection? If it has electromechanical relay contacts (not solid state), then it may be able to switch "coil" of a relay to control DC. Since second one is solid-state "relay", no coil and no inductive kick. Maybe the 250VAC contacts are fine for pilot duty DC. If controlling an actual relay with coil, put a snubber on the coil.

Yeah, that might work. So long as voltage doesn't spike above over-voltage circuit (or BMS) voltage limit before it can respond. Maybe capacitance or some other method to slow or clamp voltage long enough to disconnect.

 

[info.rivendita]

I think that will work.

I'm not sure I'm a fan of the SSR over a Contactor w/economizer.
My thoughts are voltage drop of 1-1.6v across SSR. 1v drop @ 30a = 30w even worse 1.6v drop @ = 48w.
The contactor with economizer is a constant 1.7w

The ssr would be installed between pv and charger controller, so tension passing it will be around 164v at 20amps (I’m using 430w 41v panel in two series of 4 in parallel).

 

[info.rivendita]

Second relay, contact current at 250VAC, no good for DC.

First one, 24 to 220VDC sounds good, for Voc no higher than that.

Oh, the second one is the over-voltage detection? If it has electromechanical relay contacts (not solid state), then it may be able to switch "coil" of a relay to control DC. Since second one is solid-state "relay", no coil and no inductive kick. Maybe the 250VAC contacts are fine for pilot duty DC. If controlling an actual relay with coil, put a snubber on the coil.

Yeah, that might work. So long as voltage doesn't spike above over-voltage circuit (or BMS) voltage limit before it can respond. Maybe capacitance or some other method to slow or clamp voltage long enough to disconnect.

Yes that’s my tough. Contact is VAC but I’ll use that only to interrupt a little circuit going to the ssr that will consequently close the pv supply to the charger controller. The main connection to the overvoltage relay will be 48v DC. I need to test it

 

[BradCagle]

The ssr would be installed between pv and charger controller, so tension passing it will be around 164v at 20amps (I’m using 430w 41v panel in two series of 4 in parallel).

So you'll be burning 20 - 32w in the SSR. I bet you'll need a heat sink on that thing.

 

[Hedges]

I think that will work.

I'm not sure I'm a fan of the SSR over a Contactor w/economizer.
My thoughts are voltage drop of 1-1.6v across SSR. 1v drop @ 30a = 30w even worse 1.6v drop @ = 48w.
The contactor with economizer is a constant 1.7w

Did you find specs somewhere?
Searching part number I find a listing, slightly different picture, still no specs.

Heschen Single Phase DC Solid State Relay SSR-40DD VDC/24-220VDC 40A

SSR Solid State Relay are AC relay, Triac output. The triac version of the zero switching relay is a cheap solution for resistive loads the zero switching relay switch when the AC sine voltage just crosses zero. The switch turn off when the current crosses zero.

heschen.com

I thought DC SRR was MOSFET, so has resistance and voltage drop, power loss depending on current. Potentially well under a volt, and reduced loss during times of reduced PV production.

Yes that’s my tough. Contact is VAC but I’ll use that only to interrupt a little circuit going to the ssr that will consequently close the pv supply to the charger controller. The main connection to the overvoltage relay will be 48v DC. I need to test it

Driving inductive load, something like RC snubber always helps. For SSR, probably no problem driving its electronics with AC rated contacts. If "contacts" were electronic, like triac which requires zero-crossing to turn off, then of course it would latch on.

 

[info.rivendita]

So you'll be burning 20 - 32w in the SSR. I bet you'll need a heat sink on that thing.

That’s a good point, I may be using your type of contactor placed before charger controller activated by the overvoltage relay, if working as expected. This particular ssr was 40a rated, there is another one rated for 100a, so should be warm but not that much I hope

 

[BradCagle]

Did you find specs somewhere?
Searching part number I find a listing, slightly different picture, still no specs.

Nah, I knew there must be a drop, so I did a google search on SSR voltage drop. I got 1 - 1.6v from that search.

Just hypothetical on my end as I've not used them.

 

[BradCagle]

So researching SSRs a little more. If you get one that is DC only then you should have a lower voltage drop as it will use transistor, or mosfet. The SSRs that are bidirectional, or AC will use a triac which has a higher voltage drop.

 

[Hedges]

A DC SSR could be bidirectional, would probably require additional MOSFETs.
Need data sheet to know how to use that one shown. Both input and output have + and - signs. Hopefully, "-" isn't common between them (which is what just driving MOSFET gate without isolation would do.) If so, it could only be a low-side switch. Plan here is to use voltages in battery range to drive monitor and input, with output opening circuit between PV+ and SCC input. "SSR" does imply isolated, so probably OK.

3 to 32VDC input. Monitor is for 48V nominal, while SSR input is good for nominal 24V. Some sort of regulator is necessary. IC regulator most likely, although resistor/zener/capacitor could work. Important to provide the drive strength to switch MOSFET gate fast (minimizes power consumption.) It may have its own decoupling and a boost converter, considering the minimum 3V input would never turn a MOSFET on hard enough. Or not, depending on its internal architecture.

 

[info.rivendita]

My plan was to power + and - in the input of the ssr using the main battery with a stepdown to bring 48v to 24/12v or with a stand alone 12v battery. + and - of the output seems like to be just flow indicators, + should be the way in and - the way out, but is hard to tell as I don’t find any specification. Here my goal is just to cut the pv line to also save the BMS. The battery overvoltage controller is a really nice system, but if the tension at the battery is going that high the BMS mosfet has been already fried

 

[TheAlmightyOgreLord]

Sorry to revive this thread...

I have a 150amp Daly "dumb" BMS. No temp sensor, just balance leads. I originally had an Overkill BMS but the MOSFETs melted with a green residue. Complete load failure. I switched back to my Daly BMS and instead replicated the features of the Overkill BMS (as much as I could) by adding simple normally open high current DC relays powered by the BMS itself. (This prevents long term discharge from power draw of the relays and sensors, the BMS will still cut out the entire system if left too long)

The charge current relay is between the BMS and the MPPT controller, and is powered by a solid state temperature sensor that thinks it's "cooling the battery" and turns off when it gets down to 3°C. The other normally open relay is between the inverter and the BMS, with a 3-mode boat switch paired with a resistor for pre-charging the circuit. This load current relay is powered by a solid state voltage sense board programmed to disconnect at pack voltage of 12.6V and reconnect at 12.8V (but requires pressing a remote fob to turn on the normally open load relay, its one of those anti theft remote on/off relays) which prevents reconnecting the inverter capacitors on accident before I can switch to the precharge circuit.

Now if I just simply power the solid state temperature sensor (which controls the charge circuit relay) with another solid state voltage sense relay ($12 a piece) and program it to disconnect at 13.8V and reconnect at 13.6V, would that not essentially protect the entire battery (and Daly BMS) from over voltage?

I was intending to use the extra voltage sense relay I have on hand to control the minimum balancing voltage of my Heltec cell balancer, but after reading this thread I think it's more important to protect the charging circuit from over voltage.

The only thing I see not working here is if lightning hits my solar panels, then this wouldn't protect the MPPT solar charge controller. Which I think could be done on the PV side with my setup, using a circuit breaker?

This is a 12V system by the way, although I think the same setup would work for 24V with correctly rated appliances

I like how everything is isolated, and I can easily bypass the BMS if it fails, while also barely relying on any given BMS so it lasts a lot longer. And if I add a rasPi later to automate the precharge circuit around the manual 3-mode boat battery switch, I will always have a manual fallback that's robust and hard to break. In fact, I think this might be better overall than my 120amp Overkill BMS, as much as i liked being able to program the parameters. Especially after discovering that almost no BMS on the market provides a pre-charge function.

 

[BenQ]

Damn. That sounds like it’ll do the job! Also sounds like a bitch to set up but totally worth it if you’ve got the time and inclination! Good luck buddy!

 

[TheAlmightyOgreLord]

Damn. That sounds like it’ll do the job! Also sounds like a bitch to set up but totally worth it if you’ve got the time and inclination! Good luck buddy!

Thank you! Each and every terminal contact, including the battery posts and busbars are wire brushed and coated with dielectric grease. I'm looking to go maintenance free for at least 5 years. Glad to see I'm on the right track!