Why not to use Daly BMS with MPPT controllers

[codfish]

I’d like to re-propose my Normally Closed relay solution.
Put a NC relay somewhere in the PV path. Drive the coil of the relay with a crowbar circuit running direct from the battery and set above normal pack voltage but below real damage.
Relay only needs to carry PV amps, no loss under normal operation.
Crowbar circuit only needs to carry relay coil amps ( low cost).
If circuit accidentally trips - no harm - pack was fully charged and it will reset.

This only protects the battery but could be quite low cost.

 

[Davismltc]

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

 

[Zwy]

Hi all
The video posted as the subject of this discussion was posted on YouTube by myself. Ask any additional questions. It is good to see some here have come up with similar possible solutions as I was thinking such as voltage triggered SCR or something like that. I will be investigating these a bit more. Some were asking the string voltage. It was fed with 120v. Esmart 3 cc rated to 150v. High side FET shorted, smallish 100w panels meant then current limited down to 4 amps or so and just kept going. Out of interest I capacity tested of these abused cells last night and it was 230ah out of 280ah. As seen in the video two sets of the four FETs were shorted in the BMS obviously due to overvoltage. This system now has new cells and a diybms with mechanical relays looking after things.
Jason

I've seen a video similar to this before. The first question anyone should asking is whether the charge controller was connected to the PV array without a battery at any time? In the video I saw before, the person had connected the CC to the PV array without a battery and this resulted in damaging the CC. When the battery was connected, the result was high voltage thru the CC and to the BMS.

It could be this is a fault of the CC where it doesn't safely shut down with a battery not connected. Even if a solenoid type of protection is employed, upon reconnect, the BMS could still receive high voltage.

 

[codfish]

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

Put a buzzer in parallel with the relay coil. ( make sure both buzzer and coil are rated for trip voltage).

 

[Davismltc]

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 reason. Anyways, when that happens, the SCC is now connected to the panels with no load!! I don't understand why that protection isn't already built into the SCCs offered. It seems as though the available protection is a sticker on a device or a manual that says "Don't do this or that" or "Always connect battery before connecting solar panels" LOL

 

[Zwy]

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 reason. Anyways, when that happens, the SCC is now connected to the panels with no load!! I don't understand why that protection isn't already built into the SCCs offered. It seems as though the available protection is a sticker on a device or a manual that says "Don't do this or that" or "Always connect battery before connecting solar panels" LOL

For some CC the protection is there, on others which usually are the cheaper CC's, there may not be that protection. In this case, the CC may be at fault as it doesn't offer the protection or the protection might be there but PV array has enough panels in series where cold panels produce a VOC higher than what the CC is rated for.

The fault lies with the CC in the video in my opinion. As to how or why, that needs to yet be determined. Many have tested the higher end products for this protection with no resulting failure, however we don't know regarding the CC used by the person in the video.

 

[mrzed001]

I'm new to the solar panel stuff, but it does blow my mind about how solar devices aren't better self protected.

Because they need to be cheap.
A professional charger/converter like they put into the LG RESU packs cost a lot.
OK it has galvanic separation with around 2.500V insulation potential between the inverter and the battery.
So that would survive a 1.000V solar string MPPT failure.
But would you pay $8.000 for a 10kWh pack ?

I’d like to re-propose my Normally Closed relay solution.
Put a NC relay somewhere in the PV path. Drive the coil of the relay with a crowbar circuit running direct from the battery and set above normal pack voltage but below real damage.
Relay only needs to carry PV amps, no loss under normal operation.
Crowbar circuit only needs to carry relay coil amps ( low cost).
If circuit accidentally trips - no harm - pack was fully charged and it will reset.

This only protects the battery but could be quite low cost.

I like visualization. Could you make a pic (schema, components, connection) from the concept ?

 

[Miles B]

"Why?"
Because battery is expensive (and hazardous.) A single-point failure has been identified: SCC FET shorts. So we're trying to design a failsafe for that scenario.

Yes, believe me, I get that. But the solution is not rigging up some fuse and contactor "hope it works". The solution is making the BMS do the job it is supposed to do. Protecting from overcharge is one of its core functions. Either reduce the PV Voc below destructive range, or build a better BMS. The rest of this "might" work. My application will be marine. If it doesn't work, my boat burns down at sea and I die.

 

[Davismltc]

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..

 

[Just John]

Yes, believe me, I get that. But the solution is not rigging up some fuse and contactor "hope it works". The solution is making the BMS do the job it is supposed to do. Protecting from overcharge is one of its core functions. Either reduce the PV Voc below destructive range, or build a better BMS. The rest of this "might" work. My application will be marine. If it doesn't work, my boat burns down at sea and I die.

Use a contactor based BMS.

 

[mitiempo]

Use a contactor based BMS.

Such as the Electrodacus. And make sure your solar controller is a quality one - Victron for example.

 

[Miles B]

Put a NC relay somewhere in the PV path. Drive the coil of the relay with a crowbar circuit running direct from the battery and set above normal pack voltage but below real damage.

Are you saying put the NC relay in series with the PV? Also I think you are misunderstanding the term crowbar.. crowbar refers to the short itself that is thrown across the current source. I gather you just mean something that opens the relay at a set voltage.

Relay only needs to carry PV amps, no loss under normal operation.

Importantly though, relay needs to switch PV amps without welding.. so a fairly decent relay, but not unheard of.

If circuit accidentally trips - no harm - pack was fully charged and it will reset. This only protects the battery but could be quite low cost.

I don't see any glaring problems with this. My only concern is controlling the relay.. if you reconnect the PV in bright sunlight, you will fry the BMS. The relay would need to be a decent spec, and you would have an extra device to set to your overvoltage level (make it slightly under BMS overvoltage). You would sense the voltage on the SCC output for both disconnecting and reconnecting, ensuring that the reconnect is only done under another set voltage: inside the FETs' Vdsmax. This should deal with any SCC failure. No matter how expensive the SCC, we should always be tolerant of it faulting short. Every power design like this should be tolerant of any one component faulting short and open.

A pretty simple device to make, extremely low power drain, highly reliable. I like this. Allows the continued use of the commodity BMS and solves the problem. It should also be very effective in a multiple SCC installation. As the charging bus voltage exceeds the set level, all devices should disconnect themselves. The faulty SCC output will shoot up, keeping that relay energised. The others will reset as their SCCs drop the output voltage, and reconnect to the charging bus. We would want to be sure that the SCCs are OK with this. Some may be happier with a high power load switched in as the battery is switched out.

 

[Miles B]

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 reason. Anyways, when that happens, the SCC is now connected to the panels with no load!! I don't understand why that protection isn't already built into the SCCs offered. It seems as though the available protection is a sticker on a device or a manual that says "Don't do this or that" or "Always connect battery before connecting solar panels" LOL

The BMS is there to provide fault tolerance for the SCC. The problem is the BMS was not rated for the voltage this PV array could produce in a SCC fault condition, so this choice of BMS was not fit for this circuit. It seems this maximum overvoltage tolerance number is not routinely published for these BMSs.

 

[mrzed001]

Use a contactor based BMS.

I am not sure it can handle it.
Like you have a 400Vdc solar array ... or a hybrid with 800Vdc solar array (at 10-15A).
For that you would need a relay that is designed for both:
- high Amps from battery like 200A
- high Volts from PV like 4-800Vdc
I am not sure that the contactors in the BMS relay (designed for 48V) can handle the 400V ? (even with this lower Amps)

 

[Miles B]

I'm not really aware of the prevalence of high voltage arrays, and the workings of SCCs that handle them. My preference is arrays in the ~100V range. Relays are available, though not incredibly cheap.

 

[mrzed001]

I'm not really aware of the prevalence of high voltage arrays, and the workings of SCCs that handle them. My preference is arrays in the ~100V range. Relays are available, though not incredibly cheap.

All newer MPP Solar (and its clones like Must and EASun) off-grid inverters (for EU) have 450-500Voc MPPT like MGX, the new MKX that comes soon ...
And all the real hybrid (off-grid + grid-tie) MPP Solar MPI (alias Voltronic Infinisolar) inverters have 900Voc MPPT.
Also the US version hybrid the LVX6048 has 450Voc MPPT.
And the US off-grid LV6548 has 250Voc MPPT.
All of them is used with DIY 48V packs

(They use similar high Vdc MPPT tech that the good Grid-tie inverters use like SolarEdge/Fronius/Huawei/SAJ...)

 

[Miles B]

Contactors definitely are available for voltages in that range. They are mass produced for the newest 800V EVs, which has brought the prices down (in volume). Would get pretty expensive to make yourself. I wonder what Fronius and co are doing to protect against this failure mode.

Edit: Gigavac seem to make some relatively affordable 1500VDC contactors. Made in USA!

 

[mrzed001]

Contactors definitely are available for voltages in that range. They are mass produced for the newest 800V EVs, which has brought the prices down (in volume). Would get pretty expensive to make yourself. I wonder what Fronius and co are doing to protect against this failure mode.

EV is a good example. High Vdc and mid high Amps. I will check what relays are used.
Andy's BMS had BYD relay (they manufacture EVs too) and that relay was gone fast (some months).

In the inverters are different modules: MPPT (buck-boost converter) for solar and inverter for DC/AC.
The MPPT part can have 900V but only 10-15A. The inverter part can have 48V and 100A. But nowhere both together (900V and 100A).
I think all are FET based. So no relay. Relays only there like to connect inverter to AC line (like in a Victron)

 

[Miles B]

Yep, I'm wondering what their safety measure, if any, is for short circuit failure of the FETs in their high voltage buck module. Or are they relying on the BMS to disconnect under overvoltage? I would imagine a company like Fronius have thought this through. With the number of units they sell, they would anticipate a few failing in this manner and burning houses down if no protection.

 

[Davismltc]

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 battery in my hybrid car. In that application, the battery volt monitoring was accurate to a tenth of a volt, and might be able to be improved upon.