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[noob question] will this work?
#1
  • my bms disconnected(don't know why) yesterday while pv is still connected and charging, good thing is i got lucky and my controller is not fried Big Grin . how can i prevent this from happening? like when bms suddenly disconnects also the pv disconnects any solutions how? ive got a diagram using DC SSR but i don't know if this will work please enlighten me.
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#2
Not sure what you mean by your bms disconnected. Are you saying it disconnected the battery from the charge controller?

What charge controller do you have?
What bms do you have?
How is everything currently wired up?
What were the voltages of the Battery when the bms disconnected?
Do you have any idea what the load/charge current was when the bms disconnected?
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#3
You need a bms that uses relays instead of mosfets. I had the same problem before many times, destroyed many 12 volt fans when the bms disconnected while the PV was still producing power. I never damaged the controller but any devices connected to the battery were destroyed by power surges.

I tried many ways to fix the problem, overvoltage protection relays, active balancers, voltage stabilizers, only thing that work was a new bms that uses relays. I use the chargery bms8 for the past year and it solve the problem, if the bms activates it disconnects a relay that is between the PV and controller. No more voltage surges. You need mechanical relays, the solid state relays leak voltage, I would not use those.

I recently bought a makeskyblue 60amp mppt controller that is suppose to prevent voltage surges but havent tested it yet. I don't know of any other controllers that can work with a mosfet bms without causing problems.

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#4
I don't see why relays would be preferred over mosfets. Mosfets can handle a lot more current and can actually disconnect safely under full load. Relays will start to arc and could potentially fail closed.

If you want a safe alternative to mosfets, use Contactors. They are rated for DC current and can handle successful disconnect under high loads w/o causing damage to itself; which means failing closed is very unlikely.

It is possible to rewire any bms to use contactors. Just takes a little bit of surgery. The mosfets basically would be controller the contactors, not flow of current directly
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#5
The only problem with mosfets is they leak voltage. On my system when the bms activated, they didnt completely shutoff, they still read about 11.7 volts but will not accept current. The controller seeing a 11.7 volts see's a depleted battery and tries to force a charge on it, this caused voltage surges as the bms won't let current to flow. The controller will cycle back and forth trying to charge the battery, I see voltage as high as 20 volts.

Relays or contactors will work, since I use a small 240 watt system, all I needed was a small 12 volt 30 amp automotive relay. For my 12 volt system it works good. Larger systems and higher voltages will probably need contactors.

But the best method is to try and not to reach a situation where the bms will activate but even when I made fine tune adjustments to my controller bulk voltage, there were situations where the bms activated.
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#6
(07-20-2020, 04:34 AM)jonyjoe505 Wrote: The only problem with mosfets is they leak voltage. On my system when the bms activated, they didnt completely shutoff, they still read about 11.7 volts but will not accept current. The controller seeing a 11.7 volts see's a depleted battery and tries to force a charge on it, this caused voltage surges as the bms won't let current to flow. The controller will cycle back and forth trying to charge the battery, I see voltage as high as 20 volts.

Relays or contactors will work, since I use a small 240 watt system, all I needed was a small 12 volt 30 amp automotive relay. For my 12 volt system it works good. Larger systems and higher voltages will probably need contactors.

But the best method is to try and not to reach a situation where the bms will activate but even when I made fine tune adjustments to my controller bulk voltage, there were situations where the bms activated.

Correctly connected MOSFETs don't "leak" unless they're damaged. When connected correctly with a gate voltage of zero, very little if any current "leaks". If the gate drive voltage is not done correctly, it might not be "off" properly, Vgs needs to <1V on most MOSFETs.
Sometimes a low leakage current will register on a multimeter (because meter is such a small load) but that current is so small that a 1k resistor makes it effectively 0V.

Maybe changing the config so the BMS's MOSFETs control the panel output not the charger output? No surges, no overcharging either...

Agree the charge controller should not be tripping the BMS "stop charging" setting regularly, only as last level protection.

(07-19-2020, 09:33 PM)rhedbatt Wrote:
  • my bms disconnected(don't know why) yesterday while pv is still connected and charging, good thing is i got lucky and my controller is not fried Big Grin . how can i prevent this from happening? like when bms suddenly disconnects also the pv disconnects any solutions how? ive got a diagram using DC SSR but i don't know if this will work please enlighten me.

That SSR setup isn't quite right - the SSR would be always on.
But if you connected so the PV input controlled the SSR & connected the SSR output directly to the battery + then it should work.
Running off solar, DIY & electronics fan :-)
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#7
(07-20-2020, 10:33 AM)Redpacket Wrote:
(07-19-2020, 09:33 PM)rhedbatt Wrote:
  • my bms disconnected(don't know why) yesterday while pv is still connected and charging, good thing is i got lucky and my controller is not fried Big Grin . how can i prevent this from happening? like when bms suddenly disconnects also the pv disconnects any solutions how? ive got a diagram using DC SSR but i don't know if this will work please enlighten me.

That SSR setup isn't quite right - the SSR would be always on.
But if you connected so the PV input controlled the SSR & connected the SSR output directly to the battery + then it should work.
isnt the battery+ and - must be in input of SSR so that if the BMS in battery disconnects the SSR will turn off thus PV(load) will disconnect?

(07-19-2020, 11:16 PM)Korishan Wrote: Not sure what you mean by your bms disconnected. Are you saying it disconnected the battery from the charge controller?

What charge controller do you have?
What bms do you have?
How is everything currently wired up?
What were the voltages of the Battery when the bms disconnected?
Do you have any idea what the load/charge current was when the bms disconnected?
Its an EPEVER TracerAN
it's a chinese BMS the Red one
just normal setup
i didn't measure it 
its full sun around 11AM maybe at max pv wattage can produce.
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