Saving your charge controller with a Battery Protect and a PV cut-off Solenoid

In a simple low cost shed system using 400 watt PV panel, a single 12 volt 200 amp gel battery, an EPEVER 30 amp charge controller, and a 2000 watt inverter you have low voltage disconnect on the charge controller and inverter that kicks in at 11.1v, way too low to prevent damage to the battery. One option is to add a product like a Victron Battery Protect and set it to shut down the battery at 11.8v. So if the battery protect kicks in one evening because you left a load on unattended and disconnects the battery from the controller, the next day the PV is pumping power to the controller with no battery connected and cooks or worse. Alternatively you may be fooling with the battery cables and forget to disconnect the solar first, and cook your controller.

So can we automatically disconnect the PV as well when the battery disconnects? Can you add a (normally off) car solenoid in line with the PV circuit powered using the battery voltage tapped between the battery disconnect and the charge controller? So the battery protects disconnects the battery, it powers off the solenoid and breaks the PV circuit as well protecting the charge controller, let alone maybe by preventing a fire. Is this a good or bad idea? Is it a standard practice, and I just did not read enough?

Most inverters ive used...about 10x different brands, their shut down voltage maybe 11v or 10v but they start alarming and turning off loads more like 11.8v. I would test yours with a heavy load and let the voltage sag bring it low and see.

Second, anything but $10 controllers arent damaged by a battery being disconnected. Ive done it to epevers and renogys many, many times and for hours. And thats with 400w going into a 20A controller maxing it out.

3rd a battery cutoff from a modest load is going to happen only when theres no charge coming in. Not when the panels are producing power.

Ped said:

Most inverters ive used...about 10x different brands, their shut down voltage maybe 11v or 10v but they start alarming and turning off loads more like 11.8v. I would test yours with a heavy load and let the voltage sag bring it low and see.

Second, anything but $10 controllers arent damaged by a battery being disconnected. Ive done it to epevers and renogys many, many times and for hours. And thats with 400w going into a 20A controller maxing it out.

3rd a battery cutoff from a modest load is going to happen only when theres no charge coming in. Not when the panels are producing power.

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I have read my product specs carefully and there is no mention of any system to alarm me when the voltage is sort of low but above the low voltage disconnect so I’ll try your suggestion to see what it does but doubt anything will happen until 11.1v. Will and every product instruction and you see emphasize connecting your battery first then the charge controller. I’ve assumed you don’t want PV power pouring into the controller with no where to exit. If a disconnected battery and connected solar will cause no short or long term issues with all but the cheapest controllers then why worry which is connected first the controller or the battery? Regarding the last point I did consider that in my example. Say you leave your solar shed in the evening and forget to turn off the inverter, the battery goes low, the PV stays connected, the battery protect cuts the battery power. The next morning the sun shines, PV sends power to the controller nothing gets to the battery as the battery protect stays off. It’s not going back on until that battery is manually charged a bit, the PV can’t get to the battery if the protect trips. Say you leave it for a week with PV on no battery. I’m thinking a better battery protect system cuts the battery and the PV circuit. If you can do that for $8 why not? There may be a reason why not hence the post, but if not I’ll try it.

Yeah that's true for pretty much all inverters and controllers. Theres a reason no one makes a product for this or controllers dont have it built in. Nor is anyone else, or very very few, doing it.

Thanks for the input.

These dead horses have been beaten already...

MisterSandals said:

These dead horses have been beaten already...

Trying to destory my SCC's: I can't do it!

So I took every EPEver/Victron/PWM/Renogy solar charge controller, and tried to fry it by connecting it only to solar panels, and not to a battery... guess which one survived? EVERY SINGLE ONE! I couldn't destroy any of them, and I left them connected to a high voltage array (80 volts) for 4+...

diysolarforum.com

Relays: NC vs NO or other solution?

I am hoping someone can help me find a solution. I am learning about relays and hoping to incorporate one to cut the power between my array and solar charge controller. 400w array, 45A SCC. My "thought" was to use an NO (normally open) relay but wondering how i would power a "close" in the...

diysolarforum.com

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Perfect, exactly what I needed, and is what Ped was saying. But for those who did not see these links and reading about battery first! It was logical to think a disconnected battery would screw up a controller and the answer is, it simply is not an issue.

Donald Siegel said:

Perfect, exactly what I needed, and is what Ped was saying. But for those who did not see these links and reading about battery first! It was logical to think a disconnected battery would screw up a controller and the answer is, it simply is not an issue.

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I do not think tht is is an issue but relize if your SCC does get cooked you will be voiding any warranty if their is one.
That being said I have tried to ruin my MPPT SCC's and can not. even the cheap ones.

Donald Siegel said:

One option is to add a product like a Victron Battery Protect and set it to shut down the battery at 11.8v.

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The Victron BatteryConnect is not designed to:

• Connect to or disconnect high surge loads such as inverters.
• Connect to a line that has the potential for current travelling in two directions e.g. from a battery to a load, from a SCC to a battery.

Using a BatteryProtect in these use cases will, at best, destroy the BatteryProtect. The BatteryProtect product is designed to disconnect low-priority loads from the battery if the battery voltage dips below a programmable threshold:



This topic (i.e. low voltage inverter disconnect) was discussed in great detail on the forum recently and Will has already produced a couple of videos comparing practical solutions (one uses a vehicle contactor, the other uses an opto-relay to soft-shutdown the inverter).

My personal advice is to buy a better inverter, one that has a programmable low voltage disconnect. Failing that, use the BatteryProtect's automation features to soft-shutdown the inverter. Failing that use a Voltage Sensing Relay to switch off the inverter.

Edit: Added 'topic' clarification.

Thanks for the advice and schematic. I posed this as a “shack” system, my loads never exceed 200 watts and run mostly at 110-120 watts. It’s enough to charge phones, run two 15 watt led desk lamps and a PC. There are a lot of low cost, non-programmable but decent charge controllers and inverters that are appropriate for that light load. My Battery Protect is rated at 45 amps, it never sees even 1/4 of that (no printer connected). You can always spend more and get programmable components and the problem is solved. There is a place for low cost, low load systems as well. A single used 250 watt PV is only $50. Add a $35 controller and a $40 inverter, appropriate fuses and breakers, and you can run low end systems safely and economically by staying at 1/3 their rated capacity. So why do all these low cost components decide 11.1v is an appropriate low voltage disconnect when no LA battery should go that low? I’m stuck at home like millions so I will watch Will’s videos on this topic today.

@Donald Siegel, the point being made about the BatteryProtect is that it must never be used to directly disconnect the main DC line of an inverter... doesn't matter if you're within the current rating of the BP or SBP (which will be a 65A, that's the smallest version made), that's not what the issue is. Please see the updated user manual here for clarification on how they may be connected.

The concern here is that when -and it is when, not if- the BP fails because it's being used to connect an inverter, it can fail "closed", meaning it seems to be working until you realize your batteries are dead and it never disconnected, or it can fail "open", meaning that your inverter/other loads aren't powered regardless of the battery voltage, or -worse- it can fail "half-open" which will pass a certain amount of current but not all, which will cause the unit to overheat... sometimes catastrophically. And, after all that, the BP won't be covered by warranty because it's connected in a forbidden manner... thus why it's very important, both for the sake of the BP as well as your system integrity, to immediately remove it from direct control of the DC line to your inverter, and instead use it as a remote trigger for the inverter if you require that level of control.

Thanks plenty to study and change over.

I too have read that the solar panels should never be connected to the CC without the batteries being connected. I don't understand - seems like a proper internal design should have fixed that. But in the mean time I've been reading on different sites about work around fixes with switches or relays. Relays have the potential of adding phantom loads, and both have the potential of setting up a "race condition," possibly through contact bounce, where the battery opens a millisecond before the the solar panels. My suggestion is to permanently add an imitation battery directly across the battery leads entering the CC. This would be a capacitor, possibly a super capacitor (very small compared to the actual batteries), so that when the battery is not connected, the CC sees the capacitor as the battery. A properly sized capacitor should have an unlimited life and otherwise, no effect on normal operations. Does anyone see a problem with this?

Don Solar said:

I too have read that the solar panels should never be connected to the CC without the batteries being connected. I don't understand - seems like a proper internal design should have fixed that. But in the mean time I've been reading on different sites about work around fixes with switches or relays. Relays have the potential of adding phantom loads, and both have the potential of setting up a "race condition," possibly through contact bounce, where the battery opens a millisecond before the the solar panels. My suggestion is to permanently add an imitation battery directly across the battery leads entering the CC. This would be a capacitor, possibly a super capacitor (very small compared to the actual batteries), so that when the battery is not connected, the CC sees the capacitor as the battery. A properly sized capacitor should have an unlimited life and otherwise, no effect on normal operations. Does anyone see a problem with this?

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A battery behaves different to a capacitor.
The battery will clamp the voltage to its cell voltage,
giving protection from overvoltage input. A capacitor will simply rise to whatever voltage is across it.