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benmarine avatar image
benmarine asked

Battery Protect as a MPPT 75/15 charge interrupt

Can I use a Battery Protect between the MPPT and a battery, to stop charging in the event of a high cell voltage? (the BP will be switched by a BMS). - I have read that a MPPT can be damaged if not connected to a battery while PV is attached.

I have loads connected to the MPPT. If I use a VE.Direct cable to 'turn off' the charger via Rx (instead of a using a Battery Protect as described above) will the load output also turn off?

Should I avoid either setup, and just use the BP (or a DC contactor) on my PV circuit instead?

Thanks.

MPPT ControllersBattery Protect
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4 Answers
Gary Pacey avatar image
Gary Pacey answered ·

It is perfectly valid to use a battery protect between the battery and MPPT. The only reverse current will be what the MPPT needs from the battery and that is about 1/5 of nothing. Just connect it the right way. MPPT to the "Battery" terminal and the battery to the "Load" terminal on the Battery Protect. The MPPT should be the only device connected. Never use the Battery Protect on the solar side of the MPPT.

This solution is only valid if you do not have loads powered by the MPPT. Unless you have a lighting function, move your loads elsewhere.

The reason to use the Battery Protect is to free up the VE.Direct port for use with a Venus device. If there is no Venus, then just use the Victron VE.Direct non-inverting remote on-off cable.

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Tom avatar image
Tom answered ·

I use my BP65 between my MPPT and battery as a relay switch as a back up overcharge protection.

This Victron VW project shows it used in this exact use along with the Victron MPPTs

https://community.victronenergy.com/articles/15035/victron-vw-van-electrics-install-schematic-drawing.html

I guess if you wanted to you could use the BP65 or BP100 between the PV and the MPPT charger to shut down charging in an over load protection and then the MPPT would never be disconnected to the battery except if you use a master shut off switch when your system is not in use.

I am not sure your concern is valid, but maybe someone else can pipe in.

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benmarine avatar image benmarine commented ·

Thanks Tom, I've also seen Victron describe the use of a Battery Protect in this way (between charger and battery) in the BP user manual. I guess I just wanted to reconcile that against what I'd read elsewhere about disconnecting batteries?

The BP seems to allow current to flow in the reverse direction even when 'off', so the MPPT is always powered, including any connected loads.

If anyone wants to chime in about using VE.Direct to switch the charger off instead, and how MPPT connected loads are effected, that would be great.

EDIT: I've just measured a 0.54 V drop in voltage at the MPPT (multimeter reading corresponds with the Victron app) when the BP is turned off. However, I'm bench testing this and no panels are connected. I'm not sure the consequence of this in practice, but perhaps something else to be aware of.

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Matthias Lange - DE avatar image
Matthias Lange - DE answered ·

Don't let current flow in reverse through the BP!

You should switch off the MPPT via VE.Direct non-inverting cable and connect your loads to the BP.

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benmarine avatar image benmarine commented ·

Victron illustrates the use of a Battery Protect between MPPT and Battery. In that instance, won't current flow in reverse to power the MPPT and anything connected to the MPPT load output? (at least it must whenever the PV does not provide power, ie. at night).

The manual does state that "uncontrolled reverse current will flow through at Battery Protect if Vout > Vin." But is that a problem in this application?

I am aware that I may switch the MPPT with the VE.Direct cable, but my question asked if that also turned off the MPPT load output.

Thanks.

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Justin Cook avatar image
Justin Cook answered ·

Reverse current flow through a BP will cause the BP to fail, and the FETs to weld closed, open, or occasionally half-open. This will cause heat buildup that can melt the BP casing and pose a potential fire hazard. Absolutely, 100%, DO NOT connect a BatteryProtect in any way that might allow reverse current to flow through it.

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benmarine avatar image benmarine commented ·

Hi Justin, thanks for the details, I will exclude that option then! I do think the user manual should describe the same.

I will probably still use the Battery Protect on a 240v charger, which shouldn't require current flow in the way that a MPPT does. I also have a Smart Battery Protect for actual low voltage cut-off duties.

I will probably also use the VE.Direct cable to the MPPT, but does anyone want to answer my question regarding the effect on connected loads, and also whether the use of a DC contactor on the PV circuit is appropriate. I'm having to remind myself, that this is for high individual cell voltage issues only - diagnosed by the BMS - which should be rare!


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Tom avatar image Tom benmarine commented ·

@benmarine when you connect BP100 or in your case the BP65 up to a MPPT the charging output is connected to the "In" "battery" lug and not the load. This is the proper way to install the device as a relay switch and this way you do not have a reverse charge load.

With regard to using a BP with a charger you would connect the same way for an over voltage event. The BP can not be used between an Inverter/charger and battery as in the battery cable connected to "in" "batt" and the ",out" ",load" to the inverter. This is the hazard.

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