question

Sten avatar image
Sten asked

Why can’t I use Battery Protect with reverse current?

Hi

I wonder why I can't charge through BP from output to input.

As I understand mosfet can cary the same current reverse as forward.

I do understand the problem with uncontroled reverse current but if the reverse current is controled by a solar charger that can't output more than the BP rating, whats then the problem.


In the BP manual the warning is only for use in battery to battery charging where the current can't be predikted.


"Caution: uncontrolled reverse current will flow through a Smart BatteryProtect if Vout > Vin. Therefore, never use a Smart BatteryProtect for

battery to battery charging."


Thanks.

Battery Protect
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2 Answers
Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

The battery protect limitation applies to reverse currents, whether they are from controlled or uncontrolled sources.

Please check this site for the most recently up to date manual for your product, as this was recently clarified:

https://www.victronenergy.com/support-and-downloads/manuals

I may be wrong about the reasons and am happy to be corrected. I think it’s because when the MOSFETS are off, the resistance increases, but the reverse currents are not blocked. The extra heat generated is greater that the dissipation potential of the heat sink, which is designed for the normal very low resistance dissipation situation.

There is some more discussion about it here: https://www.edaboard.com/showthread.php?258134-Do-MOSFETs-conduct-in-both-directions

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Sten avatar image Sten commented ·

It could be the reason, but try reading page 21 in this document about reverse current in mosfet.

Infineon OptiMOS Power MOSFET Datasheet Explanation - Infineon Technologies


Sorry if this get technical.

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kai avatar image kai ♦ Sten commented ·

The "body diode" in MOSFETs that permit reverse currents is a byproduct of (most) MOSFET semiconductor designs - it is not intended to carry the normal drain current. A small-ish (highly technical term) reverse current is tolerable as the heating effect on the MOSFET isn't enough to blow it up; but I guess more to the point is that this reverse current cannot be controlled by the MOSFET.

p.21 is characterising the behaviour of this body diode. I know it says that the max diode continuous forward current is equal to the MOSFET's continuous current limit. The dirty little secret here is the diode forward voltage drop. When you use the body diode path, you incur the voltage drop. So, if you were to draw say 100A through the body diode, you dissipate >100W into the body diode - and if its sustained more than a very brief period, the magic smoke comes out.

On the statement that MOSFET can carry current in both directions, yes it is technically correct but you need to satisfy the Vgs conditions to saturate the drain-source channel first. Otherwise the reverse current can only go through the body diode. This is a function of the circuit design.

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Sten avatar image Sten kai ♦ commented ·

Thanks. So to use reverse current, the mosfet must be turned on. I was'nt aware of this.

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kai avatar image kai ♦ Sten commented ·

sure. I feel I have to clarify though - when I said its a function of circuit design, i meant that in a general sense, and not applicable to the SBP design. Victron's condition of do not use SBP in reverse still applies.

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randyp avatar image randyp commented ·

Thank you for the link to the New BP, SBP manuals. Reverse current operation is excluded from BP designed usage. Wish I had known this when I bought and used a BP-100 to control battery charging over voltage. Luckily, the BP did not smoke, but it might be damaged, through no fault of my own.

Cannot find an explanation of the d mode of the BP / SBP. Wonder what it is used for ?

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Tom avatar image Tom randyp commented ·

Yes you can view the crispy BPs on the Victron Support pages.

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Tom avatar image
Tom answered ·

There are other electonic non Victron devices (switches) on the market that do what you are looking for, but unfortunately they cost $600. The BP65,100,220 just is not designed for your needs. Also a basic $40 Kilavac contactor with a precharge $2 resistor plus a $10 Chinise programable under voltage PCB will function for your needs (i am assuming not knowing your complete specs).

I removed my bp220 between my battery and multiplus and replaced with the contactor and pcb used as a switch and a third safety low voltage shut down. My BMS is my primary individual cell monitoring device and the Multiplus and mppt settings as the secondary safety.

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