How much reverse current can a Battery protect take?

Q&A and troubleshooting
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1

Hi there,

I have question in regards to my battery protect, that is set up on the charge side of my system. I wonder how much reverse current it can handle.

Some additional info:
I have a 24V 200Ah lithium battery bank, that is controlled by a small BMS NG. There are two battery protects installed, on for chargers (incoming current) and one for loads (outgoing current).
On the charge side I have an alternator (via Orion tr), a MPPT, and lastly a wind generator from Silent Wind.
Mainly the wind generator always needs some power (pbly less then 0.1A) to be operational and to be able to activate an electronic brake in case the batteries are full, or in wind speeds exceeding its capacity. In other words, the brake is a crucial part of this system.

In general, I noticed that when the battery protect switches off, it will allow some reverse current to the charging devices, this good for the the MPPT and the wind charger to manage their system, it seems to be fine like this, but I would like to know how much it can tolerate in order not to fry the battery protect and or of my wind turbine in the long run.

I am aware of the fact that reverse currents are not allowed, but from design very low loads are tolerated to my understanding. so when does it become a problem?

Ciao, Roman

2

Hi @Roman440

There is no specification from Victron for allowable backflow current.

The backfeed path is very high resistance, so backflow current generates heat in the internal components that the smart battery protect is not designed to dissipate.

There is some small amount that seems to work ok anecdotally, but Victron does not support it and doing so will void the warranty.

Most of the issues we see arise from high current loads and non-supported high inrush devices like inverters.

3

The battery protect datasheet specifies that it uses a MOSFET for switching purposes.
When the reverse current is closing or exceed the MOSFET internal reverse diode, it becomes a problem.
For a 100A battery protect, for sure it uses a MOSFET that can handle more than that.
And if you look at any MOSFET datasheet, you’ll see that the internal reverse diode is usually of the same magnitude as the MOSFET itself, from a current standpoint of view.
So, in your case, 0.1A (or up to 1A), I believe that it won’t be a problem.
But this is just me and I quote from my experience, which can be far removed from the truth… :zany_face: