Battery Protect instead of contactor

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jbakuwel asked Jun 16, '20

Battery Protect instead of contactor

Hi all,

We're considering using Victron 48V / 100A Battery Protects instead of coil operated contactors to protect LFP battery banks for under voltage and over charging conditions.

Power consumption of the Battery Protect is significantly less than contactors (0.1W vs 2W). The BMS would drive the Battery Protect via the remote switch inputs, so we'd not need to use the programmed voltages although we could conceivably use those as well.

Silicon often fails short circuited though, hence the norm seems to be to use a device that fails open circuit, such as a contactor.

Any thoughts?

Jan

Lithium Battery Battery Protect

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Answer 1 · 2020-06-18T04:39:32Z

jbakuwel answered · Jun 18, 2020 at 04:39 AM

I just noticed this in the data sheet of the (48V/100A) Battery Protect:

* The BatteryProtect is not designed for reverse currents from charging sources

It seems to contradict the over voltage protection function of the Battery Protect as that condition can only occur when charging the battery.

Puzzled...

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Answer 2 · 2020-06-18T12:31:33Z

jwfrary answered · Jun 18, 2020 at 12:31 PM

I haven’t used one in the way you describe but I expect that like most solid state relays current can only flow in one direction.

However you could presumably use one as just a charging protection device, for instance from an alternator, as long a no current flowed the opposite way.

Of course you need two of them then, which increases cost, hence there are multiple versions of the Cyrix relay designed for the purpose.

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Answer 3 · 2020-06-18T21:45:51Z

jbakuwel answered · Jun 18, 2020 at 09:45 PM

Hi @Jwfrary,

Thanks for your reply. From what I've read so far it is indeed not possible to use BPR's for both LV and HV disconnects unless you use multiple BPR's and are able to separate the chargers and dischargers. In our case, systems with AC coupled PV inverters, the Multi's can discharge as well as charge the batteries and hence we can't use BPR's for those.

Jan

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Answer 4 · 2020-06-18T23:24:52Z

Kevin Windrem answered · Jun 18, 2020 at 11:24 PM

Contractors that latch in both open and closed states are available. That way, the coil current flows only when changing state from open to closed or closed to open.

Keep in mind that the inrush current from discharged capacitors on the input of the inverter is huge. This inrush can weld contractor's contacts closed and possibly destroy solid state equivalents.

Precharge circuits are often employed that switch a resistor across the contractor's contacts to limit charging current until the voltage to the inverter is equalized with the battery voltage. Then, the main contractor is closed.

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Answer 5 · 2020-06-19T04:54:10Z

jwfrary answered · Jun 19, 2020 at 04:54 AM

@jbakuwel The multi doesn’t really need isolating in the same way if your using a lithium BMS then you can depending on the model either connect with ve.bus and the switching is done for you or connect others via a remote port on the multi and that will provide the switching.

The Battery protects are really for those smaller dc loads

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Answer 6 · 2020-06-19T06:32:09Z

jbakuwel answered · Jun 19, 2020 at 06:32 AM

@Jwfrary, I should really find the post Matthijs wrote where he states to never rely on the equipment (Multi, MPPT etc) for LV and HV protection of a Lithium battery but to always use a BMS with a contactor to isolate the battery in those conditions. A 48V/100A BPR is not really for small loads :-)

@Kevin Windrem, good point. Our main aim is to protect the Lithium bank, ie. to disconnect it as a last resort. This should under normal conditions not happen but if it does, expert analysis will be necessary before the system gets started up again: a) isolate/turn off the inverter/other equipment, b) resolve the issue (for the BMS to disconnect the battery). Then the BMS closes the contactor and then the inverter/other equipment can be turned on.

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