Unable to read Alternator current

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question

doublebubble asked Apr 8, '23

Unable to read Alternator current

I need some help please, we have had a bmv700 in our boat system for a few years now but have never been able to get the BMV to read the amps coming from the two alternators, every other load, charging system shows up nicely,, its a 12v system with one 800amp battery bank, two 85amp alternators which are grounded through the engine case to the neg side of the batteries with external Hella HPR 3 stage regulators on each alternator, my problem is I can't for the life of me work out how I can get the BMV700 to read the amps that the alternators are putting into the batteries so at the end of the day the BMV is reading a neg amp draw not knowing that the alternators are actually charging the batteries, is there anyway of getting a chassis ground alt to be connected to the load side of the shunt? I'm very confused and need some positive advise.

Rod.

alternator wiring diagram

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Answer 1 · 2023-04-08T02:32:41Z

derrick thomas answered · Apr 08, 2023 at 02:32 AM

The ONLY thing connected to the battery negative needs to be the battery side of the shunt. ALL OTHER connections needs to be connected to the load side of the shunt, then it will read properly.

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Answer 2 · 2023-04-08T04:59:22Z

doublebubble answered · Apr 08, 2023 at 04:59 AM

Thanks but that's my problem, the engine earths are connected to the battery neg, the case of the alternator is earthed to the engine to complete the alternator circuit, if I move the eng earth to the load side of the shunt the engine will not turn over, it sort of does but its as if it has a flat battery.

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derrick thomas commented · Apr 08, 2023 at 05:09 AM

you may just need to use a larger capacity shunt and go ahead and move the engine earth ground to the shunt. Victron sells a 2000 amp smart shunt. I do not know if you can get a larger shunt for the bmv700, but I would guess that is probably the issue with the slow cranking of the engine.

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kevgermany ♦♦ derrick thomas commented · Apr 08, 2023 at 07:00 AM

https://www.victronenergy.com/blog/2019/08/27/battery-monitors-and-monitoring-solutions/

@doublebubble you can upgrade the shunt, see the link.

But unless you have a monster engine, the 500A should handle your starting current. Check the cabling for resistance.

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doublebubble kevgermany ♦♦ commented · Apr 08, 2023 at 07:30 AM

Thanks for that, as I mentioned the engines are little 27hp 3cyl Yanmar 3gm30 diesels which I'd thought would be fine a bad connection could be the issue but all looks to very clean and tidy.

Rod.

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pwfarnell doublebubble commented · Apr 08, 2023 at 08:10 AM

The cable between the battery bank and the shunt will need to be at least as thick as the two engine ground cables combined to avoid volt drop. There is also a possibility that you have a poor crimp on this cable.

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klim8skeptic ♦ commented · Apr 08, 2023 at 06:56 AM

@doublebubble Thanks but that's my problem, the engine earths are connected to the battery neg,

Move the engine earths to the System side of the shunt, not on the battery.

As @derrick thomas has mentioned you may need a bigger shunt should you have large engines to start (or you start both at the same time).

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doublebubble klim8skeptic ♦ commented · Apr 08, 2023 at 07:03 AM

Thanks for the reply, the engines are yanmar 3gm30 which I understand the starter motor pulls a max of approximately 230-250amps on cranking, the engines start first pop with little to no turning over so I would have thought the 500amp shunt would be ok, but maybe I'm wrong.

Rod.

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