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miya avatar image
miya asked

Strange DC Power Values

Hi All

New yacht install - all new kit. In process of commissioning.

System -> 4x 330Ah Start Lithium, connect in parallel to Lynx Distributor, connected to Lynx Smart BMS, connected to Distributor on other side. Cerbo GX and Touch 70. No loads, no charging devices attached the the final distributor.

This is what the GX Touch is showing - and it looks normal.

just-batteries-connected.jpg

This looks as I would expect to see, battery showing -0.4A and -5W and the DC Power showing 5W - which is about right for the consumption of the BMS, Cerbo and Touch 70. So... all good so far.

I have then attached a Victron Smart Charger 5A - to the distributor on right of BMS - to slowly charge the batteries, very slowly for the first time and balance them.

I see this...

5a-charger-connected.jpgPrefect - the DC Power is now showing -59W as power is going in to batteries.

Next - I have disconnect the small 5A charger and have attached the new Multiplus II for the first time, Connected to the distributor on the right of the BMS. Connected VE.Bus and +ve and -ve DC wires only. No AC connections made at all. Not connected to mains AC and no AC loads connected.

When doing this I would expect to see the DC power be shown as a load. I would expect it to be about 5W for BMS, Cerbo and Touch.

This is where it gets strange....

multiplus-connected.jpg

DC power has changed to a -VE number - suggesting is is being charged with 6W. The AC loads look normal - 16W for the Multiplus - inverting - but idle.

Can anyone suggest / explain what is happening here ?

Kind regards, Tim

Multiplus-IIcerbo gxBMSdc system
just-batteries-connected.jpg (909.6 KiB)
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multiplus-connected.jpg (1.2 MiB)
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seb71 avatar image seb71 commented ·

I have disconnect the small 5A charger and have attached the new Multiplus II

The thing is that in this case "Has DC system" should be disabled in the GX device.

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miya avatar image miya seb71 commented ·
Thanks Seb71 - but that yacht does have DC system - so this should be on - no ??
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seb71 avatar image seb71 miya commented ·
In that case, yes, "Has DC system" should be enabled.


I was commenting strictly on that "test" you made.


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2 Answers
pwfarnell avatar image
pwfarnell answered ·

This issue keeps coming up. The DC power is calculated from the other information available, it is not measured. The Lynx Shunt measures everything flowing into and out of the battery. The Multiplus measures the AC power and from this it estimates the DC power flowing into the Multiplus. The Multiplus does not measure the actual DC power. Therefore the DC power box is an estimate, not an exact number. When you only had the battery plus system losses, i.e. only the DC system running it agreed, not because 2 measurements were the same, but simply the Lynx shunt measured 59W, there was no AC in or out, hence all that 59W was coming from the DC system so the DC box calculated 0 - 0 - 59 = 59.

You also have to realise that in your tests so far you hsve benn running very low loads, less than 1% of the measurement range of the mters, at these low loads, you are prone to zero or offset errors, to expect perfection at low loads is a false expectation.

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miya avatar image miya commented ·
Many thanks - got it.
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JohnC avatar image
JohnC answered ·

Hi @Miya

This will drive you crazy if you worry about it. But I appreciate that you might just want to understand it.

Firstly, the DC Power is a calculated figure, it's not measured separately. So if there's any errors in the readings it's derived from, they get compounded.

I've only ever seen Amps reported to one decimal point. That may not be much at 12V, at 48V the Watts make bigger steps, and at 230V the jumps are very noticeable.

And then the readings don't coincide to exactly the same time. They may only be mS apart, but can result in wild variations.. Like this..

1658062089120.pngMy system today, 48V, with a frequently-switching ac load (maybe varying 70W), no dc load/charge, and less than 60W of solar. It should show a flat zero, but the factors combine to make this. But it's still useful, as I have a dc load come on daily for a while and I can see it above the noise.

This is a 'best effort' thing, and I don't think even VRM attempts to do anything further with that data.


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miya avatar image miya commented ·
Thanks JohnC - totally get it now. Perfect.
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