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nickdb avatar image
nickdb asked

Multi Inconsistency in reported DC voltage

I have recently noticed an inconsistency between the DC bus voltages reported by the Multi (5kva 230/48) and the battery's BMS (BSL).

On VRM (and the GX BMS tab) I could see the BMS reporting just on 54V but the multi was reporting 54.5V, 0.5V higher.

The MPPT's reported the same, which makes sense as they are using shared data anyway.


I checked connections and fetched the trusty fluke meter. This reported 54.2V at the Multi and 54.1V at the battery terminals, that is what I would expect, yet the inverter is reporting much higher.

Is this an iffy voltage sensor?

I checked again this morning under similar loads and they were all agreeing on the reported voltage, so this somewhat inconsistent.

Any thoughts as to the cause? I have had some unexpected charging behaviour so went looking for reasons why.

Multiplus-II
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3 Answers
kevgermany avatar image
kevgermany answered ·

Could be, could be a Dicky connection or other internal fault. Guess you need to talk to your local friendly dealer.

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JohnC avatar image
JohnC answered ·

Hi @nickdb

I used to see this. Not so bad though, just 0.2V to sometimes 0.3V. Inside the same case of an Easy 5000. So can't be my wiring.

DVCC fixed it, and I get my V now from a Smartshunt, which was a great match with my multimeter (and the mppt). I don't have a bms, but if that were off I'd be concerned.

I have no idea why the Multi's V might be wrong. Mine may still be, but it isn't used any longer so I don't care.

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nickdb avatar image nickdb ♦♦ commented ·

Before re-doing some wiring I took some thermal images to highlight how losses can creep in. The second shot shows almost a 7C difference between the lug collar and the terminal bolt under around 70A. The negative terminal connecting to the multi is the warmest.

20220429t080306.jpg20220429t080057.jpg


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nickdb avatar image
nickdb answered ·

I am circling back to this post, having done a bit more troubleshooting after an mppt charging issue.

The differential increases under load, so this implies a wiring issue, yet there isn't any obvious heat build up (I will be doing a proper thermal check when my FLIR arrives).

Cable type and gauge is well overspecced and connections are properly torqued.

Yet if I whip out the meter and measure voltage along the same supply, I measure about 100mV per connection @60A. Over the 3 connections per +'ve and -'ve feed, this adds up to 0.6V.

I do find it hard to believe that the lugs were that poorly done, and I don't have a decent reference for what "typical" loss is - since every connection will inherently add some.

Nonetheless, it will be time to dispose of the fong kong stacking cables, and redo the battery feeds to remove as much of this as possible and hope it improves.

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JohnC avatar image JohnC ♦ commented ·
@nickdb

Could be lug crimps too. I grabbed a cheap hydraulic crimper recently for a one-off stainless steel project. Then tried crimping some Cu wires/lugs with it. Wow, it's now my favourite tool for real crimps.

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