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Victron BMV and DC-DC converter show different voltages?


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Just added a DC-DC converter so that our "dumb" alternator regulator can charge a small lead-acid start battery and then the DC converter can charge the lithium house battery with proper voltage setpoints.  All is well, except the battery voltage for both the start and house batteries shown by the DC converter are very different than what the Victron BMV shows.  This is with zero current and proper cable sizing over fairly short runs, so voltage drop shouldn't be an issue.  The DC converter shows 0.2V lower numbers than the BMV, and the BMV matches a multi-meter at the battery terminals pretty closely.  Kind of an issue, since the whole point of the DC converter is to optimize the charge voltage setpoints.  Defeats the purpose if the setpoints end up off by 0.2V.  

Any ideas?

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I suspect the converter just wasn't built that well (and has no way to adjuster readings) I would trust the DVM readings, esp if it's a quality meter (not some harbor freight type). Perhaps check numbers against another good DVM. Them apply the delta to the converter settings & double check the outputs (at the battery)

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Sounds like the DC converter LCD screen may be wired direct within the unit which may account for the voltage difference?... 

It is measuring incoming voltage minus its own power usage. 

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The DC converter doesn't have a screen, but connects to a phone via Bluetooth for display information.  Your comment about powering the unit may still be correct.  

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Could be a capacitor voltage suck or power LED or bluetooth/powered signal transmitter process voltage drop within the unit for sure..

If there is only 0.2v difference i would(unfortunately) just take note and factor it into living with it. Technology...Always “entertaining” hey.. lol Sorry and goodluck. 

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They are different as expected.  The incoming voltage is whatever the current voltage is off the starter battery, the output voltage is 0.2V lower than the house battery.  I didn't check to see if the incoming voltage was accurate as it isn't critical.  But as an example, the input voltage might be 14.6 (engine on and alternator charging) and the output voltage 14.2.  Or if I turn off the engine on requirement the input voltage might be 12.5 and output voltage still 14.2.  That doesn't work so well, since it will drain our tiny 20Ah starter battery in a hurry at 30A, but with the engine on requirements it works great.  

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Then I think you have a voltage readout that is off. You will need to monitor actual voltage into batts while adjusting 'set point' voltage in the converter

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Use a multi meter you can trust (recently check calibrated) and take measurements at the BMV and the DC-DC converter and check the differences against the displays!

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At 2mins 19seconds.... May explain the loss of 0.2V..(The converter will do the same i’d guess.)

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On 2/28/2021 at 10:32 AM, Roleur said:

BMV matches a multi-meter at the battery terminals pretty closely

Well that is 2 votes against one. Ignore the DC-DC converter.

Course you could get a 2nd multimeter and check the first multimeter.... :)

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I don't understand the setup.....  your goal is to charge the starting battery and DC-DC 30A to the lithium bank?    That seems way backwards.   The alternator should direct charge the house bank and a DC-DC for the starting battery.   You are leaving a lot of amps on the table so to speak.

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With an OEM internal regulator for the alternator, there is no good way to charge a lithium battery directly.  We started down the path of trying to just add an external regulator for the Hitachi alternator, but when Balmar said we needed to take the alternator apart to solder a wire, or take it to a shop, we bailed on that idea.  Living on an island we can't just drive over to a "shop".  The DC Converter was within my DIY capabilities.  

Yes, 30A is minimal, but for 99% of our use, it is plenty as the house battery has very little load.  For long offshore races the plan is to add a second DC converter in parallel, so we can get 60A of charging.  That plus 300W of solar will get us to minimal engine run times. 

My main concern here is avoiding overcharging or improperly charging the lithium battery.  That and weight savings.  Having a tiny start battery, lithium house battery and DC Converter, with an OEM alternator seems like one of the lowest weight options.  

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How big is the alternator?
Is the DC-DC converter set up to stop charging the house after the source voltage drops below 13.5 vdc or from the engine key switch?
What other circuit level protection do you have for your lithium batteries? e.g. disconnect on low voltage, etc?

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What make and model sailboat?

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6 minutes ago, b393capt said:

How big is the alternator?
Is the DC-DC converter set up to stop charging the house after the source voltage drops below 13.5 vdc or from the engine key switch?
What other circuit level protection do you have for your lithium batteries? e.g. disconnect on low voltage, etc?

This is on a J/111, strictly for racing.  Yes, DC converter stops when voltage drops below charging voltages.  That works well.  No LVD.  Not really concerned about low voltage as there is little load.  Just concerned about overcharging and weight.  I think the alternator is 75A or 80A.  It's an OEM Hitachi on a Yanmar 20hp.  The DC Converter has the added bonus of not maxing out the alternator when charging straight to the lithium battery without a smart regulator.

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J/111, sweet ride!  I have a 109, not too far different.

Does the BMV send out a signal to the Victron DC-DC to prevent charging a dead lithium battery?

There are so many ways lithium batteries can cause you grief. LVD is a very real consideration to include in your DIY work, as discharged lithium ion batteries need so much TLC you don't want them accidentally discharged.

You didn't ask, but every DIY needs to read this from Mr. Collins, if you have not already:https://marinehowto.com/lifepo4-batteries-on-boats/ 

I had the great joy to speak with Mr. Collins, he is an expert in the area.


 

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