zed-must avatar image
zed-must asked

Flat batteries - even on shore power

Hi, i have a new boat with a new multiplus 3,000 inverter charger + mppt solar controller. On 3 separate occasions we have returned to the boat after 10 days away to find our 550ah battery bank completely flat. Our consumption when not on board is low - around 10a. Solar should keep well ahead of that, but last time we went away we left the shore power plugged in to be sure. When we returned the batteries were again flat! The shore power was on and working with no reported outages.

The only error on the multiplus is a ‘temp sense error’ which only happens when the unit is switched on.

If we stay on board for a week or so we monitor the system daily and see that the batteries are kept at close to 100% on either shore power or engine alternator. We are now terrified of leaving the boat with even the bilge pump on for fear of returning to a dead boat again.

A separate problem is that the mppt has stopped working but we think thats a faulty unit so thats going back to victron.

Any ideas? Thanks

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1 Answer
Paul B avatar image
Paul B answered ·

Number 1. you should be looking at Battery Voltage NOT SOC. to get the correct battery status

Battery voltage will always give you a correct battery level and then compare this to the SOC and read up on how to get the SOC working correctly from There, this will depend on what battery monitor you have.

on a 12 volt lead Acid Battery system with No charging or power draw for 1 hr a full battery will read 12.7 volts, in its current condition.

down to around 12.2 -12.3 which is around 50% discharged

The Victron MPPT regulators are very solid units so this is unlikely to be faulty. if its a smart solar unit then check it out using the victron connect app, if its a bluepower unit I would strongly suggest you purchase a bluetooth dongle to suit,

These give you 30 days battery voltage history and the solar output levels and much more

Also have a look at this post re the SOC if you have a BMV bat Monitor that is

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zed-must avatar image zed-must commented ·

Thx, i dont know what you mean by SOC? See attached pic which shows voltage was good up to one point when everything went dead. I have no idea why. Again, mains power was plugged in at the time

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klim8skeptic avatar image klim8skeptic ♦ zed-must commented ·

"i dont know what you mean by SOC?"

SOC = state of charge, you state that the batteries were kept close to 100% (?).

A few things to note from the screen shot of the smartsolar.

Days 9,10,11 the batteries where never fully charged going by the lack of absorb/float state, and the min/max voltage. The solar produced about 100ah.

Day 8 the loads were turned off (?) and the batteries finally charged up. However the absorb time looks too short. And it looks like the smartsolar was disconnected from the battery (?) going by the minimum voltage.

What happened to days 1-7?

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zed-must avatar image zed-must klim8skeptic ♦ commented ·

Thanks Klim,

Yes when I said the batteries were close to 100% they were, because they were always plugged in to mains power.

Day 8 the loads were not actually turned off. Maybe what happened is the multiplus tripped and caused the loads to 'turn off' as it was no longer inverting.

The smart solar was not disconnected - again all this happened without any user intervention as nobody was on board.

Days 1-7 just show a blank graph i.e. no solar produced and max and min voltage of 0.01V i.e. flat batteries.

So whatever happened the problem appears to have occurred on day 8 - before day the batteries were charging and close to full, after day 8 they were dead.

Can you shed any light on what might be going wrong?


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