DVM to Cummins Smartcraft display .4-.5V battery drop?

[Larry Backman]

First – let me start by saying that the digital voltmeters Seaboard sells for a whopping 15$ are the cats meow. I have been amusing myself and learning about my boats battery draw and charge after putting them in this winter. I have already ducked one wiring bullet due to having them in place and seeing a weird reading before a battery fully ran down.

My boat has a single QSM 11 engine with a pair of 8D’s which charge both off a battery charger as well as from a charging relay off the alternator.

I started up today with both batteries reading 12.5-6 volts on the DVMs which is normal. While running they both read 13.5 to 13.6 volts, again which is normal.

I had a gang of kids on board and we went over to Martha’s Vineyard
shut down the engine and went out to lunch, hung out on the beach and came back to the boat about 3 hours later.

I found both batteries reading 12.4-5 on the DVM’s went to start up and got a lazy whine as opposed to a scream from my raw water flow alarm which takes power off the engine. The engine would not turn over and when I looked a second time the engine battery DVM was off, no LED.

Obviously I had a wiring problem, went to the battery and cranked the nut holding battery cable, DVM wire, hanging wires, etc. 1/2 turn tighter. The DVM lit up, the engine started, everyone of the 9 people on boat were happy except me.

It’s a 6 mile 20minutes run home from the Vineyard and I set up the Smartcraft to watch my voltage to see if it was stable or fluctuating. To my surprise, concern – the engine voltage was a steady 13.1 volts at Cruise on the Smartcraft, but read 13.5 to 13.6 volts on the DVM.

By memory as it was dark when we got back – battery -> battery cable -> battery switch -> engine cable -> starter and then power coming off the ECM(??) to give power to the Smartcraft display.

At the dock, engine idling with no load I saw the same .4 volt gap, except 13.6. Volts at the Smartcraft, 14.0-.1 at the DVM.

Two questions – is this .4 to .5 volt drop between battery and Smartcraft display “normal”or should I be tearing into my battery cables?

Second, a what if question – if my battery cable was loose as I believe was the case and it lost connectivity to the battery while underway at cruise – what happens with an electronic engine like the QSM , does it give me a low battery alarm and derate itself or since it’s electronically controlled fuel injection – does it stop firing and shut down abruptly?

1 user thanked author for this post.

Gino K

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[Rob Schepis]

Most have an option for wiring in a manual overide switch. Do it. It’s the only way you remain in total control. Will allow your battery charger to work better too.

[Larry Backman]

Nice call – ACR charging relay in the mix

I can’t cite the wiring from memory but there is an A R charging relay which balances the alternator output across the pair of 8Ds. That has to be the culprit in both batteries dropping when the big engine cranks.

[Rob Schepis]

Do you have some type of combiner that joins the house and crank bank that is not “letting go” at the drop out voltage?

[Larry Backman]

DVMs are straight run

I followed your orders exactly. 20’ straight run from each DVM to each battery. I know they are reading the battery as it is and am learning a lot watching them.

For instance explain to me why with the battery switch on 1 the engine start pulls 1 down to 10.5 volts momentarily, but also pulls 2 (house) down from 12.6 to 11.6?

FWIW, my pair of 8Ds each have a positive cable to a 1-2-both battery switch and share a common negative cable to my boat ground.

And again, is that .4-.5 volt droppage between battery and Smartcraft display normal?

[Tony Athens]

Larry,

Simple question:

Exactly where is the NEG & the POS terminals going to the “Cats Meow” DVM’s attached? Answer that!

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

That is the key to what you may be questioning what you see……………..Understand, nothing else matters, IMO, except “battery terminal voltage”…….If you take your reading from some other place then you will have a voltage drop and that has to be sorted out as to why & whether that VD would be normal…. …………………….

Tony

[Philip]

If the battery cable becomes disconnected while the engine is running you could damage the diode in the alternator reducing charging ability. If your still showing normal voltage when running then your alternator is fine.

Phil

[Larry Backman]

Mechanic says don’t worry

So I asked these questions of my Cummins mechanic, the answer was that .5 volts was normal enough in terms of drop, don’t worry about it.

He also said the alternator would take care of the ECM once the engine was running and not to worry about a sudden shutdown if the battery cable popped off (which it won’t now!)

[Rob Schepis]

Larry,
I know you thought this was lost but here it is. It was flagged by the software as “potential spam” and need moderator approval before getting posted which I just took care of. It’s here now so I will delete your followup “short version” .

[Author]

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