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Old 10-08-2020, 19:20   #1
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Battery monitor ?

I have a twin diesel m/v that has two dual purpose 8d batteries that are both starting and house batteries. I want to put a battery monitor on them. When I start the engines it can be around 1400 amps for the starter to turn the engine over.
My question is while looking at several battery monitors the shunts are rated for less amps then required for starting the engines. Is this an issue?
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Old 10-08-2020, 20:06   #2
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Re: Battery monitor ?

1. 1400 amps would be unusual, you sure?
2. Assuming it’s correct then everything excepting the engine itself should be on a separate negative bus, and there is one wire from this bus that connects either to the engine block or battery direct, that’s where you should put the shunt, it’s unlikely that the 0000 wire that connects the batteries to the engine goes first to the circuit breaker panel where all your other loads connect.
You wouldn’t capture the short starting surge, but it’s not usually that many AH as it’s hopefully a quick event.

Most shunts will take far more than they are rated for starting, but 1400 is a BIG number
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Old 10-08-2020, 21:20   #3
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Re: Battery monitor ?

If it takes 2,000A to crank an engine and the starter is engaged for 3.6 seconds, that equates to 7,200 Asec. 3,600 seconds in an hour. This example engine will draw 2Ahr during the starting process. There is no good technical reason to put a battery monitor in a start battery circuit.
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Old 10-08-2020, 22:11   #4
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Re: Battery monitor ?

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Originally Posted by a64pilot View Post
1. 1400 amps would be unusual, you sure?
2. Assuming itís correct then everything excepting the engine itself should be on a separate negative bus, and there is one wire from this bus that connects either to the engine block or battery direct, thatís where you should put the shunt, itís unlikely that the 0000 wire that connects the batteries to the engine goes first to the circuit breaker panel where all your other loads connect.
You wouldnít capture the short starting surge, but itís not usually that many AH as itís hopefully a quick event.

Most shunts will take far more than they are rated for starting, but 1400 is a BIG number

you'd miss all the alt charging current if you bypassed the engine neg.

I'm betting the manual wants a 1400cca rated battery. no way an engine would pull 1400a starting. it would be 24v instead if it needed that.

you can buy 1000a shunts. some monitors should be programable for shunt size.
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Old 11-08-2020, 04:03   #5
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Re: Battery monitor ?

@smac999
Quote:
you'd miss all the alt charging current if you bypassed the engine neg.
Most engine manufacturers and boat builders connect the alternator output to the B+ supply to the starter solenoid. This method saves time and a few dollars. Convert this to the safer and more technically correct scheme of wiring the alternator B+ to the house bank with proper OCPD where at its battery end.

And, there is still no technical reason to capture 2Ahr from my example engine starting sequence in #3.
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Old 11-08-2020, 04:13   #6
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Re: Battery monitor ?

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Originally Posted by TCM1968 View Post
I have a twin diesel m/v that has two dual purpose 8d batteries that are both starting and house batteries. I want to put a battery monitor on them. When I start the engines it can be around 1400 amps for the starter to turn the engine over.

Very common powerboat installation. Twin 715s or similar?

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Old 11-08-2020, 04:50   #7
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Re: Battery monitor ?

I would install a SOC meter (State of Charge) as it is a" gas gauge" for the battery system.


With "dual purpose" batts its especially important because they can not be discharged too deeply with out damage,,unless AGM.
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Old 11-08-2020, 20:37   #8
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Re: Battery monitor ?

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@smac999

Most engine manufacturers and boat builders connect the alternator output to the B+ supply to the starter solenoid. This method saves time and a few dollars. Convert this to the safer and more technically correct scheme of wiring the alternator B+ to the house bank with proper OCPD where at its battery end.

And, there is still no technical reason to capture 2Ahr from my example engine starting sequence in #3.
shunt's on the ground...

would only work with an electrically isolated alternator with own ground wire to the DC bus. most alts are not, and ground through the engine block, and then through the engine neg cable. which you are saying not to monitor..
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Old 11-08-2020, 20:53   #9
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Re: Battery monitor ?

Smac999
Are you lecturing me on how a battery monitor works or agreeing with me that there is no technical reason to install a battery monitor on the starting battery?

I’d say that fully half the boats I work on have isolated B-

If the OP wants to install a BM to monitor his starting battery and if he has a non-isolated alternator, it would be wise to separate the charging from the starting circuit as described.

For the record, I installed my first Coulomb counting battery monitor in 1990.
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