Starter through shunt (what size?)

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jwag956

Veteran Member
Joined
Jan 15, 2020
Messages
73
Location
US
Vessel Name
Scrimshaw
Vessel Make
1989 GB 42 Classic
Looking to replace older xantrax BMC with Victron. We have 2 house/starter 8D batteries (i.e. older style not dedicated starter batteries). I want to place a BMC on each battery since we alternate between them (there is a battery switch 1, 2, All). To get correct info it would seem that all loads - including starter go through the shunt (via the negative). Victron default shunt is 500A - I am having a hard time figuring out just how many amps my starter motors take - these are for Ford-Lehman SP135 engines.
Or is it not considered a good idea to run starter through a shunt?
 
The 135 manual does not list starter draw but the 120 manual specifies a starter running current of 650a and a locked-starter draw of 1050a. A 1000a shunt would probably be ok, but I’ve never heard of it being done.
 
Unless you crank your engines for minutes at a time, there is no need. While the current during cranking is very high, the duration is low and the amount of energy is low. Also, the software in the monitoring device will not correctly account for the energy lost, so ignoring it is no loss in accuracy.

For example 650A for 10 seconds is 1.8 AH. There are many sources of error in the monitoring system larger than that.
 
I wonder when they say it's a 500A shunt, does that mean that more will damage it or is it just not capable of reading more? I suspect the latter, I run my twin Perkins through mine and never had a problem with it. And like DDW said, if its not reading the starter draw it wouldn't really matter.
 
I use an inexpensive monitor with dual hall sensors. No shunt installation just slip over the cable.
Available on Amazon for less than $100
 
Unless you crank your engines for minutes at a time, there is no need. While the current during cranking is very high, the duration is low and the amount of energy is low. Also, the software in the monitoring device will not correctly account for the energy lost, so ignoring it is no loss in accuracy.

For example 650A for 10 seconds is 1.8 AH. There are many sources of error in the monitoring system larger than that.
Thanks - your reply is a great way to think about the entire system. And no - if I am cranking more than 5 seconds I know I have some air leak in my lines.
 
I run my starter through the shunt linked below. (Price has gone up). Works fine and never a problem. I also have my starter fused (another nautical no-no). My 80hp Lehman only draws about 500 amps, which is the maximum rating for the shunt. But what is meant by maximum? The accuracy of the shunt is listed at 1%. A momentary draw over 500 amps might be outside of 1%. Not a big issue for me. Amp hours, SOC, and voltage are what I'm concerned with. I can check it at anchor at night and see that I'm drawing 6 amps, so I know I left something on. I can check in the morning and see that I should make coffee with the engine warming to get underway.

Both legs from my two banks are fused with ANL 250 fuses. I've started with one bank, meaning that I'm drawing the 500A starter through a 250 amp fuse. The 250A ANL would blow if the 500A (.1 second) draw lasted for 10 seconds. But if I ran my starter for 5 seconds without starting I would know that something isn't right.

 
So many questions, but let’s start with:
  1. Why are you running your starter through a shunt?
  2. How are you doing so? It is the negative cable that runs through the shunt - is your starter not case-grounded to the block?
  3. What kind/size are these two battery banks? FLA?AGM?LiFePo?
 
Great answers - I've been tracing wiring - starter, battery, alternator, etc. Some updates and additional questions (possibly I have just really confused myself).
To recap - this is an older-style wiring - there is a port house/starter battery and a stbd house/starter battery - with a battery selector switch for the normal DC loads. AGM batteries, internally regulated alternators that go directly to their respective battery.

The starter (and alternator) are grounded to the engine block. There is a -ve cable that runs from the engine block to the respective battery (there is also another cable that connects BOTH blocks).

The original question was - should ALL -ve cables terminate at the shunt and then a SINGLE cable to the battery - thus in my mind, the starter current would in fact go through the shunt since that is the only path.

So - new (but I think related) - If there is circuit/path from the battery -ve DIRECTLY to the engine block then wouldn't the alternator also use that -ve path and thus also bypass the shunt (and not be recorded)??

Posed another way - if the alternator and starter share a -ve connection on the engine block then wouldn't we need to run BOTH through the shunt in order to capture the alternator charging input?
 
I have the 10 hp bow thruster running over the shunt and that doesn't cause any problems.
 
I purchased a Victron BMV 712 battery monitor and a 500 amp smart shunt for a engine/house battery bank. 4 group 31 105 AH DP flooded lead acid batteries wired in parallel 900 CCA per battery. A lot of available amps! I was concerned about the the starter pulling more amps than the capacity of the shunt. I called Victron and talked to a service representative about this. He responded with a don't worry about the shunt. Most battery terminals are not rated for that many amps. He asked me what size DC battery cables are installed? I told him 4/0 ! He then asked me what I though they were rated for? I said they are large enough for the application that is the size cable that has been in the boat since it was built. He said a 4/0 marine grade cable is rated for just under 400 amps in an engine compartment. He reassured me that the 500 amp shunt would work fine in my application. I installed it in 2021 and have put 900 hours on the engines and 8000 miles of cruising and never had an issue with the shunt installed .
Brian
 
Great answers - I've been tracing wiring - starter, battery, alternator, etc. Some updates and additional questions (possibly I have just really confused myself).
To recap - this is an older-style wiring - there is a port house/starter battery and a stbd house/starter battery - with a battery selector switch for the normal DC loads. AGM batteries, internally regulated alternators that go directly to their respective battery.

The starter (and alternator) are grounded to the engine block. There is a -ve cable that runs from the engine block to the respective battery (there is also another cable that connects BOTH blocks).

The original question was - should ALL -ve cables terminate at the shunt and then a SINGLE cable to the battery - thus in my mind, the starter current would in fact go through the shunt since that is the only path.

So - new (but I think related) - If there is circuit/path from the battery -ve DIRECTLY to the engine block then wouldn't the alternator also use that -ve path and thus also bypass the shunt (and not be recorded)??

Posed another way - if the alternator and starter share a -ve connection on the engine block then wouldn't we need to run BOTH through the shunt in order to capture the alternator charging input?
You use the term “shunt”. Can we imply you literally mean shunt (singular), not shunts (plural)? I’m having trouble picturing your configuration. If you have only one shunt, how is that wired? If two, I can imagine that, but then do you have two SoC meters? Or how do you consolidate that information into meaningful data? Sorry, it’s complicated, trying to help.

BTW, re the alternators, it depends (sorry🤷‍♂️)….if your alternators have a separate -ve post (as mine do), then they should be wired to go through the shunt. I think you mentioned they are internally regulated; I hate to make assumptions but I assume they are case ground, so if you want to be able to account for the charging current supplied by the alternator it needs to go through the shunt. Which in your case seems to mean the engine block needs to be connected to the “high” side of the shunt. As others have said, I don’t think starter draw will be an issue in your case. 500A spec means that’s what it is designed to measure at with some accuracy. It’s not going to melt down anywhere near there.
 
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The original question was - should ALL -ve cables terminate at the shunt and then a SINGLE cable to the battery - thus in my mind, the starter current would in fact go through the shunt since that is the only path.

So - new (but I think related) - If there is circuit/path from the battery -ve DIRECTLY to the engine block then wouldn't the alternator also use that -ve path and thus also bypass the shunt (and not be recorded)??

Posed another way - if the alternator and starter share a -ve connection on the engine block then wouldn't we need to run BOTH through the shunt in order to capture the alternator charging input?
I have an internal regulated alternator. This does not change the way the shunt is wired. The shunt measures all current flow out of or into the battery. The shunt should be installed as close to the battery as possible. There should be only one battery cable attached to the battery terminal. That cable should be connected to the shunt "line side" (battery side). The cable connected to the load side terminal of the shunt should be connected to a common B- terminal common strip. All DC B- cables should be attached to the terminal strip or a common B- from that strip to another B- strip. The starter draw current will be measured and any other DC current used from the battery will be measured. Any current flow going into the battery, solar charger, AC battery charger, alternator will also be measured. The important thing to remember one cable on the battery negative terminal. Any other B- cables connected to the negative battery terminal post will by pass the shunt. Any cables that are bypassing the shunt and have current flow. That flow will not be measured and the battery monitor will not be accurate. Think of the shunt as being a flow meter.
Brian
 
You use the term “shunt”. Can we imply you literally mean shunt (singular), not shunts (plural)? I’m having trouble picturing your configuration. If you have only one shunt, how is that wired? If two, I can imagine that, but then do you have two SoC meters? Or how do you consolidate that information into meaningful data? Sorry, it’s complicated, trying to help.

BTW, re the alternators, it depends (sorry🤷‍♂️)….if your alternators have a separate -ve post (as mine do), then theybshould be wired to go through the shunt. I think you mentioned they are internally regulated; I hate to make assumptions but I assume they are case ground, so if you want to be able to account for the charging current supplied by the alternator it needs to go through the shunt. Which in your case seems to mean the engine block needs to be connected to the “high” side of the shunt. As others have said, I don’t think starter draw will be an issue in your case. 500A spec means that’s what it is designed to measure at with some accuracy. It’s not going to melt down anywhere near there.
Yes - 2 shunts and 2 SOC meters, one for each house/starter battery.
Thanks for the confirmation that a) don't worry about the amp draw and b) yes - alternator and starter - if connected both need to go through the shunt.
 

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