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Battery Monitor and Shunt

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LostDog88

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Jan 2, 2021
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I want to install a battery monitor to start monitoring how many amps we use through the day and night. I thought this might be a good start as I begin to build my Solar Array for my (new to me) sailboat.
The question is that if I start with a monitor, how many amps monitor? 100? 200? 300? I guess I don't understand or know that answer.

I am guessing in the end I will need between 200 and 400 Watts of solar to recharge the batts. This would be like E=I(R) or P=E(R). right? Ohms law. E is 12vdc I is unknown and R would be all my loads. Again unknown.

I feel on a boat I only need like a 200 AMP monitor. But I am missing something.

Can anyone shed some light for me please?
 
So...If I understand the math. Somwhere around 33 AMPS. So. 100AMP monitor thing would be fine? Did I do it correctly?
 
Probably cause I have no clue what I am talking about. :)

I want to eventually end up with enough solar to power my wifes and I requirements for a day of sailing. Recharging with wind and solar. Please this is not a debate of whether wind is a good idea. I am getting wind to add to the solar. The bulk is obviously solar. Matter of fact. Idea is all of it to be solar and just have the wind to be there. You know. Working at night. Couple amps at a time. :)

So...with that in mind I was going to start with two 100 amp hour AGM. (Will try to build my own lithiums later in life) giving me 200AH at 12vdc. Install an inverter and have a cool cruising boat.

But I don't know diddly squat about what I need for solar because I don't know how many amps I use in a day. I was thinking 2000 Watts of inverter is more than enough for an icemaker and some other stuff. TV, VHF, Laptop, Charge a phone, etc.

I thought maybe getting a monitor to monitor the battery as we use the batteries and record it would be a good start to building the system from the ground up so to speak. Once I know what we spend in a day/night. I can build the system For now, the only charging available is via an alternator attached to the engine. So I am not too freaked out about running out of battery as I can just start the engine. For now.

It also has a shore power connector that charges with a 40A charger. I think it is best to save that for later. As for now I just wanna know what I use so I can overbuild a bit.

What's the best way? A battery monitor right? Whew. Hope that I am helping you so you can help me! LOL
 
Buy the Victron battery monitor with the 500 amp shunt, this will be adequate for most systems. The Victron Smart Shunt is good value and much much better than the low cost units. Your sail boat should have house batteries installed and a control distribution system for the various electrical items, usually with some indication of volts and amps.
Once you have the monitor installed a few trial runs will give you data on typical day use.
A 2000 watt inverter at full power is a very heavy loads for 200Ah of AGM battery, this will take around 200 amps on a 12v system.
You seem new to sail boats and electic systems. Start with a simple system and progress with experience. The boat engine battery should be seperated from any house battery. The basic requirements of the boat electrics is to power the boat engine and navigation/ radio equipment. All the other stuff is secondary.
You don't say what size and type of sail boat but mounting panels is a challenge as shading and space is a problem. Wind power is possible but due to space limitations for large blade area power is limited to low levels.

Mike
 
Many folks do not get the difference between using Wind on a boat versus land based. The Marine class Turbines like the Air-X from Primus when installed properly on a boat can work very well indeed. BTW Primus is one of the "few" who are tested & certified and do perform exactly as designed.

Now to help you we need to bring you back to a point.
  1. You will need figure out / estimate how much power you will need for a full day out & about.
  2. You will have to identify the surge values as that will put amperage demands. A fridge, AC, or anything with a motor has a surge at startup, quite often that momentary surge can be 3X the running amperage/wattage rating.
  3. Once you know the Power requirements, as in Amps/Watts needed to run everything, including the start surges etc (the peak demand points) you can size the battery required to support that.
  4. Next is to make sure you have an Inverter/Charger that can A) Deliver the amount of power required to your devices and B) be capable of charging the attached battery system within a reasonable time frame.
  5. Solar Controller can be considered. It will need to have enough panel to be able to charge at the Amperage required for the battery bank. Some SCC's (Solar Charge Controller) can also accept Wind Turbine power as well as Solar. These tend to be more costly but they perform very well and are optimal for managing & controlling what's going on. ** tricky see below **
  6. Then you can shop out solar panels and extras to fill the needs. NOTE: a Common MISCONCEPTION is that if you have 12V batteries, you are stuck to 12V solar panels, IT'S PHOOEY ! An SCC will take in X Volts & X Amps and convert that to whatever the battery bank is and push as much voltage / amperage it can. So you can use small solar cells to large 72 Cell 450W panels if you have the room. Depends on your boat and what you want.
** tricky **
Depends on the choices made, in some cases a "dump load" may be required due to wind generation. Quite often people will use a small Hot Water tank with 12V or 24V element (usually same voltage as battery bank) to dump excess power into while giving them hot water availability. There are other ways as well but this all depends on your needs, wants and what is available that can be used.

Solar Controllers & Inverters also have monitoring of battery state, charge state along with amps/watts going in & out. As an SCC sits "in-between" the batteries & Inverter the stats are usually quite accurate.

External Monitoring for Amps/ Watts, AH Remaining, Voltage etc can be done in various ways and there are many solutions to do so. See video from Will below as one option. There are less expensive ways and really expensive ways. A properly designed system with "aligned" compatible components often costs less in the long run.

AN IMPORTANT NOTE:
You are intending to use AGM, that's fine but be aware, you can ONLY take them down to 50% DOD, no more lest you start to harm them. So your 100AH battery will only be able to deliver 50AH usable. LiFePO can go from 100% to 0% but most people restrict that down to maximize life and cycles so will actively make use of 80% of total capacity.

 
The aili volt meter with the 350a shunt is decent enough for the price. Think I paid around £35 for mine. It doesn’t have the data logging or Bluetooth the victron does but does everything else. It attached to the negative post on your battery bank and everything is connected to that. It tells you how many amps your using, how many amp hours you have left and the voltage of your battery. Remember you can only draw a agm battery down to 50% around 12.1v. Repeatedly draining fb it below that will significantly decrease its life span.
it sounds like your new to 12v systems and I would recommend learning a lot about them and there limitations, it’s not like a house where you have unlimited electrical power.
120w of solar will at very best give 10a of power. That’s in perfect conditions. Panel pointed directly at the sun with no cloud cover or shadows on the panel.
the 2000w inverter will drain 200ah battery down to 50% charge in around half an hour. The solar panel at best will need 10 hours to put that power back into the batteries. If it’s anything like my panels and batteries probably more like 15hrs
 
Now to help you we need to bring you back to a point.
  1. You will need figure out / estimate how much power you will need for a full day out & about.
[/end quote]

This is where I wish to start. I want to tackle the other items below this after I solve what I feel is the most important item.

I ended up getting a 200 amp battery monitor with shunt. as well as some bus bars as I am sure they need fixing, replacing or something. So why not just replace them.

After I install that next week or week after I will have an accurate report on my amp usage through a 24 hour period and that should help us build my system out.

Yes I am using AGM's only because they were replaced by the previous owner and are a month old. Might as well kill them learning. :) I will eventually wish to build my own LiFePO4 battery pack. 12VDC..

As for the videos. I think I have watched every one of his beginner start here vids. Of course I need to watch again probably. LOL.

Thanks, I shall let you know soon!
Click to expand...
 
mikefitz said:
Buy the Victron battery monitor with the 500 amp shunt, this will be adequate for most systems. The Victron Smart Shunt is good value and much much better than the low cost units. Your sail boat should have house batteries installed and a control distribution system for the various electrical items, usually with some indication of volts and amps.
Once you have the monitor installed a few trial runs will give you data on typical day use.


Mike
Click to expand...
Right on.

I love my Victron Smart Shunt because it just works and it is verified to be accurate enough for my needs an presumably most others.
It is an important part of a system and I would not bother risking it with the lower cost stuff.
 
Just to push the bill even higher, I love my Victron BMV-712. Not only does it have a display, but it has a relay output you can use to switch external loads or charge sources off/on depending on state of charge. It will do the same thing for the Victron MPPT Smart Solar wirelessly using Bluetooth.

You can also use the relay output of the BMV-712 to sound an alarm before the battery goes into low voltage shutdown. This is an incredibly useful thing to know if you have any electrically controlled equipment on your boat that might create a problem for you if it just suddenly turned off.
 
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