Battery Monitors

I reached out to Victron yesterday with a few questions, and they came back with answers this morning (they were actually super-responsive).

I asked if the Victron BMV-702 can monitor SOC on two separate batteries/banks. They said NO - the 702 "can fully monitor one bank and provide voltage on a second".

That's... the same as the SmartGuage.

They said if I wanted to fully monitor two banks, "a second BMV would be needed to measure capacity."

But then said that the best use of a battery monitor is on the "start" bank, that most people find simple voltage is adequate on the "house" bank. This is consistent with the guidance in PM from a friend-I've-never-met, who said that "putting money into a meter for the reserve battery doesn't make a lot of sense".

So... that's where I'm going to leave it. I have a monitor on the start bank, I have voltage on the house bank, I'm good with that.

Interestingly, both people stressed the importance of "synchronizing" the Victron regularly... which is something I have not thought about doing. It's how the errors in the coulomb-counter type of system are factored out to ensure that the SOC measurement stays, well, in sync with the capacity of the battery. I'll be doing some reading up on this, and buying a load-tester.

The Victron guy said "If the BMV is synchronized often (every couple weeks) it’s quite accurate. This will require a full charge of the batteries which is needed anyway for the battery to have a normal lifespan. Battery aging is almost impossible to measure/guess by voltage, so discharging the battery during the course of hours and measuring how much energy it is able to provide is the only way to measure remaining capacity. The BMV is an excellent tool for measuring battery capacity over a discharge period with a controlled load. "

That actually makes sense to me. And hope it's useful here...

_/)_
Bruce

bgary said:

So... that's where I'm going to leave it. I have a monitor on the start bank, I have voltage on the house bank, I'm good with that.

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Sounds like a good decision, and another great example of people using this site to get information to make a better decision about their boat.

But it's never that easy....

You've already made the soft commitment to yourself to spend two to three hundred bucks on a new boat gadget. That money is as good as gone. Going to pass on the battery monitor? Fine, now you just have to come up with a new project and new way to spend the money.

Good luck with that!

bgary said:

....But then said that the best use of a battery monitor is on the "start" bank, that most people find simple voltage is adequate on the "house" bank.

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This is wrong. House bank is deep cycled and thus needs SoC meter. Voltage is a poor indicator of SoC so you will not know with any degree of precision how much charge is left in the house bank.
On the other hand, starter battery is at 100% at nearly all times. What is the point in looking at 100% readout? What we really want to know about starter battery is how many cranking amps it can put out but there is no convenient way of getting this information so we settle for a voltmeter that says battery is still there and voltage looks OK so it is probably (?!) OK.

bgary said:

This is consistent with the guidance in PM from a friend-I've-never-met, who said that "putting money into a meter for the reserve battery doesn't make a lot of sense"...

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IMHO reserve battery means starter battery and not house battery and then your friend is correct.

bgary said:

So... that's where I'm going to leave it. I have a monitor on the start bank, I have voltage on the house bank, I'm good with that.

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You may want to give it a second thought.

bgary said:

Interestingly, both people stressed the importance of "synchronizing" the Victron regularly... which is something I have not thought about doing. It's how the errors in the coulomb-counter type of system are factored out to ensure that the SOC measurement stays, well, in sync with the capacity of the battery. I'll be doing some reading up on this, and buying a load-tester.

Click to expand...

Your battery capacity is not what it used to be fresh from the factory, it varies with temperature, charging efficiency is not 100% and we can only SWAG at what it is, coulomb meters are actually current integrators and as any real integrator they leak/drift, battery capacity depends on rate of discharge and monitor can only make first order correction for that, etc. In other words, Coulomb counting battery monitor only works if you know how to work it.

bgary said:

The Victron guy said "If the BMV is synchronized often (every couple weeks) it’s quite accurate. This will require a full charge of the batteries which is needed anyway for the battery to have a normal lifespan. Battery aging is almost impossible to measure/guess by voltage, so discharging the battery during the course of hours and measuring how much energy it is able to provide is the only way to measure remaining capacity. The BMV is an excellent tool for measuring battery capacity over a discharge period with a controlled load. "
That actually makes sense to me. And hope it's useful here...

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I agree with Victron guy.

sharonov said:

This is wrong

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You're undoubtedly correct.

(Seriously. I don't know enough about this to know how wrong I am. Teaching me about electricity is like teaching a fish to ride a bicycle.)

My wrongness is due, perhaps in part, to the way I use my batteries. I can't think of the last time I had my 1-BOTH-2 switch set to anything but #1. I use #1 for starting, I use #1 for house, and there has not yet been a time that I have been without engine or shore-power long enough to worry about deep discharge (*)

So in my head, #2 is my "reserve", meaning, if #1 drops too low, I can always use #2 to start the motor and get things charging.

In that context, though, a couple of questions are raised in my mind:

1) what are the "best practices" I should adopt, both for power-management and to ensure battery longevity over time? Should I (for example) use #2 just for starting, and use #1 exclusively for "house" use? Am I doing harm to battery #1 (by using it for everything) or #2 (by using it for, effectively, nothing) in the current mode?

2) and, if I do the above (#2 is for starting-only, #1 is for house-only)... which should be on the battery monitor? I presume #1, but...

(there's a reason I'm a software guy... hardware hurts my brain)

Bruce

(*) When I day-sailed to the Rendezvous last summer, with autopilot and chartplotter and VHF and running lights running the whole day, I think #1 was down to 11.1v by the time I got plugged into shore power. I'd have to check the Victron "history", but that's probably the lowest #1 has been since I bought the boat. And, as mentioned, #2 has basically never been used.

In spite of all Bruce's systemic faults, I am willing to suggest that switching "start/reserve" with "house" was a simple brain fart and not a misunderstanding.

I think many people are worried about knowing their batteries capacity to several significant figures. All we really need is an indication of where we stand. Those of us who have been doing this for a long time recognize that the lengthening of recharge time indicates that the battery is getting older.

A hydrometer for flooded cells or a cheap load tester will give you an indication of how much life is left. The results won't be meaningful unless you track the readings over the life, or remaining life, of the battery. A true load test is not possible without a great expense of time and effort. It is not a simple test as some would indicate. It takes 20 hours, a variable load, and you are reducing battery life somewhat.

People have been talking about a full charge. This is really only practical at the dock. Virtually no one fully recharges their batteries when cruising because it costs too much engine run time. The recommended method is to keep them between 50 and 85%, but these numbers are nominal. If you can fully recharge your batteries every day you don't need a monitor.

bgary said:

... said that the best use of a battery monitor is on the "start" bank, that most people find simple voltage is adequate on the "house" bank.

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Went back and looked, and here's where I confused myself.

I had asked the Victron guy if it made sense to buy a second battery monitor for my second bank. He said "It depends on the function of the second bank. If it’s a start bank the 702 is fine. If it’s a second “house” bank a second BMV would be needed to measure capacity"

When I read that, I read it wrong. I read that sloppily, seeing "on a start bank the 702 is fine" and concluding that he was saying "you should have a battery monitor on your start bank"

He was actually saying "if the second bank is a start bank, (the second channel of a BMV-702, which measures only voltage) is fine. If it is a second "house" bank (meaning it will be regularly used and recharged from a deep-discharge state) then a second BMV would be worthwhile."

Sorry for the confusion. I'll now put my helmet back on and resume my seat on the short bus.

Bruce

bgary said:

.....My wrongness is due, perhaps in part, to the way I use my batteries. I can't think of the last time I had my 1-BOTH-2 switch set to anything but #1. I use #1 for starting, I use #1 for house, and there has not yet been a time that I have been without engine or shore-power long enough to worry about deep discharge (*)

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Assuming you have an ACR or some other way to automatically charge battery #2, there is nothing wrong with this setup. In fact I like it. The only thing I would suggest is once in a while start the engine from battery #2 to make sure it still has good cranking performance.

bgary said:

So in my head, #2 is my "reserve", meaning, if #1 drops too low, I can always use #2 to start the motor and get things charging.

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I use battery #2 the same way. It is always topped off and is rarely used.

bgary said:

1) what are the "best practices" I should adopt, both for power-management and to ensure battery longevity over time? Should I (for example) use #2 just for starting, and use #1 exclusively for "house" use? Am I doing harm to battery #1 (by using it for everything) or #2 (by using it for, effectively, nothing) in the current mode?

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I think you are fine. Likely battery #1 will not last as long as battery#2 since it is cycled and often sees partial SoC but there is no way around it.

bgary said:

2) and, if I do the above (#2 is for starting-only, #1 is for house-only)... which should be on the battery monitor? I presume #1, but...

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Yes, you want to monitor the battery that is cycled. That would be house battery.

bgary said:

(there's a reason I'm a software guy... hardware hurts my brain)

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In my experience software guys can make excellent hardware engineers, they just found a better way of life and won't do it ;-)

bgary said:

... I think #1 was down to 11.1v by the time I got plugged into shore power....

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11.1V is very low. Battery was pretty much dead and that is not good for its health.

Just to add some fuel to the fire, I contacted Balmar and asked the question about the Smart Gauge accuracy when two banks and a charging ACR are involved. On my E34 I have a house bank of two Group deep cell 27 wet cell batteries in parallel and single starter Group 24 (normal high CCR) wet cell battery.

Here is my question and the Balmar reply:

Is the smart gauge accurate when connected to the house bank (2 grp 27 wet cell batteries in parallel) when there is a separate wet cell starting battery and the charging system is controlled by a Blue Seas ACR?

When the batteries are being charged (and the ACR is closed, connecting the batteries) the accuracy will be about 10%. Once charging is complete and the ACR opens, after 10 to 15 minutes the accuracy should be within +/- 5% as stated in the manual. However, in my lab, under very controlled condition, I got about +/- 2.5% but that was a Lab and not real life. Do bear in mind that this was after several charge/ discharge cycles and the more cycles the unit sees the better the accuracy gets.

Best Regards,
Thomas Pusateri

I do connect the Smart Gauge to the house bank and just monitor the starting battery voltage. I agree with the comment that that is the preferred way. If the starting battery does fail to start the engine, I can combine all three batteries in parallel with the Blue Seas Battery switch.

The Blue Seas battery switch is not the usual 1,2, both but rather a single pole double throw switch, which means that in the "on" position, the starter battery is connected to the starter and the house bank is connected to the DC house panel, independently. Then there is an "emergency" position which combines all three batteries in parallel.

bgary said:

In that context, though, a couple of questions are raised in my mind:

1) what are the "best practices" I should adopt, both for power-management and to ensure battery longevity over time? Should I (for example) use #2 just for starting, and use #1 exclusively for "house" use? Am I doing harm to battery #1 (by using it for everything) or #2 (by using it for, effectively, nothing) in the current mode?

2) and, if I do the above (#2 is for starting-only, #1 is for house-only)... which should be on the battery monitor? I presume #1, but...

(*) When I day-sailed to the Rendezvous last summer, with autopilot and chartplotter and VHF and running lights running the whole day, I think #1 was down to 11.1v by the time I got plugged into shore power. I'd have to check the Victron "history", but that's probably the lowest #1 has been since I bought the boat. And, as mentioned, #2 has basically never been used.

Click to expand...

Bruce - The house battery is the one used for your normal usage in battery jargon. If you are draining your house battery down to about 11 volts you should think about adding a third battery in parallel with #1. That said, you are operating correctly, IMHO. To maximize life you should switch the two batteries for the house bank periodically. At the half season mark, annually, whatever. This means physically moving the two batteries to keep the monitor on the house battery.

Monthly, +/-, you should test your #2 battery to be sure it is in operating condition. This should be done after having it off of the charger for a couple of days.

BTW, I can ride a bicycle and a lot of what I say is a little fishy.

I use the deep-cycle "house bank" for starting and for everything else too.

I use the deep-cycle "reserve battery" never, because it's held in reserve.

I don't have a "starting battery" because calling it that would confuse me.

There seems to be a plethora of very inexpensive Chinese battery monitors on the market. Not sure whether to trust such a thing... Although some of them are so compact that you can tack one on almost any rechargeable-battery device that you have, for a few bucks. The prices seem to range from $5 to $40 on EBay.

I mean, I’m not worried about losing a few bucks to try one out, more worried about fire hazard if the smoke leaks out. Well, fuse everything and carry on!

Paralleling Batteries

If you are paralleling batteries, they should all be of the same capacity and age. One problem of Battery switches is the sequence,( OFF, 1, BOTH, 2). As BGARY mentioned his house bank was at 11.1 volts, # 2 fully charged. Where does that current surge go as the switch moves from 1 to BOTH? IS the low house bank 2 type 27's and #2 a type 24?
Is there a isolater to prevent the #2 battery from charging the House Bank? Or will the #2 discharge toward the 11.1 Volt house bank?

Vagabond39 said:

If you are paralleling batteries, they should all be of the same capacity and age. One problem of Battery switches is the sequence,( OFF, 1, BOTH, 2). As BGARY mentioned his house bank was at 11.1 volts, # 2 fully charged. Where does that current surge go as the switch moves from 1 to BOTH? IS the low house bank 2 type 27's and #2 a type 24?
Is there a isolater to prevent the #2 battery from charging the House Bank? Or will the #2 discharge toward the 11.1 Volt house bank?

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Bob - Much more important than size & age is that they be of the same type, Flooded, AGM, Gel, etc. Paralleling different battery types doesn't work. They won't charge or discharge correctly.

Paralleling batteries of the same type but different sizes is not a problem. They will charge and discharge according to size. If one is twice the capacity of the other it will take 2/3 of the charge current and supply 2/3 of the load. If the batteries are of different designs or different manufacturers there will be a small difference in the split, just as if they are the same size.

If the batteries are of the same type but different ages it is not ideal, but not a real problem if the age difference is not extreme The capacity of the combined batteries is not really known because of the older battery probably being weaker. You would not want to parallel a good battery with one on its last legs as it may hurt the good battery when it dies.

Learned a bit about batteries over the weekend. Among other things, I better understand what happens inside the battery during charge and discharge, and why letting a battery get below 12.2V (50% SOC) is a bad thing for the life of the battery.

I'm still wrestling with how to ensure that my battery monitor is appropriately calibrated in order to give me good info over time.

So here's my random question for a cold holiday Monday in front of the fireplace: Is it possible to "back into" a set of calibration settings that represent real-world condition and use?

For example, let's say I
-- Start with a good charge (100% SOC, rested open-circuit voltage ~12.6V)
-- Run the boat in a typical mode (GPS, VHF, Autopilot, AIS) for long enough to pull #1 down to 50% SOC according to battery voltage (rested open-circuit voltage ~12.2V)
-- Make note of the # of hours it took, the number of amp-hours consumed, etc.

Could I then use that data to calculate what my Cb "must be" in the Victron settings, in order for it to accurately represent my battery's real-world capacity in its calculations?

It's not an actual 20-hour capacity test, but... would it be useful?

Bruce

It's not an actual 20-hour capacity test, but... would it be useful?

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No.

You proposed a very simple load test, but performing it is not so easy.

A meaningful test should use a constant current to relate it to the 20 hour rate.

Your test current is real life, but isn't the major component the auto pilot? How do you account for wind & sea state.

How do you determine when you got to 12.2V. You have to wait 4 hours to test, allowing the battery to stabilize.

You can't duplicate the test so you have no idea if there are any test anomalies. Somewhat like writing code and not testing it.

You probably don't know how accurate your voltmeter is. My Victron measures two batteries so I know its voltmeter is not good enough. I've switched the leads and the readings are off by .08V.

You should keep in mind that exceeding the 50% SOC is not catastrophic.

I've been out of the business for twenty years, but back then people who wanted their standby generation to start bought truck batteries and replaced them every two years. Cheap insurance. They also tested the system every week.

In your case, I would add a third battery for reserve and parallel your existing two. This would double your usable power so you wouldn't have to measure SOC to multiple decimal points.

I took the plunge

The install would have been easier in an Ericson, but, I got it done.

Thought I would share a recent project. I have not been happy with the stock electrical panel when it comes to battery monitoring. Via this thread I found the Balmar, Smartgauge. I installed one for each bank. I don't have any experience with it yet so I can't comment on how well it works, but, it is better than what I had. The install was relatively easy. I ran 16 awg wire to both house banks. I used Blue Sea buss bar connectors at the batteries and installed the gauges in a small frame mounted in the circuit breaker cabinet. I was attracted to the product since it does not require the use of a shunt. The picture shows the left gauge displaying battery voltage and the right shows state of charge.

MMLOGAN said:

.... I installed one for each bank....

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I can understand the need for a second Smartgauge if you have TWO house banks (which in most cases is not recommended). However if you have one house and one starter/reserve bank I do not see the need for the second Smartgauge. I expect starter/reserve battery monitor will just sit at 100% all the time and 100% does not really tell you cranking amps or Ah capacity for the reserve battery. I recall Smartgauge has ability to measure secondary voltage and that should be enough for the reserve battery.
Hmm.. gauges are marked "port" and "starboard". Maybe you do have two house banks?

MMLOGAN said:

... I was attracted to the product since it does not require the use of a shunt...

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Actually shunt is the reason I have Victron monitor installed and Smartgauge still sitting in the box waiting to be installed. I really like the ability to see amps going in and out of the battery. How much that new LED light is taking? How much my fridge draws? When does charging current from alternator drop to the level where it does not make sense to run the engine anymore?

However I do appreciate that Smartgauge autocalibrates itself and does not suffer from the drift exhibited by all coulomb counters. That is why I want both .

I have a confession - I went to the dark side!

We purchased a power boat last spring. It's been quite the change for us. Getting to Catalina in an hour and 15 being the biggest gain. Not having all of the kids in the same small space for the same party is the biggest loss.

So, yes, I do have two house banks. I would have wired my starter battery bank to the Starboard (battery 2) gauge and my genset starter battery to the Port (battery 2) gauge but, they are AGM and my house banks are currently flooded. The install information said to not mix battery types.

The boat came with a single 150 amp gauge that is switched for consumption, engine charging and generator charging. Hard to tell how many amps any single device is drawing on a 150 amp scale.

I just need to know state of charge so I can plan when to run the genset. Mine is very quiet but I still don't want to run it unless it's needed.

Cheers,