NASA BM1

[Bow42]

How does BM1 work out my SOC. % ?
Is it a case where it measure the voltage ( 12.50 v = 80% ) or does it subtract the Amps being use and any going in to the battery's bank ? .

 

[VicS]

How does BM1 work out my SOC. % ?
Is it a case where it measure the voltage ( 12.50 v = 80% ) or does it subtract the Amps being use and any going in to the battery's bank ? .

the explanation is on the website:

How does it work?
The minimum information required to monitor the state of charge of a battery is the terminal voltage and the charge/discharge current **. The BM1+ measures both these parameters and computes the state of charge using a complex, well proven, algorithm.

A simple and intuitive display shows the service battery voltage, charge/discharge current, state of charge, and time to charge/discharge. The instrument also warns the user when the charge level is too low so that load can be shed or charging commenced to protect the battery. A resettable Amp Hour counter is provided for checking the efficiency of the charging system or keeping a track of power consumption and a second input lets the user monitor the voltage of the starter battery. The simple display format does not mean the internal software is unsophisticated. In fact two separate algorithms are used. One based on integrating amp hours over the charge and discharge cycle, the second based on an adaptive internal mathematical battery model.

A unique feature of the BM1+ is its ability to use information from both algorithms, during the charge/discharge cycles, to continually update its performance. This means that, unlike many other systems that rely solely on amp hour integration, the NASA BM1+ never needs to be re-synchronised with the battery. The algorithm includes Peukert's equation to ensure accuracy over a wide range of load currents.
The BM1+ does all this whist consuming a meagre one and a half milliamps. It is a true battery monitor, easy to install and excellent value for money.

** Some say that they can calculate the state of charge from the terminal voltage only but, as we all know, the terminal voltage falls when the battery is under load and rises when under charge. This fall and rise can be quite substantial, especially when a heavy current is flowing, so how can the terminal voltage be used to calculate anything without knowledge of the magnitude of the current involved?​

 

[Bow42]

Thank so VicS
Still trying to work out if my batteries are US or not .
Theee batteries 100 ah each
BM1 showing once the sun goes down and my panel no longer charging , 100% 12.9v
My over night Amps use is no more then say 4 Amps and hour tops , if I Call that 12 hour that 48 Amps , that's works out to around 75% 12..5 / 12.4 v .
so I can't understand why my BM1 states batteries are only 50% Volts 12.2 in the morning unless the batteries have seen better days .

 

[wiggy]

I had a similar "problem" with my BM1, until I changed the batteries and realised that th the problem was indeed my batteries not the BM1, they were shagged.

 

[RupertW]

Thank so VicS
Still trying to work out if my batteries are US or not .
Theee batteries 100 ah each
BM1 showing once the sun goes down and my panel no longer charging , 100% 12.9v
My over night Amps use is no more then say 4 Amps and hour tops , if I Call that 12 hour that 48 Amps , that's works out to around 75% 12..5 / 12.4 v .
so I can't understand why my BM1 states batteries are only 50% Volts 12.2 in the morning unless the batteries have seen better days .

You don't have to guess at the 4 amps anymore. Look at the amps discharge on the BM1 and see what the actual draw is and how much each device draws by switching them on in turn. Fridge will be a little more complicated as it goes on and off so try to estimate how much it is on per hour. Also - have you entered the 300ah capacity into the BM1 settings?

Finally - is the 12.2 V with everything off and is the 12.9 V with everything off and at least 30 mins after the last charge?

 

[ghostlymoron]

I think that the information from these devices should be treated as a guide and for this they are very useful. If you try to use them to predict to the second when your lights will go off, you're asking too much.

 

[PaulRainbow]

Thank so VicS
Still trying to work out if my batteries are US or not .
Theee batteries 100 ah each
BM1 showing once the sun goes down and my panel no longer charging , 100% 12.9v
My over night Amps use is no more then say 4 Amps and hour tops , if I Call that 12 hour that 48 Amps , that's works out to around 75% 12..5 / 12.4 v .
so I can't understand why my BM1 states batteries are only 50% Volts 12.2 in the morning unless the batteries have seen better days .

You can't just pull figures out of the air to work out your usage, that'll tell you nothing. You have a battery monitor, use it to monitor what you're using.

4a an hour during the night is miles off, unless you have a weird setup, what on Earth do you have that would use 48ah during the night ? In reality you should be using a fraction of that, unless you are leaving things like TV, laptop etc turned on overnight.

You need to check exactly what is being used.

 

[Ric]

Thank so VicS
Still trying to work out if my batteries are US or not .
Theee batteries 100 ah each
BM1 showing once the sun goes down and my panel no longer charging , 100% 12.9v
My over night Amps use is no more then say 4 Amps and hour tops , if I Call that 12 hour that 48 Amps , that's works out to around 75% 12..5 / 12.4 v .
so I can't understand why my BM1 states batteries are only 50% Volts 12.2 in the morning unless the batteries have seen better days .

Yes, I would say your batteries are shagged. But before you replace them, try renovating them with one of those smart chargers that zaps them with alternating current to break off the sulphating on the lead plates. I renovated mine three years ago and they are still like new.

 

[Pasarell]

You can't just pull figures out of the air to work out your usage, that'll tell you nothing. You have a battery monitor, use it to monitor what you're using.

4a an hour during the night is miles off, unless you have a weird setup, what on Earth do you have that would use 48ah during the night ? In reality you should be using a fraction of that, unless you are leaving things like TV, laptop etc turned on overnight.

You need to check exactly what is being used.

Depends where he is. My overall usage from when solar stops providing anything until when it starts is not too far off that 48A but I'm in the Ionian and vast majority of current goes to the fridge but if we're on board in the evening using (led) lights, computers etc it is amazing how much goes.
I have a BM1 and it is very useful but the %age left display is just a toy I think. I work from Amps out and remaining battery voltage when the fridge is not running and ignore the % display.

 

[Cantata]

From the information you give, I agree your batteries have had it.

 

[Trundlebug]

There seems to be some confusion in the posts above between Amps and Amp Hours.

The only way to tell how much you have consumed is to switch to the Ah screen, then hold the button down for 2-3 seconds to zero it. I usually do this on arrival somewhere i.e. after charging.
If you have a look at the same screen in the morning it will tell you exactly how many Ah you have consumed overnight. No need to estimate, or guess at how long the fridge has been on.

If your consumption really is 48Ah overnight from a 300Ah bank, that's significant and it's very likely your batteries are very tired. They're getting a hard life. By comparison my fridge and everything else uses much less than 10 Ah - usually about 4- 7Ah only overnight.

The % charged bar at the side, as stated above is very much an estimate and you shouldn't pay too much attention to it.
Much better to look at actual voltage, amps out, and total Ah consumption.
The same Ah consumption screen can be used with good effect to tell you if the vbatteries are tired.
If you discharge 48Ah and then recharge, with new batteries that screen will show zero (or maybe 1 Ah charged) when the batteries are fully charged.
As the batteries get older they will take 55, 60, 70Ah total to get them back to fully charged, so this screen will show +10, +20 or +30Ah charged. The bigger the difference the worse they are.

Hope that makes sense?

 

[doug748]

Hope that makes sense?

Yes, very good that Trundlebug.

I have a BM1 and am very pleased with it.
The % charge is very easily pleased though (It will show 100% after, a little charging, when I know it is not full) and easily discouraged ( It shows 80% after very little use, from fully charged).

Assuming they were all new at the same time, for a rough check, the OP might take one battery for a test if he knows a reliable shop.

 

[GHA]

Thank so VicS
Still trying to work out if my batteries are US or not .
Theee batteries 100 ah each
BM1 showing once the sun goes down and my panel no longer charging , 100% 12.9v
My over night Amps use is no more then say 4 Amps and hour tops , if I Call that 12 hour that 48 Amps , that's works out to around 75% 12..5 / 12.4 v .
so I can't understand why my BM1 states batteries are only 50% Volts 12.2 in the morning unless the batteries have seen better days .

A few thoughts....
48Ah overnight is probably on the high side, with a fridge in a warm climate plus lots of youtube on a laptop in the evening, raspberry Pi on all the time I use about maybe 35Ah overnight.
With 300Ah getting pulled down to 12.2v overnight something is amiss - with solar there's a good chance your batts weren't fully charged or your batts are too tired for this life..

This is 225Ah of 2 x 6v t105 trojans overnight last night, the voltages are direct from the battery terminals and very accurate - good chance to show off the only yesterday wired up volt logger on the Raspberry Pi , easy to see the fridge switching on and off and the laptop getting turned off. Voltage will rarely be below about 12.6v in the morning, but then trojans are great! 300W of solar which usually gets back to fully charged most days - voltage mid/top end of 14 with less than 2A going in.

 

[Richard10002]

How does BM1 work out my SOC. % ?
Is it a case where it measure the voltage ( 12.50 v = 80% ) or does it subtract the Amps being use and any going in to the battery's bank ? .

I dont know how it purports to do it, but the SOC%age on the NASA BM models is truly useless - to the point where you can be reasonably sure that, whatever it says, your SOC% is nowhere near it (even if you have input an accurate battery capacity in the settings).

I use my NASA BM2 to check voltage, Cumulative Amp Hours in and Out, and instantaneous charging or discharging rate in Amps. I very quickly learned that the SOC%age is useless.

After a few years I added a Smartguage, (£105 at Cactus at the moment), and this seemed to rperesent SOC much better.... although it is not accurate when charging, in that it reaches 100% long before the batteries are full, (as established by the Ah being drawn when the Smartguage shows 100%.

 

[Heckler]

There seems to be some confusion in the posts above between Amps and Amp Hours.

The only way to tell how much you have consumed is to switch to the Ah screen, then hold the button down for 2-3 seconds to zero it. I usually do this on arrival somewhere i.e. after charging.
If you have a look at the same screen in the morning it will tell you exactly how many Ah you have consumed overnight. No need to estimate, or guess at how long the fridge has been on.

If your consumption really is 48Ah overnight from a 300Ah bank, that's significant and it's very likely your batteries are very tired. They're getting a hard life. By comparison my fridge and everything else uses much less than 10 Ah - usually about 4- 7Ah only overnight.

The % charged bar at the side, as stated above is very much an estimate and you shouldn't pay too much attention to it.
Much better to look at actual voltage, amps out, and total Ah consumption.
The same Ah consumption screen can be used with good effect to tell you if the vbatteries are tired.
If you discharge 48Ah and then recharge, with new batteries that screen will show zero (or maybe 1 Ah charged) when the batteries are fully charged.
As the batteries get older they will take 55, 60, 70Ah total to get them back to fully charged, so this screen will show +10, +20 or +30Ah charged. The bigger the difference the worse they are.

Hope that makes sense?

Excellent explanation! Thanks for that. I was asking the same sort of questions a few weeks ago. The starter battery was shagged 100% so changed that. Then a 12 hour motor with the Sterling booster going for it soon showed up the next shagged one and two more starting to get seriously warm! One of the four of the bank was still good, it didnt even get warm. When I got them out the dates were on them Dec 2008 and 08/08 so I cant complain!
Stu

 

[Plevier]

I can't comment on the BM1, no experience of it, but here are a couple of points the OP does not seem clear about.
1 The open circuit voltage of a battery in good condition, not significantly degraded from new, will give you a pretty good indication of its state of charge and available capacity. It needs to be measured some time after taking it off charge or off load. If the battery is NOT in good condition, the OCV will NOT give you much idea of its state of charge or available capacity.
2 You cannot use the terminal voltage under load to deduce the state of charge, even of a good battery. After you remove the load, the terminal voltage will rise. If it rises a lot after removal of a load of a smallish load, it's a fair indication that the battery is not in a happy state.

 

[Bow42]

Thanks for all the info .
Replaced batteries, all good now , BM1 in the morning now says 80% 12.6v with a small load on .
Better then the 12.2v 55% I was getting .

 

[Daydream believer]

I have been following this thread as i have the older bm1.
I have 2 90 amp hour agms so checked it was set up as stated above. I have rarely looked at the time to go screen & forgot it existed. Early this morning i was woken by the fan on my triple stage charger coming on. That surprised me as apart from ais & echomax radar reflector nothing else is wired to work. The taps drip so occassionally the pump picks up pressure & that is it ( no fridge etc & been on shore power for 4 days) did use the lights last night
I guess we must have had a power cut in the night. I could see that 6 amps had been used so guess that was the lights before i went to bed. Screen showed 90% charged. Seems low for just 6 amps use. Been charging for 3 hours, initially it was charging at 7.7 amps. voltage now shows 13.7 volts as usually does when on shore power with .2 amps charge. However time to go shows 104 % charged & zero hours remaining instead of the 199 it had before.
So question is--- have i set this up wrong or is this normal?

 

[sailaboutvic]

Just taken a look at mine .
Says 104% , zone hour to charge , its charging at 9.7v as the mid day sun is on the panels and , Volts 13.7 ,
I guessing the fridge is working at the moment .
Considering the fridge take around 3.0 Amps , I can't understand if the battery's are fully charge why it's charging at 9.7 , one would think it would only charge at the rate it useing .

 

[Pasarell]

Hi Vic
First things is ignore the 104% charge. This is almost always wrong - by a long way. Ignore this screen completely!
I expect your fridge will take a lot more than 3A. Mine takes 6.2A and I expect your is similar to mine.
9.7A at midday when the battery voltage is 13.7v sounds about right to me when the fridge is running.
What solar controller do you have? Mine is a Victron MPPT controller that was new earlier this year so the latest version. It charges my sealed lead acid batteries to 14.4v then drops down to a float and absorption voltage of about 13.3v. The time from 13.7v up to 14.4v is quite long as the batteries can only take a diminishing current.
My BM1 will show %age charge at 104% long before reaching full charge, then if really at 100% and the fridge cuts in it will drop quickly to 90% or 80%. The %age charge screen should really be ignored for all practical purposes.