BM1 monitor goes haywire!!

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pcatterall

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Yes its electrics day today, more specifically batteries and charging.
I need to know that we can survive away from the luxury of shore power.
I checked the solar panels today, 2x40 watt panels were giving around 5 amps which I thought was ok.
I started to check my liesure batteries ( 2x85watts)
I have assumed that they really only hold 150 watts and this is what I have logged into the BM1
I put a fixed load on the system of 5 amps, all the charging was off, solar panels covered etc.
The batteries started from 95% charged and over 3 hours this reduced slowly at around 3% each hour, (I thought that this trial would eventually allow me to back calculate a more exact capacity figure for my batteries)
Suddenly though in the space of half an hour the meter nosedived to 1% .
There had been no extra load applied, no shorts etc.... I was mystified ( and still am)
Although the batteries should be flat ( according to the BM1) there was still plenty of power in them, the engine would crank with plenty of gusto.
The batteries are now charging on shore power at around 15 amps ( 20 amp charger) and the BM1 says it has 6 hours to go.
I dont understand the sudden drop although I have wondered about the readings being abit erratic ( hence my deciding to do todays 'investigation'
The batteries are 2 years old.
Can anyone advise me on this subject please.

In the last 15mins the capacity has gone up to 30%
The charger has dropped back on the charging rate to about 1amp and is suggesting it will take around 100 hours to fully charge.

It seems to me that the BM1 is lying about the % of charge left, this sort of thing has happened before and then suddenly the % is back up again but,as stated, this time I was monitoring the monitor. Clearly I would like to have confidence that what is shown on the scale is about right.

Help please!!
 
It is quite clear to me that my BM2 is generally clueless as to capacity, and %age capacity remaining. It suits me to know about volts and amps, and amphours, so I like the unit.

The general concensus is that, if your batteries are drawing less than 2% of capacity at 14.4V, they are more or less full. What capacity is, is anybody's guess :)

Smartguage purports to be better at capacity, but I wouldn't be too sure.
 
Getting watts and amp hours mixed up. Also how are you trying to monitor your batteries? The bm1 will only do a single battery (I think) so if you are trying to charge/monitor in parallel it will get confused. Use it with one and set it for the capacity of that battery.
 
neilf39 said:
Getting watts and amp hours mixed up. Also how are you trying to monitor your batteries? The bm1 will only do a single battery (I think) so if you are trying to charge/monitor in parallel it will get confused. Use it with one and set it for the capacity of that battery.
Click to expand...

Mine is only connected to one battery bank and the capacity was set when the batteries were new a few weeks ago. It seems to make a good stab at %age capacity remaining, but it is not consistent, and it rarely bares any relation to Ah used.
 
Richard10002 said:
Mine is only connected to one battery bank and the capacity was set when the batteries were new a few weeks ago. It seems to make a good stab at %age capacity remaining, but it is not consistent, and it rarely bares any relation to Ah used.
Click to expand...

The problem with battery monitoring is the variance in the battery voltages. Your new battery fully charged voltage could increase to 13 volt or more, peak, then slowly drop over a period of time to the normal 12.7- 12.8 volt. The same problem applies at the top end while being charged, so what is actually 100 % capacity, or the actual % capacity is not easy to identify.

The BM1 is using micro volts and amps from the shunt to calculate values, the sense wires are prone to false values due to inductance from cables running nearby.

In the OP's case, sorting out possible over charge voltage is more important, than trying to live with it.

Brian
 
I tend to ignore the percentage and time functions on my BM-1. But I do watch the volts and amps display more than any other instrument on board.
 
Thanks for all the responses. Perhaps I have been expecting too much on the percentage remaining side?
I have tried the test again with similar results...the expected drop in voltage and capacity left for a couple of hours then... zonk.. a sudden drop... the capacity reading about 40% but the volts still up at 12.8 then when I put the charger on its charging quicker than expected.
Yes I guess that I can keep my on outputs and time to get an idea of usage and check the voltage but I had hoped the % capacity would give at least a reliable guide.
 
pcatterall said:
Thanks for all the responses. Perhaps I have been expecting too much on the percentage remaining side?
Click to expand...

Percentage remaining is very inexact even when using Peukert's equation. It depends on...

Battery technology (wet, AGM etc)
Temperature history
Age
Time since calibration
Self discharge rate
Internal resistance
Charging voltage profile
The value the monitor uses for k
 
nigelmercier said:
My gauge has the facility to change the value for k, still only useful as a guide though. I must put a note somewhere so I remember to reset it when I know the battery is 100%.
Click to expand...

In what circumstances do you know it's 100%?

and why do Smartguage owners think their readings are accurate? And are they accurate?
 
A bank is 100% charged if you apply a decent charge source, Shorepower or Alternator, and the voltage rises to about the absorption voltage of 14.4v and the current going into the battery is less that 1% of the capacity. So a 100AH battery would be accepting less than 1 amp. This situation is usually impossible to find because the regulators will have switched down to FLOAT long before the batteries are fully charged, so you have to force the chargers back into absorption mode to see if they are charged. I have seen a difference on my 1000Ah bank of 15 amps charging in FLOAT mode which jumps to 60 amps when forced back into Absorption mode.

If you leave shorepower connected for at least 24 hours you may well achieve 100% as well.

I also have a Smartguage which appears to be 10-20% more accurate than my BEP battery monitor. Smarguage was recently tested by the Optima battery people and found to have an accuracy down to 0.1%. The Military and services are using it - in fact they were the ones who demanded a more accurate BM - and the Smarguage fitted the bill.
 
sailinglegend420 said:
Smarguage was recently tested by the Optima battery people and found to have an accuracy down to 0.1%.
Click to expand...

As even a Smartguage does not know the age of the battery, which has a major impact on its storage capability, that claimed level of accuracy is not believable. Believe it if you want, but it's just marketing speak.

Even a simple voltmeter or ammeter will only offer that level of repeatable accuracy if it is expensive and regularly calibrated. Some bit of mass-market built-down-to-a-price never-calibrated dubious technology - 0.1%, no chance.
 
In a way I'm pleased to see this thread and thanks for the info everyone, makes sense even to me !

I fitted a BM-1 soon after they came out, and when I moaned on here that it doesn't do all NASA claim it does - just gives voltage really, as said here the capacity display is hopeless, now I know why - lots of people said ' Oh no, the BM-1 is totally brilliant...
 
Seajet said:
In a way I'm pleased to see this thread and thanks for the info everyone, makes sense even to me !

I fitted a BM-1 soon after they came out, and when I moaned on here that it doesn't do all NASA claim it does - just gives voltage really, as said here the capacity display is hopeless, now I know why - lots of people said ' Oh no, the BM-1 is totally brilliant...
Click to expand...

For some reason people seem to give a lot more credibility to a measured figure that is in digital format than when the same information is in analogue. This is especially true if there are lots of significant figures on the display. It is a well known phenomenon, and is known as the digital deception. Marketing people love it.
 
Thanks for all that guys. Its confession time here again! althoughj I take on board the general comments re the capacity% read out my real issue was the sudden drop that I was experiencing. I started the investigation again and THIS TIME actually pulled the batteries out so I could really see into the cells rather than using a torch and mirror! It was obvious that I had not been topping up tho the right level each battery took 1 litre of distilled water ( my hands of been smacked, partly for my stupidity and partly because I have used up all the ironing water!!)
I will now repeat my trials and report back!!
 
AngusMcDoon said:
As even a Smartguage does not know the age of the battery, which has a major impact on its storage capability, that claimed level of accuracy is not believable. Believe it if you want, but it's just marketing speak........
Click to expand...
When SmartGuage first came out nobody believed it could do what it claimed - even James Hortop (manager of Merlin and ex tec editor at PBO) didn't believe it - and then he bought the company.

The SmartGuage designer is a bit of an electronics genius who is now locked in a cupboard with no contact with the outside world. He is developing new cleverer software for the US military, so someone believes in him.

The whole point about the SmartGuage software is it learns about your batteries as they charge/discharge - reading the voltage 1100 times a second. The older the batteries get the more accurate SmartGuage gets.

I've tried for two years to programme my battery monitor to make it get even close to what I think my batteries are doing, without success. My AGM batteries are now ten years old so I have no real idea what their capacity is. That is a major problem for a BM. My Peukerts value will also be changing with age, and because that has a logarithmic effect on the Ah reading a small difference from 1.15 to 1.17 can have a large effect on the displayed Ah. Also an accurate charging Ah count is affected by the “efficiency” of the battery at accepting charge. Wet cells may need 130Ah put in to register 100Ah, whereas AGMs may only need 102Ah. If all these values or not input correctly, or they change with age it will upset the Ah count accuracy.
 
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