Battery monitor

[noelex]

Are you talking about the % charge indication? Certainly the cumulative AH counter does not ' automatically synchronise' on a BM-1 .... and I cannot think of any way it could or should?.

I simply reset mine whenever I feel confident the batteries are fully charged (PV charging at 14.2v for several days) .... and also after leaving the boat for some weeks with batteries fully isolated. The inevitable inaccuracy means that there is always a totally meaningless AHr indication after such a time. (in fact better to disconnect the BM-1 and save the small power drain).

Vic

Since the battery monitor knows the net current going into the battery and the battery voltage it has all the information it needs to know when the batteries are fully charged.


The most commonly accepted criterion is when the net current drops to below 2% of the battery capacity (6 A for a 300 AHr battery) and the voltage is above 0.2v less than the absorption charging rate. Most battery manufactures regard this as 100% full.

At this stage most monitors will automatically zero the AHr and SOC counter. Some will even send a signal to the charge controller or battery charger to drop back to float.
On the best battery monitors the charge parameters that define a fully charged battery are adjustable.

The NASA unit is very good value and is fine for most boats, but its low cost is reflected in some limitations

 

[prv]

Now that is something I would be very interested to do!. If anyone wants to lend me a SmartGauge to conduct a proper technical comparison under real life conditions and over a worthwhile period I will be happy to do so

If you mean a proper engineering report akin to what Vyv Cox has done with some metal items, and you have the qualifications / experience to credibly produce such a thing, maybe you should approach Merlin Power and suggest it? There must be people who are dissuaded from buying a Smartgauge because they quite reasonably distrust its "magical" characteristics, so the publishing of an independent assessment might gain them some sales?

(I say Merlin because I believe they are the main manufacturers and sellers these days, having done some deal with Gibbo. Certainly mine has a Merlin logo on the front.)

Pete

 

[sailinglegend420]

....(I say Merlin because I believe they are the main manufacturers and sellers these days, having done some deal with Gibbo. Certainly mine has a Merlin logo on the front.)

Pete

According to Merlin boss James Horton the SmartGuage was developed because many of his BIG customers like the military and the services demanded an AH meter that worked more accurately. They are now happy, even if a few sailors amongst us can't get their head around how it could possibly work.

 

[halcyon]

According to Merlin boss James Horton the SmartGuage was developed because many of his BIG customers like the military and the services demanded an AH meter that worked more accurately. They are now happy, even if a few sailors amongst us can't get their head around how it could possibly work.

From memory SmartGauge were a firm based around Worcester, and taken over by Merlin. I have a lot of SmartGauge paperwork from 2007, If I have a minute I'll plough through it, may have more detail.

Brian

 

[crossbones]

I believe the NASA one has to be manually synchronised.

I have a BM1, very pleased with it and it doesn't need to be synchronised. If you want know how it works look at the NASA website here - http://www.nasamarine.com/proddetail.php?prod=BMN-1_Battery

 

[Plevier]

The most commonly accepted criterion is when the net current drops to below 2% of the battery capacity (6 A for a 300 AHr battery) and the voltage is above 0.2v less than the absorption charging rate. Most battery manufactures regard this as 100% full.

As an ex battery manufacturer I disagree strongly.
It's battery monitor manufacturers who regard 2% current as fully charged!
It's around that level that you need to drop to a much lower charge voltage because of the very low charge efficiency.
When it's really fully charged, the current will be more like .5A for a 300Ah battery, less for an AGM.

Re Smartgauge, I used to say it only measures voltage and couldn't work. However I've gradually found out more about it - but I don't have the full story - and I believe that by very clever gradient and trend monitoring it does work. Must admit to no hands on experience though.

I'm not saying MoD procurement is good, but it is not renowned for approving things that don't work.

 

[Barry Jones]

I've read the manual through thoroughly and to me, its more important what it doesn't say, rather than the bits it does.

There is another parallel and similar thread, which may be of interest:

http://www.ybw.com/forums/showthread.php?t=291299

At the end of the day, I have no axe to grind either way.

I don't have a commercial relationship with Smartgauge or Merlin and am not bothered whether YBW readers fit the cheaper BM1 or the Smartgauge. Obviously, people who have fitted BM1's are naturally inclined to defend the decision.

If people ask me to recommend and fit, I always supply Smartgauge (and usually a digital ammeter).

 

[noelex]

When it's really fully charged, the current will be more like .5A for a 300Ah battery, less for an AGM.

You are ignoring the voltage part of the criterion.

At 0.2v less than the absorption charge voltage (say 14.6v for a flooded battery) the charge current will never come remotely close to 0.5A for a 300Ahr battery.

Batteries on boats are charged aggressively. They cannot be left at lower voltages for long periods to charge them. Solar controllers and smart alternator regulators push as much charge into the batteries during the absorption stage as possible.
If more time was available ideally the charging voltage would drop back to lower earlier and the final charging current when the battery was fully charged would be lower.

These sort of differences of opinion are why its great to have the parameters of the battery monitor (and regulators) adjustable so you can set them at what you like.

 

[Plevier]

You are ignoring the voltage part of the criterion.

At 0.2v less than the absorption charge voltage (say 14.6v for a flooded battery) the charge current will never come remotely close to 0.5A for a 300Ahr battery.

Sorry got my decimal point wrong - more like 0.05. Not a precise figure.

Doesn't alter the fact that a battery is not fully charged at the point you specified.

The fully charged float current is not really what matters here. My point is that you cannot fully charge a battery in just a few hours by "charging aggressively" i.e. pushing lots ofc urrent in by maintaining a high voltage. As the state of charge rises, the charge efficiency drops, i.e. more and more of what you put in goes into gassing and heating. You have to terminate the fast charge well before full charge for the health of the battery. You can only do the last bit up to full charge more gently over a longer period - many hours - at typically 14V or a bit under. Over this period the current will drop and you can only say the battery is fully charged when it stabilises at the sort of figure mentioned above.

I'm not an expert on battery monitors but surely neglecting this is one of the reasons for them getting out of step and being optimistic? If you terminate your charge and go on to discharge when the BM says full (on your criteria) you are probably only at 90-95%. (Higher for AGM than flooded.)

 

[halcyon]

The fully charged float current is not really what matters here. My point is that you cannot fully charge a battery in just a few hours by "charging aggressively" i.e. pushing lots ofc urrent in by maintaining a high voltage.

When I was manufacturing chargers in the 80's / 90's we found "charging aggressively" reduced battery recherge capacity by upto 15%. As you were never able to complety convert the active material in the centre of the plate, running longer at high voltages then only increased sales of distilled water.

The problem is that most, if not all systems, be it monitors or chargers are based on the god 100%.

But you will never get a 100% charge to start with, then there are that many variables, just the production tolerence on your amphour counter's shunt is going to give you a % or 2 in a short time. If measureing voltage, what do you measure?, a battery has a no load voltage range, a on charge voltage range, and a on load voltage range. To add to the problem the on load voltage range varies with load being drawn, then they all vary with tempreture, and various other factors. If you are on a 2 week cruise the battery will be either on load or on charge, both variables voltage ranges based on amps.

What ever method gives a guide SOC, be it voltage or amphour counting, at the end of the day why is it critical to know you have 73% battery capcity ?, what is wrong with 70-75%.

The main point of battery monitoring is to warn when to charge the batteries again, around 55 - 45% capacity, minimum useable capacity. Anything above just says you are okay, and do not have problems.

Brian

 

[Plevier]

I agree completely with Halcyon on a short term view.
I would add that if you don't occasionally give that long slow charge to get as near as 100% charge as possible, that residual sulphation (unconverted active materuil in Halcyon's terms) will progress and reduce available capacity.

 

[noelex]

Sorry got my decimal point wrong - more like 0.05. Not a precise figure.

)

0.05A is less than the self discharge of some lead acid batteries

I'm not an expert on battery monitors but surely neglecting this is one of the reasons for them getting out of step and being optimistic? If you terminate your charge and go on to discharge when the BM says full (on your criteria) you are probably only at 90-95%. (Higher for AGM than flooded.)

The charge does not terminate, just goes back to the float voltage.

The most common reason a battery monitors SOC state of charge is inaccurate is that it has been wired incorrectly, with a load, or charging source on the battery side of the shunt.
The simpler battery monitors are a little optimistic, if the battery has not been fully charged for a long period of time, because they assume 100% battery efficiency and/or ignore Peukert equation. The better battery monitors allow for these factors and the best allow the efficiency to be adjusted.

 

[Plevier]

0.05A is less than the self discharge of some lead acid batteries



The charge does not terminate, just goes back to the float voltage.

The most common reason a battery monitors SOC state of charge is inaccurate is that it has been wired incorrectly, with a load, or charging source on the battery side of the shunt.
The simpler battery monitors are a little optimistic, if the battery has not been fully charged for a long period of time, because they assume 100% battery efficiency and/or ignore Peukert equation. The better battery monitors allow for these factors and the best allow the efficiency to be adjusted.

You are being very picky aren't you!

So I didn't get my decimal point wrong, I was just remembering an old rather pessimistic figure for float current in high antimony batteries; modern ones take a lot less. If you want some references look at http://www.telepower.com.au/INT95e.PDF
Fig 9 of this research paper shows float currents at varying temperatures for various battery types. Range at 25degC is about 0.1-0.6mA/Ah so that would be up to 0.2A for 300Ah (tending to increase as the battery gets older). I said "more like" 0.5A. In the context of this topic I don't think that margin of error affects the argument.

I didn't say the charge terminated, and if your incomplete recharge is followed by a no/low load with float charge voltage maintained for a day or so, fine. But if you go straight back onto discharge when the monitor says it's fully charged, then you have a shortfall, which Halcyon reckoned could be 15%, i put it a bit lower.

Peukert's equation - which is empirical rather than theoretically derived - describes the fact that at higher current a battery can deliver fewer Ah than at low current. However an Ah is still an Ah. If a 100Ah nominal battery is operated at a discharge rate such that you reach end of discharge after 50Ah, it still has 50Ah in it chemically (and at a reduced rate after a rest would be able to deliver it) and only needs the relevant amount of recharge. AFAICS Peukert is only relevant to predicting the discharge time remaining at that current. You could relate that back to a nominal capacity if you want but how would you interpret that? It's like the indicator in my car that tells me how many miles of fuel I have left. It will change according to how I am driving (but it won't attempt to express it in gallons.)

I suspect more important is failure to allow for widely varying recharge efficiency at different states of charge. At low states of charge it's nearly 100%; at full charge it's 0%; at 80% charge it's probably around 80% (don't take that as an exact figure!). Are there monitors that allow for the varying charge efficiency? The ones I have looked at just take an average figure - typically 70% I think it was - and this may also lead to a cumulative error with different discharge cycles.

 

[noelex]

Peukert's equation - which is empirical rather than theoretically derived - describes the fact that at higher current a battery can deliver fewer Ah than at low current. However an Ah is still an Ah. If a 100Ah nominal battery is operated at a discharge rate such that you reach end of discharge after 50Ah, it still has 50Ah in it chemically (and at a reduced rate after a rest would be able to deliver it) and only needs the relevant amount of recharge. AFAICS Peukert is only relevant to predicting the discharge time remaining at that current. You could relate that back to a nominal capacity if you want but how would you interpret that? It's like the indicator in my car that tells me how many miles of fuel I have left. It will change according to how I am driving (but it won't attempt to express it in gallons.)

Peukert's equation is important for battery monitors and the better ones allow for it. A simple example is if you withdraw 100Ahrs at 30A, or 100Ahrs at 1 amp in the former case the SOC of the battery will be lower than the latter. The battery monitors display needs, ideally, to reflect this difference.

 

[Plevier]

Peukert's equation is important for battery monitors and the better ones allow for it. A simple example is if you withdraw 100Ahrs at 30A, or 100Ahrs at 1 amp in the former case the SOC of the battery will be lower than the latter. The battery monitors display needs, ideally, to reflect this difference.

It all depends what you mean by SOC.

The one used at 30A will be at a lower SOC expressed as a %age of its useable capacity at 30A current than the 1A discharged one will be expressed as a %age of its useable capacity at 1A current, because the useable capacity is higher at lower rates. Those useable capacities are what are related by Peukert.

The ultimate example is a starter battery subject to a CCA test. By definition it is discharged to 100% of its 30 second capacity in 30 seconds so its SOC after the test is 0% relative to its 30 second capacity. But a (say) 60 Ah battery with a 600A CCA will only have had 5Ah taken out. It still has 55Ah of chemical energy in it.

The fundamental factor in the capacity is the amount of active material the battery contains. 1Ah output relates to the chemical conversion of a certain amount of lead dioxide (+ve plate) and lead (-ve plate) and electrolyte to lead sulphate and water. That relationship doesn't change. So the fundamental chemical capacity remaining is the same after either discharge and the recharge requirement is the same after either discharge.

So surely Peukert is only relevant to trying to predict remaining useable capacity at the instantaneous discharge rate? It's not relevant to calculating recharge (although recharge at different rates can be subject to a Peukert type relationship of its own because of varying efficiency!)

 

[halcyon]

But if you go straight back onto discharge when the monitor says it's fully charged, then you have a shortfall, which Halcyon reckoned could be 15%, i put it a bit lower.

Peukert's equation - which is empirical rather than theoretically derived - describes the fact that at higher current a battery can deliver fewer Ah than at low current. However an Ah is still an

It can be worse, but it varies that much with battery types, but at the end of the day you fail to convert all the active material in the centre of the plates.

I understand the same for Peukert's equation, more related to my time on fork truck design. If say you fitted a larger amperage motor, is was possible to get a guide to battery life reduction.

Brian

 

[halcyon]

Peukert's equation is important for battery monitors and the better ones allow for it. A simple example is if you withdraw 100Ahrs at 30A, or 100Ahrs at 1 amp in the former case the SOC of the battery will be lower than the latter. The battery monitors display needs, ideally, to reflect this difference.

You got me digging out very old notes on battery charging and use now.

I note the following:-

Battery voltage decreases with increase in discharge, due to insufficient time for acid defusion from the plates. If battery is returned to 20hr discharge, full capacity becomes availiable.

So in the case of a boat were it is generally a low level load, with odd high current peaks, it will have no effect on battery capacity. While on a fork truck were it is a perminent high load, with a 8/10 hour battery life, it will have an affect.

Brian

 

[noelex]

So surely Peukert is only relevant to trying to predict remaining useable capacity at the instantaneous discharge rate?

With respect I disagree.

The battery discharged at 10Ahrs @30A will be at a lower state of charge than the battery discharged 10Ahrs @ 1A . Period.
It does not matter what the discharge rate is in the future. The battery that has been discharged at 30A will have less capacity left.

This is the fundamental message of the Puekert equation.

 

[noelex]

You got me digging out very old notes on battery charging and use now.

I note the following:-

Battery voltage decreases with increase in discharge, due to insufficient time for acid defusion from the plates. If battery is returned to 20hr discharge, full capacity becomes availiable.

So in the case of a boat were it is generally a low level load, with odd high current peaks, it will have no effect on battery capacity. While on a fork truck were it is a perminent high load, with a 8/10 hour battery life, it will have an affect.

Brian

If there is a low level load and high battery capacity the Peukert equation is irrelevant, but modern sail boats often run high current devices such as water makers, and even boats with small battery capacity, use relatively high current devices such as a fridge and computer. At these levels of discharge the Peukert formula becomes relevant. After several charge and discharge cycles unless this is taken into account the SOC display will be incorrect. The better battery monitors recognise this, and adjust the reading to compensate for the Peukert equation.

 

[Plevier]

With respect I disagree.

The battery discharged at 10Ahrs @30A will be at a lower state of charge than the battery discharged 10Ahrs @ 1A . Period.
It does not matter what the discharge rate is in the future. The battery that has been discharged at 30A will have less capacity left.

This is the fundamental message of the Puekert equation.

We shall have to agree to disagree!

To be a bit philosophical too, the Peukert equation has no fundamental message. It is just an approximate empirical descriptor of observed behaviour resulting from internal resistance, separator porosity, plate porosity etc. What doesn't change is the chemistry.