Cable size

[Joshua]

I am still trying to get to grips with wild discrepancies in my battery monitor readings.

 

Currently low voltage readings (suggesting 55 to 60% SOC) and actual SOC readings, amp consumption and hydrometer readings (which suggest a range between 80% and 90%).

 

Could the size of cable inter-connecting my batteries have a bearing?

 

They appear to be about 10mm, I note other cables, not those connecting my batteries are as much as 20mm.

 

Could the relatively smaller 10mm cable cause false low voltage readings?

 

 

 

 

Joshua

 

 

[bottle]

How are you measuring the cables?

 

Mine are 50mm² and that is on a 24v system on a 12v system they may/probably need to be bigger than that.

[Joshua]

How are you measuring the cables?

 

Mine are 50mm² and that is on a 24v system on a 12v system they may/probably need to be bigger than that.

 

Easiest way I could Keith, 10mm is the outside diameter of the cable (including the insulating outer).

 

If they were under size, what would be the consequences, I mean how would that manifest itself in relation to battery monitor readings, if at all?

 

Joshua

Edited September 16, 2012 by Joshua

[robbio c]

I agree with Bottle deffinately upgrade the size of the cables 50sq minimum.

[bottle]

Mine are 15mm, including the insulation, so at a guess yours are a lot less than 50mm²

 

Undersized cables mean volt drop.

 

Hopefully the battery monitors are directly connected to the batteries, so no effect on the readings.

 

I have just re-read your original post and it is only the inter-connects that are 'small', they still should not influence the monitor readings .as long as the monitors are directly connected to the batteries.

 

I would still up-grade the inter links to the same size as the main feeds.

 

Slight tangent you do have your cable take off's at opposite ends of the battery bank and the monitors on those terminals as well..

 

Edit: fat fingers and syntax.

Edited September 16, 2012 by bottle

[nb Innisfree]

Mine are 15mm, including the insulation, so at a guess yours are a lot less than 50mm²

 

Undersized cables mean volt drop.

 

Hopefully the battery monitors are directly connected to the batteries, so no effect on the readings.

 

I have some 40 sq mm @ 12.2mm diameter

[Tony Brooks]

I am still trying to get to grips with wild discrepancies in my battery monitor readings.

 

Currently low voltage readings (suggesting 55 to 60% SOC) and actual SOC readings, amp consumption and hydrometer readings (which suggest a range between 80% and 90%).

 

Could the size of cable inter-connecting my batteries have a bearing?

 

They appear to be about 10mm, I note other cables, not those connecting my batteries are as much as 20mm.

 

Could the relatively smaller 10mm cable cause false low voltage readings?

 

 

 

 

Joshua

 

10mm diameter cable is about 25 sq mm which I seem to recall is the BSS minimum. 50 sq mm cable is about 14 mm diameter.

 

(I still think your amp hour counter is telling porkies).

 

Just to add that as long as there is no significant load on the system when you measure the voltage then NO the cables will not give a false low reading because voltdrop is proportional to current flowing (load) so a very low current will only cause a very low voltdrop an with 25 sq mm cable I doubt 10 amps flow would cause any significant voltdrop.

Edited September 16, 2012 by Tony Brooks

[smileypete]

I am still trying to get to grips with wild discrepancies in my battery monitor readings.

 

Currently low voltage readings (suggesting 55 to 60% SOC) and actual SOC readings, amp consumption and hydrometer readings (which suggest a range between 80% and 90%).

 

Maybe give the batts a FULL charge then find out how many amp-hours you can get, using the batt monitor, until the batts are about half way.

 

cheers, Pete.

~smpt~

[Gibbo]

If there is no current flowing, there won't be any voltage drop.

 

If you're measuring the voltage whilst there is current flowing, then you're wasting your time anyway.

[Joshua]

Maybe give the batts a FULL charge then find out how many amp-hours you can get, using the batt monitor, until the batts are about half way.

 

cheers, Pete.

~smpt~

 

That’s a good idea Pete, problem is I don’t know when I am half way, do I trust the voltage reading, hydrometer, SOC reading or amp consumption read out, they are ALL different, the range is as much as 30% or more so a big gap!

 

Here is yesterday’s data:

 

Start with fully charged batteries (until receiving an input current of between 4.5A and 3.5A, it seemed to settle there).

 

24 Hrs later we have the following readings:

 

SOC = 90.6%

 

Voltage with 8amp load = 12.04V

 

Voltage after 30 min no load rest = 12.29 and stable (experience suggests it might well rise to about 12.35 given another couple of hours)

 

CE (amps consumed since full charge) = 88ah (that’s out of a total available of 660)

 

This gives the following contradictory SOC readings.

 

 

 

 

SOC actual monitor reading = 90.6%

 

SOC based on at rest Voltage = 61%

 

SOC based on consumption = 86.5%

 

SOC based on Hydrometer reading = 75% + (3 positive disks)

 

 

 

 

One problem is that apart from the hydrometer readings, all of the others are from the same battery monitor which itself could be at fault.

 

In December I will be back at the boatyard where the system was installed and they will be able to change some parts like the monitor and shunt to see if they are at fault, in the mean time I want to do as much diagnostics as I can to help target the problem.

 

I think the best I can do is trust my hydrometer and keep taking readings while I slowly discharge the bank to a hydrometer reading of 50%, then see which if any of the monitor readings agree!

 

 

 

 

Joshua

 

 

 

 

PS can I use my multi-meter to check the battery voltage and will this be more accurate than the hydrometer, it’s a bog standard digital household multi-meter and I note the manual gives me a max DC input limit of 1000Volts, it has the usual piddly little wires and probes.

 

If I can, what is the procedure?

 

 

[bottle]

To read a voltage with a multi-meter, set the dial to 20v dc. range if your system is 12v (which it is) place red probe on positive terminal of battery and black lead on the negative terminal.

 

The only thing you need to be aware of is the accuracy of the meter, it does need to 0.5%

 

Your multi-meter and your gauge should give the same reading.

[WotEver]

SOC actual monitor reading = 90.6%

You can ignore this, because Ah counters are always wrong.

 

SOC based on at rest Voltage = 61%

You can ignore this because you haven't given the batteries 24 hours to rest.

 

SOC based on consumption = 86.5%

You can ignore this because you don't know what the actual capacity of your bank is. It will be less than the amount stamped on the batteries because they're not brand new.

 

SOC based on Hydrometer reading = 75% + (3 positive disks)

This is probably closest to the truth, but without comparing several hydrometer readings you can't be sure how accurate your hydrometer is. It's probably reasonably accurate.

 

Tony

[smileypete]

Start with fully charged batteries (until receiving an input current of between 4.5A and 3.5A, it seemed to settle there).

Out of interest, what voltage do you usually charge them up to, or at what voltage did the above current stabilise at?

 

PS can I use my multi-meter to check the battery voltage and will this be more accurate than the hydrometer, it’s a bog standard digital household multi-meter and I note the manual gives me a max DC input limit of 1000Volts, it has the usual piddly little wires and probes.

To get some idea of SoC the meter needs to be accurate to +-0.1%. With cheap multi-meters sometimes they're nigh on perfect, sometimes hopeless Best to use the meter in the batt monitor, should be accurate enough.

 

Bit baffling why the batts are performing as they do. How are they connected, in a ladder with the all the + connections at one end and all the -ve connections at the diagonally opposite end? Do they have any branding or other identification on them?

 

You could try giving them a full charge then drawing 80% of 660Ah at 30 amps or less. If the batt voltage collapses below 11V then you know they're under capacity. If they're healthy the odd 80% discharge shouldn't be too much of a problem.

 

cheers, Pete.

~smpt~

[Gibbo]

Have a look in the menus on your battery monitor. Has it calculated a really high charge efficiency? (there's a very good reason for this question).

[Joshua]

Out of interest, what voltage do you usually charge them up to, or at what voltage did the above current stabilise at?

 

 

To get some idea of SoC the meter needs to be accurate to +-0.1%. With cheap multi-meters sometimes they're nigh on perfect, sometimes hopeless Best to use the meter in the batt monitor, should be accurate enough.

 

Bit baffling why the batts are performing as they do. How are they connected, in a ladder with the all the + connections at one end and all the -ve connections at the diagonally opposite end? Do they have any branding or other identification on them?

 

You could try giving them a full charge then drawing 80% of 660Ah at 30 amps or less. If the batt voltage collapses below 11V then you know they're under capacity. If they're healthy the odd 80% discharge shouldn't be too much of a problem.

 

cheers, Pete.

~smpt~

 

Hi Pete,

 

 

 

 

I am about to conclude a lengthy 6 hour charge that began when the batteries were already showing 95% SOC.

 

I have left the generator running pending any suggestions Forum members might have as to tests I could usefully now run?

 

Does anyone have any suggestions before I stop the charge?

 

Currently the charge voltage is indicating 14.26/28 and the current is oscillating between 4.4 and 3.6 amps.

 

I have checked the batteries and can give you the following info:

 

They are simply marked “Dual Purpose Marine Battery” and are “Multicell”, there are no other markings.

 

All the negatives are connected and from one end of the row of 6 bats lead to one side of a shunt, all the positives are also linked and connect from the other end of the row to the other side of the shunt. There is also a 94ah starter bat independently wired to the shunt and an isolation switch.

 

If anyone has any ideas as to what tests I might now run, now would be a good time to say.

 

As soon as I turn the genny off, I will start by taking a hydrometer and voltage reading using my multi meter, should I let the batteries calm down first ?

 

 

 

 

Joshua

 

 

 

Have a look in the menus on your battery monitor. Has it calculated a really high charge efficiency? (there's a very good reason for this question).

 

My monitor, Gibbo, is set with the following settings:

 

Bat Capacity = 0660 (6 x 110ah bats, 6 weeks old and almost constantly under charge since new with never more than about 15amp max current drawn at any one time).

 

Charged Voltage = 13.2 (default)

 

Tail current = 0.8% (on 660ah bank that equates to a charge current of 5.28 amps for 3 minutes before full charge registered, default is 4% based on a 200ah bank)

 

Charge Efficiency factor = 95% (default)

 

Peukert exponent = 1.2 (obtained on specific verbal request from the battery supplier)

 

Those are the basic variables upon which the monitor synchronises itself.

 

Bat capacity is based on a 20h discharge rate at 20c.

 

 

 

 

Joshua

 

 

[nicknorman]

 

All the negatives are connected and from one end of the row of 6 bats lead to one side of a shunt, all the positives are also linked and connect from the other end of the row to the other side of the shunt. There is also a 94ah starter bat independently wired to the shunt and an isolation switch.

 

 

If that were the case, there would be a big bang as the shunt would represent a near short circuit across the bats!

 

Anyway, sounds to me like the shunt is not measuring all current in and out, due to something bypassing the shunt. From your description I suspect it might be the starter battery -ve. If that is connected to the domestic bank -ve it represents an alternative current path to/from the domestic bank. Make sure the starter battery -ve is not connected to the domestic bank -ve.

 

I would also set the tail current a little higher, maybe 2%, at least until you have got the monitor working reasonably well.

[Gibbo]

So your battery monitor doesn't calculate the charge efficiency itself? Some do, some don't. Of those that do, you can override it. Is the 95% your manually entered one? Or a monitor calculated one?

 

I agree with Nick (this is why I asked about the charge efficiency figure). I think some of your loads are bypassing the shunt and you're actually discharging much lower than you think.

 

I also think your batteries are in a much worse condition than you think.

 

PS. I also suspect your starter battery is connected to the wrong side of the shunt. I only say this because almost everyone does it wrong. That includes boat builders, "marine engineers" and especially "marine electrical engineers".

 

PPS. I know, literally, hundreds of "marine electrical engineers". About 5 of them know how to wire a shunt up properly.

Edited September 17, 2012 by Gibbo

[Joshua]

So your battery monitor doesn't calculate the charge efficiency itself? Some do, some don't. Of those that do, you can override it. Is the 95% your manually entered one? Or a monitor calculated one?

 

I agree with Nick (this is why I asked about the charge efficiency figure). I think some of your loads are bypassing the shunt and you're actually discharging much lower than you think.

 

I also think your batteries are in a much worse condition than you think.

 

PS. I also suspect your starter battery is connected to the wrong side of the shunt. I only say this because almost everyone does it wrong. That includes boat builders, "marine engineers" and especially "marine electrical engineers".

 

PPS. I know, literally, hundreds of "marine electrical engineers". About 5 of them know how to wire a shunt up properly.

 

While you were writing that, I was in the engine room doing some more checks and I wrote this before I read your reply, I am exited that we may have discovered my problem but now also worried about the consequences, am I in danger of a big bang as Nick puts it and if so why hasn't it yet and could this wiring have damaged my batteries?

 

The following is what I was typing while you were:

 

Turned genny off. Left batteries to calm down for 30 min or so then took hydrometer and voltage readings.

 

First surprise, the bat bank (6 new bats) show on the hydrometer 3 disks = 75%+

 

Old starter battery of unknown age and condition but at least 2 or 3 years old shows 4 disks = 100%!

 

 

 

 

Next, multi-meter reading, domestic bank shows 13.07v while battery monitor indicates 13.09v

 

Same with starter battery.

 

 

 

 

One other curious factor. There does not appear to be any + and – signs on the starter battery terminals, so noting which of the terminal cables from the starter battery went to the side of the shunt into which all the domestic bank + terminals lead, I treated this as the + lead. However this gave me negative value on my multi-meter so I switched them round and got a positive reading.

 

In other words, it appears that the negative terminal lead from the starter battery, goes to the side of the shunt where the positive leads from the domestic bank go. In other words, not to the side where my domestic bank negative lead is connected. On one side of the shunt there is a single cable and that is the cable joining all my domestic bank negatives.

 

Is this possible/OK?

 

The positive cable from my starter battery goes first to an isolation switch and then some other bus bars (I think that’s what they are called) and other Victron electronics.

 

 

 

 

Is any of this relevant?

 

 

 

 

Joshua

 

 

 

[Gibbo]

No it's not possible.

 

As Nick said, if you had negative to one side of the shunt, and positive to the other side, something would (not might, not possibly, not perhaps, but absolutely 100% certainly would) have gone bang.

 

You're mis-seeing something. Badly.

[Joshua]

If that were the case, there would be a big bang as the shunt would represent a near short circuit across the bats!

 

Anyway, sounds to me like the shunt is not measuring all current in and out, due to something bypassing the shunt. From your description I suspect it might be the starter battery -ve. If that is connected to the domestic bank -ve it represents an alternative current path to/from the domestic bank. Make sure the starter battery -ve is not connected to the domestic bank -ve.

 

I would also set the tail current a little higher, maybe 2%, at least until you have got the monitor working reasonably well.

I just reread your post Nick and it would appear that my starter negative lead is correct in not being wired with the Domestic bank. I take it when you say big bang you mean if the positive terminals are on the other side of the shunt, I must recheck this now.

Joshua

 

 

[Gibbo]

Joshua where are you? Perhaps a forum member can come and have a look for you?

Edited September 17, 2012 by Gibbo

[Joshua]

If that were the case, there would be a big bang as the shunt would represent a near short circuit across the bats!

 

Anyway, sounds to me like the shunt is not measuring all current in and out, due to something bypassing the shunt. From your description I suspect it might be the starter battery -ve. If that is connected to the domestic bank -ve it represents an alternative current path to/from the domestic bank. Make sure the starter battery -ve is not connected to the domestic bank -ve.

 

I would also set the tail current a little higher, maybe 2%, at least until you have got the monitor working reasonably well.

 

Just checked, in fact the positive terminals from both the domestic bank and starter go to the same 'Bus' bar.

 

The starter negative goes to the opposite side of the shunt to the dom bank. Apologies, there is a mass of cabling of different sizes and colour and it is difficult to track the path. I see of course, now what you mean by a big bang, even my idiot brain can understand that. My father who was an electrical engineer will be groaning in his grave!

 

As far as my problem is concerned, I am now disappointed that I am non the wiser.

 

Could it simply be that these are rubbish batteries and I need to get a better quality?

 

I can understand that might lead to poorer performance but would it throw the monitoring into such a spin?

 

 

 

 

Joshua

 

 

 

Joshua where are you? Perhaps a forum member can come and have a look for you?

 

Saul junction on the Sharpness and Gloucester.

 

Joshua

 

So your battery monitor doesn't calculate the charge efficiency itself? Some do, some don't. Of those that do, you can override it. Is the 95% your manually entered one? Or a monitor calculated one?

 

I.............

 

By the way, the efficiency factor is a manual setting, the default is 95% but I can set it to whatever I like.

 

I have no idea how to calculate it, would the supplier have this information?

 

In any event, my hydrometer readings suggest that my dom. bank is not charging fully, the starter battery reading is a good bench-mark.

 

Given my end of charge voltage and current readings, should they be fully charged by now?

 

Is this a sign of poor quality batteries or do I have to charge for even longer?

 

 

 

 

Joshua

Edited September 17, 2012 by Joshua

[Gibbo]

What I meant was, the battery monitor can check the charge current and state of charge all the way through an entire discharge and recharge cycle. From this it can automatically calculate the charge efficiency. It is almost impossible to do so by hand (you can do it if you have a month to spare). Some monitors do it, some don't.

 

I still think your shunt is wired up wrong. I'm rarely wrong on these matters

 

Richard Edwards was a good one in your area. But he's retired now

Edited September 17, 2012 by Gibbo

[nicknorman]

Just checked, in fact the positive terminals from both the domestic bank and starter go to the same 'Bus' bar.

 

The starter negative goes to the opposite side of the shunt to the dom bank. Apologies, there is a mass of cabling of different sizes and colour and it is difficult to track the path. I see of course, now what you mean by a big bang, even my idiot brain can understand that. My father who was an electrical engineer will be groaning in his grave!

 

 

 

Sorry but that description doesn't really make much sense because you are saying that the starter and engine bat +ves are connected, and the -ves are connected via the shunt. So really, that would effectively make them one bank. I think it's likely that you are mis-tracing the wires. As Gbbo says, the money is on an incorrectly wired shunt, so perhaps you should take some time out to carefully draw the connection diagram on paper, then scan or photograph it and post it on the internet with a link here.

 

In my limited experience I agree with Gibbos comments on the ability of "professionals" to wire a shunt up correctly - I recently had to fix the shunt installation on a friend's boat who had just had it installed "professionally" but incorrecly.