Ammeter - advice needed

[blackrose]

3 hours ago, jradley said:

I'm assuming you have a separate battery for engine starting, is that right ? The question then becomes how is the charging of the starter battery and the domestic bank managed ? If you have a an automatic split charging system wired between the two, which side of the shunt is it - battery side or domestic side ? Depending on how it is wired it is entirely possible that an automatic split charging system allows charge to flow from the starter battery to the domestic bank and vice versa - it shouldn't, but that doesn't mean it's impossible. A wiring diagram would be really, really useful to have.

 

Charging of the separate start battery is from the same charger which has 3 X 12v outputs. 2 of the outputs are linked and go to the domestic bank (as per charger instructions) and the 3rd output goes to the start battery. The start battery is connected to voltage sensitive relay which sends charge current to the bow thruster bank at the bow when the start battery reaches 13.6v. 

 

The BEP shunt meter only monitors the domestic bank. It's fairly simple, and I understand it so no wiring diagram required.

2 hours ago, WotEver said:

My thoughts wandered along the same lines. Either the meter is telling porkies or there is a variable charge flowing. Between battery banks would seem the most likely. The quickest way to know is to short out the sense wires. If the meter shows 0.0 with them shorted then it would suggest that two battery banks are somehow linked. Bow thruster bank?

I'll do it later today and report back.

[jradley]

1 hour ago, blackrose said:

 

Charging of the separate start battery is from the same charger which has 3 X 12v outputs. 2 of the outputs are linked and go to the domestic bank (as per charger instructions) and the 3rd output goes to the start battery.

 

The BEP shunt meter only monitors the domestic bank. It's fairly simple, and I understand it so no wiring diagram required.

So there is a link (the charger) between the domestic bank and the starter/thruster combo.  There is a potential path right there for one bank to charge/disharge the other.

 

There is still the question of the alternator - I would expect it to be somehow connected to both the domestic bank and starter/thruster combo, so that when the engine is running it can charge both. It is important to know how the output from the alternator is wired and what devices it goes through. I would expect perhaps another voltage sensitive relay for example, but it could be a more "intelligent" system. Knowing what it is and how it's wired would help.

 

If the -ve's of all batteries are well connected together at the batteries then I wouldn't expect any drain from one bank to the other to go through the domestic circuitry (and hence shunt). However, if they are not or the connections are poor then it is possible that the -ve's are effectively connected via the domestic circuitry afterall so could go through the shunt.

 

One little trick, but might not be conclusive either way, would be to run the starter motor whilst watching the meter reading (I bet you can't do that unless there is another person to hand). If there is some phantom drain between the two battery banks then you might expect to see some fluctuation in the reading when the engine is being cranked. If the domestic and starter circuits are electrically completely separate then you shouldn't see any fluctuation when drawing current from the starter or thruster.

 

John

Edited July 26, 2019 by jradley
typos

[Chewbacka]

On 24/07/2019 at 20:43, WotEver said:

Beaten me to it. I was just about to make the same observation. 

Yes and yes. All the meter does is to measure the voltage across the shunt. If there’s no current flow then there’s no voltage and it should read zero. Shorting the wires together ensures there’s no voltage across them. If the meter reads 0 when the wires are shorted together then it’s accurate and the reading of 0.2 is the 200mA taken by the meter. 

you can also check the current meter accuracy by measuring the voltage across the shunt with a volt meter that can measure uV.  Assuming it is a 200A/75mV shunt then a current flow of 0.2A will give a voltage of 75uV.  The shunt rating is usually stamped on the side of one of it’s little feet.

 

no need to disconnect the current meter from the shunt when you do this as the voltmeter impedance is very much higher than the shunt impedance.

Edited July 26, 2019 by Chewbacka

[WotEver]

Just now, Chewbacka said:

you can also check the current meter accuracy by measuring the voltage across the shunt with a volt meter that can measure uV.  Assuming it is a 200A/75mV shunt then a current flow of 0.2A will give a voltage of 75uV.  The shunt rating is usually stamped on the side of one of it’s little feet.

Indeed, but how many cheap multimeters read that low?  My Fluke only reads down to 0.1mV. Sure, there are better meters but how many folk own one?

[blackrose]

1 hour ago, jradley said:

So there is a link (the charger) between the domestic bank and the starter/thruster combo.  There is a potential path right there for one bank to charge/disharge the other.

 

 

No, there is no link. As with all multi output terminal chargers, they are separate outputs.

[blackrose]

1 hour ago, jradley said:

There is still the question of the alternator - I would expect it to be somehow connected to both the domestic bank and starter/thruster combo, so that when the engine is running it can charge both. It is important to know how the output from the alternator is wired and what devices it goes through. I would expect perhaps another voltage sensitive relay for example, but it could be a more "intelligent" system. Knowing what it is and how it's wired would help.

 

 

Which alternator are we talking about here? The domestic alternator charges the domestic bank and that start alternator charges the start/BT banks. There is no charge connection between the domestic and start/BT systems and no other relays apart from the one I've already described between the start/BT banks. I appreciate your time in wanting to help but I really think you're on a bit of a wild goose chase here.

[jradley]

14 minutes ago, blackrose said:

 

No, there is no link. As with all multi output terminal chargers, they are separate outputs.

Without an internal circuit diagram of the charger it is impossible to know if it could leak current between outputs. It would be poor design if it could, but I wouldn't be surprised.

10 minutes ago, blackrose said:

 

Which alternator are we talking about here? The domestic alternator charges the domestic bank and that start alternator charges the start/BT banks. There is no charge connection between the domestic and start/BT systems and no other relays apart from the one I've already described between the start/BT banks. I appreciate your time in wanting to help but I really think you're on a bit of a wild goose chase here.

OK, so you have 2 alternators. That helps as we now know that there isn't a smart splitter or whatever on the alternator output between the 2 banks of batteries, just your charger.

 

There is still a very distinct possibility that the domestic alternator is draining the battery bank. If there is no isolator between the alternator and batteries then it's hard to disconnect the alternator to find out if the 0.2A diappears when disconnected. I thought you had an isolator switch becasue I was looking at the photos posted by Johny London at the start of the the thread - coming late to the party I hadn't realised you weren't the same person

 

I may well be on a wild goose chase, but in the absence of you actually finding the cause of your parasitic drain or mysterious charge current, anything which is easy to to do test is surely worth a go. I work on the engineering principle that anything that possibly can be at fault, until proven to be not at fault must be treated as as a supect.

 

John

[blackrose]

On 24/07/2019 at 20:43, WotEver said:

 

Yes and yes. All the meter does is to measure the voltage across the shunt. If there’s no current flow then there’s no voltage and it should read zero. Shorting the wires together ensures there’s no voltage across them. If the meter reads 0 when the wires are shorted together then it’s accurate and the reading of 0.2 is the 200mA taken by the meter or whatever. This is where a clamp meter can help to find out where it’s going. 

 

Sorry if I'm being a bit thick but am I supposed to connect the two sense wires while in situ? (i.e. while connected to the shunt?) Isn't this just going to bridge the shunt. I'm trying to avoid damaging anything by putting currents across where they shouldn't go so would you mind explaining in a bit more detail exactly what you mean please?

[nicknorman]

Yes, just “bridge” the shunt, effectively taking it out of circuit. ie short the two sides of the shunt together. No significant currents will flow unless you somehow include the battery positive - so don’t do that!

 

edit: actually what you need to do is disconnect one of the sense wires and connect it to the other sense wire. Otherwise the resistance of the shorting wire will still result in voltage drop.

Edited July 26, 2019 by nicknorman

[blackrose]

6 minutes ago, nicknorman said:

Yes, just “bridge” the shunt, effectively taking it out of circuit. ie short the two sides of the shunt together. No significant currents will flow unless you somehow include the battery positive - so don’t do that!

 

edit: actually what you need to do is disconnect one of the sense wires and connect it to the other sense wire. Otherwise the resistance of the shorting wire will still result in voltage drop.

Thanks. So if I disconnect one sense wire and connect it to the other sense wire on the other side of the shunt does it matter which side of the shunt?

 

Also when I reconnect does it matter which sides of the shunt the black/red sense wires go to?

Edited July 26, 2019 by blackrose

[blackrose]

Ok, with both sense wires connected to one side of the shunt I'm still getting a "0.3amp Charging" reading on the monitor

Edited July 26, 2019 by blackrose

[blackrose]

45 minutes ago, jradley said:

There is still a very distinct possibility that the domestic alternator is draining the battery bank. If there is no isolator between the alternator and batteries then it's hard to disconnect the alternator to find out if the 0.2A diappears when disconnected. 

 

It could be the alternator draining the batteries but then why would I also see a small charge? Also I fitted this battery monitor about 12 years ago and this has never happened before so if it was the alternator why wasn't it doing this before?

Edited July 26, 2019 by blackrose

[blackrose]

On 24/07/2019 at 10:29, WotEver said:

Easily confirmed by shorting the two sense wires together. If it still reads 0.2A then it’s meter drift, if it reads 0.0 then there’s a 0.2A drain somewhere. Car radio on standby? Phone charger plugged in but not charging anything?

 

So it's definitely meter drift then?

[WotEver]

1 hour ago, blackrose said:

Ok, with both sense wires connected to one side of the shunt I'm still getting a "0.3amp Charging" reading on the monitor

 

32 minutes ago, blackrose said:

 

So it's definitely meter drift then?

Yes, definitely. With the two sense wires connected together there cannot be a voltage across them and that’s all the meter reads. So definitely meter drift. 

 

Sorted

I know you’ve already done this but for the sake of anyone else reading this thread.,,

1 hour ago, blackrose said:

if I disconnect one sense wire and connect it to the other sense wire on the other side of the shunt does it matter which side of the shunt?

Nope. 

 

1 hour ago, blackrose said:

when I reconnect does it matter which sides of the shunt the black/red sense wires go to?

Yes. Otherwise the meter will read backwards. 

[blackrose]

7 minutes ago, WotEver said:

 

Yes, definitely. With the two sense wires connected together there cannot be a voltage across them and that’s all the meter reads. So definitely meter drift. 

 

Sorted

I know you’ve already done this but for the sake of anyone else reading this thread.,,

Nope. 

 

Yes. Otherwise the meter will read backwards. 

Yes, I've just discovered this! ?

[blackrose]

Thanks everyone for your help with this. I fitted the monitor about 12 years ago and haven't touched it since so forgot how it worked. Anyway I guess I'll go into the monitor settings and see if there's any way to correct the drift 

[WotEver]

38 minutes ago, blackrose said:

Thanks everyone for your help with this. I fitted the monitor about 12 years ago and haven't touched it since so forgot how it worked. Anyway I guess I'll go into the monitor settings and see if there's any way to correct the drift 

If it’s drifting from -0.2 to +0.3 I wouldn’t touch it. 

[blackrose]

So a +/- 0.5a error is acceptable? I've never noticed it to have this much drift before. It'll be interesting to see if the error reduces when it gets cooler 

[blackrose]

6 hours ago, jradley said:

Without an internal circuit diagram of the charger it is impossible to know if it could leak current between outputs. It would be poor design if it could, but I wouldn't be surprised.

 

 

Just to go back to this, I have a circuit diagram of the charger in the manual and they are definitely separate outputs. I'd be very surprised to find any multi- output charger that could leak current between terminals. What would be the point of multiple outputs if they were all connected? ?

[blackrose]

Anyway, we got to the bottom of it eventually and it was nothing to do with any current leakage.

[nicknorman]

3 hours ago, blackrose said:

So a +/- 0.5a error is acceptable? I've never noticed it to have this much drift before. It'll be interesting to see if the error reduces when it gets cooler 

Well it’s not great, but the point is that if you could adjust it down to zero under specific circumstances (ie specific temperature) then it might be worse under different circumstances (temperature).

 

In terms of AH-counting, 0.3A over 24 hrs is worth 8AH which is pretty inconsequential. Your Smartgauge is the thing to use to determine when the batteries are getting excessively discharged. The ammeter is useful to determine when the batteries are charged but again, a 0.3A error is insignificant for a large battery bank.

 

So not ideal, but equally not a big deal.

Edited July 26, 2019 by nicknorman

[TheBiscuits]

24 minutes ago, blackrose said:

What would be the point of multiple outputs if they were all connected?

None whatsoever.  As a diagnosis of failing equipment upstream though, it could be useful ... if it's not supposed to leak and it is leaking, the magic smoke is trying to get out.  From experience, it will find a way out at the most inconvenient time ...

[blackrose]

5 hours ago, TheBiscuits said:

None whatsoever.  As a diagnosis of failing equipment upstream though, it could be useful ... if it's not supposed to leak and it is leaking, the magic smoke is trying to get out.  From experience, it will find a way out at the most inconvenient time ...

 

Yes I guess so, but in that case it's unlikely that access to the charger's circuit diagram will stop the outputs leaking into each other! ? 

 

Perhaps I'm too casual about these things, but I generally assume that parts designed to do certain jobs are doing it. Otherwise I'd have to check that every bit of electronics on my boat weren't somehow leaking into each other.

 

When problem-solving I tend to investigate the most likely causes first rather than the least likely.

Edited July 27, 2019 by blackrose