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When's 110a/h not 110a/h?


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On our trip I have had to replace our leisure batteries and purchased four Numax XV31MF batteries and after a short while it became obvious there was something wrong.

 

Our monitoring system shuts down the 240v system when the batteries get to low voltage, a friend I was travelling with is conversant with batteries and charging systems so spent the next two weeks checking the boat system for a fault.

 

No fault was found with electrical or charging system, so we started monitoring the charge against amp/hr on our Victron BMV 700 and despite only using 25% of amp/hrs the batteries went so low the system shut down.

 

I read some where somebody had a similar problem and I am not sure if it was on this forum, so if anybody can help I would be grateful.

 

Regards

 

Andy

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On our trip I have had to replace our leisure batteries and purchased four Numax XV31MF batteries and after a short while it became obvious there was something wrong.

 

 

Sounds to me as though the fault that caused you to decide to replace the old batteries might have minifested itself on the new set too.

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No fault was found with electrical or charging system, so we started monitoring the charge against amp/hr on our Victron BMV 700 and despite only using 25% of amp/hrs the batteries went so low the system shut down.

 

 

Can you expand on this please? I don't understand. Do you have actual figures from the BMV rather than percentages?

 

And how did your friend conclude there is nothing wrong with your charging system? Sounds like a poor charging problem to me on the face of it.

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Yes you'll need to give a lot more information.

 

So when charging, what is the voltage on the batteries and typically how long do you charge for?

 

What is the low voltage cut-off for the inverter set at?

 

Low voltage cut-off isn't a particularly good way to control max discharge because it depends on how much current is instantaneously being taken. Also there might be some voltage drop in the cabling between the batteries and the inverter / poor connection / resistive battery isolator etc.

 

If you have a meter it would be advisable to compare the voltage actually on the battery terminals, with the voltage at the inverter, when your usual inverter loads are on.

 

Of course it is possible that your new batteries are duff, but we are suspicious that you obviously had a problem previously, that new batteries has failed to address, so maybe the problem is somewhere else.

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I suspect that you've been relying on the BVM to Inform you of the State of Charge, am I correct?

 

If so, you will have been severely under-charging your batteries.

 

The figures requested in previous posts will confirm or disprove this suspicion.

 

By the way, there is no such thing as amps per hour, but no need to go into that at this stage.

 

Tony

 

Another question: What is the charging current at the time you generally stop charging?

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I suspect that you've been relying on the BVM to Inform you of the State of Charge, am I correct?

 

If so, you will have been severely under-charging your batteries.

 

 

How does that happen then?

 

I thought the BMV needed to be told the battery capacity, so if/when the true capacity falls below what the BMV thinks it is, overcharging would result if anything. Assuming the BMV is an AmpHour counter.

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How does that happen then?

 

I thought the BMV needed to be told the battery capacity, so if/when the true capacity falls below what the BMV thinks it is, overcharging would result if anything. Assuming the BMV is an AmpHour counter.

 

Because the percentage of the charging current that is converted to "charge" is a variable figure with too many variables to give anything but a best guess number. This means that for a lot of the time such meters tend to overestimate how much currant has been converted to charge so may read 100% charged when only (say) 80% charged. It is not helped by (I understand) by the fact that the tail current setting such gauges use to infer "fully charged" and then do a reset is set a bit too high so it resets to 100% charged when the batteries are less than that so the reading gradually drifts out of synch with the batteries.

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How does that happen then?

 

I thought the BMV needed to be told the battery capacity, so if/when the true capacity falls below what the BMV thinks it is, overcharging would result if anything. Assuming the BMV is an AmpHour counter.

The default condition for resetting the SoC to 100% is a current of 4% of nominal capacity. That is really high, and becomes even higher when the actual capacity becomes less than the badged capacity, which it inevitably does. Of course if you change the 4% to say 1% you will get a better charged battery at 100% indicated SoC, but most people don't fiddle with the settings.

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I do use the BMV to get the information but yesterday was a good example:-

1) In the morning BMV showing low voltage

2) Ran engine and charged via alternator and Genset

3) BMV, Panel and I have a charge and discharge indicator all showed fully charged at 12pm ish

4) Trent and Mersey Canal declared open and we set off 1.30pm everything showing fully charged

5) Cruised for five hours Genset used again everything showed full charge whole trip

6) This morning the BMV showed (we have a 24v system):-

22.73v charge

-57.9a/h

7) Today we have cruised 4.4 engine hours and the BMV showed 26.7 charge

Where we purchased the batteries from arranged for an engineer to meet us at Nottingham to check the batteries and he arrived one hour after we moored and they showed fully charged across all four batteries.

He also checked for any current leakage but found none, he also checked our meter against his meter and this was ok.

He then suggested that the batteries may have been on the shelf for a while and that we should go on electric hook up for 16hrs to charge the batteries to their full capacity.

So next day we booked into Nottingham Marina and was on hook up for 24hrs and then cruised for three hours after leaving the marina.

I did a manual sync. of the BMV when I took the hook up off as now the batteries were supposed to be at their ultimate charge.

When we moored the BMV showed 26.6v charge and at 10pm that evening the BMV showed 24.1charge and – 34.4amp/hrs.

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So, you have a 220Ah bank, giving you 110Ah usable. That's not a whole lot.

 

What charging current is the BMV showing as it approaches 100% SoC? This is important.

 

Your example of yesterday from flat to fully charged in 4 hours is an impossibility. The engineer who came out just after you'd taken the batteries off charge would have been reading surface charge (as confirmed by the 26.6V). As they're sealed batteries he couldn't take a Specific Gravity reading so he couldn't know if they were fully charged or not. I suspect they weren't.

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I do use the BMV to get the information but yesterday was a good example:-1) In the morning BMV showing low voltage2) Ran engine and charged via alternator and Genset3) BMV, Panel and I have a charge and discharge indicator all showed fully charged at 12pm ish4) Trent and Mersey Canal declared open and we set off 1.30pm everything showing fully charged5) Cruised for five hours Genset used again everything showed full charge whole trip6) This morning the BMV showed (we have a 24v system):-22.73v charge-57.9a/h7) Today we have cruised 4.4 engine hours and the BMV showed 26.7 chargeWhere we purchased the batteries from arranged for an engineer to meet us at Nottingham to check the batteries and he arrived one hour after we moored and they showed fully charged across all four batteries.He also checked for any current leakage but found none, he also checked our meter against his meter and this was ok.He then suggested that the batteries may have been on the shelf for a while and that we should go on electric hook up for 16hrs to charge the batteries to their full capacity.So next day we booked into Nottingham Marina and was on hook up for 24hrs and then cruised for three hours after leaving the marina.I did a manual sync. of the BMV when I took the hook up off as now the batteries were supposed to be at their ultimate charge.When we moored the BMV showed 26.6v charge and at 10pm that evening the BMV showed 24.1charge and – 34.4amp/hrs.

You've mentioned "showing fully charged" or the like, several times. But what exactly do you mean by that?

 

If you mean that the BMV has reset to 100% SoC then what is the tail current (It) set to? If you don't know, we'll presume it is the default setting of 4% of capacity. This means that when the BMV first reads 100%, in fact the batteries are still a fair way off being fully charged.

 

To know when the batteries are reaching fully charged you need to monitor the charging current and the voltage. Assuming the voltage is up around 28.8v or preferably more, the batteries can only be considered fully charged when the current has fallen to below 2A or so.

 

If you are in shore power there may also be an issue with the charger going to float before the batteries are really fully charged. Again, only by monitoring the "raw data" ie charge voltage and charge current, can you be certain that the batteries are in fact being fully charged.

 

Also, don't forget that when you quote the morning voltage and AH used, the voltage figure is only relevant if there is no load on the batteries (ie everything turned off, and left off for a while). Any load will reduce the natural resting voltage and make the voltage figure meaningless as an indication of state of charge.

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... he arrived one hour after we moored and they showed fully charged across all four batteries.He also checked for any current leakage but found none, he also checked our meter against his meter and this was ok.He then suggested that the batteries may have been on the shelf for a while and that we should go on electric hook up for 16hrs to charge the batteries to their full capacity.

Just to mention that it would be very difficult for the engineer to know the batteries were fully charged or not. Sounds like a bit of BS to me. And it also has to be said that if the batteries "had been on the shelf for a while" then they should have been charged in the shop and not left to sulphate.

 

Anyway the important thing is to be quite sure your charging equipment and strategy is adequate before definitely blaming the batteries, although it does rather look as though they have about 1/2 capacity.

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So, you have a 220Ah bank, giving you 110Ah usable. That's not a whole lot.

 

What charging current is the BMV showing as it approaches 100% SoC? This is important.

 

Your example of yesterday from flat to fully charged in 4 hours is an impossibility. The engineer who came out just after you'd taken the batteries off charge would have been reading surface charge (as confirmed by the 26.6V). As they're sealed batteries he couldn't take a Specific Gravity reading so he couldn't know if they were fully charged or not. I suspect they weren't.

We have 220a/h at 24v nothing is making sense at present.

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Andy, I have asked twice and Nick has also asked twice:

 

"What is the tail current at the point where the BMV indicates 100% charged? And at what charge voltage?"

 

Tony

We have 220a/h at 24v...

If the batteries have full capacity, yes. Of which 50% is usable, so 110Ah available. You are getting flat batteries after using around half that which suggests to me, as I said before, that you are under-charging your batteries.

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Hi Andy

 

Al the above is very technical and correct.

 

Just to possibly confuse you more, I have six batteries (130 amp.hr each) wired to give 24v so my bank is in theory, 390 amp.hr @ 24v.

 

The usable capacity,in theory is 195 amp.hrs

 

The BMV SOC reading was way out because the default battery capacity is 200 amp.hr, the BMV battery capacity has to be set correctly for your bank but without a proper test procedure you cannot know the true capacity of your battery bank and it is a constantly changing amount due to ageing and lack of complete charging. that is why the BMV is not the best to give accurate SOC readings.

It will read amps in and amps out but as you know you have to put more back than you take out.

 

So I suggest that you have been told fibs by the BMV as to when your battery bank is charged.

 

The best way as per mentioned by others is to,use the volt and amp reading to have some guide as to when your batty bank is charged.

 

High voltage and low amp, so for me that is 29.6v and about 4amps, it takes hours and hours so I compromise but give them a good charge once a week..

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Andy, I have asked twice and Nick has also asked twice:

"What is the tail current at the point where the BMV indicates 100% charged? And at what charge voltage?"

Tony

 

If the batteries have full capacity, yes. Of which 50% is usable, so 110Ah available. You are getting flat batteries after using around half that which suggests to me, as I said before, that you are under-charging your batteries.

This ^^^^

 

We've presently got a nice new battery bank, 452Ah nominal, and with the light nights and efficient electrics, we're presently only using a few per cent of capacity each day. It was down to all of 96% and 12.7V this morning, for example. But it still takes a couple of hours run time to get those watts back in the box, charging until the tail current is around 4A and the voltage at 14.5V.

 

I'm actually thinking of going down to 85Ah batteries next time, or maybe reducing the bank to 3 units.

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As I off-topically mentioned in another thread recently, we are good at 'help, mystery battery fault' threads.

 

They always resolve into 'your batteries are goosed because you've not been charging them enough'.

 

This thread seems a good example!

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As I off-topically mentioned in another thread recently, we are good at 'help, mystery battery fault' threads.

 

They always resolve into 'your batteries are goosed because you've not been charging them enough'.

 

This thread seems a good example!

 

Yes, it seems if you are wondering if they are wrecked, they already are

 

Richard

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Yes, it seems if you are wondering if they are wrecked, they already are

 

Richard

 

This is partly because there seems to be no simple and practical way of establishing the actual capacity of your battery bank.

 

Or IS there?

 

I started a thread on this years ago and all the methods suggested were complex, awkward and time consuming to apply and/or involved big approximations

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This is partly because there seems to be no simple and practical way of establishing the actual capacity of your battery bank.

 

Or IS there?

 

I started a thread on this years ago and all the methods suggested were complex, awkward and time consuming to apply and/or involved big approximations

Once you get your BMV it will be easy. Simply fully charge the batteries, then run them down as normal until the Smartgauge reads something easy like 75%. Then note the AH used on the BMV, multiply by 4 (if you chose 75% SoC) and job done, to within a reasonable accuracy anyway. Take it down to 50% SoC for better accuracy. Typically you don't need to know whether your 110Ah battery has a capacity of 105AH or 100AH, you are more interested in whether it has 50AH or 10AH!

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