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 I'm after picking your brain setting up Victron BMV-700 it asked amp hours my batteries are AH115 each X4 do I put in 115 or 460 because I have 4 batteries Thank you for helping 

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460 if your batteries are brand new and in perfect condition - it wants the capacity of the whole battery bank.

 

If they aren't you'll need to accurately measure their actual capacity and put that figure in.

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5 minutes ago, Russ9294 said:

 I'm after picking your brain setting up Victron BMV-700 it asked amp hours my batteries are AH115 each X4 do I put in 115 or 460 because I have 4 batteries Thank you for helping 

 

 

Normally you would put 460, but because of the treatment they have received (allowing them to discharge down to 11v and not recharging properly)  they will be considerably less than that.

You need to do a measured controlled discharge and measure what the are actually achieving NOW.

Then use that to programme the BMV

 

 

Do you know how to work out the capacity ?

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I’m guessing I brought the boat 3 yrs ago and the batteries I was told was new just before I brought it I have added 2 x solar panels 710 total

No 

No idea at all

Does this help

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13 minutes ago, Russ9294 said:

Does this help

 

No.

 

To test the battery bank you need to get it FULLY charged - that means charging until you show a current of less than ~6amps at 14.4 (or higher) volts with no change in either volts or amps for at least an hour.

 

Now the batteries are fully charged. turn EVERYTHING electrical off so there is zero discharge.

Wait 1 hour for the surface charge to disperse.

 

Take a known electrical load of around (say) 15 amps, or 10 amps (4 or 5 amps is fine but will take twice as long) connect this load to the batteries and note the time.

 

Regularly check the voltage and when it gets to 12.2 volts switch off the load, wait a few minutes and check the voltage (which will have risen to - maybe - 12.3 volts)

Put the load back on and repeat until the rested voltage remains at 12.2 volts.

 

Lets say (just to make the maths easy) it has taken 10 hours to get from 'full' to 12,2 volts.

12.2 volts is the battery 50% discharged so you battery capacity is 

 

Time (10 hours) x 2 x discharge rate (10 amps ?)

 

Your battery capacity is 200AH.

 

Substitute your own figures

Edited by Alan de Enfield
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Alternatively fully charge as Alan suggests and zero/recalibrate the BVM. Use overnight or so and then note the Ah out and the alleged percentage charged before daylight the next morning. Then for example if the BVM says 40 Ah out and 50% charged the capacity will be 40 x 100/50 which is 80 Ah battery capacity when 100% charged.

 

Only believe the % of charge for one discharge after fully charging and calibrating the meter. From then on it is likely to tell larger and larger lies by understating the amount of discharge.

Edited by Tony Brooks
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You can't rely on the calculations the BMV uses battery capacity for (such as % charge) anyway, so a best guess of remaining capacity and take anything but volts and amps with a pinch of salt would be my advice. I tend to use the voltage display whilst discharging and amps display whilst charging. If you aren't already familiar, look up 'resting voltage' to assess your state of charge and 'tail current' to know how to assess when your alternator has fully charged your batteries. For me, with roughly 400Ah now, that's a steady reading a bit less less than 2 amps, rather than the oft quoted 2% of battery capacity. Before I fitted solar, that would be my target when cruising and charging by alternator, and it's still my guide in winter when solar is unlikely to finish the job.

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27 minutes ago, Sea Dog said:

You can't rely on the calculations the BMV uses battery capacity for (such as % charge) anyway, so a best guess of remaining capacity and take anything but volts and amps with a pinch of salt would be my advice. I tend to use the voltage display whilst discharging and amps display whilst charging. If you aren't already familiar, look up 'resting voltage' to assess your state of charge and 'tail current' to know how to assess when your alternator has fully charged your batteries. For me, with roughly 400Ah now, that's a steady reading a bit less less than 2 amps, rather than the oft quoted 2% of battery capacity. Before I fitted solar, that would be my target when cruising and charging by alternator, and it's still my guide in winter when solar is unlikely to finish the job.

 

The Ah discharged will be accurate and so will the percentage charged immediately after fully charging and resetting, but after that I agree with what you say abut percentage charged being incorrect. Alan did say charge until the voltage fails to fall and the voltage fails to rise over an hour, so that may even give a 1% of capacity current flow.

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1 minute ago, Tony Brooks said:

The Ah discharged will be accurate...

Yes, that also ought to be right and, to be honest, I do glance at that myself first thing to check my usage overnight.

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Time for abject apologies. I got it wrong and @Sea Dog got it right.

 

Even if zeroed and recalibrated the % discharged will be incorrect unless the battery capacity the meter thinks is there actually is and that is unlikely.

 

After the discharge overnight (no solar input) the state of charge needs inferring from the rested voltage and from that the capacity can be calculated to be near enough. Especially if you accept that the % charged etc. and such like will be wrong most of the time and learn to use rested voltage instead.

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3 minutes ago, Tony Brooks said:

Even if zeroed and recalibrated the % discharged will be incorrect unless the battery capacity the meter thinks is there actually is and that is unlikely.

 

After the discharge overnight (no solar input) the state of charge needs inferring from the rested voltage and from that the capacity can be calculated to be near enough. Especially if you accept that the % charged etc. and such like will be wrong most of the time and learn to use rested voltage instead.

 

My BMV712 seems to do a very good job of providing what seems like accurate info on the SOC of my Lithium bank. Nominal capacity is 520Ah but, after a number of tries, I have found that setting a capacity of 485Ah seems to provide accurate enough info on SOC.

 

In a similar way, with my Lead Acid bank and a NASA BM2 monitor, I would estimate capacity by fully charging, and watching Ah used compared to voltage. This enabled me to gradually home in on the actual capacity - although the SOC on the NASA was never even close, (except in the same way that a stopped watch is correct twice a day).

 

 

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9 minutes ago, Richard10002 said:

 

My BMV712 seems to do a very good job of providing what seems like accurate info on the SOC of my Lithium bank. Nominal capacity is 520Ah but, after a number of tries, I have found that setting a capacity of 485Ah seems to provide accurate enough info on SOC.

 

In a similar way, with my Lead Acid bank and a NASA BM2 monitor, I would estimate capacity by fully charging, and watching Ah used compared to voltage. This enabled me to gradually home in on the actual capacity - although the SOC on the NASA was never even close, (except in the same way that a stopped watch is correct twice a day).

 

 

 

As we are told Lithiums don't sulphate like LA your frst paragraph is as  I would expect.

 

I also agree with the second paragraph, in my view that is the way to do it unless you can take the batteries out of service and check as @Alan de Enfield explained above. That is not so easy for a live-aboard.

 

 

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20 hours ago, Tony Brooks said:

Time for abject apologies. I got it wrong and @Sea Dog got it right.

Absolutely no need for apologies Tony, and certainly not abject ones! If the batteries are taken to full charge, the gauge would be right if it was reset to 100% - it's after that it all goes down hill! The algorithm is probably pretty good, but the variables in it cause the issue. I like the BMV, and the % charge can be a handy indicator I suppose, but the key point for most readers to hoist in is not to rely on the % figure for their state of charge assessment, and it'll be worse than useless if they haven't been pretty fussy about the set up. Of course, you're fully aware of all that, but it is useful to the discussion. :)

 

 

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Setting your battery up you use the C20 reading for your battery’s supplied by the manufacture which is slightly less than the battery one. Roughly if your battery’s are 460 it’s about 420, you need to check with manufacture. All I do then is charge battery’s fully then with nothing charging battery’s let them drop below 85% to synchronise. After this I just watch the voltage reading and amps reading to monitor the battery charge 

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2 minutes ago, chevron said:

Setting your battery up you use the C20 reading for your battery’s supplied by the manufacture which is slightly less than the battery one. Roughly if your battery’s are 460 it’s about 420, you need to check with manufacture. All I do then is charge battery’s fully then with nothing charging battery’s let them drop below 85% to synchronise. After this I just watch the voltage reading and amps reading to monitor the battery charge 

 

That may be the correct method for new batteries, but the OP has old batteries that have been down to 11 volts and not properly recharged, they will have some amount of sulphation which means their capacity is reduced by 'an unknown amount'.

I don't see how setting their capacity 'as new' will help.

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36 minutes ago, Alan de Enfield said:

 

That may be the correct method for new batteries, but the OP has old batteries that have been down to 11 volts and not properly recharged, they will have some amount of sulphation which means their capacity is reduced by 'an unknown amount'.

I don't see how setting their capacity 'as new' will help.

In fact it is most likely to render all readings based on the set capacity inaccurate to say the least.

  • Greenie 1
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On 03/03/2021 at 20:45, Tony Brooks said:

 

The Ah discharged will be accurate 

The BVM  has a correction factor for peukert which is going to distort that figure, it results in a good educated estimate for lead acid batteries not the exact number of amp hours. If you set the peukert factor so as not to affect the amp hour count (as you should for lithium) you will find it not as accurate with the varying loads large and small for lead acid. There is also a charge efficiency factor built in which alters the charging amp hours so again the raw amp hours are not displayed, again this results in a good indication for a lead acid battery, but needs changing for lithium.

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