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SoC for Batteries - with a lot of data!


Dr Bob

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Guys,

I want to try and understand where I am with my batteries and SoC. Apologies for the long post but keen to understand the data I am looking at.

There were 3 * 110 Ahrs bog standard cheap lead acids when we bought the boat (63ft 2002 NB). “Almost new” said the broker but no receipts. Guess they are probably less than a year old but seemed to be ok in the first few days (and certainly not poor). We then added 3 more 110Ahrs, that was a couple of months ago – so we now have 660Ahrs. Since then we have cruised most days between 4 to 8 hrs and also have 2*100W solar panels with an MPPT controller. We've done at most 7 days on shore power. The SoC has never dropped below 80% (??????) based on amps counted out on the battery monitor and the voltage 'at rest' which I am guessing (probably badly).

I am interested to see if this 80% is just a figment of my imagination (given Tony and Wotever's comments to me in previous post)!!! So some real data.

On Tuesday pm I assumed we were 100% SoC after 5 hrs engine running and then 3 hrs of strong sun. Tuesday night we used approx 50 Amp hrs according to the BMV battery monitor and the voltage in the morning was 12.50V (but taken with a small current draw alternating between 1.7A and 5.2 A depending on whether the fridge was running (240V via a Victon inverter). I have not tried a 'at rest' voltage as the inverter is always on so I am assuming the 12.5V is slightly higher and could be 12.6V at rest. This seems to suggest I was nearly at 100% SoC on the Tuesday afternoon.

Wednesday we ran for 5 hrs but then it was cloudy but I assumed we were almost full and reset the amp counter to zero.

Thursday morning (today) the Amps out were again 50Ahrs and the voltage with inverter and fridge was 12.48V. I turned everything off and after 10 mins the voltage was 12.55V. Ok, maybe 85% SOC?? but would the voltage have been higher if it was at rest for an hour?

Thursday I did a bit of monitoring viz

Time          Voltage    Amps     Amp hrs

8.00am       12.50         -1.7        -50 (not quite at rest with inverter on but no fridge – but coffee machine had taken 80A for a few mins not long before)

Engine on 8.15

8.35            13.58         30          -40

10.00          13.67        16.1         22.8

10.45         13.71        13.9          33.1

11.15         13.87        10.8          47.7

11.45         13.88         8.3          46.4

12.15         13.89         6.5          49.9

14.05        14.00          4.6          58.5

Engine off, but strong sun

14.10        14.15         4.7           58.9

gave up and went to the pub.

All the above were measured with the inverter on powering the fridge and a variety of laptops, tablets and phones charging.

I have a few questions on the above data:

  • I was expecting to see the amps stabilise at 2% of battery bank (therefore nearly 14A) and hold steady for 45 mins once full. Problem is that the voltage never got to 14.4V (or 14.8V etc). Can the Amps be used at these lower voltages to determine SoC.

  • The charge is coming from the engine alternator, I think a 90A one – Engine is a 2002 Beta 43. Should Voltage be higher in the float stage? Not a clue how to alter it.

  • The Ahr counter is interesting. It starts at -50Ahrs (what I think I used last night) and then went positive by nearly 60Ahrs – which suggests 110Ahrs had gone into the battery. Even if you reduce this by 20% you still get 90Ahrs which is a bit above what I thought was in the batteries (from the voltage at rest – or not as it was).

  • The solar continued to charge most of the afternoon at 9-10amps or so, with 5A being used by the inverter – voltage shown was 14.1V. On the MPPT controller it shows that the Adsorption voltage is set at 14.4V and the Float at 13.6V. I assume these settings were put in by the installer 2 years ago. Should I set the float higher? I assume the 14.1V measured is the 14.4V less the drain from the inverter.

  • With a battery bank this size, what is the likely voltage at rest if the voltage with say a 2A drain is 12.50V. Any guesses?

  • Did I ever get to 100% SoC?

 

Tomorrow early I will look at the Amp hrs out overnight and the voltage at rest (with everything off for at least 30mins).

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Unless I have missed something, your batteries are always working, charging or running fridges etc. Therefore they will never be at 100% SOC. 

However,  looking at those readings, there is nothing wrong with them either.

Best reading was ....

.gave up and went to pub.

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  • Haha 1
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The large over-run on AH seems odd. Are you sure the AH counter (BMV) is installed correctly? It seems quite common to install them incorrectly so that something either a charge source, or in you case a discharge source, is bypassing the BMV shunt. Have a look at the domestic battery negative terminals. They should be connected to each other but only connected to one other thing - the shunt. If there are any wires going anywhere else then this represent something bypassing the shunt which will make the BMV read garbage. Remember NO other wires. No, not even that one! No, not one going to the engine battery negative. Nope, definitely not that one... get the general idea?

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I agree with Nick, the BMV isn't seeing everything.

My BMV goes negative overnight, then gradually goes positive as the battery charges, and usually only needs 10% more put in than taken out to getvback to 100% SoC.

The voltages look fine.  A 5.2 amp discharge on a 660ah battery will not affect the voltage significantly. 

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1 hour ago, Dr Bob said:

With a battery bank this size, what is the likely voltage at rest if the voltage with say a 2A drain is 12.50V. Any guesses?

12.50V :)

Okay, maybe 12.51V ;)

1 hour ago, Dr Bob said:

Problem is that the voltage never got to 14.4V (or 14.8V etc). Can the Amps be used at these lower voltages to determine SoC.

Sadly not, no. At least not without a lot of experimentation to draw up a graph or table to determine what a 'full' battery will draw at say 14.1V. 

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1 hour ago, Dr Bob said:

I assume the 14.1V measured is the 14.4V less the drain from the inverter.

Never assume. What's the voltage at the output of the controller?

Do you have more that one method of measuring voltage? An error of 0.3V is not unheard of with a cheap multimeter. 

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2 hours ago, matty40s said:

Unless I have missed something, your batteries are always working, charging or running fridges etc. Therefore they will never be at 100% SOC. 

However,  looking at those readings, there is nothing wrong with them either.

Best reading was ....

.gave up and went to pub.

......and the pub was a cracker ...the Globe at the top of the Ashby. Food was excellent and the Timothy Taylor, not a bad pint. Dont care about the batteries this evening.

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2 hours ago, nicknorman said:

The large over-run on AH seems odd. Are you sure the AH counter (BMV) is installed correctly? It seems quite common to install them incorrectly so that something either a charge source, or in you case a discharge source, is bypassing the BMV shunt. Have a look at the domestic battery negative terminals. They should be connected to each other but only connected to one other thing - the shunt. If there are any wires going anywhere else then this represent something bypassing the shunt which will make the BMV read garbage. Remember NO other wires. No, not even that one! No, not one going to the engine battery negative. Nope, definitely not that one... get the general idea?

thanks Nick ...and Cuthound. I am not sure. I have not yet done a deep dive into the engine bay to check out the terminals, but will schedule this for the next time I get down there. I seem to remember on the yacht we lived on for 3 years had a similar problem of 'jumping' amphrs in or out - and that was wired up right. I tend not to take much notice of the amp counter other than what has gone out overnight.

I do understand what you are saying as I did install one correctly in the past. I will check.

33 minutes ago, WotEver said:

Never assume. What's the voltage at the output of the controller?

Do you have more that one method of measuring voltage? An error of 0.3V is not unheard of with a cheap multimeter. 

I am just using the voltage on the battery monitor for the voltages, but it does seem to tie up closely with the voltage on the MPPT controller. I have no idea how accurate the voltage shown is and if it is worth quoting it to two decimal places.

Ok, some good input so far. Now I know I cannot use the amps in to assess SoC with the low charging voltage.

......so is my Tail voltage ok? Should it be 14.4V and if so, how do i get to that. Can you adjust the alternator output?

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21 minutes ago, Dr Bob said:

I tend not to take much notice of the amp counter other than what has gone out overnight.

Until you establish the cause of the discrepancy it is equally as likely that the Ah used is the one that's incorrect. One of them (used or replaced) would appear to be wrong so I'd suggest that fixing that would be a good first step. 

24 minutes ago, Dr Bob said:

I am just using the voltage on the battery monitor for the voltages, but it does seem to tie up closely with the voltage on the MPPT controller.

So do they both read 14.1V? Or does one read 14.1 and the other 14.4?

39 minutes ago, rusty69 said:

It makes an Ass out of u an me, and mtb with his two dodgy Smartgauges:)

But MtB likes his Smartgauges like that. Otherwise he'd either get them calibrated or do it himself. 

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26 minutes ago, Dr Bob said:

thanks Nick ...and Cuthound. I am not sure. I have not yet done a deep dive into the engine bay to check out the terminals, but will schedule this for the next time I get down there. I seem to remember on the yacht we lived on for 3 years had a similar problem of 'jumping' amphrs in or out - and that was wired up right. I tend not to take much notice of the amp counter other than what has gone out overnight.

I do understand what you are saying as I did install one correctly in the past. I will check.

I am just using the voltage on the battery monitor for the voltages, but it does seem to tie up closely with the voltage on the MPPT controller. I have no idea how accurate the voltage shown is and if it is worth quoting it to two decimal places.

Ok, some good input so far. Now I know I cannot use the amps in to assess SoC with the low charging voltage.

......so is my Tail voltage ok? Should it be 14.4V and if so, how do i get to that. Can you adjust the alternator output?

With our AH counter, the charge required to replace the consumed charge is roughly 6% more. Which is not a huge difference between AH out and in. There is no reason why your AH counter shouldn't be similar. Unless it is mis-wired!

Regarding your alternator charge current, if it's only starting out at 30A for such a big battery bank, it is either a very small alternator, the engine revs were too low, or there is a blown diode in the alternator. By comparison, when I start the engine with our 450AH bank I get an initial current of 140A or so, settling at perhaps 100A after 10 minutes. We have a big modern alternator though. What alternator do you have?

Your low charge current and voltage could simply be because it's an ancient alternator. But whatever, if the voltage is only reaching 14.05v at a very low current, you are not going to be able to,properly charge your batteries with your engine, especially in winter when the solar won't be helping much. Even 14.4v is on the low side for modern leisure batteries.

All that of course presumes that you are correctly measuring voltage and current and in fact we can already be pretty sure that you aren't measuring the latter correctly (because the AH counter measurements don't balance).

Most alternators can't be adjusted, but with some it is possible to fit a regulator that is set to a higher voltage. Also, low voltage and current could be due to poor wiring between alternator and batteries.

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1 hour ago, nicknorman said:

 

Regarding your alternator charge current, if it's only starting out at 30A for such a big battery bank, it is either a very small alternator, the engine revs were too low, or there is a blown diode in the alternator. By comparison, when I start the engine with our 450AH bank I get an initial current of 140A or so, settling at perhaps 100A after 10 minutes. We have a big modern alternator though. What alternator do you have?

Your low charge current and voltage could simply be because it's an ancient alternator.

The alternator is the original on the Beta 43 from 2002. I think it is a 90A one. There is also an engine battery alternator.

When the engine is started it has been around 40A for the first 4 mins or so with the SoC around 90% so i dont think it is far off in initial current but the tail voltage seems low.

1 hour ago, WotEver said:

 

So do they both read 14.1V? Or does one read 14.1 and the other 14.4?

 

I will have a look tomorrow and report back.

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17 minutes ago, Dr Bob said:

The alternator is the original on the Beta 43 from 2002. I think it is a 90A one. There is also an engine battery alternator.

When the engine is started it has been around 40A for the first 4 mins or so with the SoC around 90% so i dont think it is far off in initial current but the tail voltage seems low.

I will have a look tomorrow and report back.

The current seems very low to me. Even though the batteries may be at 90% SoC, with such a large bank I'd expect the surface charge demand to be the max alternator output.

Just for clarity, there is no direct relationship between present SoC and charging current - it depends on "how you got there". So a battery discharged to 50% and then recharged, might well have quite a low charge current by the time the SoC gets to 90%. But a battery that has just been discharged to 90% SoC will have a very high charge current when first put on charge.

I'd check for any voltage drop in the charging circuit - ie check the voltage across the alternator vs the voltage across the battery, during the early stages of charge when the current is maximum.

If there is a significant difference between the voltages, that points to a poor / high resistance connection or inadequate wiring.

if there isnt, I'd suspect a partial alternator failure such as a blown diode.

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From your figures, the alternator voltage was still rising when the engine was switched off and the charging was left to the solar. This would imply that your batteries were not fully charged.

The Ahr counter may be out of sync, especially if it has been told that the batteries were fully charged when they weren't.

The Amp figures seem low, unless there is a current drain at the same time, so that not all the current is going to the batteries. The tiddly alternator on Gamebird (50A, I think) does better than that, and would be putting out about 15A at around 14V, when my batteries would be about 80% charged. Both alternator and solar appear to be going through the BMV shunt correctly.

There may be a bit of resistance somewhere: how is the split charging arranged; diode or relay?

 

 

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10 minutes ago, Iain_S said:

From your figures, the alternator voltage was still rising when the engine was switched off and the charging was left to the solar. This would imply that your batteries were not fully charged.

 

 

 

That is what I was thinking and why I posted the thread.

 

11 minutes ago, Iain_S said:

 

The Ahr counter may be out of sync, especially if it has been told that the batteries were fully charged when they weren't.

 

 

...and I am sure this was the case...when I reset the counter the previous day.

12 minutes ago, Iain_S said:

 

There may be a bit of resistance somewhere: how is the split charging arranged; diode or relay?

 

 

Not a clue. It is just the original alternator. I will do some checking of voltages over the weekend.

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Have only read this quickly, and have had a few beers, but it looks like you are saying that after 6 hours of engine running (8:15 to 14:05) you have 4.6 amps going into the batteries with a voltage of 14.0volts. 14.0 is much much too low, your batteries will never get fully charged. (or at least not in a realistic time).

You need a higher charging voltage and a Smartguage.

................Dave

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8 hours ago, Iain_S said:

how is the split charging arranged; diode or relay?

Keep up at the back :D

It's a twin alternator set up, no split charging. I agree, though, that initial current is way too low. With a 450Ah bank at 90% in the morning, I see an initial current in three figures at around 14.2 V, swiftly dropping off as the surface charge builds up.

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25 minutes ago, NB Lola said:

 Not wanting to derail this but if I got lithium batteries would I still need to understand this, what is to me, mumbo jumbo?  If I do need to learn this given I will be replacing with lithiums next year, where is the best place?

No, lithiums have a number of advantages over lead acid including not minding not being fully charged, not minding being fully discharged, left flat etc. The disadvantage is of course the cost! And that they don't fade out when fully discharged, they just stop working.

Edited by nicknorman
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1 hour ago, NB Lola said:

 Not wanting to derail this but if I got lithium batteries would I still need to understand this, what is to me, mumbo jumbo?  If I do need to learn this given I will be replacing with lithiums next year, where is the best place?

They do look very attractive but don't look anywhere close to cost effective at present, but if you enjoy been at the "bleeding edge of technology" then go for it.

..................Dave

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1 hour ago, dmr said:

They do look very attractive but don't look anywhere close to cost effective at present, but if you enjoy been at the "bleeding edge of technology" then go for it.

..................Dave

Roughly how much for say 440 a/hours? though I understand from previous threads that less may well be needed as they can be run down so to speak.

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4 minutes ago, philjw said:

Just to give you the idea of the price, here's a 90Ah at battery megastore for£997.00

http://www.batterymegastore.co.uk/victron-bat512900400-bms300200000.html

Berlimey!!!!!! They are as yet then pie in the sky much like electric cars etc etc..............

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4 minutes ago, mrsmelly said:

Roughly how much for say 440 a/hours? though I understand from previous threads that less may well be needed as they can be run down so to speak.

Have a search on www, I found something a couple of weeks ago and did a rough mental calculation but its all slipped from my memory, probably too much beer drinking.

Preety sure it was a link on this forum.

Its never trivial comparing batteries, Trojans can go down to 20% but its probably best to keep them above 40 or 50%. I think Lithium can go to 0% but maybe 20% is recomended. Trouble is its probably not good to go down to 20% cus if you have a crisis (engine won't start or need to go out urgently) then you have nowt in reserve, so 50% is probably wise regardless of battery requirements.

I think Lithium came out about 3 to 5 times as expensive. I reckon they will become ideal boat batteries, but I suspect its a few years off yet.

............Dave

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2 minutes ago, dmr said:

Have a search on www, I found something a couple of weeks ago and did a rough mental calculation but its all slipped from my memory, probably too much beer drinking.

Preety sure it was a link on this forum.

Its never trivial comparing batteries, Trojans can go down to 20% but its probably best to keep them above 40 or 50%. I think Lithium can go to 0% but maybe 20% is recomended. Trouble is its probably not good to go down to 20% cus if you have a crisis (engine won't start or need to go out urgently) then you have nowt in reserve, so 50% is probably wise regardless of battery requirements.

I think Lithium came out about 3 to 5 times as expensive. I reckon they will become ideal boat batteries, but I suspect its a few years off yet.

............Dave

Yes agreed. They are going to be good as will electric cars someday but we are certainly not there yet.

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