Maximum amount a healthy battery can recover after rest?

[Joshua]

I am trying to get to know my batteries better to establish a management schedule that suits my demands.

 

My current focus is on voltage readings.

 

I understand (GIBBO) that the only reliable reading is after the batteries have been rested for at least 4 hours and preferably 2 days.

 

The latter is impractical because I live on my boat.

 

Is it possible to estimate (either in percentage or voltage terms) by how much a bank of batteries will recover after being properly rested?

 

Battery bank is new.

 

For example: If my battery bank (660A) is giving a reading of 12.10 volts immediately after everything is turned off and say 12.24 volts after say 4 hours rest, by how much more are they likely to recover? Is there a maximum that they are unlikely to recover by?

 

The above readings would suggest my batteries are at about 55% SOC.

 

After proper rest could they reveal that in fact they are say 75% or 80% by returning a voltage reading of 12.50 volts or is that impossible?

 

I am particularly interested in these figures because at the same time as giving me the above voltage readings, my Battery monitor is returning a reading of 81% SOC and a hydrometer test seems to support this by reading all cells consistently over 75% charge.

 

I would like to know which is the more likely so that I can reliably run my batteries to below 65% SOC without risking discharging below 50%.

 

65% is the lower limit my battery monitor needs to recycle/calibrate itself.

 

 

Joshua

[Gibbo]

You need some maths. You have two voltage readings, project an exponentionally decaying recovery. That's what they will finally get to.

 

No I'm not going to explain how to do it. But unfortunately it's the only way. It's not spot on, but it's damned close.

[smileypete]

I am trying to get to know my batteries better to establish a management schedule that suits my demands.

 

My current focus is on voltage readings.

 

I understand (GIBBO) that the only reliable reading is after the batteries have been rested for at least 4 hours and preferably 2 days.

 

The latter is impractical because I live on my boat.

If they don't recover back above 12.2V then it's probably a good time to charge them, but if they do then it's OK to use them a bit longer. This was partly covered in a thread recently, linky:

 

http://www.canalworld.net/forums/index.php?showtopic=49899&view=findpost&p=930815

 

I wouldn't worry about whether they're exactly 50% SoC instead of 45% or 55%, it won't make a big difference to batt life. If you do really want to know whether they're exactly 50%, and funds allow, then just buy a Smartguage.

 

cheers, Pete.

~smpt~

[Joshua]

If they don't recover back above 12.2V ...................

~smpt~

 

 

Stop there, that is the question, knowing when they are in a recovered state. Say after 2 hours the battery voltage returns 12.2 volts and after 24 hours 12.5 volts, if I keep recharging my batteries based on the 2 hour rest reading, I will in fact never be letting my batteries discharge below 80% not as you say 50%.

 

That's a huge difference and the 2 hour rest data would not be much use in building a good battery management routine.

 

If on the other hand, the extra amount of recovery after 2 hours was always relatively small, I could rely on the 2 hour rest readings.

 

That was my question.

 

 

 

 

 

I wouldn't worry about whether they're exactly 50% SoC instead of 45% or 55%, it won't make a big difference to batt life. If you do really want to know whether they're exactly 50%, and funds allow, then just buy a Smartguage.

 

 

cheers, Pete.

~smpt~

 

I didn't say I was.

I already have a battery monitor. My problem at the moment is that its SOC and voltage readings are wildly out of sync.

I need to establish first whether there is a problem and second where it lies.

 

The answer to my thread question ' How much does a healthy battery recover' will answer the first question.

 

Assuming there is a problem, as far as I can see, there are 3 main suspects, batteries, gauge and shunt.

 

[1] The batteries are new and hydrometer readings indicate healthy.

 

[2] The gauge may well need to synchronise itself (see the thread you linked to and Gibbo's confirmation of this) and that is why I need to accurately know when my batteries are at or below 65% (the lower limit needed by my monitor to begin synchronisation) but without any trustworthy readings at the moment, hard for me to establish and therefore at the risk of me over discharging my batteries when trying to find it.

 

[3] As to the shunt, I will worry about that once/if I have established the innocence of the battery monitor!

 

 

 

Joshua

 

 

[Gibbo]

That was my question.

 

And it was answered in post #2

[smileypete]

Stop there, that is the question, knowing when they are in a recovered state. Say after 2 hours the battery voltage returns 12.2 volts and after 24 hours 12.5 volts, if I keep recharging my batteries based on the 2 hour rest reading, I will in fact never be letting my batteries discharge below 80% not as you say 50%.

 

That's a huge difference and the 2 hour rest data would not be much use in building a good battery management routine.

 

If on the other hand, the extra amount of recovery after 2 hours was always relatively small, I could rely on the 2 hour rest readings.

 

That was my question.

OK a few questions,

 

What is your situation...

 

Is your boat normally on a shoreline? do you live aboard?

Do you use a high power inverter for power hungry items?

What sort of batt bank do you have?

 

In answer to to your questions, the bigger the load the longer the batt voltage takes to recover. It just takes a bit of experience to guage how much it's likely to recover with heavy loads or light loads.

 

The limitations of shunt based batt monitors is that they don't work so well with partial batt charging, even so they need setting up properly and recalibrating as the batt bank ages.

 

Maybe do a trial if you really want, put a heavy load on the bank for a while, see how long it takes to recover.

 

cheers,

Pete.

[Joshua]

And it was answered in post #2

 

Thank you, I appreciate that, I was answering smileypete who seemed to misrepresent my original question.

 

By the way, I assume the maths is too involved to be worth posting? I thought perhaps I would take a progression of readings over regular intervals but within a more convenient duration and then extrapolate from that?

 

 

 

 

Joshua

 

 

[Joshua]

OK a few questions,

 

What is your situation...

 

Is your boat normally on a shoreline? do you live aboard?

Do you use a high power inverter for power hungry items?

What sort of batt bank do you have?

 

In answer to to your questions, the bigger the load the longer the batt voltage takes to recover. It just takes a bit of experience to guage how much it's likely to recover with heavy loads or light loads.

 

The limitations of shunt based batt monitors is that they don't work so well with partial batt charging, even so they need setting up properly and recalibrating as the batt bank ages.

 

Maybe do a trial if you really want, put a heavy load on the bank for a while, see how long it takes to recover.

 

cheers,

Pete.

 

We live onboard and tour extensively.

 

Never on shoreline.

 

We have a victron 12/3000/120 + 12vDC diesel generator.

 

6 x 110 domestic bank + starter bat.

 

 

 

Pete, as far as the recovery is concerned, I obviously am not being very clear.

 

I can't keep turning off my system for 2 or 3 days while I wait to see what the rest voltage is. I need to be able to calculate/estimate it from the rest voltage given after just a few hours.

 

And before you suggest again that I buy a smartgauge, I already have a battery monitor, a victron.

 

It may well be that the Victron gauge is crap but I have at least to try and calibrate it before I give up on it, chuck it away, and buy a smartgauge, especially as Gibbo himself tells me that his gauge will have to be recycled a few times just like my victron.

 

My problem is trying to get accurate enough data (With or without the aid of my un-calibrated battery monitor) in order to calibrate that same monitor (a catch 22?), without having to shut my boat down for several days every time I do it.

 

Another way of putting this is: Given that my battery monitor 'appears' to be returning contradictory data and I don't know how to test my shunt, how can I reliably discharge my bank to 60% SOC?

 

 

 

 

Joshua

[Tony Brooks]

We live onboard and tour extensively.

 

Never on shoreline.

 

We have a victron 12/3000/120 + 12vDC diesel generator.

 

6 x 110 domestic bank + starter bat.

 

 

 

Pete, as far as the recovery is concerned, I obviously am not being very clear.

 

I can't keep turning off my system for 2 or 3 days while I wait to see what the rest voltage is. I need to be able to calculate/estimate it from the rest voltage given after just a few hours.

 

And before you suggest again that I buy a smartgauge, I already have a battery monitor, a victron.

 

It may well be that the Victron gauge is crap but I have at least to try and calibrate it before I give up on it, chuck it away, and buy a smartgauge, especially as Gibbo himself tells me that his gauge will have to be recycled a few times just like my victron.

 

My problem is trying to get accurate enough data (With or without the aid of my un-calibrated battery monitor) in order to calibrate that same monitor (a catch 22?), without having to shut my boat down for several days every time I do it.

 

Another way of putting this is: Given that my battery monitor 'appears' to be returning contradictory data and I don't know how to test my shunt, how can I reliably discharge my bank to 60% SOC?

 

 

 

 

Joshua

 

 

But if you had studied the myriad topics here about this sort of problem you would have known it is almost 100% certain that your amp hour counter (battery monitor) is telling lies AND unless you can provide the conditions that allow the monitor to very regularly re-sync itself AND have its parameters correctly set then the only battery monitor option at this time is a Smartguage.

 

As long as the shunt is the one supplied to match the monitor, all terminals are in good order and it is in a place where all the load and charge goes through it (again diagrams in a number of threads here) it will not be the shunt. Basically one end of the shunt needs to be connected to the battery (Ideally and possibly mandatory negative) and all the other "boat circuit" wires at the other end.

 

You say that you have a 12v DC generator. Well if its like the petrol one recently advertised in Towpath Telegraph then please do not believe the add. It is untrue and you will have to run that generator for as long as you would the engine to fully charge the batteries so the monitor can be synced - that is in excess of probably 10 hours. If you are talking about the 12V output on a mains generator please look at its specifications. The Honda ones would take days to recharge a bank.

 

At the least you need a mains generator feeding the Victron (assuming its a combi-unit) for many hours or a shoreline.

 

Livaboards can get a fair estimate of battery state of charge from the voltage providing the are not running any heavy loads at the time and it has been perhaps an hour under a small load after charging has stopped. As far as recovery of "state of charge" is concerned my advice is to forget it, just monitor the bank voltage now and again. In common with so many people with your type of equipment I suspect this all comes down to insufficient charging and maybe sulphated batteries.

[Gibbo]

Thank you, I appreciate that, I was answering smileypete who seemed to misrepresent my original question.

 

By the way, I assume the maths is too involved to be worth posting? I thought perhaps I would take a progression of readings over regular intervals but within a more convenient duration and then extrapolate from that?

 

http://en.wikipedia.org/wiki/Exponential_decay

 

[smileypete]

I can't keep turning off my system for 2 or 3 days while I wait to see what the rest voltage is. I need to be able to calculate/estimate it from the rest voltage given after just a few hours.

OK thanks for the info.

 

What sort of batt voltage do you typically see first thing in the morning?

 

The batts will have had a rest overnight, then the reading will give some idea of how soon they need to be charged.

 

The odd discharge some way below 50% isn't going to do any great harm.

 

cheers,

Pete.

[bottle]

Assuming that your monitor is theBMV type you c an set calibrate/synchronise it in two ways:

 

from the manual

 

1.2 Synchronising the BMV

 

For a reliable readout, the state of charge as displayed by the battery

monitor has to be synchronised regularly with the true state of charge of

the battery.

 

This is accomplished by fully charging the battery. In case of

a 12 V battery, the BMV resets to “fully charged” when the following

“charged parameters” are met: the voltage exceeds 13.2 V and

simultaneously the (tail-) charge current is less than 4.0 % of the total

battery capacity (e.g. 8 A for a 200 Ah battery) during 4 minutes.

 

The BMV can also be synchronised (i.e. set to “battery fully charged”)

manually if required. This can be achieved in normal operating mode by

holding the + and – buttons simultaneously for 3 seconds, or in setup

mode by using the SYNC option (see sect. 3.4.1).

 

The BMV does not synchronise automatically

 

One possibility is that the battery never reaches the fully charged

state: this will dramatically reduce service life!

The other possibility is that the charged voltage setting should be

lowered and/or the tail current should be increased.

See sect. 4.3.

 

I will admit I have not read all the manual recently but cannot remember it requiring the batteries to be discharged.

 

The regime for charging your batteries is:

 

Get the gauge synchronised, as above.

 

When it reads 50% or any other number you wish, recharge the batteries. till it shows 95 to 100%

 

Once a month re synchronise you gauge.

 

I live aboard, have a Smartgauge, I roughly use about 35% of my bank capacity in a day, I know that when the gauge shows 65% I can recharge my batteries to 100% in about 3½ Hrs.

 

No synchronising, no voltage readings, no recovery times.

[Joshua]

...................

You say that you have a 12v DC generator. Well if its like the petrol one recently advertised in Towpath Telegraph then please do not believe the add. It is untrue and you will have to run that generator for as long as you would the engine to fully charge the batteries so the monitor can be synced - that is in excess of probably 10 hours. If you are talking about the 12V output on a mains generator please look at its specifications. The Honda ones would take days to recharge a bank.

 

At the least you need a mains generator feeding the Victron (assuming its a combi-unit) for many hours or a shoreline.

 

.......................... In common with so many people with your type of equipment I suspect this all comes down to insufficient charging and maybe sulphated batteries.

 

Its one of these - My link

 

Its very unlikely to be sulphating, the battery bank was new at the beginning of August, for the first 2 weeks we cruised on average 9 hours a day so they really were well charged every day. Since then we have moved every 2 or 3 days and I have charged the batteries to an indicated 95%+ before we set off just so the engines alternator could get the batteries well and truly charged.

 

I have also tested the batteries with an hydrometer and they appear perfectly healthy.

 

The problem is my monitor tells me, for example that my SOC is 80% and that my voltage is 12.2, they can't both be right. So which is, if any?

 

I am led to believe, by all the advice here that probably the voltage is the most reliable reading (my bank is at 50%), but a hydrometer test (75%+) suggests not, and just a bit of common sense tells me that with the light loads involved, 80% is probably right and the low voltage readings are bogus.

 

If they are bogus, what could cause such?

 

I have followed all the advice referred to in the manual as outlined by Keith below.

 

As far as I can tell, I have all the right settings programmed in.

 

I can tell from the recorded history data on the monitor that the battery monitor has in the last week, automatically synchronized 3 times. It also tells me that it has never experienced a charge cycle (defined as discharge to below 65% and recharge to above 90%).

 

I take it from this that my battery bank has never been below 65% because it certainly has been above 90%. I assume the latter from the input amp readings that I get after prolonged full charging, namely in the region of 5 amps. (despite the fact that those readings come from the same battery monitor that I now mistrust, simply because of the lengthy hours of charging with some very good charging equipment).

 

By the way, our bank has so far had some pretty light loads to deal with, most of the time around 5amps with radio and lights, rising periodically to 10amps when the fridge turns on. Very occasionally, we watch a DVD via PC/LED Screen/HiFi Amp and then we draw about 40amps, for everything else (washing machine, vacuum etc.) we put the generator on. We haven't yet stopped anywhere long to have had the time to give the batteries a decent workout!

 

 

Joshua

 

OK thanks for the info.

 

What sort of batt voltage do you typically see first thing in the morning?

 

The batts will have had a rest overnight, then the reading will give some idea of how soon they need to be charged.

 

The odd discharge some way below 50% isn't going to do any great harm.

 

cheers,

Pete.

 

Problem is they don’t get a rest because of [a] inverter – 2amps and fridge - periodic 5amps.

 

Next week I am planning to arrange matters so that I can turn off everything at least overnight, give them say 8 hours and hope that will provide some better data. Got to eat my way through a freezer compartment full of Salmon first!

 

 

 

 

Joshua

 

 

 

Assuming that your monitor is theBMV type you c an set calibrate/synchronise it in two ways:

 

from the manual..............

 

 

Thank you for the info Keith, see my reply to Tony above.

 

Joshua

 

 

[smileypete]

Problem is they don’t get a rest because of [a] inverter – 2amps and [b ] fridge - periodic 5amps.

Yeah but that's only about a 1% draw on a 660Ah bank, can't see it making a huge difference.

 

So given that, what sort of batt voltage do you typically see first thing in the morning?

 

I live aboard, have a Smartgauge, I roughly use about 35% of my bank capacity in a day, I know that when the gauge shows 65% I can recharge my batteries to 100% in about 3½ Hrs.

 

No synchronising, no voltage readings, no recovery times.

Seems to be almost essential equipment for a 'mobile power station', unless one can be a bit laissez faire about batt SoC

 

cheers, Pete.

~smpt~

[bottle]

The problem is my monitor tells me, for example that my SOC is 80% and that my voltage is 12.2, they can't both be right. So which is, if any?

 

Joshua

 

They could be both correct but there would have to be a large load on.

 

Have you checked the voltage given on the gauge with another voltmeter (accurate to0.5%)

 

Next week I am planning to arrange matters so that I can turn off everything at least overnight, give them say 8 hours and hope that will provide some better data. Got to eat my way through a freezer compartment full of Salmon first!

 

You should be able to do that now, check your freezer spec. it should be able to cope for at least 12Hrs without power, if you do not open the door.

[Gibbo]

POST #2 CONTAINS THE ANSWER!!!!!!!!!!!!!!!!!

[Tony Brooks]

Its one of these - My link

 

Its very unlikely to be sulphating, the battery bank was new at the beginning of August, for the first 2 weeks we cruised on average 9 hours a day so they really were well charged every day. Since then we have moved every 2 or 3 days and I have charged the batteries to an indicated 95%+ before we set off just so the engines alternator could get the batteries well and truly charged.

 

I have also tested the batteries with an hydrometer and they appear perfectly healthy.

 

The problem is my monitor tells me, for example that my SOC is 80% and that my voltage is 12.2, they can't both be right. So which is, if any?

 

 

snip

 

 

Joshua

 

 

It is almost certainly the monitor - as I said. Unless you are sure they resyc they will gradually tell you they have a higher and higher charge than you actually have. The reason is they can accurately count the amps going out of the battery and can accurately count the amps going into the battery but they can not know how many of those amps going in are being converted to battery charge.

 

The amount being converted to charge depends on a vats range of internal and external factors and such instruments seem to tend to overestimate the amount of amps being converted to charge so you get the symptom on voltage showing a low state of charge and the monitor showing a high state of charge - does that seem familiar to you? Once you have drawn a little charge off a battery and the load is low then volatge will not lie to a significant degree.

 

Trust the voltage and ignore the monitor or if you must have a monitor then buy a Smartguage.

 

I would not agree that your batteries can not be sulphated based on the reasons you give and past experience but I doubt the degree is significant at present.

[Joshua]

Joshua

 

They could be both correct but there would have to be a large load on.

 

Have you checked the voltage given on the gauge with another voltmeter (accurate to0.5%)

............................

 

Where do I buy a voltmeter, what type do I need and how (very generally) do I use one?

 

The odd voltage readings I am getting are not when under great load, typically for example, I may have an unloaded reading (just 2amps for the inverter) of say 12.24 which will drop to 12.10 when the fridge comes on then return to 12.18 quite rapidly and after 10 minutes 12.24, at the same time giving me a SOC reading of 80%+ and a hydrometer reading of “over 75%”.

 

 

 

 

Joshua

 

 

 

POST #2 CONTAINS THE ANSWER!!!!!!!!!!!!!!!!!

 

 

 

Its OK, I get it and will be having a go as soon as I have built up the energy to tackle the maths.

 

I think there may be other issues though, that I would like to eliminate.

 

 

 

 

Joshua

 

 

[bottle]

Where do I buy a voltmeter, what type do I need and how (very generally) do I use one?

 

The odd voltage readings I am getting are not when under great load, typically for example, I may have an unloaded reading (just 2amps for the inverter) of say 12.24 which will drop to 12.10 when the fridge comes on then return to 12.18 quite rapidly and after 10 minutes 12.24, at the same time giving me a SOC reading of 80%+ and a hydrometer reading of "over 75%".

 

 

 

 

Joshua

 

SNIP>>>>>>>>>>>>>>>>>>>>>>>>

 

 

 

That is exactly to be expected.

 

When a load is applied the voltage will drop and recover when the load is removed.

 

The % SOC will only change if the load is on for some time.

 

The % change is affected by the size of the load and the length of time it is applied.

 

The above voltages do seem to be a little low though.

 

The other issues you are trying to find could be bad connections.

 

You may wish to check all connections, including the cable ends.

Edited September 13, 2012 by bottle

[nb Innisfree]

OP.

About 3 years ago we had probs with our battery bank voltage crashing even though the BMV monitor said they were at 78 or 80% SoC, we had already established this was a pack of lies and had been ignoring it and instead looked at the voltage first thing in the morning (BMV voltage reading is very accurate btw) and using that as a guide. We had already been following the Victron book on boat electrics which advised an equalisation charge once per month, this turned out to be insufficient to stop battery deterioration, thinking they were on their last legs we started as a last resort to giving them regular long absorption charges with our genny and occasional landline via our Victron 24-3000-70. This recovered the batts to a reasonable degree and we decided to buy a Smartgauge but because the batts were down to less than 50% of their capacity the Smartgauge proved to be useless so for two years we relied on morning voltage readings to decide when to do an 8 hr absorption charge. (btw we have 8x125 ah AGMs)

 

So that's where we finished up, doing daily bulk charges with the engine and alternator and weekly 8 hr absorption charges with the genny and Victron and waiting until we need new batts before we can use the Smartgauge.

 

As off grid liveaboards this charging regime is probably the best you can hope for.

[smileypete]

The odd voltage readings I am getting are not when under great load, typically for example, I may have an unloaded reading (just 2amps for the inverter) of say 12.24 which will drop to 12.10 when the fridge comes on then return to 12.18 quite rapidly and after 10 minutes 12.24, at the same time giving me a SOC reading of 80%+ and a hydrometer reading of “over 75%”.

If the batt monitor can read amp-hours then give the batts a FULL charge then see how many amp-hours you get until they're about half way. This thread is worth a look for more on doing a full charge:

 

http://www.canalworld.net/forums/index.php?showtopic=50218&st=0

 

About 3 years ago we had probs with our battery bank voltage crashing even though the BMV monitor said they were at 78 or 80% SoC, we had already established this was a pack of lies and had been ignoring it and instead looked at the voltage first thing in the morning (BMV voltage reading is very accurate btw) and using that as a guide.

Congratulations you're now a batt expert!

 

cheers, Pete.

~smpt~