Battery charging, with 12V sockets and a multimeter

[zubeye]

Sorry if this has been covered many times before....

As a short term solution, I've bought a multimeter to assist in battery charging. It truly is a dark art to me, and I am very randomly running the engine each day at random revs.

I've tried all sorts of spots around the battery bank but can't get a reading at all. I think that the positive terminals are inaccessible. I could unstrap the battery pack but this doesn't seem like a practical daily option so I havn't done this yet.

The only place I've found where I can get a reading is the 12V cigarette lighter socket. This morning, with only the 12V fridge running, I had a reading of 12.4 V. I turned the engine on at 1200 revs, and this measured 13.50, increasing to 14 after about 10 minutes.

Can anyone advise if it's possible to use these readings to manage my battery charging, i.e. set my rev count and a rough idea of time?

It's not a problem to turn the fridge and all electrics off for the testing.

[Arthur Brown]

Buy and fit a Smartgauge this is a little black box that translates dark art into numbers on a display. It is likely the best available indicator of state of charge available. However still you need to do things with the readings it offers -usually start the engine, or wipe the solar panels (clean means more electricity.

 

Yes you can use volts readings to interpret the battery condition, but this requires some familiarity with what you call the Dark Arts!

 

Are you on shoreline, off grid or cruising daily?

[zubeye]

I am happy to install guages etc, but I'm pretty tied up for the next month or so, and therefore looking for an interim solution for the next couple of months.

 

I'm off grid, cruising one day a week, so 6 days stationary, followed by a full day cruise.

[Arthur Brown]

A fair approximation to a charging regime is to run the engine for about an hour every day plus a six hour session at the weekend. Unless you want to learn the "dark arts" then try this til you get a Smartgauge fitted.

If you have say 250w of solar then this will save you fuel and engine time in the summer, more will be needed to satisfy your needs in the winter.

[zubeye]

I'm totally sold on the smartguage.

 

But in the meantime, is there a way to use the 12V socket to inform my rev count, and charging time?

[nicknorman]

Using a 12 ciggy lighter socket somewhere on the boat will be fine, provided other loads are off. This is because the meter draws virtually no current. However if another device eg fridge, lights etc is switched on and drawing some current, this will drop voltage in the wiring between the batteries and the 12v ciggy socket and thus cause the meter to mis-read.

 

So you can certainly use your meter to check the battery's state of charge prior to charging (ie to tell you when you should be starting to charge) and typically you don't want to let the zero-load voltage drop below about 12.2v (having allowed a few minutes of zero load for the batteries to settle, longer if you have just made a high current discharge (inverter etc).

 

Where a voltmeter is bad is that it doesn't tell you when to stop charging. As you say, you can see the voltage rising from say 13.5 to 14 and hopefully beyond. But once the batteries are in absorption charge, their voltage will remain constant, determined by the alternator's regulator. But at this stage they are far from fully charged and the only way to know when the are fully charge or nearly so, is to monitor the current going into them. You cannot do this with a conventional meter but you can with a DC clamp meter such as the UNIT UT-203 available on ebay for around £30. This has to be put around the thick wire from the alternator to the batteries in order to measure the charging current.

 

There is no precise definition of "fully charged" but I would be looking for perhaps 1% or less of the AH capacity of the bank, eg 4.4A for a 440AH bank. This will take quite a long time to achieve and so some folk charge to less than fully charged daily, but do a long charge perhaps once a week to reach fully charged and ward of sulphation.

 

If your batteries are "shagged" (technical term) it may be difficult to reach 1% but anyway, by looking at the current every 30 mins or so, you can see when the current stops reducing significantly and that is the time to say they are fully charged.

 

Ed: If you are thinking of getting a smartgauge (good idea!) bear in mind it is very easy to install temporarily - you just need the two wires attaching to the battery posts, via the supplied fuse and don't really even need to mount the gauge anywhere, it can just sit on a shelf or whatever until you have time to install it properly. I would still recommend getting a UNIT UT-203 though, they are dead useful gadgets.

Edited August 25, 2014 by nicknorman

[smileypete]

Alan de Enfield posted this handy table on the original thread:

 

Looking at the 12V batt column, as long as the batt voltage recovers above 12.2V (aka 'resting voltage') you're doing OK.

 

For batts in daily off grid use, a full charge at least weekly is best, which can take 6-8 hrs, sounds like you'll be doing that anyway.

 

As NN says, a clamp meter is the easiest way to get a current reading without installing things. When doing the long weekly charge, once the charge current tails off to a steady low level (aka 'tail current') the batts are fully charged.

 

Worth getting at least a basic understanding of what's going on, can be very handy if problems arise in future.

 

(ETA: Nick has said this all in a more detailed and eloquent way, just thought I'd add a basic summary to help show it's not horribly complicated.)

 

cheers, Pete.

~smpt~

Edited August 25, 2014 by smileypete

[zubeye]

Awesome thanks so much. My resting voltage is well above 80% so it looks like all is well.

[valrene9600]

As NN says, a clamp meter is the easiest way to get a current reading without installing things. When doing the long weekly charge, once the charge current tails off to a steady low level (aka 'tail current') the batts are fully charged.

 

 

Could you give a guide figure?

[nicknorman]

As NN says, a clamp meter is the easiest way to get a current reading without installing things. When doing the long weekly charge, once the charge current tails off to a steady low level (aka 'tail current') the batts are fully charged.

 

 

Could you give a guide figure?

As I mentioned earlier, maybe 1% of the AH capacity ie 4.4A for a 440 AH bank. But that is just a guide. For new batteries it probably should be less, for old and knackered batteries maybe more. Just keep an eye on the current and stop when it no longer decreases after say 30 mins.

 

Or when you get bored or run out of fuel!

[valrene9600]

Thankyou.

[lampini]

 

Sorry if this has been covered many times before....

 

Indeed, many thanks for this thread - even though it has indeed been covered many times before - i just had one of those "lightbulb" moments as i never really GOT it before!

 

Edited August 25, 2014 by lampini

[Ricco1]

As an interim solution I'd suggest buying a cheap 12v plug in volt meter. Yes, you have to learn to interpret the readings when things are running but you'll soon get to grips with that. Mine serves all my needs in terms of knowledge of the state of the batteries. Can't remember exactly how much is cost but I think around 3 quid delivered, off Ebay.

[Alan Saunders]

 

Sorry if this has been covered many times before....

As a short term solution, I've bought a multimeter to assist in battery charging. It truly is a dark art to me, and I am very randomly running the engine each day at random revs.

I've tried all sorts of spots around the battery bank but can't get a reading at all. I think that the positive terminals are inaccessible. I could unstrap the battery pack but this doesn't seem like a practical daily option so I havn't done this yet.

The only place I've found where I can get a reading is the 12V cigarette lighter socket. This morning, with only the 12V fridge running, I had a reading of 12.4 V. I turned the engine on at 1200 revs, and this measured 13.50, increasing to 14 after about 10 minutes.

Can anyone advise if it's possible to use these readings to manage my battery charging, i.e. set my rev count and a rough idea of time?

It's not a problem to turn the fridge and all electrics off for the testing.

 

As others have said any load, e.g. refrigerator, TV, lighting will reduce the actual battery voltage. The 'rest voltage' with all loads disconnected for many (24) hours after charging or, maybe, twenty minutes after a major discharge may indicate the state of charge (SOC). Measuring the battery voltage with a DVM anywhere on a circuit that has no other load will be the same as measuring at the battery terminals.

 

You can certainly use a simple volt-meter plugged in wherever is convenient to optimise engine RPM for maximum charge rate even if the circuit is shared with active devices. e.g. Plug a voltmeter in wherever you can see it from the the engine throttle position, increase revs, note maximum Voltage, reduce RPM to barely maintain that Voltage. If you have a tachometer note the reading, in future that rpm will be your optimal charge rate.

 

Likely, your alternator will mostly charge your batteries at < its rated capacity but absorb all other loads up to its maximum current capacity.

 

If your voltmeter reads 14V+ by 8pm you are doing well but still need a long charge from your engine or better from shore power or PV to optimise the relative costs.

[ditchcrawler]

 

 

If your voltmeter reads 14V+ by 8pm you are doing well but still need a long charge from your engine or better from shore power or PV to optimise the relative costs.

I thought you shouldn't run the engine after 8pm sorry I have the coat already.

[dontpanic]

The State of Charge graph that Smiley Pete posted earlier in this thread has thrown me.

 

The graph shows a 12 volt battery at 100% at 12.73!

 

I assumed a full charge for a 12v battery would have been 14 volts as my 'maximum battery voltage' read out from my Victron Energy Monitor has given a reading above 14v (14.84) and the gauge on my volts meter gauge on my engine panel goes up to that. Can anyone explain what I am missing?

 

I have recently had the battery monitor fitted so I am trying to get a handle on the dark art!

[smileypete]

No problem, the chart is for 'resting voltage' when no charging, or no loads are applied.

 

Same as if the batt is disconnected and it's just sitting there not doing anything.

 

When the batt is being charged it's voltage will rise and can be anything up to 14.4V to 14.8V.

 

Clear as mud ?

 

cheers, Pete.

~smpt~

Edited August 30, 2014 by smileypete

[nicknorman]

Yup, when the battery is charging, the voltage is more about the charging device than the battery. Only when the battery is not charging and hasn't been charging for some hours, and is not discharging either, is the voltage a reflection of state of charge.

[Alan Saunders]

With respect to AdE, smpt etc., that chart of battery 'rest voltage' versus State of Charge (SOC) is missing 0% ~ 11.35V. Some lead-acid batteries might have a different rest/voltage range. A sulphated battery may recharge quickly to maximum volts, fail to provide larger loads but continue to supply small loads, lighting, TV etc. for many hours down to their minimum voltage, maybe 9V.

 

Immediately after a battery charger is turned off, whatever the SOC, you may see 13.7V generally known as 'surface charge'. With no load it will take many hours (8,12, 24?) for this to reduce to the true voltage measured to two decimal points. Some vehicle maintenance manuals recommend a large load e.g. main beam headlights (2x60W=10A) turned on for a minute or two, turn off, wait five minutes before measuring the battery voltage.

 

Similarly, if you are checking the voltage after drawing current, e.g.in the morning when the 'fridge has been running on/off, you need to wait a while (after turning off all loads) maybe twenty minutes before measuring the 'rest voltage'.

 

It has been emphasised many times here (thank you Gibbo) that a minimum 50% SOC is a design guideline intended to compromise the cost of replacement batteries versus the cost of running the main engine to recharge those batteries. Availabilty of mains power and lower cost of Solar Power skews those figures.

 

My cheap 110A FLA batteries (£70 inc VAT) are claimed to last only 50 100% discharge cycles to 0%, maybe 500 cycles to 50%. Both, I believe, imply a 100% charge between cycles and battery reduced to 80% of its original capacity. Trojan may be good but many (all?) other batteries would benefit from their regime.

 

Finally, some forum members report that ther batteries have lasted 5 years or more but they do not say how often they 'cycle' their batteries. When my boat (gas 'fridge, flourescent lighting) was only used for holiday and weekend cruising 5/8 hours per day and an occasional stay for two days (without running the engine) a single 110Ah battery 'lasted' for two years. Back then I only knew that the battery was 'low' when the 9V CRT monochrome TV stopped working, halcyon days! Cruising or moored, a typical live-aboard partially recharges their batteries every day, possibly the worst regime.

 

HTH and has not panicked 'dontpanic',

Alan

[dontpanic]

Brill, great replies. I have been in doubt that my monitors SOC read out was incorrect as it was reading 100% when the volts were around the 12.8 volts mark rather than 14v. I am feeling much more confident in it after your replies. Thank you.

 

Am I right in understanding that I should give my Victron energy battery monitor some time to settle in before I conclude if it is working correctly? It was fitted about 10 days ago. Some of the readings are not what I expect for example TTG and current have not shown a reading or at least I have not seen one!

[smileypete]

Brill, great replies. I have been in doubt that my monitors SOC read out was incorrect as it was reading 100% when the volts were around the 12.8 volts mark rather than 14v. I am feeling much more confident in it after your replies. Thank you.

 

Am I right in understanding that I should give my Victron energy battery monitor some time to settle in before I conclude if it is working correctly? It was fitted about 10 days ago. Some of the readings are not what I expect for example TTG and current have not shown a reading or at least I have not seen one!

 

If it's a BMV the SoC reading can be worse than useless if not set up properly, and the chance of that is usually slim.

 

Doesn't help that the manual doesn't explain the settings very well, and that some of the defaults are daft.

 

So just in case it's not set up properly, I'd recommend just using the voltmeter and ammeter, as explained in post #7.

 

If you'd like to check it's set up OK, please post all of these settings from the BMV: Battery capacity, Charged voltage, Tail current, Charged detection

time, Charge efficiency factor.

 

Then also post the size of your batt bank in Ah, whether sealed/AGM or non sealed batts, and whether you have solar charging or not.

 

There's another topic running on a BMV batt monitor, might be of interest:

 

http://www.canalworld.net/forums/index.php?showtopic=69564

 

cheers, Pete.

~smpt~

Edited August 31, 2014 by smileypete

[Arthur Brown]

The big issue is that a fully charged battery on a float charge is at 13.8 ish volts but a fully charged battery rested for an hour has lost nothing but the voltage will be one volt less at 12.8v. It takes a human to interpret the meter readings, and training and experience to do it well.

[dontpanic]

Hello Smiley Pete I have taken a reading/snapshot of my Battery monitor whilst I had my 2000w inverter on charging my DAB radio and my 12w solar panel attached to the batteries but it was dark when I took the reading . I was quite excited that for the first time I had a Current reading and a TTG reading. I couldn't find all the readings you listed but have listed what I could find on the BMV600S. If it would be possible to check whether it is set up correctly from them that would be fantastic . I have two AGM 125 Ah batteries for leisure and one AGM 90Ah starter battery for the engine.

 

"If you'd like to check it's set up OK, please post all of these settings from the BMV: Battery capacity, Charged voltage, Tail current, Charged detection
time, Charge efficiency factor."

 

Battery capacity 250Ah (2 x leisure batteries)

v 12.75v volts

I -0.96A (current in and out)

CE -4.2Ah (consumed energy)

SOC 99.1%

TTG 240h (time to go)

 

Historical readings

H1 -7.9Ah (The depth of the deepest charge)

H2 -4.2Ah (The depth of the last discharge)

H3 0.0Ah (depth of the average discharge)

H4 0 (number of charge cycles, counted as below 65% to above 90%)

H5 0 (number of full discharges)

H6 15.7Ah (cumulative amp hours drawn from the battery)

H7 12.25v (minimum battery voltage)

H8 14.84v (max battery voltage)

H9 1.34 (number of days since the last full charge)

H10 1 (number of times the BMV has automatically synchronised)

H11 0 (number of low voltage alarms)

H12 0 (number of high voltage alarms)

 

Sorry if the list is OTT but thought it might all help.

 

 

There's another topic running on a BMV batt monitor, might be of interest:

http://www.canalworl...showtopic=69564

 

Thank you I will take a look at the thread.

 

Thank you Alan yes it's good to get a feel for these things and finding out what is normal helps!

 

Thank you in advance Smiley pete for you help

[smileypete]

Hello Smiley Pete I have taken a reading/snapshot of my Battery monitor whilst I had my 2000w inverter on charging my DAB radio and my 12w solar panel attached to the batteries but it was dark when I took the reading . I was quite excited that for the first time I had a Current reading and a TTG reading. I couldn't find all the readings you listed but have listed what I could find on the BMV600S. If it would be possible to check whether it is set up correctly from them that would be fantastic . I have two AGM 125 Ah batteries for leisure and one AGM 90Ah starter battery for the engine.

 

 

Battery capacity 250Ah (2 x leisure batteries)

v 12.75v volts

I -0.96A (current in and out)

CE -4.2Ah (consumed energy)

SOC 99.1%

TTG 240h (time to go)

 

Historical readings

H1 -7.9Ah (The depth of the deepest charge)

H2 -4.2Ah (The depth of the last discharge)

H3 0.0Ah (depth of the average discharge)

H4 0 (number of charge cycles, counted as below 65% to above 90%)

H5 0 (number of full discharges)

H6 15.7Ah (cumulative amp hours drawn from the battery)

H7 12.25v (minimum battery voltage)

H8 14.84v (max battery voltage)

H9 1.34 (number of days since the last full charge)

H10 1 (number of times the BMV has automatically synchronised)

H11 0 (number of low voltage alarms)

H12 0 (number of high voltage alarms)

 

Sorry if the list is OTT but thought it might all help.

 

 

Not quite the ones I had in mind, there's a manual on this link, page 9 & 10 shows how to get into the settings, so the parameters in post #21 can be obtained:

 

http://www.victronenergy.com/upload/documents/Manual-BMV-600S-600HS-602S-EN-NL-FR-DE-ES-IT-PT-SE.pdf

 

BTW are the leisure batts ones that can be topped up, or sealed ones? Also worth knowing, is there a solar setup or will one be added in future?

 

cheers, Pete.

~smpt~

Edited August 31, 2014 by smileypete

[dontpanic]

My batteries are 2 x long life, deep cycle, AGM sealed 125Ah batteries. I am currently using a 12w solar panel that I clip on to the batteries with crocodile clips. I am intending to buy a 100w solar panel for a permanent set up, hopefully in the next few weeks.

 

 

 

If you'd like to check it's set up OK, please post all of these settings from the BMV: Battery capacity, Charged voltage, Tail current, Charged detection
time, Charge efficiency factor.

 

Battery capacity 250Ah

Charged voltage 13.2v

tail current 04.0%

Charged detection time 03 mins

Charge efficiency factor 095%

 

Thanks Pete