Battery charging

[Wild Goose400]

Hi first time on this forum and I’m not technical.

I’ve spent two hours with the engine running to charge my boat domestic batteries today.

The battery monitor said they were down to 45% and we causing fire up the heater.

After 2 hrs charging they were still only at 56% according to the monitor.

Went ashore for two hours and came back having left only the fridge on and everything else off.

The monitor now shows 100% charged!

Any ideas what’s going on? Is the battery monitor faulty?

 

[Tranona]

The important figure is the resting voltage. a fully charged battery should be above 12.7 volts. the %age charged figures are only a guide as it depends on how the monitor is set up. surprising though to go from 45 to 100 in only 2 hours - 45-56 is more like it. Give it 3 hours with no load and measure the voltage.

 

[Refueler]

Hi first time on this forum and I’m not technical.

I’ve spent two hours with the engine running to charge my boat domestic batteries today.

[/QUOTE]

Are you with old fashioned 1-2-both switch or VSR ?

If VSR - then engine most likely was spending most of its time charging Engine Start battery before domestic battery.

The battery monitor said they were down to 45% and we causing fire up the heater.

[/QUOTE]

Sorry - don't understand about 'heater' ???

After 2 hrs charging they were still only at 56% according to the monitor.

Went ashore for two hours and came back having left only the fridge on and everything else off.

The monitor now shows 100% charged!

Any ideas what’s going on? Is the battery monitor faulty?

Fridge was probably at temp and therefore reduces down to very low demand and I assume this charge was from mains charger - not engine ?? If nothing else on - charge will be good and directed where you decide ..

 

[PaulRainbow]

A VSR doesn't work like that. Within a very short time of starting the engine the alternator voltage will trigger the VSR. There is no way for it to prioritise one battery.

 

[Neeves]

It might help if you define what you mean by 'battery monitor' and what brand it is. Or take a picture of it. How big is your battery bank? Some monitors allow you to scroll through various options, amps in, amps out, % state of charge, volts - what are your readings. When did you last fully charge the batteries and rest the monitor to 100%. How big is your engine and alternator

Usually you get to the battery being near full and then it takes an age to fully charge to 100%. But if you battery bank were small it would be quicker.

Basically the more data you novice the more useful will be the answers.

Your third sentence 45% and heater - needs explanation.

Jonathan

 

[noelex]

Battery monitors can drift out of calibration. They will reset to100% SOC when certain parameters are met.

I suspect in your case that your battery monitor adjusted its SOC to 100% reasonably rapidly when these conditions were satisfied.

 

[William_H]

Battery monitors come in 2 styles. Sophisticated type measures accumulated current into batteries and subtract current out. So giving a % that relates to the conditions when monitor is set up or zeroed. (100%)
The simpler monitors just interpolate battery voltage as a typical charge percentage. Battery voltage varies from high when on charge medium when fully charged and left standing for hours while lower when being discharged or finally flat. My opinion on this type is that they are useless and a simple volt meter will tell you what you need to know.
However I would recommend an amp meter which indicates charge current from engine driven alternator. This will indicate when a battery charge engine run is doing useful charge and when due to battery voltage rising only a small charge is going in. ie waste of engine run. Having said that another battery added will get you more charge in from engine run. (not closer on charged but rather more total) ol'will

 

[Ostara24]

Welcome to the forum, I am kinda new here myself.

Are you aware of how a battery is charged in stages? ie BULK/ABSORBTION/FLOAT?

45% is very low for a battery unless it is lithium, I assume it is not?

At that starting level(assuming your monitor is calibrated correctly), the initial charge on engine would be bulk, BUT, do you have a smart alternator?, if not it would of been a low charge rate if only running engine at low revs, with an “dumb” alternator, hence the small increase.

If you then plugged into shore power after turning off engine, your shore power battery charger(which is “smart” will have instantly noticed the low battery charge rate and gone into max charge ie BULK, which would have speeded things up until absorption charge was reached and the next charging state was initiated.

It is worth an hour of your time to check the required charging voltages asked for by your batteries (written on them) and setting your charger appropriately, and finding out if your monitor can be reset when you have allowed your batteries to go through a complete charging cycle on shore power for say 24hours or when the amps going into your batteries are around 1% ofyour total battery voltage at ABSORBTION voltage not FLOAT OR BULK. It is worth resetting your monitor every few months as things can get out of whack for all sorts of reasons that are too detailed to mention here.

.

 

[Refueler]

A VSR doesn't work like that. Within a very short time of starting the engine the alternator voltage will trigger the VSR. There is no way for it to prioritise one battery.

OK .. I have always considered it as charging up first battery then once voltage is up - switching in 2nd ... but thinking about it ... of course the alternator will be pumping out over 14v ..

 

[Wild Goose400]

It might help if you define what you mean by 'battery monitor' and what brand it is. Or take a picture of it. How big is your battery bank? Some monitors allow you to scroll through various options, amps in, amps out, % state of charge, volts - what are your readings. When did you last fully charge the batteries and rest the monitor to 100%. How big is your engine and alternator

Usually you get to the battery being near full and then it takes an age to fully charge to 100%. But if you battery bank were small it would be quicker.

Basically the more data you novice the more useful will be the answers.

Your third sentence 45% and heater - needs explanation.

Jonathan

Thanks everyone for your replies. The domestic bank is 3 batteries which are showing 11.9 volts and 12volts with everything switches off on the monitor (see photo).

There doesn’t appear to be enough charge to start the diesel heater.

The engine battery is consistently reading 12.7volts.

 

[Wild Goose400]

I have an engineer coming later and will report back.

 

[Tranona]

That suggests bad batteries, but first check voltage at the terminals. The engine battery has an easy life so is usually fully charged as soon as the engine starts and runs a few minutes.

As you can see from this chart 11.9v is entering the danger area and not surprised it can't cope with a high demand load such as starting the heater.
octopusasia.com/battery-soc-to-voltage-chart/

 

[Neeves]

If Tranona is correct, and he is seldom wrong, then you will need new batteries. But if you have an engineer coming he will probably want to sell you batteries. The question you have to ask him, or yourself, how did the batteries 'destroy' themselves. You don't want to buy new batteries until you are sure you can keep them sufficiently charged to offer a long lifespan.

You said originally that the batteries were at 45%, then after 2 hrs charging 56% and then 2 hours later at 100% - but the picture of the mon seems to suggest 56% or did it drop from fully charged to 56% - in which case the batteries have given up the ghost.

If the engineer is coming 'later' and this gives you the time - describe your battery practice/usage, size of batteries, and the assembled members will be able to offer you advice on what they might do in your situation. You can weigh up the answers offered here with the suggestion from the engineer. ......That's what the forum is for . But if you have a big bank you may need to rely on shore power or solar and the latter might need some imaginative solutions as to where to instal - so details of the vessel might also be useful.

I'm not prying - but we are groping in the dark.

Jonathan

 

[B27]

I suggest ignoring the battery monitor for now, either the batteries are pretty dead, or there is a circuit problem.

When the charging and batteries are basically known to work, then you can see if the monitor is doing its job.
It may be fitted or set up wrong.

 

[Fr J Hackett]

This may help the OP if he hasn't already got one.

https://www.nasamarine.com/wp-content/uploads/2015/12/Clipper-BM1BM2.pdf

 

[Scubadoo]

In my experience I've never trusted the NASA BM battery % readings and pretty much ignore it, instead I focus on the voltage and amp readings, that tells me all I need to know.

 

[Graham376]

NASA BM1 monitors are pretty accurate as far as amps and volts are concerned but take the capacity bar graph as a rough indication only, it's (in my experience) not very accurate. Diesel heaters take a large load when starting and batteries need to be close to fully charged so no surprise yours won't run. We sometimes had to start engine to start ours. What do amps display when running your engine?

 

[simonfraser]

'Diesel heaters take a large load when starting and batteries need to be close to fully charged so no surprise yours won't run'

they def need a good voltage when starting up e.g. 12.5V when being used to start the heater, (not at rest)

 

[B27]

IF the 11.9V in the image is correct at 300mA of discharge, then the battery is virtually flat.
It may also be 'dead', incapable of being recharged.

Possibly the alternator is not charging the battery.

Starting point should be to get the battery charged, presumably by shore power.
Then the whole system needs checking out.

 

[Boater Sam]

State of charge monitors tell lies unless they have been reset recently with FULLY charged batteries.
You have not been fully charging them for a while and I would expect them to be shot - sulphated.
The only way to ensure that they are fully charged is to measure the charge current going in. Keep charging until the current is less than 2% of the total battery bank capacity, preferably 1% even better. This will take many hours, a couple of hours is nothing like enough.
Taken off charge and no load for several hours the voltage of a fully charged lead acid 12v battery should be at least 12.7v.
For best life you should never discharge to below 11v. And even then, recharge immediately.