Battery charge current vs voltage?

[jimp45]

I was wondering if it is possible to determine, for a given size of battery bank, the amount of charging current the batteries will accept at a given voltage.

Eg: I’m at anchor, my batteries are at 12.0v therefore I need to run my engine. How long do I run my engine for before the law of diminishing returns take over and the amount of current accepted is so low that my solar panels can complete the charging process. Can this be estimated from the battery voltage?

For example I imagine it would work like this:
12.0 v = 40A
12.1v = 30A
12.2 v = 25A
12.3v = 20A
Until
12.8v = 5A at which point I would turn off my engine and let the solar panels continue to charge.

Is there a formula or graph that would provide this info for a given sized of battery bank and alternator output.

 

[halcyon]

I was wondering if it is possible to determine, for a given size of battery bank, the amount of charging current the batteries will accept at a given voltage..

Voltmeter and ammeter, when the voltage is upto your alternator regulation point, and then watch the ammeter till it falls to a point you think is not worth the effort.

That is to say, voltage reading 14.3-14.4 volt, amps down to say 5 amp, at this point your not going to get much more into your batteries.

Once you reach regulation volts, amps will fall faily quickly.

Brian

 

[sarabande]

There's a whole section here

http://www.ibt-power.com/Lead_acid_tech/lead_acid_tech.html#

and in Battery University

http://www.batteryuniversity.com/

if you want some bedtime reading so that you can converse on equal terms with your voltmeter and ammeter.


Different types of battery need quite different treatments to reach the nirvana of full charge (like wives and husbands, I suppose)

Best to leave all the thinking to one of those clever battery chargers Halcyon knows about.

 

[jimp45]

Thanks Brian

I've just realised I would be reading the regulator voltage and not the battery voltage when charging. I looks like I will have to spend £100 on an ammeter and shunt after all.

Jim

 

[Vonasi]

Forget about voltage as any sort of signal to indicate battery charge. It is only useful if the battery has been disconnected from any input or output for a very long time. Unlikely situation on a boat.

An ammeter is the only way of knowing whether you should give up charging because it is becoming inefficient. I would strongly recommend installing some sort of battery monitor. They may seem expensive at £100-£200 but you will save this back in extended battery and engine life from not abusing your batteries or running your engine unnecessarily.

We used to have to use a generator sometimes to charge our batteries (we are much more efficient now and so don't need it) but the battery monitor allowed us to know when the 330 amp/hours of batteries were 50% discharged, below this and you are really killing your batteries. When we started the generator the full 30amps from the 4-stage charger would go in, but would slowly tail off until after about an hour and 15 minutes less than 20 amps would be going in. The batteries were only 75% charged at this point but continuing would just be a waste of fuel as the amps drop off rapidly. The last 20-25% of battery charge can take forever to put back.

On another note, using an engine to just charge batteries is very inefficient and environmentally bad. Why use a 36hp (in our case) engine to drive a 1/2 hp alternator?

 

[jimp45]

Vonasi I agree that using the engine to charge the batteries is to be avoided. I will have 240-watt solar panels. However I can foresee on cloudy days I may have to use the engine and in that case I do not want to run for longer than is useful. It is estimating when the last 20/25% is reached is what I am trying to calculate. As you say probably after approx. 1hr

Jim

 

[Vonasi]

Vonasi I agree that using the engine to charge the batteries is to be avoided. I will have 240-watt solar panels. However I can foresee on cloudy days I may have to use the engine and in that case I do not want to run for longer than is useful. It is estimating when the last 20/25% is reached is what I am trying to calculate. As you say probably after approx. 1hr

Jim

The amount of time will vary wildly dependent upon the max output of your alternator and the size of your battery bank, plus the type of battery charger you are using. The only way to know for sure is with a battery monitor or ammeter, believe me it will pay for itself. Plus a battery monitor will give you something to stare at all day, 'oooh look 4 amps going in and 95% charged, will it make 100%' can often be heard from the crew on our boat (sad but true)!

240 watts is a heck of a lot of solar power - I'm jealous. We only have 120 watts and are able to run a fridge at anchor 24/7 in the Med for two weeks between charges. With your sort of power we could sell some power to other boaters! What are you planning to power up with all that?

 

[vyv_cox]

Forget about voltage as any sort of signal to indicate battery charge. It is only useful if the battery has been disconnected from any input or output for a very long time. Unlikely situation on a boat.

An ammeter is the only way of knowing whether you should give up charging because it is becoming inefficient. I would strongly recommend installing some sort of battery monitor. They may seem expensive at £100-£200 but you will save this back in extended battery and engine life from not abusing your batteries or running your engine unnecessarily.

I fitted a DP meter this season http://www.dpmeters.co.uk/ considerably less expensive than £100 - 200, I think I paid less than £40. It has been most useful in monitoring battery condition.

I have 120 Watts of solar panel, in Greece, where they keep batteries topped well up without running the engine. Refrigerator running continuously and all usual drains on power. Voltage is usually down to 12.1 or 12.2 first thing in the morning, up to 13.5 by mid-day. We only engine charged once this season, end of September when sunlight was notoceable by its absence. One hour is usually sufficient to reach float current with a Sterling unit, 55 Amp alternator, 330 Ah of battery bank.

 

[jimp45]

Vonasi
I’ve bought a Solaris Sunrise cat, currently in Preveza, which I plan to live aboard and cruise the eastern med. for the next 2 years. I am assuming an efficiency of around 25% of the wattage over 10 hrs for my solar panels. The front opening fridge will use quite a bit of power, as will laptops WiFi etc. I like to be over engineered as they say.

Vyv
Thanks for the info. Is 12.1v a bit low, less than 40% as I understand.


Jim

 

[vyv_cox]

Vonasi
I’ve bought a Solaris Sunrise cat, currently in Preveza, which I plan to live aboard and cruise the eastern med. for the next 2 years. I am assuming an efficiency of around 25% of the wattage over 10 hrs for my solar panels. The front opening fridge will use quite a bit of power, as will laptops WiFi etc. I like to be over engineered as they say.

Vyv
Thanks for the info. Is 12.1v a bit low, less than 40% as I understand.

Jim

My experience is that 25% is a very conservative estimate. By about 1000 we are getting 5 Amps, increasing to 8 by around midday and still 5 amps at 1800. AFAIR my theoretical maximum is 9. In September it becomes a little more difficult to keep full charge, but although I made my panels to tilt for maximum exposure I rarely bother do it.

12.1 doesn't bother me too much, it often used to be 11.7 with smaller panels and only 220 Ah. They are 5/6 years old so don't seem to have suffered from this treatment.

 

[Vonasi]

Jimp45, now I see why you have so many solar panels, in fact I would buy more if I were you or better still bin the front opening fridge and fit a top opening one.

We ran for four years in the Med with a front opener and it was a battery killer. This last year we have had a top opener and the difference has been nothing short of astounding.
Not only do things stay cold but the power consumption reduction is massive. The front opener used to rule our life with having to recharge, as fast as we could put it in, the fridge took it out. Not any more though. I used to have to pull the generator out to charge up after 3 or 4 days but this last summer I didn't need to get it out at all, plus we were able to sit at anchor for two weeks at a time and use the laptop. We would never have done this before as every ounce of electricity had to be dedicated to that damn front opening fridge.

 

[mitiempo]

I second the purchase of a good battery monitor. One of the best is the Victron (BMV-600). Once you have one you'll wonder why you waited so long. You always know the state of your batteries, no waiting as with a voltmeter. One of the keys to fast charging is a separate 3 stage regulator for the alternator, as with the built in reg the charge current tapers off very quickly. By the way an alternator uses approximately 1 hp / per 25 amps.
If you have solar panels a MPPT controller will increase efficiency by about 30%. What it will do is if your panels are producing 18 volts at say 12 amps, it will convert this to 14+ volts at a higher amp rating for more efficiency. And if your panels are producing 12 volts at 6 amps for example, not enough voltage for charging, the controller will convert this to 14+ volts at a lower amperage to get some charging current.

 

[Vonasi]

Mitiempo... I am so jealous, I have only got the BMV-501. Having the 600 must be at least 99 better!

Seriously, thanks for backing up the 'get a battery monitor' side of this debate. I still find it fascinating that people live on complicated boats with power going in and coming out and yet most of them resent paying for a battery monitor. Then every so often they swear at the battery manufacturer for making something that only lasted five minutes - that's because they abused them while they were sleeping!

 

[mitiempo]

Actually I have a Xantrex Link 10, but suggested the Victron because it's a UK product and I know it to be a good one. I think the first upgrade to an electrical system should be a monitor as without it you'll never know where you stand. Even going through the boat and turning one item on at a time to chart usage is very educational and gives you a good idea what to make more energy efficient - lights, etc.

 

[ccscott49]

Another vote for a battery management panel, transformed my charging regime, I now know exactly what's going in/out and the state of charge. Gives me an in depth knowledge of consumption of individual components also.

 

[Heckler]

My experience is that 25% is a very conservative estimate. By about 1000 we are getting 5 Amps, increasing to 8 by around midday and still 5 amps at 1800. AFAIR my theoretical maximum is 9. In September it becomes a little more difficult to keep full charge, but although I made my panels to tilt for maximum exposure I rarely bother do it.

12.1 doesn't bother me too much, it often used to be 11.7 with smaller panels and only 220 Ah. They are 5/6 years old so don't seem to have suffered from this treatment.

Vyv
nice to see some practical experience and figures. After reading that 12.xx is half charged and 12.x is flat I looked at my voltages and worried then thought hmmm whilst the "experts" are predicting gloom and doom if the voltage is 12.x In fact as you say, I used to see less than 12v on Sonsy Lass and it didnt seem to bother the batteries even after 5 years!
Stu

 

[mitiempo]

If you're taking the batteries down to 12 volts very often you're shortening their life a great deal. Another reason for a battery monitor. 12.2 volts is 50%. See state of charge below. This chart is for Trojan but others will be close.

 

[Plevier]

12.2 volts is 50%. See state of charge below. This chart is for Trojan but others will be close.

I'm sorry you just can't say that!

It depends on many factors including the fully charged specific gravity the battery designer has chosen (generally ranges from 1.220 to 1.300), the volume of electrolyte relative to capacity, the temperature, the general health of the battery and other factors. At best it will be approximate, and only if measured when the battery has been off charge for a while and there is no current flowing.

You really do need a battery monitor (but even that won't be absolutely precise) if you are cycling the batteries, and a well controlled smart charger.

I'm intrigued by the Smartgauge http://www.smartgauge.co.uk/sgadesc.html. On conventional theory it shouldn't work but you hear convincing references for it. The manufacturers are very coy about it, the "technical" pages really tells you nothing and are misleading in some areas. Anyone know any real detail?

 

[Heckler]

If you're taking the batteries down to 12 volts very often you're shortening their life a great deal. Another reason for a battery monitor. 12.2 volts is 50%. See state of charge below. This chart is for Trojan but others will be close.

Point I was making, Ive seen these things before, doom and gloom, batteries knackered if you let them go tooo low, load of bollox, mine on my previous boat lasted 5 years with full cycling, and were still going strong when I sold her! Good enough life for me!
Stu

 

[mitiempo]

Willow
The chart I posted was for a specific Trojan 12 volt battery and you can say that as Trojan did. I didn't say it would be precise for all but close. I agree a monitor is by far the best way as without one you have to rest the batteries before checking voltage - not really practical in real life. A battery monitor tells you what is more important than voltage in reality - state of charge or % of charge. As well as amps in or out. And I think you'll find a battery monitor is quite precise after you've programmed it for your specific batteries. The reason I posted it is that a previous poster mentioned taking his batteries to 12 volts which it too low for long battery life.
Stu
Full cycling - what is that exactly? And a good battery should last a lot longer than 5 years - more like 8-10 if not taken below 50% of charge.