When to Charge--Volts vs Amps?

[rtbates]

That's what we all did BEFORE the age of electronics.. Some of us STILL do it that way.

[Cheechako]

I might add that many elecrtronics (GPS, DS) have a built in volt meter... to verify your main voltmeter...

[cwyckham]

Why would I want to shut down all my electrics for an hour a day to check my state of charge if I have a battery meter. It might not be dead accurate (though I bet it's close), but it's gotta be at least as accurate as the voltage check and a heck of a lot more convenient.

Using the amps in as a check of when to shut down the generator makes a lot of sense, though.

[btrayfors]

You don't have to shut down for an hour.

All you have to do is know your boat. I've used a digital voltmeter and an ammeter ONLY for more than 23 years to measure the state-of-charge of my batteries. A rough approximation is all that's required to tell you when to charge and when to stop.

If you know your boat, you'll have a good idea of what voltages to see with "normal" loads. For example, when you awake in the morning and you see 12.4V with the frig running (6A draw) and after you've turned off the anchor light, you know that the SOC is greater than 75%. No need to charge, yet.

Later, after breakfast and inverter loads with charging cellphones and computers and things and, maybe, the toaster, you see 12.2VDC with just the frig running and the VHF radio. Hey, it's just about time to charge. The batteries are just above 50% SOC.

By contrast, when you awake one morning to see 12.1VDC with just the frig and anchor light running, you know it's high time to charge. The batteries are probably hovering around 50% SOC.

Maybe they're actually at 55% SOC. Or 44% SOC. What does it matter?

And that, my friends, is ALL you need to know.

IMHO, knowing your boat, and using a calibrated digital voltmeter is all you need to determine SOC and when to charge.

A good digital or, better, analog ammeter is all you need to tell you when to stop charging. I say "analog" because you can see the needle from a distance. When it's down to almost zero, it's time to stop. When you start charging, it should be way up (you'll know the usual number....mine is 85 amps).

I do boat electrics for a living. I design and install all kinds of equipment, including battery monitors (like the excellent Victron 600 sitting on my desk at the moment). But, a secret....you really don't need them, if you know your boat and are half-conscious.

Worse, even a great device like the Victron can be really misleading if it's not properly calibrated and recalibrated continuously as the health of the house battery bank deteriorates over time or, worse, if it's not installed correctly.

Bill

[Mark Johnson]

Originally Posted by mestrezat

While talking to another cruiser(not a wharf rat), we got into a discussion about when he charges his batteries while out cruising. He told me that he never believes the amount of negative amps his meter tells him as the day goes by. He only looks at the volt level.
With 12.7 a fully charged battery, he turns off every thing on his boat and waits 30 minutes and looks at the volt reading on his meter. If it reads 12 volts, he starts up his generator and recharges.
When my amp meter tells me it's at about -90 hours, I typically start up my generator. I only have 330 amp hours in my banks. I never looked at the volt level of the batteries. When I charge them, I know I won't ever get back to 0 amp hours(unless I'm tied up to a marina). Running my generator till I achieve 0 is a huge waist of time and gasoline. My friend waits till his amp meter says it's only putting out 15 amps and then he shuts down his generator because running it any longer is a waste of gas versus what he will get amp wise for his battery.
While I think my fellow cruiser has a good idea on when to charge his batteries, I feel that letting it get down to 12 volts is pushing the limits of draining a battery till it's totally drained(never a good thing to do).
Is my fellow cruiser on the right path as to when to charge the batteries? Throw out my amp meter and only go with what my volt meter says? Is a battery level of 12 volts going too low and damaging my batteries?
Hey, battery stuff can be a form of the black arts and I could use the input of other cruisers out there that have their own opinions.

I have a set of 2@6V Trojan L-14s, (340 Ah...) and a Link 10 monitor. I trust it implicitly... after 17 tears on the boat, and 12 as full time liveaboard, "mostly self sufficiently". I expect 15 - 20 years from a set of batteries. The key is that I only average about 40 Ah per day, (with a VERY energy efficient set up), and bring the batteries back to 100% daily with my solar panels, (usually by noon, with a 3 stage regulator)

If you got just one large panel, and run your generator for the first 50% of what you need that day, (when the batteries are down where they will accept a strong charge), then your batteries would last much longer, the devices onboard would be happier, and you would have an emergency backup. Also, since the Link 10 (or similar), will re "0"itself after "full", every day, the running algorithm will never get an accumulated error.

It is money well spent!

Oh yes... the batteries' Voltage is a very poor indicator, unless the batteries have no charge in or out, for hours before the reading. Only specific gravity can come close. I'd bring the batteries up daily, and trust a good charge indicator.

Mark

[Masquerade]

This has been an interesting discussion. I watch my voltmeter and ammeter like a hawk. The main reason I do this is because not too long ago I installed 3x80 watt solar panels and an MPPT 3 phase controller to charge 3 x 105 amp batteries. At about the same time I installed a new fridge unit and that drains about 6 amp when it kicks in.

The interesting thing is that with the panels charging things up and only the fridge running on a relatively low setting I can see what is happening. At rest and in the day the batteries can get well up to 14.4 volts. Fridge on in the day time and there is almost no voltage drop thanks to the panels. In the morning the battery voltage is down to about 13.7 volts. So you can imagine how happy this makes me. No need to start any motor!

I have watched the system when I turn extra things on during the day and night and this has proved particularly interesting. As a result all lights are slowly being replaced with LED's as the existing little things chew up watts at an incredible rate.

On the weekend I was out playing in 30 knot winds and 3 metre swell with the auto pilot on while mostly reaching and running. It too managed to chew up a bit but in the day time there was very little movement in voltage because the panels were supplying most of the juice to the whole system. Mind you, changing course just a little and or changing the sail setting really makes a difference with the demand made by the autopilot.

In combination, the ammeter and voltmeter are a great way to monitor load and supply and this has made me feel more comfortable about battery life and maintenance.
I also have a Xantrex smart regulator and echo charger for the cranking battery so I always have the back-up of a fully charged 120amp starting motor.

I should add that this system has evolved by reading many of the discussions posted on the Forum and some specific advice offered by some generous and knowledgeable members.

I retire in 40 days, but who is counting. I will spend a few months doing more stuff to the boat and some contract work and then I am off for six months cruising from Sydney to Cairns and back. Got to have good electrics.

[Cheechako]

Dont you have a device that stores hours worked "IN" and Days off "OUT". How in the world do you know if you are retired or not....?

[Masquerade]

I will have to wait till I am retired to know

[onestepcsy37]

what btrayfors says. over time you get to know your batteries, your load, and your charging cycles. i know that under ordinary circumstances i will have to charge my batteries every third day. i run the 35 amp charger until it has stepped down to about 10 amps. then i'm done for the next three days. i even know that i will need about one quart of gas in the honda eu1000 to run the charger to that level.

if i occasionally check the voltage level in the middle of the night it's just to confirm that my charging schedule is on track.

[sailinglegend]

Originally Posted by btrayfors

All you have to do is know your boat.

I love this quote - but to really "know your boat" you have to regularly monitor your battery monitor or your volt and amp meter.

I agree you don't really need a battery monitor because the voltage and the current drain will give you a very good idea of the state of charge. You can't tell much about the state of charge if there is charge current going in - unless it is approaching full charge.

I would suggest to those with a battery monitor which can become VERY inaccurate that they don't just look at the Ah count or % left, but they also check the voltage when there is a "small" current drain. Depending on the size of the house bank 12.5v @ -5 amps is about 75% charged.12.2v @ -5 amps is about 50%. If your Ah count is telling you anything very different then you have problems with the calibration.

Over time the max capacity, Peukerts constant and the charge efficiency parameters all change from the original settings. Also if there was an error on the installation or extra kit has been added and not wired via the shunt then other large errors will accumulate.

Check both Ah and voltage - get to know your boat - and make sure the crew does also!

If you really want to get to know your boat then do a proper "open circuit" test at 12.2 volts with -5 amps. To make this very easy just disconnect ONE battery by lifting the negative stud and leave for 4 hours and re-check the voltage on that battery. It may have come back to 12.25v or 12.3v. It you want to get more accurate then let the voltage @ -5 amps go down to 12.1v and redo your open circuit test and if it recovers to 12.2 volts then 12.1v @ -5 amps is your 50% state of charge point.

Having said all this as batteries age their actual 50% SOC voltage will probably be less than 12.2v, so nothing that most of us say here can be written in stone!!!!!.

[noelex 77]

If looking at battery voltage to predict SOC don't forget to take temperature into account. It makes a significant difference.

This is a good resource, with graphs for soc verses voltage for different charge and discharge currents.
http://www.scubaengineer.com/documen...ing_graphs.pdf

I still much prefer a good battery monitor however. A lot easier and more accurate.

[laforge24]

some intresting facts,
even a 100% charged batt. can indicate 12.4 !
determine the 'real' voltage after disconnecting all at least 12hrs
deep cycle batt. like to be discharged deep; 75%
lead-acid batt. like to be discharged lots in short time, and immediate charge.
all batts need at times to reach a boiling point to avoid sediment settling.
the best way to have long-life use off batt. is to have 2 sets! ;;
charge the one while using the other and vice versa,
because a batt. doesn't like to be discharged while charged,
voltage is not a good level indicator,
when batt. on charge and charge(Amp) indicator is high=batt. low,
Amp. indication low= batt. still suff. charged
WE KNOW MORE WITH AN AMP.INDICATOR THEN VOLT.

[laforge24]

ie; if onboard you have 4x 120Ah, make 2 groups, use one, charge other
when converter/or other disconnects you switch over to reserve/back-up
there are some handy devices that disconnect your circuit when bottom reached!
sometimes unhandy when all lights go out, but you could connect it to only one main light, call it the batt.saver,
they are sometimes included in solar charge regulators.
in my personal 15yrs experience it is better to discharge batt. and charge correctly
while using other batt.
WHY DOES A DISCHARGED BATT. FREEZE A CHARGED BATT NOT.

[noelex 77]

Originally Posted by laforge24

deep cycle batt. like to be discharged deep; 75%
.

Deep cycle battery cope with deep cycles better than ordinary battery's, but lead acid batteries will always give their longest life when the discharges are kept shallow.

A maximimum of 50% discharge is often quoted, but this number is not absolute. Less is better for battery life and health.

[laforge24]

True Noelex, could discuss better when aboard, love to be invited on Alu,:-)
cheers experience