Battery charging

[FF]

Some folks are under the illusion that a 100A alternator will stuff 100A into a 200A batt set in an hour and they are done.

This months Home Power , avilable on line has a great simple article Battery Instalation anf Maint starting on page 74.

Most interesting was the Surette formula for the Absorbtion phase ,

the one AFTER* the Bulk Charge* that brings the batts back to 80% SOC .

Absorb time =(0.42 x batt 20 hour capacity ) Divided by charge current

So an 800A bank with a 75A charge

(.42 x 800ah) divided by 75 = 4.5 hours of Absorb time!!

*

Sure 800 ah would be a large boat setup , but run the numbers for your house set .

My guess is almost everyone that does not top off with solar is constantly undercharging .with short set life and lousey performance the result.

This constant undercharging (never getting to 100% full charge) would be easy to see with a SOC meter.

For most cruisers the daily long charge time is simply unacceptable , the solution is to operate 50% to 80%SOC* , just bulk charging .

This will degrade the set so a larger* bank and more frequent replacement is the norm.

The price of quiet.

*

*

 

[Fighterpilot]

Ok, help me out here. SOC?? State of Charge??? or ??? I have a load meter--is that the same as a SOC meter? Thanks

 

[FF]

www.bogartengineering.com/

Here is one that will do the job ,made for RV's so don't mount it outside in the rain. Trace makes nicer ones $$$$ for boats.

The SOC meter (most brands) will observe your battery thru a few charge and discharge cycles.

It will then know the capacity of your bank, regardless of what the batt advertisement claimed.

By simply watching the SOC and setting the alarm to go off at say 50% you can keep up with what is happening, and never kill the set.

You must get back to 100% full as often as you can.

Simply observing what happens with the fridge or other loads overnight will be Eye Opening!

If you have wind ,solar, 2 noisemakers ,and a engine alt , that's fine the SOC will tell you where you are at any time.

For a cruiser the SOC should be installed before the battset is changed out , so you can see how the charge system is doing on a really hard case.

 

[timjet]

FF, I believe the bulk charge rate to 80% you're referring to pertains to wet cell batteries, correct? Then as you mentioned it takes a long time to refill the remaining 20%.
However with AGM batteries it's my understanding they will accept a bulk charge rate to nearly 100%.

 

[bobofthenorth]

FF wrote:
www.bogartengineering.com/

Here is one that will do the job ,made for RV's so don't mount it outside in the rain. Trace makes nicer ones $$$$ for boats.

The SOC meter (most brands) will observe your battery thru a few charge and discharge cycles.

It will then know the capacity of your bank, regardless of what the batt advertisement claimed. .................

Its not quite that simple with the Bogart/Trimetric but that's the basic principle - you can use the meter to tell you what you really have for battery capacity.* And it won't be as simple as taking the number from the glossy battery brochure and multiplying that by the number of batteries onboard.

We have a Trimetric 2025 on the bus and there was a Link 2000R on Gray Hawk when we bought her.* In theory the Link did everything that the Trimetric does but it did it in such a clumsy fashion that I never trusted it and never really used it.* Fortunately the regulator went out on the Link so I had a good excuse to replace it with a Trimetric this year.

To flesh out the answer to Fighterpilot, a SOC meter like the Bogart Trimetric uses a shunt in the negative line immediately before the battery to measure all current in and out of the battery.* From that information it gives you a fairly accurate estimate of how much of the actual capacity of your battery bank you have used.* Its not a perfect science because SOC is influenced by how rapidly the discharge and recharge happens and the meter has to make some approximations but overall its a lot better than trying to use voltage and WAY better than not having any monitor system at all.* I like to set my SOC meter up with a very conservative estimate of the bank capacity and then we have a very hard and fast rule about never discharging past 50%.* Lifetime battery cycles vary depending on the depth of the cycles and they vary a lot.* You need to look at the charts of cycles vs depth of discharge but you can easily double your battery life by limiting your discharge depth to 60-70%.* We occasionally go below 60% on the meter but because I have set my capacity conservatively the real SOC likely never gets below 60%.

 

[Fighterpilot]

I may have a problem using a SOC since the negative to the battery comes to the batteries from the starter on the port engine which in turn connects across to the starter on the starboard engine. The two batteries were 8Ds and there was a negative connecting cable connecting the two--so a shunt before the batteries couldn't tell which battery was being used. Am I correct on that assumption? When I am in position 1 I assume only one bank has the circuit completed, position 2 the other bank, and off--no circuit completed. I don't think a shunt on the negative side before the batteries could determine which battery was being used, but could it give a number for the overall condtion of the batteries?

Now I have two 34s in parallel for start and two 6volts in series for house, perko switched 1, 2, and off. The negative lead connects to both banks. One of the 8Ds had failed and it was a bear to get out being outboard of the 3208 so with the fuel tank out we took out the other battery as well and replaced them with lighter weight batteries. Thanks

 

[bobofthenorth]

The Link system had provision for monitoring 2 battery banks but the Trimetric does not.* Link accomplished that with a fancy multi-shunt contraption.* You are correct that the monitoring will be in reference to the total battery capacity controlled through that shunt.*

I'm unclear exactly what is being combined through your combiner switch but that switch will be in the positive side of the circuit.*

 

[FF]

To get folks to contemplate the big picture I asked the question ,

What is the difference between a fuse and a shunt? The answer is,,,, LOCATION.

The easiest way to monitor the boat is to forget about the starts ,
they will lave little hooked to them and are usually cycled between 100% and 99.5%, as in your car.

A volt meter across the start solenoid will give all the info you can use , standing V, charge V ,and crank V.

Put the shunt into the return line of the house bank.

 

[bobofthenorth]

FF wrote:
To get folks to contemplate the big picture I asked the question ,

What is the difference between a fuse and a shunt? The answer is,,,, LOCATION.

And I remember when you asked and answered the question - - its an airplane analogy as far as I'm concerned - as in "way over my head".* Care to explain what you mean?

A fuse is designed to break the circuit in the event of a catastrophic failure.

A shunt is a very precise resistance designed to monitor current flow.*

I fail to see how they are related and in point of fact they should be located in more or less the same location IMHO.

Please explain what you mean because this dummy doesn't get it.