Charging batteries by idling engine

[Steve O]

I need to charge my house bank by running my engine while at anchor (solar not working for me in rain following Florence). I have four 6 volt - 235 amp/hr batteries set up in a parallel / serial configuration to give me 12 volts and 470 amp/hrs. My alternator gives 116 amps at 2500 rpms cold. Since my alternator is belted at 4.3 to 1 I figure about 600 rpms on the engine. Does that mean I will need to idle my engine for about 4 hours (470 amp hrs / 116 amps) to fully charge my batteries (in a perfect world)?

Thank You

[SVTatia]

I'd guess the time to charge depends on the SOC which was not given.

[jamhass]

Steve:

In a perfect world yes and no.

In the real world:

1. You should never run your batteries until dead, only to 50%. Thus half the "ideal" time

2. Your alternator will heat up and give less amperage.

3. Real world charging becomes asymptotic as the batteries near full charge, thus extending charging time.

There are lots of other issues affecting charge time, but these are the major ones.

[Steve O]

OK Then it seems like I am better understanding the principles here. I know that the alternator will reduce output at higher temperature and that I am calculating this assuming the batteries are in 0% charge. Previously I was only running my engine for an hour or so to charge up the batteries from 12.2v to 12.5v. That charge did not last long but now I am living onboard it is even more critical. I am struggling to comprehend all the factors involved in maintaining the system.

[sailorboy1]

Forget about fully charging your batteries with your engine on cloudy days. That’s just a crazy waste of fuel and engine wear and tear. Instead only run till the acceptance amps start decreasing, then STOP.

It even applies to charging with a generator. This mourning I ran the generator till the battery charger stopped putting out its max amps. Then I turned it off even though that still let me -50 ah out f the batteries.

This is the old school presolar cruiser operation.

[sailorboy1]

Of course to be able to do this you need to be able to measure amps via either a panel ampmeter of a battery monitor

[Cheechako]

Most my boats were charged this way. I doubt they were ever fully charged except if I motored on passage a lot.

I mostly had 100 amp small frame alternators and external regulators. Some regulators worked the alternator a lot harder than others. The charge curve seemed to be 15-20 minutes at 80-90 amps, then would back off slowly, reducing to minimal charging after maybe another 40 minutes.

I charged at high idle, maybe 1100-1200 rpm ... especially for the first half hour or so. It's nice to have a good ammeter installed so you can tell if it's worth continuing to run AND to find your best rpm. You don't have to have a fancy measurement device though.

With refrigeration in the tropics I had to charge about one hour twice a day.

[roverhi]

Diesels like to be run against a load and the alternator is a poor load after the initial high amperage output. The Yanmar 2GM in my newest boat had the exhaust elbow clog up with a little over a 100 hours on it. Because of the incapacity and then death of the owners engine had only been run no load regularly at the dock for a number of years.

Only running the batteries down to 50% capacity will maximize the life of the batteries but may be an ultimate waste of charging energy especially if you are using the engine to charge them. Running the batteries completely down is not good especially if they are not charged back up immediately. Running them down to 12v or so will still give long battery life and more efficient charging. That's basically what we did cruising pre solar and still got 4 years out of the batteries using them 24/7365 for two years of dock free cruising.

[Maine Sail]

Originally Posted by Steve O

I need to charge my house bank by running my engine while at anchor (solar not working for me in rain following Florence). I have four 6 volt - 235 amp/hr batteries set up in a parallel / serial configuration to give me 12 volts and 470 amp/hrs. My alternator gives 116 amps at 2500 rpms cold. Since my alternator is belted at 4.3 to 1 I figure about 600 rpms on the engine. Does that mean I will need to idle my engine for about 4 hours (470 amp hrs / 116 amps) to fully charge my batteries (in a perfect world)?

Thank You

Charging flooded golf cart batteries from 50% to 100% SOC takes roughly 7-10 +/- hours depending upon the health of the batteries. Declining charge acceptance, as SOC increases, and low charge efficiencies at high SOC's means the last 15% of charging takes a very long time.


LiFePO4 batteries have charge efficiencies approaching 100% but lead acid does not....

[barnakiel]

Running the alt for 4 hours at max output without extra cooling might kill the poor thing.


Very often you will see alts sized 1/4 to batteries (60 A alt on 240 Ah battery etc.).


I think you will find it hard to charge any battery in 4 hours from 'empty' (50% discharged). This is because as the battery charges up, it can only accept smaller and smaller number of Amps from the alt.


In the perfect world, 4 hours is viable with a powerfull 'full rat' alt. But in practical terms it often takes a bit more time. Up to 6 hrs, I think.


b.

[a64pilot]

This is what the little Honda is for, run it for a few hours first thing I in the morning, and let Solar finish the job.
Even on sunny days, you only get about 5 or 6 hours of good charge this time of year where your at, which falls short of Maine Sails 7-10 hours, meaning that even on good Solar days you weren’t really getting fully charged.

However what Sailorboy was saying is also true, that is going from about 50% to 85% charged can be done in just a few hours and is the efficient time of charging, the closer to fully charged you get, the less efficient charging is, You don’t want to run your engine during inefficient charging times, that is when Solar pays for itself.

Your alternator is not going to charge your batteries at max capacity for four hours, not if they are lead acid, after the first 30 min or so your charge acceptance rate will begin to diminish slowly, after a few hours, your only charging at a much lower rate, the last couple of hours at a very low rate.

[Maine Sail]

Originally Posted by barnakiel

I think you will find it hard to charge any battery in 4 hours from 'empty' (50% discharged). This is because as the battery charges up, it can only accept smaller and smaller number of Amps from the alt.

This is what it the charging process looks like even with super efficient and faster charging AGM batteries. It took six hours and six minutes to go from from 50% to 100% with a Lifeline AGM at a .15C charge rate. Keep in mind this was holding absorption all the way to 100% SOC. Your average marine charger, solar controller or alternator regulator will usually drop to float way before it should so it will take even longer to reach 100% SOC in a real world application. Deep cycle flooded batteries charge even slower than the AGM depicted below....

[hellosailor]

Steve-
You will typically lose 10-20% of the amperage being used to charge a battery. More than 10% of the amperage is mainly wasted as heat, often boiling the electrolyte. So if you wanted 470A put into the batteries? You'd probably need to put in 525-550A in order to get 470 "stuck" in there.
Using an ammeter, even a simple clamp meter, to see what's actually happening is a good idea. The alternator will cut back and derate as it heats up, but there are other "dumb tricks" that alternators and regulators can surprise you with. Or not. So, seeing the real numbers means a lot.
Alternators all have an output curve, more power at more RPMs, and while ~100A is all you need to charge at a maximum rate (often quoted as 1/5th of the capacity for wet lead cells) you may find that running the alternator faster compensates for the normal output reduction as it warms up. And, if there are the usual fan(s) on the alternator, the extra speed may actually help keep it cool.
If you run some numbers, you may find that having a local machine shop make up a custom alternator pulley for you is worthwhile. Look at the rpms for your engine. Do you normally idle at 600? Or 800? Do you normally cruise at 2500? With "battle speed" around 3000?
So you take those numbers, and take a look at the output curve for your alternator. It may not put out full power until 3500 shaft rpm. It may be rated for operation at 10,000 shaft rpm "forever" or just intermittently.
Ideally you want a pulley size that will let your alternator run at full output when your diesel is idling, or cruising, but never pushing the continuous duty speed even when your engine is at "battle" speed. Compromise and adjust for reality as needed. There are a variety of pulleys available for many alternators, a machine shop may ask $150 to run one off custom. (Or $20 in the third world.)

[Pizzazz]

To summarize, charging the batteries with the engine in idle is what the majority of sailors do anyway and in pre-solar days, it was the main method... for me, it is still the main method.

Diesels like to run at close to max power, so that is a minor concern (i.e. if you idle often, and do not run the engine at max rpm frequently, carbon will build up).

Alternators produce less current as they heat up - it is best to monitor the current with an ammeter to see how it impacts your situation. It will also help with the min rpms where you produce max amps. In my case, alternator rpms above 2,000 gives rated output when cold, so most likely you are fine.

Batteries charge fastest between 50% and 80% state of charge, with about 90% efficiency. Above 80% state of charge, the efficiency begins to drop, the current begins to drop, so you are better off not idling the engine above 80% state of charge.

Your method of observing the volatage from 12.2 to 12.5V is not very accurate but it is good enough if you do not have an ammeter.

SV Pizzazz

[David M]

Most alternators are capable of their maximum charge by running the engine a little above idle. If you don't have an ammeter on the output then look at the voltage. That will give you some idea of when it has reached its max output.