How long to charge batteries at idle?

diggin2day1

While staying at anchor in my FV270 for a few days, how long should I let my boat run to charge up the batteries? This question has some obvious variables..... But if I were to idle for say 15-20 minutes, would it make any difference at all? I don't have an onboard generator, but I will have my Honda 2000 with me just in case.

Black_Diamond

It will help, but the alternator needs to be running ar high rpm to put out the rated amps. 

Michael T

@D2D1

Depends somewhat on alternator amperage - easy for you to find out with your PN or google your engine at Mercury sites or it should be stamped on the alt.

Next, it really depends on your battery:

First - what size and type is it. A series 31 AGM is a very strong battery that's easy to charge. A series 31 Thin Plate Pure Lead AGM is (for non NASA applications) THE beast of charging

Second - what is the battery's condition, its voltage state and how long it will hold it.

Third - If you are talking about two or more house batteries, how have they been wired together? The proper way or not? Huge difference that compounds geometrically with the more you have together.

Fourth: - The battery construction/chemistry is a MAJOR factor.

****Flooded accept charge at 25% of their rated amp hours

****Gel accept charge at 25% of their rated amp hours

****AGM accept charge at 40% of their rated amp hours

*****Thin Plate Pure Lead AGM (the best by FAR) accept charge at 300% of their amp hours

Many newer chargers can handle all of these charging profiles. Some can handle more than one battery chemistry at a time.

P.S. you'll get better charging at 1000 to 1,200 rpm than idle. Check your alternator's out putprofile.

 

diggin2day1

Now that's some great info right there!!! I'll check my alternator and do some math, thank you. Heading to Block Island next week and we're anchoring for 4-5 days. After thinking about it, it's probably way more economical(and quieter) to just set up and run the Honda to charge them up(I'll even let the admiral blow-dry her hair one day while charging batteries)... 

pault1216

My experience supports using the Honda with an adequate charger. OEM alternator rating are usually at a much higher RPM. I tried running my FV260 for 30 minutes at cruise for 2 group 31s that were close to full discharge and it didn't put a dent in it.

MarkB

Check your voltage at idle and then start to Rev up slowly.  You should find around 650 Rom at idea voltage is low and at some point as you Rev up the voltage picks up to 14V.

I think that happened at 800 rpm on my boat. I was told that's when the alternator was pushing out adequate amps to charge the batteries. 

Dano

Glad I found this thread.  I believe I may have either a regulator or alternator issue.  Start/run battery is at 14V/idle (800rpm) and the house is 13.5.  At 2800, the Start/run is at 15.  They are wired correctly, or so the mechanic says.  Time to get a multimeter?  I am on shore power so they start/run fine.  2000 727 CC 454 7.4   Alternator is a Mercury part number 862031

212rowboat

some alternators are not alternators at all... they're generators... the difference, so far as we can concern ourselves, is that a generator is single coiled and will make power stepping with RPM and hitting a max... an alternator is three coiled and will make power at a steady output, so long as the base RPM is achieved.... Cars with alternators usually don't kick in until 1800 rpm... 

1800 RPM for the alternator, that is... 

the engine could be turning much much lower than that, and all through the magic of gears... the crank wheel turns all the accessories and is usually the biggest wheel/pulley in the mix... the size of the alternator/generator wheel is mucho smaller, and turns at least (in most cases) twice the speed of the engine.  

1800 RPM on the alternator is usually translated at around 900 RPM on the engine... or, 850.. or even 800... 

an alternator pushing 15vdc is overcharging the battery- on purpose... think of it like pouring water out of a pitcher... if the pitcher is pouring 12vdc at a volume of 100liters(amps) and hour, if the alternator is only pouring 12vdc at a volume of 100amps per hour the power stored in the battery is only holding steady.  if the charge is greater- say 14.7, the voltage out the other end is 12vdc and without fluctuation (regulated)... 100 amps/hour at 14.7vdc is greater volume than 100a/h at 12vdc... so... the battery fills... on more modern rigs w/ PCM/ECU's, there is a circuit specifically for power/battery/alternators which interacts with the alternators built in regulator and only draws the juice it needs (may pump @ 14.7vdc when it senses the batteries are discharging/discharged, but back down to 12.7 when the batteries are full).. 


yadayadablah..... for your purposes: get a solar charger.... a 100w/12vdc panel mounted on your arch or your bimini, or even on your bow only requires 16/20ga wire to run back to the battery, and will charge at reasonable rate to keep your starting battery topped up enough to turn your engine and get you home... you can pick up a solar panel for really cheap nowadays, and it will come with a regulator/isolator to keep it from draining when it's dark.  park your rig in the sun (you're going to anyway, right?) and you're good to go... it will trickle and exercise (most have LED's) while the boats being stored.. they just make huge sense for boaters.  

Michael T

I missed the most logical question - it isn't a bad idea to check the output of your charger and also what type of battery chemistry it is set for.