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Old 06-19-2016, 02:08 AM   #1
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what charges the genset battery?

Hi all, I thought I had a lot of questions before I bought a boat, little did I know that was only the tip of the ice berg. My genset battery is dead, which I just assumed was impossible I guess and never really thought about it until yesterday. I went down there today and it looks like there isn't anything to charge it. I think it's pretty old and probably needed replaced anyway, but it got me wondering how often I should plan on this happening.

I don't have shore power because the new power stations where I have to dock for the summer always trip when I plug in, but is shore power the only thing that would otherwise charge the genset battery? I have an inverter, and while I'm still not entirely sure of everything it does, it appears that it does not charge the generator battery. The only cables on the genset battery are the ones that start the genset. Is there an easy way to tap into the main batteries to charge this one too? As far as I can tell, the generator charges the main batteries through the inverter, but I'm not even sure about that. Fortunately (?) the asparagus my wife planned on cooking with the stove was mistakenly put in a bag with herring that spilled (oops) and she wasn't too excited about eating them after that. We do plan on going out for a few days next week though so I need to replace or recharge it. Which brings me to another potentially silly question- if I had jumper cables on board could I have jumped the genset battery off of the mains? Thanks
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Old 06-19-2016, 02:46 AM   #2
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My genset battery died of old age right after I bought it. On my boat, the only thing that charges the genset battery is the alternator on the genset. Not a bad setup if you do run the genset periodically. My guess is that is how yours is setup.
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Old 06-19-2016, 05:06 AM   #3
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Common setup to have the genset battery charged only by the genset alternator. The genset could have been started by the use of jumper cables and many boats have hardwired a connection between the main battery bank and the genset for this purpose.

If your genset battery is your sole isolated battery (the rest being house bank / engine starter combination) then it would be prudent to replace the genset battery before it dies on some sort of maintenance schedule. I toss my isolated battery every five years. It is the cheapest battery on the boat.

Concerned that your boat is tripping the shore power breaker. This narrows your options too much. You need to investigate and solve this problem.
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Old 06-19-2016, 05:36 AM   #4
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Agree with the above. Once you get your genset running again, check the battery voltage while it's running. It should be around 14 volts which will indicate the alternator is changing. I prefer not to have the genset battery tied to anything else. If everything is working properly, running the genset once a month for at least half an hour will keep it charged, also good for the rest of your genset.

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Old 06-19-2016, 08:00 AM   #5
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"I don't have shore power because the new power stations where I have to dock for the summer always trip when I plug in,"

This is something you want to check, since if you go to a dock without a GFI , you might kill a swimmer or two, among other hassles.
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Old 06-19-2016, 08:03 AM   #6
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With all due respect, you are confusing what inverters and chargers do. I would hire a marine electrician and spend an hour or two with him. Let him trace wires and tell you what does what.

Some gensets have no DC alternator and depend on the shore power (or genset AC output) charger to keep the genset battery charged.

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Old 06-19-2016, 08:21 AM   #7
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With all due respect, you are confusing what inverters and chargers do. I would hire a marine electrician and spend an hour or two with him. Let him trace wires and tell you what does what.
He may have a battery charger built into his inverter, such as a Magnum Energy.

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Old 06-19-2016, 10:49 AM   #8
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Thanks for the replies everyone. I have used the generator several times over the last month, so my guess is that this battery is just past it's lifespan. I'll replace it before we head out in a couple days.

My understanding is that the inverter does charge the main batteries. I'm not 100% on this, and it's towards the top of the list of things I need to learn soon.

I have had the only marine electrician in town look at it regarding the breakers and apparently this is a pretty common problem. Fortunately, it's only the new transient dock built this year that trips, all of the 4 main harbors are set up with older power and the boat works fine there (don't worry FF, we only use 2-3 swimmers daily. After the summer dies down, I should be in one of the old harbors and have shore power. Unfortunately, the marine electrician is pretty much booked for the summer with fishing boats. I guess that's what happens in small fishing towns, so the GFI fix is going to have to wait a while. Thanks for the replies, I figured the generator alternator was probably all that charged it's battery but feel much better getting some reassurance from people with experience.
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Old 06-19-2016, 12:52 PM   #9
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With regards to the GFI issue. We have discussed this numerous times . Do a search you'll find methods to track the issue on your own.
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Old 06-19-2016, 07:40 PM   #10
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My Onan charges the battery.
And the converter on bank 3 charges the battery when the generator is off.
I have a relay, when gen starts breaks bank 3 charging wire to open circuit.
However, I added that circuit myself after the boat had several decades of usage by previous owners. My guess, it may not matter too much, but I suppose disconnecting bank 3 charging gives bank 1 and 2 a little more energy.

Other thing is the converter charges at 13.7 vdc. When onan runs, it charges at mid 14's, so that would to bank 3 converter's charge circuit appear like a full battery. So my adding that relay has no effect, which is why it worked fine for several decades.
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Old 06-19-2016, 07:46 PM   #11
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1. When running,the Onan genset alternator.
2. When not running, a dedicated regulated 25 watt solar panel. There was enough sun in Sitka when we were recently there, year round I don`t know, panels don`t need permanent blazing sun to work.
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Old 06-19-2016, 11:47 PM   #12
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Larger gensets have an alternator, smaller ones either have a 12 volt winding for a charger or some type of built in charger when making power. They're not always connected. Using a diode or a commercial product with a diode in it, you can connect your genset battery to the in port charger. The diode keeps the battery separate. A diode only allows electricity to move in one direction.
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Old 06-20-2016, 12:46 AM   #13
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Fortunately, it's only the new transient dock built this year that trips, all of the 4 main harbors are set up with older power and the boat works fine there .
No, that is most definitely not fortunate. You have a problem and the properly wired and protected new transient docks points it out. As to the extent of the problem we can't know. Maybe will just kill your anodes faster, if in fresh water, could kill a swimmer so that's really not a joke, or it may just lead to a fire on your boat. You need to find someone with the equipment and the knowledge to figure it out.
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Old 06-20-2016, 12:50 AM   #14
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The GFI issue is a serious one and potentially lethal to people in the water around the boat and to people on the boat. With a little electrical knowledge It should take about five minutes to track that down and it is something your surveyor should have checked. The issue is either reverse polarity in the power post (possible) or a neutral ground connection on your boat (most likely).

Your inverter may vert well be charging your batteries as many units are charger/inverters with both capabilities, look at the data plate on the unit and it will tell you.
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Old 06-20-2016, 01:22 AM   #15
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Funny I should see this first thing after coming back from 5 days aboard. Funny, because; my gen set batt went kaput first try I did to start gen set. Found the batt read 10.0 on multi meter. Sooo - I charged it over knight with 1 amp trickle charger. Read about 12.3 in morning with charger on its terminals. Soon as I took charger off batt.... the 12.3 reading dropped very quickly as I watched. Hit starter on gen set and got about 2 revolutions before dead again. Purchased new starter batt, installed it and went merrily on with boating. My gen set alternator charges its battery.

PS: Don't use dual purpose batt, use starter batt only for this... lasts much longer, for internal batt reasons!
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Old 06-20-2016, 10:31 PM   #16
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I pulled the battery from my skiff, which read 12.4 volts. Swapped out with genset battery and it started right up. However, while it was running it only showed 12.3 volts. Apparently my genset is not charging. I also don't see an alternator on it, or at least not what I'd expect an alternator to look like. I see a starter, a fuel pump, and a solenoid. For the immediate future I guess I'll bring some jumper cables on board until I can delve in further
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Old 06-20-2016, 11:07 PM   #17
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You might want to have a stater/generator shop look at that starter.
Some units were a combination starter/generator in one unit.

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Old 06-21-2016, 12:24 AM   #18
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Maybe it`s there but I can`t find brand and model for the culprit genset.
Also, if the batt is ancient, replace it and see if it lasts, that will tell you if you have some form of charging built in and working, there has to be something for the batt.
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Old 06-21-2016, 12:55 AM   #19
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you didn't miss it, I never mentioned it. It's an Onan 4kw. I actually just finished downloading some manuals I was able to get from the PO so hopefully I can get this straightened out sooner than later. Thanks again to everyone for the help!

Also, regarding the GFI- after the terrifying posts, I called the electrician again today to make sure I understood him correctly when he was inspecting the boat. He confirmed that there is no risk of death, fire, electrocution, etc with my current situation. He has decades of marine electrician experience and is local and knows the situation of my boat and our harbors very well. I am going to trust that he knows what he's talking about, but thanks to everyone for the information
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Old 06-21-2016, 01:56 AM   #20
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you didn't miss it, I never mentioned it. It's an Onan 4kw. I actually just finished downloading some manuals I was able to get from the PO so hopefully I can get this straightened out sooner than later. Thanks again to everyone for the help!

Also, regarding the GFI- after the terrifying posts, I called the electrician again today to make sure I understood him correctly when he was inspecting the boat. He confirmed that there is no risk of death, fire, electrocution, etc with my current situation. He has decades of marine electrician experience and is local and knows the situation of my boat and our harbors very well. I am going to trust that he knows what he's talking about, but thanks to everyone for the information
Trust all you want. How then did he explain your issues? I'm just glad you're not docked next to me with the problems. Decades of experience is only good if accompanied by knowledge. A lot of people dismiss GFI. Home electricians too. A lot of fires to show for it, but the probability is great that any single boat won't have a fire. You need to do some more reading in other places before putting all your trust in one person. Did this same electrician fail to note your battery issues? He is the electrician who left you unable to have electric. What are you going to do at other marinas when you cruise?
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