On my onboard charger, it says my deep cycle is full charged, and that my cranking battery needs charged. This is after having it continuously plugged in for a couple of days. I noticed this after my trim would not work. I used an external battery charger on the cranking battery and now it’s charged enough to use trim. I live up north, and it’s to cold to get the boat out. Would the cranking battery be low as a result of my outboard not being ran in a couple of weeks, or does my onboard charger have an issue?

Welcome to Bassresource.  This could be a bad battery or a bad charger.  You need to test both with a meter to find the real answer.  There are people on here that could provide you with more info if you included the make/model of your on-board charger.  

It is a Noco Genius 8 amp 2 bank. 

First welcome to the forum. If I get this right, Your primary battery charges with a external charger but not with your on board charger. If that's right then there's a problem with your on board charger.

I would remove your battery and set it up on a charger and see if takes a full charge. That way you'll know your problem is with the boat charger. Make sure to check all your connection on your batteries. Also make sure you don't have anything running on your system. If you have battery switches turn them off so you won't get any drain.

If this is the case, what are some good chargers that aren’t too expensive?

 

9 minutes ago, Gibby said:

If this is the case, what are some good chargers that aren’t too expensive?

 

 

You can get a Minn-Kota on board 2 bank 10 amp for $299 at TW. There are cheaper ones but I just with MK because I know there good. There maybe cheaper ones put there. 

What kind of boat bass/kayak/small fishing? I only ask in case your planning on upgrading to a larger trolling motor.

Just saw a MK charger 2 bank for $156 Hope this helps

I have an 18 ft tracker. Right now it’s a 12 v system but I’m looking to upgrade to a 24 v system in the future, but for the upcoming season I think I’ll just stick with 12, so I would only need a two bank charger. Thanks for the help!

 

A couple if things you can do.  If you have a volt meter, with the charger plugged in and charging, take a voltage reading across each battery after several minutes.  If it's in the charge mode, you should see a voltage of over 14 VDC. but not more than 14.6 VDC.  

If the battery is fully charged and the charger has gone into the maintenance, the voltage is going to be below 13.6 vDC, based in the type maintenance mode it has.  Some chargers use a trickle charge which should be between 13.2 and 13.6 VDC.  Some use a mode that cuts off and turns back on when the battery has self discharged to a set level so you could see anything between 13.8 and a little over 12 VDC.

After taking the readings across each battery, unplug the charger and swapped the charger leads going to the batteries, so each battery has the opposite bank charging it and redo the voltage test.  Give the charger several minutes to have time to cycle before taking the readings.  If you get similar readings after swapping the charger leads, you most likely have a bad battery.  

If the reading changes between the batteries, depending on what you did read, you may have a bad bank in the charger.  

Post what your findings were and I might be able to give you a better diagnoses.

Are the contact areas on the battery and the charger terminals clean?

Load test the cranking battery. 

How old is the cranking battery?

Simple to switch the charger leads from trolling motor to cranking batteries to test the charger if you don’t have meters to test with.

You have a good brand name onboard charger, that is why switching leads should resolve the charger question.

You should have a 50amp DC switch to isolate (switch off) charging to your outboard engine during charging your batteries.

Tom

One thing I should have mentioned.  A lot of quality (smart) chargers will not charge a battery if the voltage is not within a certain range.  This is to prevent trying to charge the wrong voltage battery with them.

This is not the case with the hot box type chargers, they can even be used for a power supply. 

So, your battery could be junk and and not be turning your on board charger on, where the cheap hot box could care less.  

Most parts stores will test your battery for free.