Battery problems

[Berndahle]

Something strange happened to me yesterday and I just can't figure it out. I took the boat out for the first time this season (2009 Hunter 36). Over the winter I had the battery switch off and just topped them off from shore power every once a month. Victron monitor said they were at 98% when I got there yesterday. We sailed for two hours or so with the fridge and instruments on. When we wanted to head back, the engine didn't even turn over. Fortunately, I installed a second emergency backup battery as per MainSail's recommendations, so we were able to start the engine and head home.
My house battery bank consists of two 4D AGMs with a total of about 400 Ah. Not only can I not understand why my batteries would be dead, but the monitor showed 98% when we headed out. Also, now the monitor is showing voltage and charging amps while on shore power (around 70 Ah which tells me it's on bulk charge), but the state of charge, consumed energy and time-to-go displays show nothing. Not 0, just dashes "--".

I can't image something all of a sudden shorted out causing my house bank while I was underway. Does anyone have another explanation for the battery issue as well as the monitor display?

 

[boodey@wansw.net]

My refrigerator/freezer draws about 7 amps. That is enough to draw my battery bank down in a very short time. The only time I can run it is when either the engine is running, I am on shore power, or have sun to run my solar panels.

 

[Berndahle]

I think mine pulls 4.4 Amps when it's cycling (which it doesn't all the time, even in the summer), but at 400 Ah I can run my fridge for 45 hours before I get to 50% state of charge in my bank. So that can't be the issue.

Someone suggested to me it may be a bad cell in the battery. Could that be an explanation?

 

[Joe2954]

Are your engine/start battery and house bank combined in some way? The Victron iwill track current for either or both since it is on the ground, however, the voltage signal may only come from the house battery (wherever the red wire is hooked).
My charger charges both banks with the same setting but batteries are normally separated except when combiner relay closes.
I had similar circumstance to yours - checked the start battery and found it almost dry. Topped it with water and had it back up to 13 volts off a separate voltmeter but it still would barely turn over the engine. Checked all connections for loose or crudded up. Ended up replacing start battery and switching charger down to lower voltage setting.
I will add a switch to the charger line to the start battery and plan to switch the combiner separately from the "ignition" switch.
My Balmar probably puts out too much juice for the start battery.

In the garage the old start battery would charge for a few seconds on my auto charger before clicking that it was full. It would show almost 13 volts, but run down to nothing if I put any load on it.

 

[Berndahle]

They're separated by the battery switch. 1 is house bank, 2 is backup. I don't have a combiner. The monitor shunt is only connected to the house bank so it won't register anything from the backup if I'm switched to #1.

I checked all connections and they seem fine.

At this point, the bad cell theory seems to be the most logical explanation to me but I'm hoping to hear more from others.

 

[Benny17441]

The drain caused by the refrigerator should not have been of consequence in just a two hour sail. I would first ascertain the health of the batteries under a load test. Yes, a bad cell could be a possibility but having the batteries bench tested will determine if they are at fault.

 

[Dave602]

also make sure the monitor is properly synchronized with the battery and monitor. You have to make sure the battery charger is in float mode and turn it off and sync the battery to the monitor so it knows it is at full charge. That is if all your cells are good.

 

[Stu Jackson]

Benny and Dave gave good advice. With two 4Ds, I would try to test the cells with a hydrometer first before lugging them out of the boat.

 

[Steve Dion]

Stu: Don't think the hydrometer is going to work on an AGM. Maybe it would be a better bet to buy/borrow a load tester. These batteries are probably still covered by a warranty.

I would put them on a charger and be sure that they are fully charged before going much further.

 

[Don12364]

If your batteries really are good and they really were 98% charged, then to consume all that battery power over 2 hours means something was drawing 200 amps. 200 amps at 12 volts implies whatever was causing this loss of power must have been producing 2.4 kilowatts of heat. The only thing on a boat which can do this would the your immersion heater in the water tank via a static inverter - if you have one.
Otherwise you would see smoke!
Also, if it was bad batteries, it would need at least one dead cell in EACH battery to give those readings.

If none of the above applied then , IMHO your original conditions were not as you thought, or your batteries have too little water, or your engine start circuit has some high resistance - including the starter motor brushes and solenoid.

 

[Berndahle]

That's true, Donalex, that's a good thought. I'm positive the water heater wasn't on along with the inverter and there wasn't any smoke or heat being produced. So it does seem as though the batteries were almost dead before I even left shore but then I still can't explain the 98% reading.

Engine started right up on the backup battery so there's not unusual resistance there.

 

[kloudie1]

The Monitor does not measure internal battery current so it is probably telling you that one or both of the AGM's have a bad cell.. Are all of your chargers outputs tailored to AGMs ??

 

[Berndahle]

Claude, are you saying that even with one or two bad cells, the monitor would still indicate full state of charge? I would assume a bad cell means that no current flows through it anymore, in which case the monitor would show 0% Amps, or is my thinking flawed?

I don't have any sophisticated alternator regulator but I don't do a lot of motoring. Mostly I charge from shore power through my Xantrex Freedom 25 and it is set correctly for AGMs. I've had this setup with these batteries for 3 1/2 years now without any problems. Perhaps something went wrong over the winter and a cell or two died.

Is there an easy way to test the batteries for dead cells, given they are sealed AGMs? Stu suggested a hydrometer but I've never used one and not sure I can even get inside an AGM.

 

[Dantodd]

I think your best bet is to fully charge the batteries separately and then test them. I believe the Victron measures current in and current out and bases the state of charge on that data, so any battery will show 100% SOC after a time on the charger.

I would leave one battery on the charger for a couple days and then disconnect it and let it sit, undisturbed, overnight. Measure the voltage at the post. If you have good voltage (not sure what an AGM should be at full charge) put the battery back in the circuit without the charger and adjust the Victron for the new capacity. Now, watch and see if the amperage out and voltage track that of a healthy battery. There are good charts available on the 'net to show amperage draw v. voltage to determine state of charge on a healthy battery. I have the charts for flooded batteries but I'm sure they're out there for AGMs as well.

If one or both batteries do not perform acceptably they will need to be replaced.

 

[Berndahle]

Thanks everyone for your input! I'll try what Dantodd recommended and report back in a week or two.

 

[kloudie1]

DanTodd is right on.. and yes, an internal short of one cell will not be read by the monitor .. it should see the volts drop, but no amperes will cross its shunt as the battery dies .. There is no way to get a hydrometer reading on an AGM .. it has to be all voltage related.. the bleed down speed of the surface charge is one indicator.. but most AGM's have no access to the individual cells conductors.. so ya won't be able to know which cell is bad but ya should be able to quickly spot which battery is bad. You are on the right track, Berndahle..

 

[Berndahle]

Quick update: After charging them for a few days and letting them rest I measured 12.7v on one and 10.7v on the other. So one perfectly healthy, the other perfectly dead. Talked to a rep at Deka who asked me to have it tested. They confirmed it was dead and offered me a new AGM 4D for $421. I said no thanks since I can get a new bank of four 6V Duracells at Sam's Club for less plus I won't have to schlepp the 4D back to the boat and install it. After having taken that one out of the boat I'm looking forward to new batteries that are about half the weight.

Now the question is what do I do with my other 4D which is still in great shape. I can't really keep it in my bank because I'd be mixing AGM and flooded. i could keep it as a backup but I have no good place to put it. I may be able to add another battery box. Thoughts?