batteries drop from 13v to 8v randomly

[jpmccros]

All, I've got a pretty old 40' sailboat with 2 house banks and 1 starting bank all at 12 volts. I have a 3 bank 30 amp battery charger with red/green leds for charging indications.

Here's my issue. Batteries normally are at 13 -14.7v when everything is working. Then all of a sudden we notice the lights are dim and the water pump won't run. I look at the DC volt meter and it shows the voltage at 8 volts. When I go look at the battery charger, all 3 banks show green indicating they are charged.

To correct this I turn my battery charger off for 30 seconds and then back on. Now the lights are red and the voltage on the DC voltmeter climbs back to 13 Volts I'm a matter of a minute.

Details.
House Batteries are the basic 6volt water filled batteries connected in series to get parallel. 2008

Starting batteries are 2 12 volt deep cycle with a battery isolator (old style). I think these are newer.

Also there is a 150 w solar panel charging.

I'm suspecting there is a bad battery or cell that somehow is draining the system. I plan on doing a major DC upgrade and replace all batteries to GEL cell and change out the panel As well as the starting battery isolator. Previous owner (s) have made a mess of things so I don't know what all may be causing this issue.

Any pointers on what specific piece may be causing this issue?

[skipmac]

Hard to tell but I might suspect an issue with the charger. If you are connected to dock power then the charger should be able to run the electric loads on its own unless you have a really bad battery. The fact that you turn the charger off, back on and it works again indicates to me something with the charger. If it was a dead or shorted cell in a battery then resetting the charger wouldn't fix that.

Some other things.

Why have two deep cycle batteries for cranking the engine? Not that they won't work but one properly sized starting battery would do a better job. Like using a wrench to drive a nail. It will work but a hammer would do it better.

Also, why switch to gel batteries? They are a lot more money and a lot more sensitive to charge problems. Gel batteries need to be fully charged on a regular basis or they will almost certainly die an early death. The main benefit of gels is they can be charged more rapidly than a liquid filled battery. In fact they prefer it. With your 30 amp charger you won't be able to take advantage of that.

[barnakiel]

Is this possible that you are actually on shore power charger most of the time and when it cuts off due to (overheat?) the batts take over and they are actually shot? So you would see say 13 Volts while the charger is on, then the charger switches off and you very quickly drop to 8 Volts (batts shot at 10 minus some load = 8 Volts).

Mind a charger off at override (e.g. overheat may still seem ON, unless yours has blinking overheat signalling LEDs).

This could explain too why you think you are fine when you switch off (reset) and switch on the charger.

What happens when you switch off all consumers then charge the batts full, then switch off the charger? How long can your batts alone deliver?

Can your batts be actuall kaput? How old is this bank?

b.

[jpmccros]

Skipmac,

The only reason I'm wanting to go to gel cell is that I know I won't perform the normal inspecting required for topping them off with water.

What I don't understand is how is the charger able to drop the entire bank down to 8v?

Also the batteries do not come back to 13 if you keep the charger off for a while.

[jpmccros]

barnakiel,

The batteries are 2008 from what I can tell.

We take it out at least once a month and anchor overnight. Batteries remain 13 v.

Also we spent 40 days (and nights) in a boat yard and the solar charger was enough to keep the lights on every night. We had no battery issues the whole time.

any recommendations on a new charger?

[DeepFrz]

Maybe a bad power connection? What is the voltage reading at the battery posts themselves with an accurate DVM when the panel meter reads 8 volts? If okay, what is the reading at the terminals and then work your way up to the panel ensuring. When the volt meter reads 8volts what happens if you shut the charger off? And the solar charge controller as well? Start eliminating things and verifying things until you find the problem.

[barnakiel]

I went back to your post and I am thinking about the fact that you are 2 x 6 making 12 Volts. If one 6 V batt is blank, it will pull the system down to 8, I think.

In any case be VERY cautious. Try to find and correct before any further damage comes along.

Can you inspect all cells? Is any of them dry? Is any of them NOT losing water at regular rate?

2008 is getting old in regular use - 7 years. I have seen Trojans (6V into 24 bank) killed in 3 years when the guy who was supposed to be the captain forgot to do the regular checks and maintenance.

If you can charge them all then let them sit some and then test them with a load tester, you are very likely to find a bad apple.

Remember, a load tester, not any tester. But start with a visual inspection if you can open the cells and peek in (do wear gogles !!!). Make sure you you ventilate the area very well before any tests and checks!

Let us know what you find.

b.

[Lake-Effect]

Originally Posted by DeepFrz

Maybe a bad power connection? What is the voltage reading at the battery posts themselves with an accurate DVM when the panel meter reads 8 volts? If okay, what is the reading at the terminals and then work your way up to the panel ensuring. When the volt meter reads 8volts what happens if you shut the charger off? And the solar charge controller as well? Start eliminating things and verifying things until you find the problem.

+1

If your batteries have actually dropped to 8v total... and they're 7 years old, they are possibly toast, or at least will have a much reduced life now.

[savoir]

Originally Posted by skipmac

Hard to tell but I might suspect an issue with the charger. If you are connected to dock power then the charger should be able to run the electric loads on its own unless you have a really bad battery. The fact that you turn the charger off, back on and it works again indicates to me something with the charger. If it was a dead or shorted cell in a battery then resetting the charger wouldn't fix that.

Some other things.

Why have two deep cycle batteries for cranking the engine? Not that they won't work but one properly sized starting battery would do a better job. Like using a wrench to drive a nail. It will work but a hammer would do it better.

Also, why switch to gel batteries? They are a lot more money and a lot more sensitive to charge problems. Gel batteries need to be fully charged on a regular basis or they will almost certainly die an early death. The main benefit of gels is they can be charged more rapidly than a liquid filled battery. In fact they prefer it. With your 30 amp charger you won't be able to take advantage of that.

You are thinking of AGM batteries.

[dohenyboy]

Having been in the same situation this is my advice:
First of all go through all your connections and make sure they are all clean and tight. Use battery anti corrosion spray or whatever on all connections. That is, remove all wires and cables and reinstall. Look for problem wiring as you do this.
It is correct, your charger should be capable of carrying the load alone.
Those batteries are old and you don't know how they could have been abused, so you are going to be replacing them anyway...but the last thing you want is a bad charger to ruin the new batteries.
I would replace the charger. New one is not expensive. Look at defender.com. At worse case you have a backup charger.
I have the pro mariner pro tournament 30 amp. Yes its about 400 dollars but it has been very dependable.
If you have solar and just do overnights you might get away with a good quality automatic car charger or two (blasphemy, i know).
The solar can top up your batteries. Electrons is electrons.
Anyway..
I always carry an automotive charger with me. Yes I know they are not "smart" but its a cheap emergency backup. Plus, if your batteries get severely discharged, some smart chargers won't start charging till you bring up the voltage.
Try equalizing your house bank. It might extend you.
Calder's 3rd edition on boat electrical and mechanical systems makes a very good case for only having 1 house bank, not two. I would combine the banks when you install new house batteries.
I would also, when the time comes, replace the deep cell engine batteries with starting type batteries. You should be able to find out how many cold cranking amps are specified for your engine. As for me, I get the biggest that will fit, at Walmart or costco. At least 50% more than required in CCAs. The price differential is nominal.




Why can't things remain where i carelessly left them?

[bletso]

Sounds like a partially open or shorted cell. Will look fine and nothing and then all over again. I have had the seen the same problem in automotive many times.

[Terra Nova]

Sounds like a loose connection, probably a ground.

And your batteries have outlived their useful lives.

[jeepdoesrubicon]

Lots of good advice here. i would also get a bigger charger. It clearly is the charger overheating. Do maintenance on the batteries, but replace the charger. I have a 160 amp alternator that has no trouble keeping the batteries afloat until they fail!

P

[hamburking]

Turn everything off, then check the voltage of each battery with a digital voltmeter right at the posts. Hopefully this will show you the bad battery. However, it is just as likely a bad connection. This is harder to find. I've had cables broken inside the sheathing that looked fine, but would work only intermittently. Have someone watch the voltage while you jiggle each battery cable, watching to see if you can induce the voltage loss.

And finally, maybe its something like the motor for your fridge or bilge pump coming on, giving a huge drop in voltage because of a big power draw? Do you have anything like that?

Good luck. I'm searching for my own wiring gremlin myself.

[skipmac]

Originally Posted by jpmccros

Skipmac,

The only reason I'm wanting to go to gel cell is that I know I won't perform the normal inspecting required for topping them off with water.

What I don't understand is how is the charger able to drop the entire bank down to 8v?

Also the batteries do not come back to 13 if you keep the charger off for a while.

I think a critical bit of information, the charger shows fully charged but the batteries have 8 volts. But you turn the charger off and back on and it charges the batteries back to 13V.

Barnakiel had a good suggestion about this. Does your charger have an error code, that is does a different light come on or does it blink a certain way? Are you certain the charger is saying the batteries are fully charged when you see low voltage on the battery?

Back to the gel batteries. They are much more expensive than liquid filled batteries and they do have their own maintenance requirements. They don't need liquid checked but they must be fully charged on a regular basis or they will die very soon. They should also be charged at a high rate to do best.

So you can still kill gel batteries, just a different way of killing them than liquid batteries and if you do, you just killed about three times more money.

Here's another test. When the batteries show 13V what happens if you turn off the charger and run just on the batteries? How long will they power your lights?