Finding the batteries in my bank to keep

[Warby12]

Hi, our boat has 5 x 100 Amp hour batteries of an elderly age. They came with the boat and don't seem to have much life left. We have just bought a 25 amp charger which seems to be running hotter than I would like considering all we are powering is a fridge and lights in a mild climate. So I think retiring some of the batteries in the short term will ease the load on the charger. (We are Marina bound for now)

(long intro sorry)

So the question is, how can I easily sort the best two or three batteries from the five? Specific gravity test on the electrolyte in each cell?

[a64pilot]

That or I'd disconnect them over night and check the voltage in the morning. Dead batts won't hold voltage after a few hours. Be sure you electrically isolate the battery's from each other when you disconnect them of course.
Since your marina bound, you only need one battery to act as an accumulator / capacitor, assuming you have a constant source of 120VAC of course, so you can shop around for a good price for replacements.

[Warby12]

Even overnight would be quick enough? Great idea. I could put the charger on the start battery, while waiting, then I could give it a couple of days. Thanks for that.

[btrayfors]

Warby12,

Unfortunately, the ability of a battery to "hold a charge" does NOT equate to the remaining capacity of the battery. These are two different things. It's perfectly possible to have a battery "hold a charge" overnight, yet have extremely diminished capacity. Think of a car battery which seems OK, but can't start your car.

The only ways to accurately test the remaining capacity of a LA battery is to either:

1. do a controlled 20-hour test, placing a C/20 resistive load on a fully-charged battery; or

2. use a sophisticated conductive/resistive tester such as the Midtronics series testers.

Additionally, if you have a very old battery bank, as you stated, it doesn't make sense to replace just one or a few of the batteries. You'll really need to replace them all.

While you're at it, you might think of changing the configuration of your batteries. If you have the space and inclination, golf-cart batteries are a much better deal for a cruising boat. They are true deep-cycle batteries with thicker plates which can take more abuse -- for many boats these are the most economical way to go to achieve a robust house battery bank.

Bill

[skipmac]

Second the recommendations from Bill.

I would add that 25 amps seems like a small charger for a 500 amp hour battery bank.

[a64pilot]

A battery that will hold voltage is a battery that won't be draining down your charger, but as stated it may have very little capacity. I've always been able to find a battery that has lost capacity by using one of the automotive capacity testers, looks like a heater and most shops will test for free. Now that does not tell you long term capacity, just how well a battery can handle a large short time load, but it's always spotted bad batteries for me in the past. You can do a long time test by using something as simple as a car headlight for a load bank if you want to spend that time, but again, I think he has already accepted his bank is past due for retirement.
I think he knows his batteries are shot and need replacing, I think he trying to come up with a way to keep from cooking his charger or smoking a battery, I have to assume cost is an issue of he would otherwise have already replaced his bank?
I second the golf cart battery, makes me wonder why you don't see more of them used.
I'm talking value here, not money is irrelevant, I want the best battery.

[Warby12]

Thanks heaps all

Yes I am trying not to cook the charger so just want to remove any dead batteries from the bank and hopefully keep a bit of capacity for a weekend cruise. This is all a temporary solution as we will be stuck in the marina for a couple of years and I will choose a cruising configuration just before going so the batteries are fresh then. Waste of money to do it now.

I really appreciate all of your help

[btrayfors]

With respect, I don't think you got the message. Let me make it clear.

1. With an ancient battery bank consisting of 5 12-volt batteries (6 2V cells each), it makes no sense to try to pull out one or two or three "bad ones" and just keep the rest, whether of not you're in a marina. This is likely to be unsatisfactory, and could even be dangerous (think shorted cells, maybe why your charger is overheating).

2. If you're truly marina bound and have a modest load, then a small battery bank -- say two 6-volt golf-cart batteries in series giving you 225AH capacity -- would be plenty.

3. The size of the battery charger isn't really important if you're in a marina, so long as your average load doesn't exceed the capacity of the charger.

4. Your charger should not be getting hot. You didn't say what you're using for a charger, but a smart marine charger is a good investment. For marina use, again, even a smallish one would be OK. If you're a diesel boat, one of the Iota DLS/IQ4 chargers is likely to be most economical. They come in all sizes from about 15A capacity to 90A capacity. Best one out there for worldwide use these days is the Sterling Pro Ultra (AKA Professional Mariner Pro series) which come in various sizes up to 60A.

Bill

[sparrowhawk1]

First check electrolyte level. If you don't find a bad battery that way(dry cell) check the voltage with the batteries separated after they set for 24 hours as already suggested. And its also true putting a load on will be the best indicator of capacity. It makes sense to me to save the good batteries and not buy new ones until you're ready to cruise.

[forsailbyowner]

I bought a battery load tester at harbor freight for $20. It will give you a reading in 15 seconds and the relative load capacity of the batteries will be easy to spot.

[Cheechako]

Originally Posted by Warby12

Even overnight would be quick enough? Great idea. I could put the charger on the start battery, while waiting, then I could give it a couple of days. Thanks for that.

Yes disconnect all from each other and from any other draw. Then measure voltage in the am on each. If you have a bad battery it should be substantially lower than the others.

Not sure what you mean by HOT, but all chargers get pretty warm at times as a function of how they work.

[Warby12]

Hi btrayfors, I do indeed appreciate you taking the time to ensure I understand your point the charger is a CTEK 25 amp output and max bank of 500 amps hours.

Our longer term goal with the boat is to build a really robust dc simple 12 vlt dc system but also not too big. Fridge (will be tropics cruising often), lights, nav and a small inverter, maybe 250/300 AH if possible. Wind and some solar already have. Not large scale at all. Very limited AC I will get a Honda petrol genset for power tools. Like I said robust and simple.

Golf cart batteries do sound the way to go but that is for the future. All I'm after now is minimising what I already own. I agree with you, last thing I want to do is leave a dead battery in the mix. What I will do is follow everyone advice and do a simple load test (like a headlight) and find out exactly what I have on board, rather than just guessing, which is a shame as guessing is easy and fun

[Warby12]

Yes disconnect all from each other and from any other draw. Then measure voltage in the am on each. If you have a bad battery it should be substantially lower than the others.

From cheechako above, this sounds like the trick thanks again all

[SnowT]

Steps I would do..

Isolate every Battery...

-check it's voltage..
[If it a 12v Battery and the voltage is less than 11v you have a possible Dead cell in the battery..]
each cell is about 2.2-2.4v
1.9v discharged

-- depending on what size voltage..
-- Even if your Batteries are less 12v they can be charged with a higher rating charger[Higher voltage] you just need to watch out for the voltage of the batteries so they don't go over their Max rating..

-- Let the Battery rest for 30min to an Hour then check the Voltage..

If the voltage has dropped quickly and continues to drop you have an issue..

A voltage 10v or Less in a 12v Battery means it's Dead..

A Dead cell will discharge a Battery quickly.. A Battery that has a dead cell will effect every cell around it..


Juergen

[forsailbyowner]

Originally Posted by SnowT

Steps I would do..

Isolate every Battery...

-check it's voltage..
[If it a 12v Battery and the voltage is less than 11v you have a possible Dead cell in the battery..]
each cell is about 2.2-2.4v
1.9v discharged

-- depending on what size voltage..
-- Even if your Batteries are less 12v they can be charged with a higher rating charger[Higher voltage] you just need to watch out for the voltage of the batteries so they don't go over their Max rating..

-- Let the Battery rest for 30min to an Hour then check the Voltage..

If the voltage has dropped quickly and continues to drop you have an issue..

A voltage 10v or Less in a 12v Battery means it's Dead..

A Dead cell will discharge a Battery quickly.. A Battery that has a dead cell will effect every cell around it..


Juergen

Doesnt always work that way though. Just got over a starting debacle. Battery read 12.8 cold after sitting overnite with no charge. Then it failed the load test miserably. That battery was the reason i bought the load tester to isolate the problem.