Replacing single house battery

I have six 6v house batteries and discovered one that appears to be failing. I have been told to replace all six rather than just the one that has failed. Why is this being recommended? What might happen if I only replace the failed battery? With the cost of these I hate to replace five good batteries because of one if it is not necessary. Is this really necessary to replace all?

Greetings,
Welcome aboard.

That is what is the usual recommendation. Not sure if I would do 5 extra if they tesl good. How old are they? Worst case, the old ones drag the new one down to their level and eventually you have to replace all 6 and do not get all the life out of the new one.

Test the other 5 with a hydrometer if they are unsealed lead acids and maybe a load tester, or just the latter if sealed type. If they test ok, I`d punt, replacing just the one. You might get another year or so out of the other 5, and I doubt they will kill the new one, though they may drag it down to their level.

Usually batts are installed as a group, so the 5 existing ones are probably ready for the recyclers .

Batts in a string will all be killed as one fails.

Take care of them , install a SOC meter, keep them at the majic l 100% charged as often as you can , and enjoy the next 6-7 years.

Be sure to purchase deep cycle batts , not starts or 50/50 style.

Trojan golf cart are cheap and reliable .

Here in SW Florida there about $75 each, with core.

So this applies to a single bad cell or a single bad battery in a multi battery bank:

say you have a 12v bank of 6 cells (either two or one batteries) and you charge it up to 14.1v during acceptance phase. Now one of those cells is dead, lets say 1V. So the remaining 5 cells are charging at 2.62 volts (13.1/5), which if you had all good cells would be the equivalent of charging you battery at a whopping 15.7 volts. That's not good for them - you are basically equalizing them every charge.

If that only happened a little while, a few times, and other cells or batteries are newer, I'd punt as well, but if its been that way for a dozen or more charge cycles... I'd probably think about buying all.

FF said:

Trojan golf cart are cheap and reliable .

Here in SW Florida there about $75 each, with core.

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Wow! Are you sure about that? I'm seeing around $150 each here in TX for T105's. I'm looking at replacing a 10 cell house bank in few weeks....I'd love to buy them at $75.

Here's our "did not replace the whole battery bank story". We have 10 Trojan T-105's in our hose bank. I found one of the batteries in January had a seam that leaked acid at the top. A factory defect. We had replaced the entire bank in April, 2014.

I read/called/checked around and a lot of the information said since I was using them at 12 volts, if I was replacing one, I should replace the pair. The only down side would be the new batteries would seek the capacity level of the remaining 8. After 3 plus months all the batteries are healthy. It looks like we have the same capacity as we did back in January.

FYI: We paid $115/battery, with core exchange, at a golf cart shop in JAX.

If budget wasn't a factor, I'd recommend replacing all. However, I have a bank of 6 GC batteries and chose to replace only the bad battery. Time will tell if this was the right move for me.

I normally am one that says replace all but my opinion has modified somewhat over the last few years.

A lot depends upon how old the existing batteries are and what condition they are in.. If the whole bank is fairly new , less than two years, it may be worthwhile replacing the one only.

It is very possible, most likely , you will not get a long life out of that battery as the others will likely drag it down to their level. Also when recharging the charger will overcharge the older ones while undercharging the new one. Stay on top of the water.

However, you MAY get several years out of them if they are in good shape.
I would definitely do a proper load test on the rest of the batteries. If they are down on capacity enough then you have a warning that , although you have bought some time, you are going to have to face the music sooner than you like.

Take a good look at how they are all connected/charged as poor connecting/charging practice can damage even the best batteries.

greysailor said:

Wow! Are you sure about that? I'm seeing around $150 each here in TX for T105's. I'm looking at replacing a 10 cell house bank in few weeks....I'd love to buy them at $75.

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That was my reaction as well. I've never seen them under about $110.

FWIW, the last time I bought 6v GC batteries I got them from Costco. Even they were more than $75 each.

The other option is to sell 4 of the remaining good ones and buy 6 new. Get a quote for the core charge for the 4 you want to sell and $50 per battery. Know it sounds crazy, but you would be surprised how many people will buy used batteries. I sell the start battery out of my charter boat every few years and there's a line to see who gets it. Personally, I'd rather have 6 brand new batteries. If someone is going to give me $200 plus the core charge, that just makes it a little easier.

Ted

Thanks all for your input and suggestions. The batteries are 6 volt Trojan L16H-AC 435 AH @20hrs, big guys. The first place I checked was asking $375 per battery so price is a big factor. All the cells on the questionable battery were low. All the rest of the batteries looked very good on both the hygrometer and load test. I don't know how old they are since I just got the boat. The bad one had a loose connection so I tightened things up and will see what happens with a charge. More to come...

I would contact Trojan or one of their dealers. Likely there is a stamped code in the case that should equate to a month and year of manufacture. Think it's premature to make a plan without knowing if the batteries are 1 year or 7 years old.

Ted

Battery life isn't measured in years, it's determined by discharge/charge cycles at a given Depth of Discharge (DOD). The higher the DOD, the fewer cycles the battery will provide before failure.

A battery routinely discharged to 60% or more will probably only provide half the cycles as a battery that regularly sees 40% DOD, with the caveat that the charging cycle is completed to a full charge. There are so many variables that affect battery life that without extensive testing, it's almost impossible to determine battery condition at any given time. The age of the battery is essentially meaningless. That explains the wild variation in battery life as reported dockside measured in years.

If you replace a single battery in a bank, the replacement battery will surely have a lower internal resistance than the existing bank, so when it's put into the circuit, it charges at a higher rate than the remaining batteries. It overcharges. The remaining batteries undercharge. The new battery will heat because it's absorbing more amperage than the rest, and it will gas off more than the rest. It creates a very unpredictable set of circumstances that rarely work out as planned (to incur the cost of only one battery vs. a whole bank).

If budgetary concerns dictate that you replace only a failed battery in a bank, IF you can find a used battery that approximates the condition of the existing bank it's replacing a member of, you may be able to see the bank through the remainder of its anticipated life expectancy, but it's a crap shoot at best. But you'd be far better to replace a single battery with a used one, NOT new! So you're further ahead with the budget as well as your chances of bank survival with a used battery!

So, all that being said, keep in mind that a single battery failure in a bank is not typically a rogue, it's usually a sage, as it's telling you there's something wrong in the bank, and if one fails, chances are another one will be following soon, so think a bit before replacing that single battery! Given that you found a bad connection, you may be able to correct that and the affected battery may be salvageable.

To buy some time, you might be able to take 2 batteries out of the parallel and run on 2/3 of the bank if you can tolerate the reduced capacity. Either way, check your series/parallel wiring in the bank to insure that all the jumpers are precisely the same length, same gauge, scrupulously clean terminals, and that the main pos & neg connections come off opposite ends of the bank with respect to the length of the circuit path.

"with the caveat that the charging cycle is completed to a full charge."

Good info, but for cruisers that prefer to anchor out , that "full charge" 100% does not come often enough to create the longest life.

The usual solution is to oversize the house batts , so if they loose 15% 20% of their ability over time , the boat still has the daily power it requires.

So back at the boat today. As I expected tightening the terminals made no difference on charging the bad battery. I did discover that my pair of house batteries are separate from the inverter bank and the same age. I plan on taking one of the house batteries and replacing the bad battery in the inverter bank and putting two new batteries in the house bank. I suspect I will need to replace all soon, but this will spread out the cost a bit. Thanks for all of your suggestions and thoughts.