Replacing one battery in a bank of house batteries

[gstent]

My electrician advised that one battery in my bank of six 120AH batteries was dead and needed to be replaced. In the meantime he isolated it from the rest. I have ordered a replacement and it has come with this advice from the supplier - what do you think?

*"if the customer is changing only one battery in the bank there will be no warranty.* The reason behind this is you have 1 new battery and 3 old batteries and your new battery will only work as good as the worse battery in the bank.* In our opinion it is a waste to spend the money on just 1 battery as its like putting a used battery in the system.* Plus doing this can make you have problems with the equipment on board some times."

[smac999]

The general rule is replace the whole bank at once.

[Jeanneau 45.2]

FWIW, I just replaced my entire house bank consisting of 4 GRP 31 DC FLA, even though 2 were still serviceable. I received the same advice wrt changing out the entire bank ... and as a full time cruiser mostly at anchor I needed optimum performance. If you are plugged in most of the time and you are only expecting to keep the boat for a short time, you might get away with mixing batteries. As Click and Clack said "in the end the stingy person spends the most money".

[thomm225]

We get basically the same reply from the Companies that maintain our large Liebert and APC UPS Units.

These things have anywhere from 24 to 48 twelve volt batteries or more.

If the batteries are only 4 or 5 years old though, we will replace individual bad batteries as necessary rather than replace the whole string which can cost $9,000 or more.

Last time we did this, I believe we replaced 5 batteries out of 36 and that was a couple years ago. We are still running that UPS on those batteries without a battery alarm out battery problem

We all know it's best to replace them all but sometimes it costs way too much. Taking a chance on replacing just the bad one can sometimes pay off

[a64pilot]

Think of a large flashlight with four or more batteries in it.
Do you take you multimeter out and find the bad Cell and only replace it?
Of course not, cause you know the other three almost dead batteries will drag the good one down to their level as they are all connected together.
Same for your boat bank.

That UPS bank is largely not used, they are all in standby, waiting on a power failure of course, the Boat bank is in use, that is different.

[thomm225]

Originally Posted by a64pilot

That UPS bank is largely not used, they are all in standby, waiting on a power failure of course, the Boat bank is in use, that is different.

Actually, we have quite a few power outages here and the UPS Units are used especially in Summer.

Plus it has a battery alarm, if the voltages are incorrect.

These points can be argued forever.

I had a starter battery in parallel with as deep cycle battery for about 5 years because I wasn't going to just throw away the starter battery after my diesel failed and I went with a pull start outboard

So I used it as one of my house batteries and we all know not to do that also (especially those two in parallel) but I got about 5 years out of those batteries which BTW were charged with one 65 watt solar panel thru a $12.00 Chinese Controller that held the voltage at 14.4 volts max

I'd monitor the panel side of the controller until the voltage neared VOC and then disconnect the controller. I had the wires to the batteries on alligator clips

It was an interesting experiment...

[john61ct]

Replace all at once.

[S/V Illusion]

My advice is follow the manufacturer's advice rather than from a bunch of strangers, myself included.