Replacing 2 batteries instead of 4?

I would change them all. You’re in Australia, don’t you guys go out into the middle of nowhere at times? Why run the risk of having a problem?

 

^^^^^^^^X2

 

Hello mate, nice to see that's just not an American attitude, but there is some merit to that. A battery can short out, and you may not know it, but it drains the good ones. Replacing just 2 is like taking a shower, and putting dirty underwear back on ( turning them inside out, doesn't count) You'll ruin the new ones in short order. BTW, I always got along fine with 3 batteries.

 

Charge them all up. Disconnect them. Wait over night. Check voltage on each battery. If they are all exactly identical (probably 12.60 volts) then you’re good.
If the old ones are less than the new ones, they’ll cause a situation where the new ones are effectively charging the old ones constantly and they’ll all drain down over some period of time.

 

I'm gonna disagree with the parts changers on this one. If they test good why not get some more life out of them? Mark the dates so you know thier age so you can retest them on your next PM.

While their is a certain amout of relibility from changing all four, they never all fail at the same time. When my dad was running his Pete and I was running the shop and keeping it up, the battery on main cables would always fail first. Sometimes at a rate of 3 to 1 of the other 3. I started rotating an older battery to the main cables and putting a new on on the other end. He was running a inverter for the fridge. These were Interstates.

 

I have 2 banks of 2. I've always tested with load test and hydrometer every year and rotate out what doesn't pass like Goodysnap said. I have noticed that the one battery my inverter (Battery 3) was connected to went first. My inverter runs fridge and microwave. One time battery 3 was shorted, I found a silver dollar sized spot on the bottom of the battery where the plastic case melted.

I have recently heard of testers that can reveal how much cranking amps are left in a battery, looking into that.

 

Typically a battery group is only as strong as its weakest battery. Personally I would do all 4 unless the remaining batteries are relatively new. I normally don't run batteries longer than 3 years in anything I own either.

 

Those digital testers are trash IMO. I haven't found a test method yet that beats the tried and true carbon pile load test.

 

The midtronics capacitance type testers are very temperamental and depend highly on a good clamp to battery connection at the battery post, specifically the lead of the post. Clamping to the threads on a group 31 will not yield an accurate test because its stainless. False fails are very common when connections are poor. Like everthing else the tool is greatly depended on the user to follow instructions.

I agree it is hard to beat the old carbon pile. Pretty reliable......