when does a battery do more harm than good?

[Tariq0101]

I have a small 12v batter bank using 3 batteries connected on parallel:
100 Amp, 3 years old
120 Amp, 2 years old
70 Amp, 1 year old
I basically just add whatever 12v battery I get my hands on on parallel.
and now I'm thinking of buying a 4th battery because winter is coming (not a game of thrones reference)
but the 100 amp 3 yo battery is worrying me, I'm not sure that it's holding any charge but I worry that some energy is wasted there on internal resistance, is there any harm from keeping it or should I remove it? and if I remove it and found that it's still holding some charge, is there a way to calculate if that charge is more than what the battery wastes on internal resistance?

 

[time2roll]

Separately test all the mix matched batteries at least once a year.

Fully charge and let rest disconnected 48 hours and check voltage Is good. Verify full charge with a hydrometer. If still in doubt have it load tested.

While in use the parallel is fine. If you store them without a float charge I would separate them.

 

[Texas-Mark]

Some will be here shortly to say you should never mix different age batteries, even in parallel. But I have been doing it for years. I have one battery with a 4 year difference.

But as posted above, just check the specific gravity on all of them, Maybe even load test them all separately.

 

[Porch]

You can use a Battery Analyzer to get some idea of the battery condition.

Digital Battery Analyzer

Amazing deals on this Digital Battery Analyzer at Harbor Freight. Quality tools & low prices.

www.harborfreight.com

But you are correct. If you have a really poor battery, it will be turning your charging power into heat and gas and prevent the other batteries from getting a full charge by holding the voltage low. So I would separate and test each one.

 

[Texas-Mark]

You can use a Battery Analyzer to get some idea of the battery condition.

Digital Battery Analyzer

Amazing deals on this Digital Battery Analyzer at Harbor Freight. Quality tools & low prices.

www.harborfreight.com

I question the ability of that sort of meter to accurately test a battery. How is is loading it more than a few amps? IMO one of those simple resistor based testers would be better than that.

While a hassle to have to take them somewhere, places like Walmart will put it on the good analyzer and print out the results.

 

[Porch]

I question the ability of that sort of meter to accurately test a battery. How is is loading it more than a few amps? IMO one of those simple resistor based testers would be better than that.

While a hassle to have to take them somewhere, places like Walmart will put it on the good analyzer and print out the results.

I questioned it too. It's not loading up the battery, but doing a resistance and other checks. There is more expensive models that do the same thing. When comparing it to other "professional" testers at the shop I worked at, the little HF was surprisingly accurate.

 

[Tariq0101]

Some will be here shortly to say you should never mix different age batteries, even in parallel. But I have been doing it for years. I have one battery with a 4 year difference.

But as posted above, just check the specific gravity on all of them, Maybe even load test them all separately.

So if the battery didn't pass the load test I should get rid of it?

 

[Texas-Mark]

So if the battery didn't pass the load test I should get rid of it?

As long as you are sure it was fully charged when you did the test.

When any of my batteries start to look "tired", I pull it out of my bank and put it on a stand alone charger that can do equalization. After maybe 5-10 cycles of that, it usually comes back to life. If not, it gets replaced.