Can I add a fourth battery?

[AJ-SeaChange]

Here's the story.



June 2018 - 3 x 95ah 800a Varta 12v batteries were installed.


Noticed a problem in October 2019. Checked batteries, two of them were toast at 10.2v each even after a full day of charge from 600 watts of solar. One of them still shows 12.6v of charge.



Bought 3 x 100ah 800a Varta 12v batteries and replaced all three original batteries. All working fine now.



Here's the question.


Would it be OK to add the June 2018 - 95ah 800a Varta 12v battery into the chain with the 3 new batteries? Does the 5ah difference cause issues?


Thanks in advance. Baz

[john61ct]

Yes you can but not ideal.

Every 12V added in parallel means more likely to have balance problems.

More importantly, what was the cause of the previous failure, and was that fixed.

Hypothetically, would buying a whole new set every year be a problem for you?

[AJ-SeaChange]

Hi John, the problem was traced and fixed.


Buying a new set every year would not be good.


You say that adding a fourth battery wired in parallel could cause balancing issues, what would be the best way to add more storage capacity?

[S/V Illusion]

I’d like to see an 800 amp 12v battery.

[AJ-SeaChange]

Originally Posted by S/V Illusion

I’d like to see an 800 amp 12v battery.

Nobody mentioned 800amp batteries.


Original post says - 95ah 800a Varta 12v batteries.

[a64pilot]

You can add the fourth battery. Many successfully have a LOT more batteries than four, you do need of course to ensure your neg and pos are at different ends of the bank.
However what is your voltage when you wake up in the morning and how are you determining SOC?

If what is killing your batteries is the gradual walk down of SOC over time and it likely is, adding another battery may not do much.

Not all of them being the same age and capacity is not ideal, but it will work.
Ideal is of course to buy four all new batteries of the same capacity, but as assume you like most have budgetary constraints that get in the way of perfection.

[AJ-SeaChange]

We have a Victron BMV 700 battery monitor and I check the voltage on that every morning.


To recharge we have 600 watts of solar. Or if cloudy we run the engine at 1,500prm until we are back to 100% battery capacity.


Thanks for your advice.

[john61ct]

Originally Posted by AJ-SeaChange

You say that adding a fourth battery wired in parallel could cause balancing issues, what would be the best way to add more storage capacity?

Best is to use fewer strings in parallel, each unit being higher Ah capacity, in your case 400Ah cells, 6*2V is best.

3*4V is also possible.

The usual approach, but yes a compromise compared to the above options, is 6V GCs, @200+Ah commonly available, so 2P2S, total two strings gives redundancy, units not too heavy to manage, total well over 450Ah.

Obviously advice for "next time", best to replce the whole bank at once.

Meantime here's wiring guidance on ameliorating those imbalance issues from using too-small 12V units.

http://www.smartgauge.co.uk/batt_con.html

[Rapanui]

I would say yes but not ideal as others have said, provided the one remaining battery from the original set is in good health. My concern is that presuming all 3 were in parallel, why would the the one have survived and the other 2 have not. At the least, I would put the survivor on a battery tester and conduct a discharge test.

[witzgall]

Given what happened to the bad batteries, it is highly unlikely that the health of the remaining battery did not take a hit. It will quickly draw the health of those new batteries to it's level. Don't do it.

Chris

[MoodyS]

Don't use the old battery, you could find yourself replacing all 4 next year. You only mention solar and engine charging, give your batteries a break and go into a marina for the weekend every 6 months at least and let them soak up a full charge and sit at float voltage for at least 48 hours. They will appreciate it.

[kas_1611]

If you have space for 4x 100a-hr batteries could you instead have 2x 200a-hr batteries? Saves multiple wiring and any potential imbalance issues?

I know Varta do a 140a-hr as I have 2 of them myself and I've found the proMotive 220a-hr online with the SuperHeavyDuty version giving an extra 5a-hr. The AGM version is a 210a-hr. Plus there is also a 260a-hr Deep Cycle AGM in the Professional range.

Maybe these are an option instead of 4x smaller ones.

[john61ct]

Getting to 100% Full only every six months (!!!) will not do much to help longevity.

Monthly maybe, weekly much better.

If you have both solar, and ICE sources at high amps, no need for marinas, just use ICE powrr to get to 80-85% SoC before a bright solar day starts, and keep consumption down, maintain Absorb V until your endAmps spec is reached.

Ideally, do this most cycles.

[john61ct]

This is not three new ones added to one old, but the opposite.

Note I misread the "new" bank of three strings as being July 2019, not 2018, bit more marginal call there.

Just adding one more battery, as long the other three are testing OK, yes the lifetime of that new one will be reduced.

[a64pilot]

Yeah, it’s one new to add to a newish existing bank.
He has three options.
1. Learn to live with existing capacity.
2. Increase it by adding a battery
3. Replace the whole bank.

Assume 1. Isn’t viable, and he doesn’t have the funds or maybe just doesn’t want to spend the funds to replace the whole bank, that only leaves option 2.
Even if the new battery is drawn health wise to the existing three, which I think it will be, he should still have plenty of life left, even if he only gets one year out of the one battery, it’s only 1/4 as much as buying 4 all new batteries.

2 volt cells, 4V batteries (never seen one of those myself) are really sort of unicorns for most people. Yeah they exist, probably won’t fit, and you have to special order them, paying exorbitant shipping rates etc. and if you get a bad one, there isn’t another one on the shelf.

Odds are what he has now isn’t a deep cycle battery anyway, so buy one more and take some time to decide what is available to buy for next time, and have a plan and maybe save up for it so it’s not an unexpected expense.
Primarily take the time to learn about the care and feeding of a LA bank, so that you will be the right kind next time and if you take care of it, you may get more than 5 years out of it.