AGM Battery Age & Replacement

[haycord]

Scenario: Combining batteries of a different age.
If I have four AGM 12V 200AH batteries that are 3 years old and one fails - what will be the issue with replacing only the one other than shortening its life a bit? Assume the group has been taken good care of in regard to Charging, DoD, Ambient Temp, etc.
Thanks

 

[grizzzman]

If the voltage does not match the old batteries will take the new battery down. Also due to the higher resistance of the older batteries the new one will be pulled down faster.

 

[Hedges]

Series or parallel?

If parallel, can you get by with three?
If parallel, why did one fail, and are the others subject to same issue?
Were they wired to balance current draw?

If series, they may have needed to be charged individually or otherwise "equalized"; one that is low will degrade faster.

 

[haycord]

The existing system is three 12V 200AH in Parallel. I'm doing some system changes and want to go to four batteries in a 2S2P configuration providing 24V 400AH. I was using the described scenario to try to better understand simply because replacing three $350 batteries that are still perfectly good is a bit much on a budget.

So if someone had a 12 string battery bank that's just out of warranty and one fails is the SOP to buy all 12 again? Like I say I'm just trying to understand this a little better. Thanks

 

[seneysolar]

The existing system is three 12V 200AH in Parallel. I'm doing some system changes and want to go to four batteries in a 2S2P configuration providing 24V 400AH. I was using the described scenario to try to better understand simply because replacing three $350 batteries that are still perfectly good is a bit much on a budget.

So if someone had a 12 string battery bank that's just out of warranty and one fails is the SOP to buy all 12 again? Like I say I'm just trying to understand this a little better. Thanks

SOP would be to replace the bad one and move on, they will always have subtle balance issues.

Yes the new battery will be pulled down to the level of the older batteries; an equalization charge can help. Most agm manufacturers dont recommend equalization charge, but sometimes its necessary once or twice in their life.

 

[haycord]

I understand the sacrifice regarding the new battery being added. Generally speaking I often hear 'don't do it' like its a really bad thing. Appreciate the comments.

 

[Q-Dog]

Do you have a way to capacity test these batteries? 3 year old AGMs are awful close to end of life. Kinda depends on the brand and how they have been treated too.

 

[haycord]

Do you have a way to capacity test these batteries? 3 year old AGMs are awful close to end of life. Kinda depends on the brand and how they have been treated too.

This is my plan for testing. See any holes in it? Thanks

Battery: (one/test) 12V 200AH (2400Wh) AGM

Monitoring Device: Victron BMV-712

Charger: Victron BlueSmart IP65

Ambient Temp: 60 – 70 DegF

Charge to 100%

Let it rest for 24 hours

Connect to Inverter w/ 700W Electric heater

Operate until 1200Wh has been consumed

Let it rest 1 hour

Record data. Expect 12.07V (50% SoC)

 

[Hedges]

A hole could be if SoC and voltage drops too low to complete test.
Will you have a way to record time until shutdown? I plugged in an AC mechanical timer (good for maybe 15 minute granularity)
Good also to monitor or spot-check voltage along the way, check against Peukert chart.

After charging to 100% and resting, check voltage. That should indicate if some cells are low, or all fully charged.
For my batteries, there is an equalize process which brought up voltage a bit, but I didn't confirm effect on capacity. (they were quite old, quite degraded.)

How does usage compare to expected life? As Q-dog says, could be near end of life at 3 years (or beyond) if cycled deeply every night. Or could be expected to have 70% remaining.

12V x 200 Ah is 2400 Wh. Testing at 700W (plus how much inverter no-load and efficiency loss?) is about 0.33C
Data sheet capacity is likely 0.05C, 20 hour. But your loading would better match usage.
I used about 625W for what had once been 48V 100 Ah, 0.125C. With 12 year old batteries, I got 40% capacity.

 

[Mattb4]

The existing system is three 12V 200AH in Parallel. I'm doing some system changes and want to go to four batteries in a 2S2P configuration providing 24V 400AH. ...

Having old and new in series is not a great idea. You would be better to buy 2 new (series connect) and than parallel with 2 of the old leaving out the worse performing old battery of the 3 after testing all of them.

 

[seneysolar]

SOP would be to replace the bad one and move on, they will always have subtle balance issues.

Yes the new battery will be pulled down to the level of the older batteries; an equalization charge can help. Most agm manufacturers dont recommend equalization charge, but sometimes its necessary once or twice in their life.

I re-read this and i realized the scenario changed from having 4 batts and replacing a bad one in the first post; to having 3 and adding a 4th to grow the system a few posts down

IMO This is a different decision to make with replacing 1 to maintain the current bank size VS to grow the bank while mixing old/new. I wouldnt do it on purpose, only if you absolutely need to.

Honestly, Your money is better spent with all new when building your new 24v bank. Surely you can find another purpose for the 3 old batteries...

 

[Sundog33]

Do you have a way to capacity test these batteries? 3 year old AGMs are awful close to end of life. Kinda depends on the brand and how they have been treated too.

They can last a real long time if you treat them well.
Mine had 70% capacity after 10 years of just sipping them down 20%

When I got my Lifepo4s I hatted to throw them out. Now they just sit in a corner on a trickle charger waiting for some odd emergency.

 

[Sundog33]

This is my plan for testing. See any holes in it? Thanks

Battery: (one/test) 12V 200AH (2400Wh) AGM

Monitoring Device: Victron BMV-712

Charger: Victron BlueSmart IP65

Ambient Temp: 60 – 70 DegF

Charge to 100%

Let it rest for 24 hours

Connect to Inverter w/ 700W Electric heater

Operate until 1200Wh has been consumed

Let it rest 1 hour

Record data. Expect 12.07V (50% SoC)

DOn't do it! Theses are not lithium batteries. Drawing them down low can damage them permanently.

Buy yourself a car Battery Tester for $35 bucks. They are not perfect but will give you a pretty good feel to where your batteries are.


something like this

MOTOPOWER MP0515A 12V Car Battery Tester Automotive 100-2000 CCA Battery Load Tester Auto Cranking and Charging System Test Scan Tool Digital Battery Alternator Analyzer​

 

[Sundog33]

 

[haycord]

Thanks big time for the good advice. I can see I was going to far exceed the C Rating. Old age is taking its toll on me - I often tell friends I played with too much Lead and Mercury when I was a kid. Lol

 

[Hedges]

So did I.

The C you propose probably matches typical utilization (when not idle) so takes care of the Peukert calculation for you.
And 50% DoD, immediately recharging afterwards, should only be damaging to the extent of 1/300 or 1/700 of lifespan, whatever specs say. So long as that is 50% of present capacity.