Understanding Battery Aging

[fust]

Hello DIY Solar folks, I'm a n00b looking for some better understanding.

I understand that it's a bad idea to mix old and new batteries, but I'm wondering what the reasons are, besides the basic idea that capacity is reduced over time, so mixing an old battery rated at 120Ah with a new one rated the same Ah is a no-no. But I'm wondering if there's more to it.

Here's a scenario (partially hypothetical for simplicity/clarity):

• I have a 12V flooded lead-acid deep cycle battery rated at ~120Ah that is more than one year old.
• I tested the actual Ah, and determined that the battery now has a 100Ah capacity.
• I need to add an additional battery to reach the capacity I need for my inverter/load.
• Is it safe to add a brand new 100Ah battery in parallel with the older battery, since their capacities match, or are there other factors to consider?

Thanks in advance for any advice!

 

[Steve_S]

With Lead Acid it is NOT good to mix new with old. The lowest common denominator rules so the older battery will pull down the new one's capacity. This applies to all Lead chemistries, FLA, AGM etc.

Lithium Based batteries do not have this issue at all. You can add packs to your battery bank over time and there is no issues with that.

Also a tidbit: All lead batteries can only be discharged to 50%DOD otherwise harm is being done. With LFP (LiFePo4) you can safely discharge to 99% regularly without ill effect. Consider that at present, a prebuilt 12V/200AH LFP Battery pack in the US can be had for <$350 and will last for 10+ years, that may be worth your consideration.

 

[fust]

@Steve_S, thanks for the reply!

• I already have the lead-acid batteries (two actually... they were free, and not abused, still in great shape), so while I know Lithium is better, I'm looking to expand what I already have instead of invest in completely new batteries. I'd like to run the lead acid batteries to the end of their life and then change to better batteries.
• Aware of the 50% rule, but good lookin' out, and relevant for others who read this post, so thanks!
• Re: mixing old and new batteries of different capacities for FLA and other lead-acid chemistry batteries, I know that's generally considered bad. Thanks for pointing that out clearly, same as above re: good info to post in a n00b thread!
• My question isn't about mixing batteries with different capacities though. It's about mixing batteries of different ages but with the same capacity as tested.
• So can I reference the actual tested Ah capacity of the old batteries in their current state of decay to add new batteries with Ah ratings that match the the old batteries at their current tested Ah capacity.
• If the old batteries test at 100Ah, and the new batteries are rated at 100Ah, it seems essentially the same as if I bought all new batteries rated at 100Ah each.
• Or does it work that way, and if not, why?

Thanks again for the answers!

 

[Steve_S]

If your current FLA 120AH's test out at 100AH and you add in another 100AH FLA that should be okay. If you added another 120AH, you'd lose that extra 20AH - simply put. Adding a battery will not extend the life of the others but it will reduce stress from charge/discharge cycles. Be cautious with wiring your batteries up and extending the bank, one can create problems with battery balancing. Parallel & Series wiring gets trickier with more batteries. To that end, I suggest having a good look at this document from Victron which is an excellent resource on wiring up battery systems.

https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdf

 

[DIYrich]

Hello DIY Solar folks, I'm a n00b looking for some better understanding.

I understand that it's a bad idea to mix old and new batteries, but I'm wondering what the reasons are, besides the basic idea that capacity is reduced over time, so mixing an old battery rated at 120Ah with a new one rated the same Ah is a no-no. But I'm wondering if there's more to it.

Do a web search for the details. Some stuff I read is that the underlying cause is different internal resistance has the new batteries charging faster, and the higher internal resistance of the old batteries will cause them to heat up and possibly boil.

When you add capacity, make sure you are still charging the batteries to 100%. This is really important for lead acid.

On the flip side, slow charging could mitigate the heat effect.

Another general statement is that the new batteries will inherit the age of the old batteries. A variation of the "capacity reduced over time", but easier to conceptualize (combine with 2 year old battery knocks 2 years off the life of the new battery).

 

[Mattb4]

Hello DIY Solar folks, I'm a n00b looking for some better understanding.

I understand that it's a bad idea to mix old and new batteries, but I'm wondering what the reasons are, besides the basic idea that capacity is reduced over time, so mixing an old battery rated at 120Ah with a new one rated the same Ah is a no-no. But I'm wondering if there's more to it.

Here's a scenario (partially hypothetical for simplicity/clarity):

• I have a 12V flooded lead-acid deep cycle battery rated at ~120Ah that is more than one year old.
• I tested the actual Ah, and determined that the battery now has a 100Ah capacity.
• I need to add an additional battery to reach the capacity I need for my inverter/load.
• Is it safe to add a brand new 100Ah battery in parallel with the older battery, since their capacities match, or are there other factors to consider?

Thanks in advance for any advice!

The lead acid battery that has the degraded capacity does not equal a new battery of that capacity. It is suffering from the effect of more than a year of chemical reaction.

Nothing stops you from using a new battery with an old battery other than you are shortening the life of the new one. As a strategy, like when you have a battery that has gone bad in a existing setup, to add a new one it can carry you along for awhile. There are many folks that swap in and out old batteries that they get for little or nothing to eke out some production.

 

[DIYrich]

One possibility I came across was: replace 120 Ah with two 120 Ah. Put the old battery on the shelf plugged into a battery tender. When the new battery have been used the same length of time as the old battery (1 year?), you can add the old battery and have 3. Ideally, buy the same make/model of the current battery.

 

[Q-Dog]

After running LiFePo4 batteries for 2.5 years, I don't know if I would go back to flooded lead even if free. Maybe, if someone gave me a truckload of new ones I would think about it, but probably not. LiFePo4 doesn't need watering, doesn't care if it isn't fully charged regularly, doesn't offgas or heat up when charging.

Nope, not even if free.

 

[Zwy]

Hello DIY Solar folks, I'm a n00b looking for some better understanding.

I understand that it's a bad idea to mix old and new batteries, but I'm wondering what the reasons are, besides the basic idea that capacity is reduced over time, so mixing an old battery rated at 120Ah with a new one rated the same Ah is a no-no. But I'm wondering if there's more to it.

Here's a scenario (partially hypothetical for simplicity/clarity):

• I have a 12V flooded lead-acid deep cycle battery rated at ~120Ah that is more than one year old.
• I tested the actual Ah, and determined that the battery now has a 100Ah capacity.
• I need to add an additional battery to reach the capacity I need for my inverter/load.
• Is it safe to add a brand new 100Ah battery in parallel with the older battery, since their capacities match, or are there other factors to consider?

Thanks in advance for any advice!

I put 2 new Costco deep cycle 210Ah in my golf cart with 4 of the 90% plus recovered Trojan 220Ah it came with. I did this as I wanted to swap from a resistor to a controller and decided to install the controller first ($1300), run the lead acid for a few years, then install some LFP. As long as you understand the old batteries are what the actual capacity is for the bank with batteries in series, then it isn't a problem.

For batteries in parallel, it gets a little tricky because one battery (the oldest and least capacity) will tend to boil from my experience and draw from the better batteries even when not under use. One can get away with charging/discharging them often such as everyday if you were to only pull way less than 50% capacity and slow down charge rate to a crawl but that comes with it's own set of problems regarding never reaching full SOC.

If this is for a solar system, I'd go with LFP and move on, use the lead acid for something else. I've written quite a few posts about the subject and debated with other members about this, but for a solar system, lead acid doesn't make any sense. https://diysolarforum.com/threads/renogy-100ah-agm-renogy-100ah-hybrid-gel.62240/post-790120

 

[robby]

AGM batteries do have a place.
If your batteries are being used only for power outages its fine to have AGM batteries fully charged for long periods, that is not something You want to do with LFP batteries.

 

[Tomthumb62]

Consider that at present, a prebuilt 12V/200AH LFP Battery pack in the US can be had for <$350

Where are you seeing these prices? 100Ah for under $350, yes, but not 200Ah. Cheapest I've seen for 200Ah is about $550-600 for something like a Chins.

 

[Zwy]

AGM batteries do have a place.
If your batteries are being used only for power outages its fine to have AGM batteries fully charged for long periods, that is not something You want to do with LFP batteries.

Charge the LFP to 80% and forget the AGM would be a better option.

100Ah AGM working capacity= 50AH
100Ah LFP charged to 80% working capacity = 60Ah

If this really was just a backup for power outages, it still might be beneficial to charge LFP over 90%. Cycle life won't matter with very low usage. Personally, I purchase batteries to use them.

 

[Q-Dog]

AGM batteries do have a place.
If your batteries are being used only for power outages its fine to have AGM batteries fully charged for long periods, that is not something You want to do with LFP batteries.

Are you saying the OP should add AGM to the FLOODED LEAD ACID ? Or replace the flooded lead acid with AGM?

 

[Steve_S]

Where are you seeing these prices? 100Ah for under $350, yes, but not 200Ah. Cheapest I've seen for 200Ah is about $550-600 for something like a Chins.

SHNOOTZ, sorry TYPO. I meant 12V/100AH LFP for < $350 USD.

 

[robby]

Are you saying the OP should add AGM to the FLOODED LEAD ACID ? Or replace the flooded lead acid with AGM?

No it was a general statement in Reply to post #10

 

[fust]

Thanks everyone for the discussion and all your insight!

For the record, this is for an on-the-cheap, off-the-grid solar ground install setup that feeds my RV parked at a creek camp on our property for weekend getaways and possibly a month or so stay if I get the urge to write the Great American Novel or Cousin Eddie comes to stay ? It's almost like a large-scale portable system that won't move much, panels and possibly the whole system won't actually travel with the RV. I'll have about 1400W of 24v panels into a 60A MPPT controller (might wire them as 48v though).

I can get the two 100Ah FLA batteries for around $200 total. Since it looks from the posts here like there is no risk of some kind of accelerated degradation of new batteries when mixed with old as long as the Ahs (as tested, not rated) match, it seems to make economic sense to just follow the FLA path for a bit until it dead ends. I'll have usable 200Ah total for ~$200, and when those wear out, I'll switch to LifePo4. They won't be rocked too hard all the time, but I want to be able to occasionally run the microwave (700W), toaster oven (1200W) or coffee maker (1000W) when I'm down there, and have the 300W absorption fridge on and cold the whole time (40% duty cycle, with propane backup if the batteries hit the 50% cutoff). 200Ah usable/400Ah rated, over 4 batteries into a 2000W(sustained)/4000W(peak) inverter, should get me there and be less taxing on the system, yes?

Does that all seem reasonable, or am I way off over here? This is the first time I've delved into solar on this scale, so please bear with me if I'm calculating things wrong...

Thanks again!

 

[Zwy]

Think in watts and how long you will run something.

200Ah x 12 = 2400Wh

Take the wattage times the length of time you will run the item, for example 15 min equals 1/4 hour. That will tell you how long you can run something plus the lost efficiency of the inverter.

An example, 300W x 8 hours = 2400Wh

You will want to run as many loads during sunlight as possible when PV power is available. I suggest a shunt for battery SOC monitoring and determining how many watts an item is drawing at any time if you do not have a shunt already.

 

[fust]

Thanks! I'm more worried about the amp draw of a particularly high item like the toaster oven making toast out of the batteries instead!

I bench tested the setup with the two batteries I have in parallel, and the voltage on the batteries drops to 11.3v while the toaster load is applied while plugged directly into the inverter. There will be more wire runs in the final setup.

Isn't that bad for the battery, and will it be easier on the battery bank if I add two additional batteries in parallel (as described above, by matching the tested Ah capacity)?

 

[Tomthumb62]

Thanks! I'm more worried about the amp draw of a particularly high item like the toaster oven making toast out of the batteries instead! I tested the setup and can run it now with the two batteries in parallel I have, but it drops the voltage on the batteries to 11.3v while the load is applied. Isn't that bad, and will it be improved if I add two additional batteries in parallel?

Lead acid will suffer voltage drop worse and more easily than lithium. Is 11.3v bad under a heavy load? It’s not great, but it’s what you have to work with. It can lead to a quicker death to your batteries and devices, depending. But the alternative is to not run heavy loads or, ya know, just stop using your batteries. Jk

I think you said your batteries are flooded lead acid? If so, do you have them on a maintenance schedule?

 

[fust]

They are not in service yet in my system (but they are used - and not abused). I'm still cobbling together my hillbilly solar array, but have been keeping them charged regularly. What would the schedule normally entail besides checking the water level? I have an automotive electrolyte tester. Should I use that as well?

 

[fust]

Actually, I also have a refractometer (calibrated in brix) and a glass floating hydrometer. Is one of those better, or is the bulb-squeeze automotive kind sufficient?

 

[Tomthumb62]

Actually, I also have a refractometer (calibrated in brix) and a glass floating hydrometer. Is one of those better, or is the bulb-squeeze automotive kind sufficient?

Sorry I don’t know much about maintaining flooded batteries. I know there are some experts on this forum, perhaps one of them will chime in.

 

[Zwy]

Actually, I also have a refractometer (calibrated in brix) and a glass floating hydrometer. Is one of those better, or is the bulb-squeeze automotive kind sufficient?

Refractometer is best.

As you are not well versed in flooded lead acid batteries, I suggest starting with videos Trojan put out. There are quite a few but this video is very important, look at their channel for more.

I can not stress enough you should consider 2 banks where one bank is being used and the other is charged thru absorption, then once absorption is done on that bank and the bank reaches 100% SOC, then the banks are reversed and the other bank is now charged. At least once a month, equalization should be performed to reverse sulfation and stratification. To perform EQ, you will need a capable charger and the refractometer.

Flooded lead acid can last a very long time or only a few short years. This is determined by how deep the batteries are discharged, full charge thru absorption and routine equalization.

There is more information on the https://diysolarforum.com/forums/lead-acid-batteries.20/ forum. I suggest you become a sponge and start reading absorbing the knowledge presented. Some very good information has been posted over the years.

 

[fust]

OK great, thanks!

Another related question:

When I add the additional batteries, I'd like to directly connect them with busbars. When determining the cross sectional area needed for the busbars, am I calculating using the highest possible load the bank might see (in my case would be the 4KW peak my inverter is rated for) divided by the number of batteries? So here 4000W between four batteries would mean the busbar calc would be done with 1000W. Or do I plan it so every interconnect between every battery could handle the full load on their own, and therefore use 4KW as the operative for all connections?

I'm using this:

Bus Bar Size Calculator

Current carrying capacity and budget as under size busbar can cause heating and damage in busbar while over size busbar can affect the cost of project. By using BUSBAR Size Calculator we can prevent these issues by predicting them in the first place.

www.allumiax.com

 

[Tomthumb62]

OK great, thanks!

Another related question:

When I add the additional batteries, I'd like to directly connect them with busbars. When determining the cross sectional area needed for the busbars, am I calculating using the highest possible load the bank might see (in my case would be the 4KW peak my inverter is rated for) divided by the number of batteries? So here 4000W between four batteries would mean the busbar calc would be done with 1000W. Or do I plan it so every interconnect between every battery could handle the full load on their own, and therefore use 4KW as the operative for all connections?

Fuse and wire to rated power, not surge power. So if your inverter is rated for 2000W, 4000W surge, your fuses and wires should be rated for continuous power at 2000W. This includes your bus bars. And it’s never a bad idea to go a gauge or two bigger, if it’s not financially prohibitive and can physically fit in the space. Bigger gauge everything means less resistance, heat and voltage drop.