Dear Dave,
Did you answer a question recently about mixing batteries of different sizes, chemistry and age? I believe it was you but I can’t find it in the RVtravel.com archives. —Ed, 2005 Holiday Rambler Ambassador
[NOTE: This is actually a two-part question. Ed sent the first question listed above, and after I sent the rough draft to him, he sent a follow-up on how he added a third lithium battery two years later. That question and response are added to the bottom.]
Dear Ed,
I did not write an article specifically on mixing battery sizes and chemistry; however, we have touched on adding a new battery to an older array or “bank” in some of the posts.
Although I know the basic answer is not to mix different sizes and ages, I thought it would be good to research what battery manufacturers had to say about the chemistry and such from a technical point of view. Several battery manufacturers have posts regarding installation of different batteries and they all recommend not to do it.
Good information from the Battle Born Batteries website
When you’re building a battery bank for your mobile or off-grid application, it can be tempting to mix and match different battery sizes. Simply put, the answer is NO.
For the longer explanation on why this is a very bad idea, keep reading.
Mixing Battery Sizes: What Happens?
You cannot connect batteries of different amp-hours in series with good results.
We strongly recommend you DO NOT attempt to mix battery sizes (amp-hours) and connect together. Due to differences in battery management systems and battery cell counts, there may be a charging and voltage discrepancy between batteries.
This will cause the highest voltage battery to charge the lowest voltage battery attempting to create balance in the circuit. The greater the difference in voltage, the higher the probability of battery failure.
Can I Connect 100 Amp-Hour Battle Born Batteries to the 270 Amp-Hour Battle Born Batteries?
Here at Battle Born Batteries, we offer two different lithium battery sizes: our 100 Amp-hour BB100 series and GC2 series, and the new 270 Amp-hour batteries.
These two different sizes of batteries should never be connected. Any attempt to connect Battle Born Batteries of different amp-hours will result in a voided warranty.
You should only ever connect batteries of the same size.
This was posted on the Dakota Lithium website
Why Can’t You Mix Battery Sizes and Chemistries
Mixing battery sizes and chemistries is generally discouraged because it can lead to various safety and performance issues, such as battery leakage and sub-optimal device performance. For best results, it is suggested that you replace all batteries with the same brand, chemistry, voltage, and size when the device performance becomes unsatisfactory.
It is best to be consistent because there will be differences in how each brand is made and performs. For example, some batteries are made for specific applications (such as Dakota marine lithium batteries), and this should be borne in mind when purchasing your battery. Only consider incorporating batteries of different brands for your electronics if the brand you are using isn’t available in the necessary size, voltage, etc.
Mixing Battery Types and Sizes
Here are some of the key reasons why you should avoid mixing battery sizes and chemistries:
Voltage Differences: Batteries of different sizes and chemistries may have different voltage ratings. Mixing batteries with different voltages can lead to uneven power distribution, damaging electronic devices, or, in extreme cases, posing a fire hazard.
Capacity Mismatch: Batteries come in different capacities (measured in ampere-hours or milliampere-hours), and using batteries with significantly various capacities together can result in an imbalanced discharge and overcharging, which can shorten the lifespan of the batteries and potentially lead to damage or overheating.
Discharge Rates: Batteries with different chemistries often have different discharge rates. (Battery discharge rate, also known as discharge current or discharge rate, refers to the rate at which a battery releases its stored electrical energy as usable power. It is typically measured in amperes (A). It is an important parameter to understand when working with batteries in various applications, such as portable electronic devices, electric vehicles, and backup power systems.) Combining batteries with different discharge rates can result in inefficient energy use and, in some cases, overheating or other safety risks.
Chemical Incompatibility: Mixing batteries with different chemistries, such as lithium-ion and alkaline, can result in chemical reactions that produce heat, gases, or other dangerous byproducts. In extreme cases, this can lead to leakage, rupture, or even explosions.
Bottom line
So the bottom line is that it is best to match batteries in size, chemistry, and even age. Installing a new battery parallel to an older one is not recommended due to the change in charge/discharge characteristics of the old battery. Flooded Lead Acid (FLA) and Absorbed Glass Mat (AGM) batteries also have different charge/discharge characteristics, which means one battery will most likely either be overcharged or undercharged.
Follow-up from Ed
I looked back and found I installed 2 SOK LiFePO4 batteries in parallel in the spring of 2021. I added one more of same ah rating this spring. I’m trying to decide what to do. I’m wondering if there is a way to test them for how close they are in voltage and charging characteristics and with that info, am I OK to keep all three. If I can’t keep them together, my worst fear is the new one has ruined all three. —Ed
My response to Ed
I contacted my technical guy at Expion 360 with your question and here is what I got back:
Thank you for reaching out with this insightful question. Our support team has provided an initial response, but I wanted to personally address it and provide a bit more context.
When it comes to lithium batteries, we generally advise against mixing different chemistries, capacities (Ah), or physical sizes within the same bank. Even when batteries are the same model, differences in age and usage history can affect their performance and internal resistance over time, potentially impacting the efficiency and lifespan of the entire battery bank.
If batteries of the same type are added but are of different ages, slight variations in cell performance can impact the charging profile. The battery management system (BMS) may have difficulty balancing cells accurately, leading to issues such as uneven charging/discharging, which can result in additional strain on the older batteries. This may cause them to discharge faster or heat up more, potentially reducing overall bank performance.
For optimal performance, we recommend ensuring that all batteries in a bank are similar in state of charge (SOC) before combining them. Ideally, it’s best to purchase a matched set of batteries together or to replace an entire bank when upgrading. This helps the BMS maintain uniform control and ensures that each battery operates as efficiently as possible.
I hope this provides the clarity you’re looking for, and we appreciate your commitment to delivering accurate information to your readers. Feel free to reach out if there are any more details I can provide. —Carson at Expion 360
You might also enjoy this from Dave
Will RV’s house batteries “play nice together” if I replace chassis battery with lithium?
Dear Dave,
It’s just about time for a new battery in the truck portion of my motorhome. I’ve been thinking of upgrading to lithium. I added a second house battery wired in parallel about two years ago when I replaced the factory battery. They are both in great shape. Question is: Is it OK to have a lithium truck battery and regular house batteries? Will they play nice together? —Judith, 2018 Coachmen Freelander Micro
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Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”
RVDT2815
Connecting different sized batteries of the same voltage in series will result in the maximum performance of the bank being equal to the lowest performing battery so yes, that is not a wise idea but that is simply not the case when connected in parallel.
Even in parallel, of course battery manufacturers are going to say “for best results, you should only replace in sets and never mix brands”. It sells more batteries.
It’s as technically accurate but absurdly disingenuous as saying “milk’s freshest right at the udder”.
They leave off the reality that everyone running lithium batteries probably has a lead acid under the hood…..Just sayin’
If it is not ok to mix different batteries then why do manufacturers do it when they build a new RV. House batteries and chassis batteries are different amp ratings completely different batteries yet the charging system will charge both kinds of batteries from one source with no problem. Why would putting two different house batteries really matter if it doesn’t matter for chassis and house? How do you actually ask Dave a question, I can not find a link? I am a retired ASE master mechanic and know how the battery charging system works.
The house batteries and coach batteries are not actually connected together. While the converter and alternator ‘may’ charge the all the batteries, they are isolated from each other.
If you press the boost switch, it will connect the house batteries to the chassis battery for emergency starting.
As a retired mechanic and owner of several Motorhomes I can say that there is a solenoid located between the main battery and the house battery that anytime the motorhome is running that sono just engages a circuit making a straight wire between the two groups of batteries. It charges all batteries equally at the same time. It is different when the unit is plugged in as only the house batteries charge off of the converter in the motorhome. The chassis battery does not and the button that you push that connects both batteries if you need it for starting all it does is activate that solenoid which is activated all the time anyway when that chassis engine is running.
And that solenoid only control is what should be updated in order to properly charge lifepo4 batteries. Read up on lithium battery vs lead acid charging profiles to comprehend the differences.
The house and chassis batteries may be charged from the same source, inverter/charger or alternator, but they have separate charging devices that adjust charging profiles for the different battery banks.
As a retired master mechanic all I have to say is you are wrong. I could show you wiring diagrams to show you how the charging system on a motorhome works, but we would have to be talking in person. Can’t do it in a simple way in a short comment if you will not listen to a expert you would not believe anyway.
If you have lead acid chassis batteries and lithium house batteries, even though they may be charged from the same source there should be some component leading to each bank that provides a different charging profile. I don’t remember the specifics. Please avoid the snide comments.
Dave,
I sent the original question. Ultimately, I contacted SOK and their answer was “no problem” with different ages. I understand the points given in your response today. So far there seems to be no problem almost 4 years later but I have no way to accurately test for the issues brought up.
Ed