Ten Lithium Battery Myths and Answers: How can I get the longest life from lithium batteries? After you spend lots of money on lithium batteries you want them to last. It would be a shame if you killed them early.
I have been an early fan but not an early adopter of lithium iron phosphate batteries for RVs and have multiple friends credit me as at least part of the reason they spent lots of money on a lithium battery set up for their RV. Most of these friends purchased their lithium batteries before I did. I just found out another friend purchased lithium batteries for his fifth-wheel this week.
We do not get compensation from any company to promote their batteries. These truths are provided only to help our friends use their batteries in the most effective way.
These battery secrets that are not well known
So, this post is to make sure my friends are informed about how to treat their lithium batteries and the best way to achieve the longest life from their investment. So, this post is to address Ten Lithium Battery Myths and Answers
I started writing about my next battery set up a full year before getting going with lithium. Part of the reason was that I was determined to have real-life experience not just relying on other people’s experiences to make my decisions. So even after knowing plenty of reasons to get lithium, I hesitated and made do with my lead-acid batteries, if for no other reason is that I could make them work – well enough. Other people who know me well, say it is all based on me being really, really cheap.
So even before spending what still seems to be a king’s ransom on a new battery set up, I already knew from studying the subject, that lithium likes to be treated differently than lead-acid.
Here are Ten Lithium Battery Myths and Answers that I needed to answer to re-align in my brain.
- To know the state of charge, the only thing I need to know is the voltage.
- Always fully recharge batteries.
- Charge batteries slowly over a long period of time.
- Never fully discharge batteries.
- Never discharge batteries quickly.
- Recharge the batteries with a high voltage/amperage and taper off as the battery gets closer to full.
- Keep charging full batteries with a small trickle or float charge to maintain their health.
- Once the batteries are fully recharged I should remove them from the charger.
- Always fully charge a battery before you put it in storage.
- Avoid heat, avoid cold.
I have already covered many Ten Lithium Battery Myths and Answers in my previous posts in the following articles, so some of this is a re-hash of old details. Here are some of the posts regarding the subject starting from oldest to newest.
Starting with Lead Acid Batteries
Then introducing lithium batteries
- Lithium/Lead Acid Final Analysis
- Which First, Solar or Lithium
- Bash Solar Data Collection
- Bash Solar Test Data Analysis / Observations
- Hybrid Mongrel Battery
- Massive Electrical Bucket
Important note: I currently charge my lithium batteries using only the inverter/converter. In the future, I will be able to charge my batteries using either the inverter/converter or solar or even both at the same time.
Ten Lithium Battery Myths and Answers
Myth #1: To know the state of charge, the only thing I need to know is the voltage.
Lead = No / Lithium = No
It is a complete falsehood, for RV owners that they can read the battery voltage, and determine the state of charge for a battery. Battery voltage is related to the state of charge but doesn’t tell you nearly anything. First, if you are charging the battery then you are reading the charging voltage, not the battery voltage. Secondly, if you turn off the charger and then immediately read the voltage, then you are reading a voltage called a surface charge, not the true battery voltage. The only way to read the true voltage is to wait after recharging and after a small discharge then read the true voltage.
For lithium batteries, the problem is even worse. Here are the lithium state of charge numbers:
- 13 volts = 30%
- 13.1 volts = 40%
- 13.2 volts = 70%
- 13.3 volts = 90%
- 13.4 volts = 99%
These voltages versus state-of-charge were obtained from a post by a Battle Born Batteries employee on their website and are not “official” documents.
Remember, you can only use the above numbers if you are not charging or discharging.
Volts mean nothing (almost nothing)
As you can see the difference between almost fully discharged (30%) and almost fully charged (90%) is 3/10 of one volt. The range between 40 and 70% is completely unknowable. So you need a better method. The only way to truly know the state of discharge from a lithium battery is to measure discharge in terms of amperage used.
Here is the Victron 712 that I use to determine the state of charge.
The Victron BMV-712 records current as it enters and exits the battery and keeps track of battery state of charge based on amp-hours used not voltage.
The Victron BMV-712 showed me my discharge and recharge events over the last week (and all last year) as represented on this graph. Here is the same graph, based on volts. All readings were taken while charging or discharging. All numbers greater than 13.3 volts were taken while I was charging and are artificially high. Voltages taken while discharging are artificially low. All voltages taken during large discharge rates were lower than voltages taken with minor discharge rates.
The purpose of the voltage graph is to show that you cannot determine the lithium battery state of charge by looking at the voltage.
When looking at the screenshot (the Victron BMV-712 links this data to my phone) in the next picture, you can completely disregard the voltage, because the screenshot was taken while I was charging, the voltage does not relate to the state of charge.
Published voltages versus state-of-charge numbers may be wrong
The above Battle Born voltage versus state-of-charge numbers may be wrong. I have allowed my battery bank to drift slowly lower from a full charge at last created by charging at 14.4 volts. The monitor read 100% until last night. Here is the voltage versus state-of-charge taken today from my Victron BVM-712.
If the numbers from the Battle Born website were correct, then I should be at 90% state of charge at 13.3 volts. This would mean that I had consumed nearly 100 amp hours from my battery bank — not just 15.6 amp-hours. Testing of this style, drawing only 8 watts could take forever so don’t expect too many more of this real-life voltage versus state-of-charge readings. I think that the 8-watt draw comes from running the LED status lights on my inverter. I took another reading at 99% state of charge this time after consuming 30 amps volts in this reading was 13.29 (the draw was still 8-watts)
Myth #2: Always fully recharge batteries.
Lead = Yes / Lithium = No
Lithium batteries do not need to be fully recharged after every discharge. Holding lithium at full charge reduces the life span. Holding lithium at a full charge when the battery is hot reduces the life of the battery dramatically. The best practice is to fully charge when you need it and then allow for gradual discharge down to a lower charge as you use it. If you cycle the battery between 25 and 75% (13 to 13.2 volts) state of charge the lithium battery may outlive you. I don’t ever expect to replace my lithium batteries.
In order to cycle between 25 and 75% effectively, you need a big enough battery so that it doesn’t have to be full to provide enough energy for you to use it in the middle of the charge range.
The way I plan to use my lithium batteries is to increase the charge voltage to 14.4 volts for a couple of days before leaving a campground with shore power and going to a campsite without shore power. This will bring the battery up to a full charge. Thus when I depart a campground with full hookups I will start with a full battery and then as I camp allow the lithium state of charge to float up and down associated with the available energy. For right now, this means letting the state of charge drop for a couple of days before I recharge them using the generator.
Control for full charge by limiting voltage
After I install the solar panels I will allow the available sunlight to recharge the batteries as it is available up to full, as it is available, thus never having to run the generator. When off-grid my solar will be programmed to deliver 14.4 volts.
Then after going to a campground with shore power I will drop the charge voltage to about 13.3 volts which should produce a charge level of less than 90% of a full charge.
Reduction of about 10%, in terms of capacity, results in nearly double the lifespan of a lithium battery.
Myth #3: Charge batteries slowly over a long period of time.
Lead = Yes / Lithium = gradual is better
At this point, I need to introduce what is called a C-rating. The C-rating applies to both charging and discharging. One “C” means from zero to full in one hour. For discharge one “C” means from full to zero in one hour. Thus if you have a one hundred amp hour battery bank, then a one “C” charge would be 100 amps per hour.
A slow charge rate would be less than 0.5C per hour. Lithium life expectancy increases with charge rates of less than 0.8C. The slower the charge or discharge rate, the longer the life of a lithium battery. The way to control the rate of charge is to limit the amperage of the charge source.
In terms of C-rating, my charge C-rating was 0.135 and my discharge C-rating was 0.0148 last week. In terms of charge and discharge rates, my batteries should live longer than I do.
Control the speed of recharge by limiting the amperage (if needed)
For me, one 0.8C means that I could recharge my 900+ amp hours of lithium at up to 720 amps per hour and still extend the life of the batteries. My maximum potential charge rate, after I put solar on my RV is 225 amps per hour (0.25C). Given this, I will be well below the long-life charge rate.
The way to control the level of charge is to limit the charge voltage. For me, this means that when I am using my lithium batteries when off-grid I will be charging at 14.4 volts to get a full charge. When I have shore power I will be charging at 13.3 volts or perhaps even lower to limit the charge to about 70%.
A higher charge rate will result in a shorter life span for the battery but may be necessary given some circumstances. The theory says that I could recharge my batteries at 900 amps per hour… I don’t know how I could ever create such a charge rate so this isn’t even an option. The reason high charge rates are bad for the battery is that the cells do not have time to dissipate extra heat during the recharge.
Myth #4: Never discharge batteries quickly.
Lead = Yes / Lithium = No (to a point)
For lithium battery discharge this means to discharge at a rate of equal to its full capacity — over the period of one hour. Thus a one hundred amp hour battery would be fully discharged in one hour. Lithium batteries are willing to discharge fast, but fast discharges also create internal heat in the battery and prolonged high discharge rates should be avoided. The theory says that I could discharge my 900+ amp hours all in one hour. However, I could never achieve such a discharge rate. My fuse is rated at 300 amps per hour thus this is the maximum I could ever achieve. Plus I don’t have anything that could draw anywhere near 300 amps per hour through the fuse.
For lead-acid batteries, this really isn’t a question because of what is called the Peukerts’ law which I described in my post of Lithium Lead-Acid Final Analysis. Simply stated Peukert’s law states that “If the rate of discharge increases, then the available capacity of the battery decreases.” Peukert’s law kills the ability of lead-acid batteries ability to quickly discharge.
So since quick discharge only applies to lithium what is the best way to discharge a lithium battery and the answer is slowly as possible, assuming it meets your needs. If your system design requires a large sustained load, you need more batteries. For me, this isn’t a problem because the battery’s total capacity is large and my total use is limited to the output of my inverter. My inverter is small compared to my battery size.
Myth #5: Never fully discharge batteries.
Lead = Yes / Lithium = No (to a point)
The safe discharge level for a lead-acid battery is about 50%. The safe discharge level of a lithium battery is equal to the design of the BMS. (Battery Management System) The BMS is built into a lithium battery which limits such things as discharge level, and many other things. The result is that the BMS will stop discharge before damage to the cells occurs and thus you can use all the energy in the battery until the BMS puts a stop to the discharge.
However, the depth of discharge does affect the life span. What good is it to buy a battery and then never use it? So here is a basic table to understand discharge related to life in a lithium battery.
|Depth of Discharge||Cycle Life|
Cycling the battery between 10 and 90 percent charge is a formula for long life. Cycling the battery between 25 and 75 percent charge is a formula for an extremely long life.
If you made your own lithium battery without a BMS you will need to limit your discharge to a safe level otherwise you can kill your lithium battery by over-discharge.
Myth #6: Recharge the batteries with a high voltage/amperage and taper off as the battery gets closer to full.
Lead = Yes / Lithium = No
This is a 100% true statement for lead-acid batteries and not a factor for lithium batteries.
My chargers are programmed to maintain a constant voltage (at 14.4 volts) until the end of the charge cycle. My charger also provides constant voltage and current for the entire charge cycle. The only time I adjust this is when I switch to shore power for a couple of weeks, then I will drop the charge voltage to 13.3 volts or perhaps even lower, to prevent holding the battery at a 100% charge for long periods. See also the myth about heat and cold below.
Myth #7: Keep charging full batteries with a small trickle or float charge to maintain their health.
Lead = Yes / Lithium = No
Lead-acid batteries leak and lose charge over time. Always maintain the charge level of a lead-acid battery at 100% which is optimum for a long life of the battery.
For lithium batteries staying at 100% charge can be bad, again refer to the discussion on heat and cold below. Lithium batteries also don’t leak and are happiest when allowed to float between 25 and 75% state of charge.
Myth #8: Once the batteries are fully recharged remove them from the charger.
Lead = No / Lithium = Maybe
For lead-acid batteries, you want to leave them connected to get the trickle charge.
For lithium batteries, it depends on how smart your charger is. A new word I have learned in the last month is “rebulk” which isn’t a real word but rather describes when the charger kicks in to recharge the battery. My rebulk number is going to be 13 volts or perhaps even lower when we are on shore-power and 13.3 volts when off-grid.
It is starting to sound like I’m going to be adjusting my charging numbers frequently but I put notes on my calendar and will only have to adjust them three times this summer. I don’t think it will be too big a burden.
Myth #9: Always fully charge a battery before you put it in storage.
Lead = Yes / Lithium = No
For lead-acid batteries store the battery at full and always trickle charge the battery when in storage at least every couple of weeks. Lead-acid batteries will lose charge unless topped off when in storage.
For lithium do not store a full battery, instead, discharge it to about 50 – 70% and then store it. Lithium batteries do not lose charge when in storage. When a lithium battery is full for a long period of time it degrades the performance of the battery. Millions of people with laptop computers that are plugged in all the time (including me) know this to be true.
#10 (not a myth): Avoid heat, avoid cold.
Lead = Yes / Lithium = Yes
Heat kills batteries, all batteries. If you want to kill your car battery, let it sit outside in Phoenix, in the direct sunlight, get real hot. Then after it is nice and hot, put a high amperage recharger on it. Car batteries in Phoenix have a low life span.
Same for lithium. If you want to charge a battery – cold is better than hot, way, way better than hot.
Lithium batteries hate being held at a high charge level especially when they are really hot. Try to keep your batteries cool. For RVers, to me this means, to try to find moderate temperatures using your wheels and go someplace other than where it is not so hot.
|Temperature||40% Charge||Full Charge|
|32’ F/ 0’C||1% drop in life expectancy||3% drop in life expectancy|
|77’ F/25’C||2% drop in life expectancy||10% drop in life expectancy|
|104’F/ 40’C||8% drop in life expectancy||18% drop in life expectancy|
If you need to store your lithium batteries and expect them to be uncomfortably hot (uncomfortable for you) then store them at a low state of charge (not empty). If you don’t have a choice with regard to temperature, then keep your lithium batteries at a low state of charge when it is very hot.
Pertaining to cold, all batteries don’t like real cold temperatures (neither do I). Lithium batteries have a BMS and are unwilling to charge when the temperature is too cold. Warm them slightly and then they will accept a charge. Charging gradually creates heat inside the battery and as it gets warmer then the BMS will allow higher charge rates.
For lithium batteries, if it is too hot or too cold for you, then it is too hot or cold for your battery.
Ten Lithium Battery Myths and Answers — summary:
- Voltage readings cannot identify the state of charge.
- Do not always fully recharged lithium batteries.
- Slow recharging is better than fast recharging. I am unable to recharge my batteries so fast as to damage them.
- Batteries BMS protects from over-discharge, yet deep discharge causes stress that is good to avoid.
- I am unable to discharge my batteries quickly enough to damage them.
- There is no reason to change the voltage or amperage as I recharge my lithium batteries.
- It is better to allow my batteries to discharge than it is to keep them fully charged.
- Batteries automatically are removed from my charger after the charge is complete.
- Do not store a full lithium battery. Do not ever store an empty lead or lithium battery. Store a lithium battery at about 50 – 70% charge.
- High temperatures (more than 122′ F /50’C) kills all batteries. Do not charge a lithium battery to full in high heat. Do not ever hold a lithium battery at full during high heat. Avoid charging when cold, but the BMS will not allow recharging when a lithium battery is too cold.
Summary of best management for lithium batteries
- Keep your batteries at moderate temperatures.
- Do not store fully charged batteries. Store batteries in a cool place.
- Avoid deep cycling.
- Avoid rapid discharge and recharge.
Quick Numbers Summary
- Charge at 14.4 volts for full capacity.
- If full capacity is not needed charge at 13.3 volts or maybe even less to increase the life span.
- Start recharge at 13.3 to keep the battery near full. Start recharge at 13 volts to allow for deeper discharge, especially during storage.
I will update any number in the blog, on the fly, if I learn that something needs to be changed to give you the best information. All of the above is according to the best information available.
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