Since this thread has gone more into AGM to LiFePO4 I thought I might put down some data as I know it.
The benefits of Lithium batteries are substantial, and you need to take all of them into account when planning to switch.
Lifespan
Lithium batteries usually specify a lifespan of 2500 to 5000 cycles (at 75% to 80% of capacity). This figure is generally derived from testing done in a lab environment, as there just isn’t enough data out there to support real life data. Even the best AGM battery will give you somewhere between 500 – 1000 cycles. For comparison sake this would also be considered somewhat in a laboratory environment. We do have more data about led acid batteries and the effects of temperature on life. As a rule of thumb AGM battery life decreases by 50% for every 10°C rise. So, a battery consistently used at 35°C would be 250 – 500 cycles
Usable capacity
Lithium batteries suffer no capacity degradation down to 80% discharge. The number above for AGM batteries is based off 50% discharge. While you can take AGM down to 80% you reduce the expected number of lifecycles if you do.
Output voltage and current.
Lithium batteries have almost a flat discharge curve compared to lead acid, so the voltage output at 20% discharge is the same as 80% discharge. While lead acid the voltage output drops greatly between fully charged and 50%. Remember power (watts) = amps X voltage, so as the voltage drops the current needed to power a device goes up. Also, there is minimal Peukert’s losses for high current loads, so lithium’s are better for high draw inverter uses.
Charging
Lithium does have its own charging profiles and can be charged faster than most standard AGM systems. This means solar chargers can maximize there time in good sun.
Temperature
While much is said about lithium’s low temperature charging issues, their output characteristics far exceed that of a lead acid battery at lower temperatures. AGM batteries produce less than 50% of their room temperature (77°F) capacity at 32°F. And as capacity gets better at higher temp, life span decreases. As a rule of thumb AGM battery life decreases by 50% for every 10°C rise. So, a battery consistently used at 35°C would be 250 – 500 cycles. It should be noted that we do not really have a lot of information on the effects of temperature and charging on lithium batteries in situations like RV’s There has been some data that suggest higher temperature charging does degrade capacity and the charge voltage also has an impact on capacity.
Mounting
Generally, lithium batteries store 3.6 more amp-hours than a AGM battery bank of the same weight. They also don’t out gas so there is no need for venting, this allows us to find a location inside the rig, most likely reducing cable lengths and other things.
Battery Management Systems
Lithium batteries for our rigs generally come in two designs, the first would be a lithium battery bank that you build for your size requirements, and then you would need an external battery management system or BMS. The BMS is the very important part of the system that controls the charging and discharging of the battery.The second system has the BMS integrated into the battery, if you put multiple battery banks into your system, they each have their own BMS system. One of the advantages of the external system is that the parts can be replaced or repaired as needed. An internal BMS that goes bad generally means the battery needs to be replace. There are some batteries with replaceable BMS integrated module.
In either case many of the integrated BMS modules have become known as “drop-in modules”, in fact many are put intro standard lead acid battery size containers. Unfortunately, the term “drop-in” or “12 volt compatible” have been used to describe the ease of changing over to a lithium battery. It has also been exasperated by many dealers trying to sell these drop-in modules. To be clear, while you may be able to get away with just dropping them in your existing setup, you could be sacrificing one of the bigger selling points in extended cycles compared to lead acid.
Chargers
We generally can have three possible charging sources in our existing lead-acid rig; shore power, solar, and via the vehicle alternator. Shore power chargers tend to follow the standard Bulk, Absorption, and Float stages in charging, some may say the have a 4th stage called equalization. Solar charges also follow the same three stage charging process. The alternator is kind of in its own world, it generally does not have the defined stages.
Lithium batteries prefer two stages constant current and constant voltage. If you wonder how that could be drop in compatible you need to look deeper. In general the Bulk phase of a three stage charger is a constant current phase, the charger puts out a set charge rate until the charger hits it absorption voltage. After that the charger goes into absorption phase and puts out a constant voltage while slowly reducing the current. This of course is very similar to the constant voltage phase the lithium requires. We really run into a problem when we reach float, lithium batteries don’t need or want float
So how do you make your standard 3 stage charger work with lithium, a lot depends on your existing charger capabilities. Many chargers actual support a lithium battery or CC/CV charge out of the box, some may just call it a two-stage charge bulk/absorption. The first is relatively easy, we need to set the absorption set point. You need to check the charger, but it will generally give you a voltage that it puts out for different battery types. Even better, is if it allows programming you can put in the recommended voltage from your lithium battery manufacturer. Generally, lithium like somewhere between 14.2 and 14.6. For shore power you need to look at the charge current available, we like to set the maximum charge current at 0.5C or half the Ah capacity. That would be 50 amps for a 100Ah battery. This will also apply to your solar controller, although it may be limited by its output.
For Float we find that a voltage 13.6 and below will not affect the battery. Our goal is to hopefully find a battery type that matches our absorption voltage, and the float voltages we require. Again, some units are fully programmable making this easier.
If your battery has an equalization stage, then you must disable it, if you can’t disable then you must be able to set the voltage to your absorption level.
The next one is absorption time, you thought we already had that done by setting the voltage. Actually now we need to set how we end absorption. This unfortunately can be done in a multitude of ways and/or combination of events. Typically, the charger determines ending the absorption charge by when the current reaches a certain percentage of the battery capacity. For Lithium that would be around 5% to 10%. Occasionally some chargers will determine done by a timer, in most cases 2 hours would be fine for lithium. Other chargers may use combination of both time and current.
Since lithium doesn’t really like to be charged to 100% SOC a lot could be said for just ending your charge when it reaches the absorption voltage, this usually gets you to about 90% .In any event contact your battery manufacturer, they should be able to give you the best advice for your charger.
This gets to the alternator, while it is possible to just drop it in and expect the alternator to adequately charge the battery it can be fraught with errors. Lithium generally has huge current appetite for charging, as you increase the battery bank it get worse. It can very easy to max out the alternator. While alternators have a max current, they were not actually designed to run at max current all the time, this will result in early life failure of your alternator. You also need to monitor the voltage output of your alternator, they have generally a large working range. To high of a voltage could damage the battery or the integrated BMS, to low of a voltage would prevent charging. There are various devices intended to help with some of these issues. One simple device that helps prevent some of these problems would be the
LI-BIM 225. This could easily replace your existing separator or ACR.
- Under normal charging conditions, the BIM will connect for 15 minutes every 35 minutes. That means that the BIM will connect for 15 minutes, disconnect for 20 minutes, and repeat this cycle until the coach battery is charged.
- If the coach battery resting voltage exceeds 13.4V than the BIM will disconnect. A resting voltage greater than 13.4V indicates a fully charged battery. Note that “resting voltage” means that no current is flowing to the coach battery. The BIM will disconnect if the alternator voltage exceeds 14.4V. This protects the coach battery from over charging.
- The BIM will disconnect if the voltage difference between the alternator and the coach battery is less than 0.1V. If the voltage difference is too low, then there is a negligible charging current, and no need to connect to the coach battery.
- The BIM will disconnect if the alternator voltage drops below 13.3V. If the alternator voltage is too low, then it cannot adequately charge the coach battery, so there is no reason to connect.
- Made in the USA.
The other product that has been made to be between the alternator and lithium battery is a
DC to DC charger. DC to DC charger take power from your starting system and convert it to a 3-stage charger or for lithium a CC/CV charger.
Sterling power makes both a 30 amp
and 60-amp versions. There are some such as KISAE
DMT-1230 or
DMT-1250 that add the solar controller into the same package. DC to DC chargers are becoming more and more popular so you can also find other manufacturers jumping in here. The best thing to do here is work with your battery manufacture and their support team to see if one of these chargers work with their product and what they believe the best settings are.
While we hear of the cold temperature charging issues, it is important to make sure if you are using an existing three stage charge that temperature compensation normally available for lead acid is disabled.
So before just dropping hard cash for some drop-in AGM to LiFePO4 batteries make sure you understand the existing equipment you have, and what will work with the battery you are purchasing. The BMS will take care for a lot of the evils, but care should be made to make sure you are not just covering bad charging procedures.
To get the most out of your LiFePO4 batteries - it is best to keep the battery temperature under 45°C (under 30°C if possible). This will be one of the hardest things to accomplish, it really means that we should move the batteries inside the rig, rear they can be in a controlled temp most of the time, especially when charging. If you can charge you rig at night in cooler temperatures do so. This also kind of rules out the standard under the van positions that sportsmobile does. The temperatures reflected from the road and exhaust can max temperatures out relatively quickly. Remember lead acid batteries don’t fare well in heat either
- Keep charge and discharge currents under 0.5C (0.2C preferred), this should be relatively easy to meet for most systems.
The two batteries in the beginning of this thread,
Lifeblue and
Battleborn, both would be considered drop in replacement batteries, they both have the BMS integrated in the battery. Battleborn appears to offer 50Ah and 100Ah version similar to group 31. The Lifeblue can come in 100 Ah and 125Ah group 31 size, they also have 150 Ah and 200Ah in a 4D size, they round out their line with 300Ah version that is close to a 8D size. Lifeblue also have low temperature version in 100, 150, 200, 300 Ah sizes.
One thing to look at when building battery banks with integrated BMS, is that they will have limits on the charge current and limits on the discharge current. Take a Lifeblue 150 Ah battery and a Lifeblue 300Ah battery. They are both considers High Current models and support 150 amps discharge current. If you were using a 2000 watt inverter at max power you could go over the 150 amp limit, so it would be better to have two 150 Ah batteries in parallel that would give you 300 amp limit, versus the one 300 amp battery at 150 amps. The Battleborn batteries have 100-amp limit. This limit is because of the BMS.
It should be noted that the Lifeblue xxx-HC models can be connected in series up to 48 Volts. The xxx-HCLT can only be parallel connected due to the way the internal heating system works. The LT modules have an internal heating mechanism designed to allow more effective charging in extreme environments. The Lifeblue low temp module will take charge power to run the internal heating mechanism, to bring it up to an acceptable temperature before charging.
Battleborn batteries offer a electrical thermal blanket that is thermostatically controlled.
Lifeblue also offers Bluetooth connectivity, for a smart app. For a battery that is supposed to last that long, do we think Bluetooth will also.
Another option might be the
Expion360 VPR PowerMod; a group 24 size 120Ah battery that has a removeable integrated BMS.
There are more drop-in batteries hitting the market every day. The benefits of lithium are real, they don’t have to make them sound better than they are.