Moving to LiFePO. This topic comes up a lot when people are looking to replace there existing battery which is most likely AGM. The price of LiFePO batteries has come down considerably, manufactures have gone out of their way to limit your upfront cost by excepting a host of charging parameters. They still tell you of a high life cycle count that they have validated inside a laboratory (think how much power you get actually out of your solar panel compared to the laboratory method for sizing). So are we really going to get the numbers they are providing ?
The following is a list of things to do for long happy life of LiFePO batteries from a article on Solarcity.com How to Find Happiness With LiFePO4 (Lithium-Ion) Batteries
Keep the battery temperature under 45 °C or 113°F (under 30°C or 86°F if possible).
- Keep the battery temperature under 45 °C or 113°F (under 30°C or 86°F if possible) – This is by far the most important!!
- Keep charge and discharge currents under 0.5C (0.2C preferred)
- Keep battery temperature above 0 Centigrade when discharging if possible – This, and everything below, is nowhere near as important as the first two
- Do not cycle below 10% – 15% SOC unless you really need to
- Do not float the battery at 100% SOC if possible
- Do not charge to 100% SOC if you do not need it
Everybody seems to know about cold weather issues with Lithium, but very few think of the high temps effects on lifespan. This is one of the big reasons that I highly recommend that Lithium batteries get installed inside of the van. The traditional under the van rack, even the fiberglass boxes through the floor don’t provide the insulation against the road heat, imagine the heat from the roadway in 100 + degree days. Depending on your battery or BMS they might even shut down at a higher temp setting.
Below is day in my van, the temp probe in the air conditioner (rooftop) is usually the hottest. The other temp probe is in the cabinet that houses my Truma Combi, it does have some venting. I was working on the van most of the summer, so the battery cabinet was open, and I didn’t have a probe installed. But you can see the benefit of having your battery inside in a cabinet.
Keep charge and discharge currents under 0.5C
Size your battery bank correctly, the general recommendation for discharge and charge rates is 0.5C. Most of the life cycle data is based on 0.5C charge and discharge rate. So if you are running an inverter on a consistent basis you are easily bumping needing over 150 amps of discharge capability. This puts you in the 300 amp-hr battery size. More importantly it is not hard to find a single 300 amp-hr LiFePO that only has a BMS that supports charge and discharge rates of 100 amps. So be careful on sizing your battery or batteries. Many times, you may need multiple batteries to get your discharge rate high enough.
Don’t discharge below 0°C or 32°F.
This is the most well-known issue with LiFePO, and to be clear it does have an impact. But to be clear you can start out by moving the battery inside, also many LiFePO’s can be purchased with internal heaters. Some will be powered off of the battery itself, others can be externally powered, I have the heater for my battery attached to two battery sources, the LiFePo itself, or if need be I switch over to power provided by the Transit battery. For those of you worried about losing the power to heat your battery, remember you were losing significant amount of power to cold with your AGM battery,
Don’t discharge below 10% to 15% SOC
. Although many LiFePO’s may advertise 100% discharge capability it is generally considered good not to go below 10%.
Do not float the battery at 100% SOC if possible
. This is one of those that appears to be contradicted by the battery manufacturers themselves. They now appear to be happy to support a low float voltage. But again, that was a decision that was made to lower the cost of entry into the Lithium world. If you do have a charger that has a pure Lithium profile or 2 stage charger it might be a good time to use it, especially in the heat of the summer.
And while we are talking about charging profiles, let’s address the Absorption phase. With AGM batteries you generally have longer Absorption phases, some chargers even gave you the ability to increase the time in absorption depending on the depth of discharge. For LiFePO, the absorption phase should be relatively short. I believe BB says 30 min per 100 amp-hr battery.
The other phase that you should disable is automatic equalizing, LiFePO’s do not get equalized, if you can’t completely disable, at least set the equalize voltage the same as your absorption setting.
Finally, if you have any temperature compensation setting, they should not be used for LiFePo.
Do not charge to 100% SOC if you do not need it
. This is one of the great features of Lithium compared to AGM. You don’t need to get if charged to 100% SOC. Remember partial SOC is one of the bigger killers of AGM battery life. When sizing your battery bank, you might want to look at 70% to 80% capacity of the battery for your target. This will give cover your #4 and #6. Generally during the summer my solar panels provide enough energy to replenish what I lose overnight. If I am leaving the rig parked in the driveway for periods of time, I may disconnect the solar for a day or two, this then allows solar to replenish power to the rig on a daily basis but not go over say 90% SOC so you don’t have to get into float. This doesn't mean you can't charge it to 100%, if your trip may need that extra battery time.
Speaking of SOC, a SOC meter is imperative to understanding the health and capacity level of your battery. The voltage drop-off for LiFePo is minimal, so if your use to using voltage for remaining capacity you will be in trouble. A SOC meter for AGM batteries is a good thing also. Currently the Victron 712 is a good SOC meter, that does well with Lithium. They also have the wireless Bluetooth unit, I recommend the 712, because it also has Bluetooth but also includes a programable relay that you might be able to use to say shut down your solar or DC to DC charger if you have reached a certain SOC.
I would only use one charging source at a time, so using a combined DC to DC/ solar charger is a good way to eliminate. In any advent you may want the ability to turn off your charging devices remotely. This is even more important if the combined units may force you over maximum charging current.
At some point your battery BMS might disconnect. While some conditions may reset themselves others may require some effort to get reset. The battery is basically disconnected from your charging devices, and many of these devices need to be connected to a battery to work. Most of the devices that have been developed for LiFePO have some procedure to work around this issue. Make sure you know what that procedure is what it takes to get your BMS reset if needed. So many LiFePO batteries come with an internal BMS. You might inquire what the possibility of changing out BMS if it goes bad on your battery. It should be cheaper than replacing a whole battery.
If you are installing with a large inverter, you may need a pre-charge circuit to make sure the capacitors are charged slowly before turning on.
While many of these best practices sound like a pain, they really are not that difficult to add to basic battery health procedures. Even if you don’t perform the last four, your battery cycle life will probably still far exceed your expectations and anything close to what AGM’s could deliver. And remember the specified cycles for an AGM were also best-case lab environment produced numbers.