Is lithium-ion really the solution to all our battery woos? Well, not quite…
LFP batteries too have their limitations. A big one is temperature: You cannot charge a lithium-ion battery below freezing, or zero Centigrade. Lead-acid could not care less about this. You can still discharge the battery (at a temporary capacity loss), but charging is not going to happen. The BMS should take care to block charging at freezing temperatures, avoiding accidental damage. This is a Big Deal in our Canadian climate!
Temperature is also an issue at the high end. The biggest single cause of aging of the batteries is use or even just storage at high temperatures. Up to around 30 Centigrade there is no problem. Even 45 Centigrade does not incur too much of a penalty. Anything higher really accelerates aging and ultimately the end of the battery though. This includes storing the battery when it is not being cycled. We will talk about this in more detail later, when discussing how LFP batteries fail.
There is a sneaky issue that can crop up when using charging sources that potentially provide a high Voltage: When the battery is full the Voltage will rise, unless the charging source stops charging. If it rises enough the BMS will protect the battery and disconnect it, leaving that charging source to rise even more! This can be an issue with (bad) car alternator Voltage regulators, that need to always see a load or the Voltage will spike and the diodes will release their magic smoke. This can also be an issue with small wind turbines that rely on the battery to keep them under control. They can run away when the battery disappears.
Then there is that steep, steep, initial purchase price!
But we bet you still want one!..
The BMS (Battery Management System) that is build into the batteries functions as a simple on-off switch, switching the batteries off when Voltage, current, or temperature parameters get to the edge of what is safe. Contrary to what many think, the BMS does not change the charge or discharge current, it really is just an on-off switch; if you have a charging source that can push hundreds of Amps into the battery the BMS will not prevent you from doing this, but it will sense the large current, and switch the battery off when the upper safe limit is reached!
Lithium Batteries with Build-In Heater
The issue of not being able to charge LiFePO4 batteries below freezing is now being overcome by versions that have a built-in heater and a thermostat that senses when the battery is being charged at a low temperature. It will kick in a heater to get the cells above freezing before actually charging the battery (incidentally this is also how an electrical car works in winter).
That may look like a good solution to the no-charging-below-freezing problem, but it too comes with its own potential issues: When the heater is running the battery cells are not charging (and vice-versa). This could cause problems when your charging sources can deliver more current than the heater needs, which in turn causes the ‘charge Voltage’ (though note it’s not actually charging, just heating) to rise rapidly. Generally the charge controller will limit the Voltage, and keep it from rising too high. However, this may erroneously cause the charge controller to believe it’s done bulk-absorb charging and go to its float stage before it is done heating, at which point the batteries will not actually be charged at all! The work-around is to create a custom charging profile for the charge controller, with a suggested value of 14.2 or 14.4 Volt for bulk-absorb (for a 12V battery bank, multiply to fit your situation for 24/48V batteries), and 14.0 Volt for float. Absorb time should still be kept low, 0.5 or 1 hour is suggested, and temperature compensation should be set to zero (or off). This way the batteries will get charged to at least 95% State-Of-Charge even when the heating cycle leaves the charge controller at float by the time it is done.
Because the battery cells are switched off while the battery heater is running, there is in essence no battery. That leaves the system with very little Voltage-buffer, in other words a small current changes cause large Voltage changes, something that may not work with (for example) wind turbines that expect to see a large load to keep them under control.
If your system has a battery monitor that uses a shunt to measure the Amp-hours going in, and coming out of the battery, this will lose track of the State-Of-Charge when a heated battery is involved. Any Amp-hours going towards heating are not charging the battery, but they are counted by the battery monitor! There is no work-around for this, you will just not get good SOC readings from a shunt-based battery monitor when battery heating is involved.
Heated lithium-ion batteries should NEVER be connected in series! Unless the BMS’s in both batteries communicate, there is no way the heaters will be switched on/off at the exact same times, and that will cause all kinds of issues. So no series connection for heated LiFePO4 batteries.
The long and short of this is that you should think long and hard about heated LiFePO4 batteries. If at all possible it may be a better idea to stick with regular (unheated) ones, and use a well-insulated battery box to keep the temperatures above freezing.
Jump-Starting The BMS
For most conditions (over-current, over-Voltage, under-temperature, or over-temperature) the BMS will automatically switch back on again, either after a set amount of time has passed, or once the conditions are safe. However, there is one case where the BMS will NOT switch on by itself, the battery will stay off: When any cell within a LFP battery falls below the lower safe Voltage limit the BMS will switch off to protect the cells from over-discharge. It does this with still a little charge left in the cells, so the battery can sit for a while and self-discharge before damage to the cells occurs. The important part is that the BMS will not switch the battery back on by itself! When this happens the battery simply “goes away” and produces 0 Volt.
To make the BMS switch on again after a low-Voltage disconnect event the battery needs to see a charging Voltage. How much exactly varies from brand-to-brand, but generally this means 14.0 Volt or up (for a 12V battery). Keep in mind that inverter-chargers won’t work without a battery, nor will solar charge controllers. They need to see regular battery Voltage to function. That means you cannot switch the battery BMS back on by charging from a generator (via your inverter-charger) or your solar panels. To make the BMS switch on again you either need a 120V AC charger that can do “dead battery charging” as it is usually called in the brochure, meaning it puts out a charging Voltage even if it does not sense a battery. Alternatively you can “jump start” the switched-off battery by taking another battery of the same nominal Voltage, even a lead-acid battery, and connect it in parallel with the dead battery, and then charge via solar or your inverter-charger. As soon as the Voltage reaches high enough the BMS will sense it and switch the battery back on again. At that point you can disconnect the extra battery, but please keep charging so the empty battery does not immediately switch off again with the slightest load.
Balance & BMS
Another source of confusion, and potential problems, is when multiple single lithium-ion batteries, each with their own BMS, get connected in series to create a battery bank with a higher Voltage. For example, by connecting four 12 Volt batteries in series to create a 48V battery bank. Lithium-ion batteries do not at all self-balance! This is very different from lead-acid, where charging gets progressively less efficient as the battery gets more fully charged; this makes it so when multiple batteries are connected in series the ones that have less charge in them will automatically “catch up” to the fuller battery. Not so for lithium-ion batteries! Any difference in charge between series-connected batteries will persist from charge-cycle to charge-cycle and wreak havoc: Say a half-full 12V battery is connected to a fully charged 12V battery, in series, to create a 24V battery bank. When discharging the half-full battery will reach empty first, and at some point the BMS will intervene and switch that battery off, causing the entire battery bank to “go away”. Moreover, the empty battery will not switch on again until it sees a charging Voltage. The second battery will at this point be about half full, and if the entire bank is charged in this state the situation will be exactly the same as before; one battery will reach full while the other is only just half-full. Worse still, the fully charged battery will continue to rise in Voltage until the BMS intervenes and switches that battery off, causing the entire bank once again to just disappear (though it will switch back on again by itself, not needing a “jump start”). This situation will persist forever, unless manually corrected.
Victron Battery Balancer
Victron Battery Balancer
Victron Battery Balancer
Victron makes a product that can help balance multiple series-connected 12 Volt lithium-ion batteries. Their Battery Balancer measures the Voltage of each 12V battery during charging, and bypasses 1 Amp of the battery with the higher Voltage, to the battery with the lower Voltage, until they are equal. It takes one Battery Balancer for a 24 Volt battery bank, two for a 36 Volt bank, and three for a 48 Volt bank.
That means you HAVE to make sure that all batteries are at the same State-Of-Charge (SOC) before connecting lithium-ion batteries in series! The easiest way to ensure this is by fully 100% charging each battery before they are connected in series. For a set of 12V batteries that means (for example) using a 120V AC charger plugged into the grid or a generator, set it to an absorb Voltage of 14.4V, and let it charge until no more current goes into the battery. Repeat this for each battery, and only then connect them series. Depending on how well they match this may need to be repeated every now and then (once a year or so) though reports are encouraging that a group of series-connected batteries will continue to behave well over time, as long as they started out at the same SOC.
Note that this is only a factor for series-connected lithium-ion batteries. Parallel-connected batteries do not have this issue, they all see the same Voltage and eventually will arrive at the same SOC.
It is normal for series connected lithium-ion batteries to each have a slightly different Voltage (even if they are at the same SOC), and because the BMS will switch off batteries that exceed an upper safe value, it is a good idea to set any charging sources in the system to use the lower limit for the bulk-absorb Voltage that will still fill up the battery. A good value would be 14.0 Volt, with a 1-hour absorb time (on a 12V basis, multiply as needed). That will hopefully prevent any individual battery from rising above the upper cut-off limit. If this still causes the BMS of one or more batteries to intervene, try a lower absorb-time value such as 1/2 hour. If needed absorb time can be set all the way down to zero (or as low as settings allow), by the time a LiFePO4 battery reaches 14.0V it is already at least 95% full.
The bottom line is that when at all possible you should avoid connecting lithium-ion batteries in series! There now are 24V and 48V batteries that avoid the whole issue altogether. Our advise is to use those.
BMS Induced Power Limits
Unlike lead-acid batteries that can (briefly) deliver hundreds and even 1000+ Amps in current, there is a hard limit to what the BMS in a lithium-ion battery will allow. There usually are several stages; this much for a fraction of a second, that much for a few seconds, and some lower limit for long periods of time. Exceeding those limits means the BMS will switch off, and the battery “goes away”. This affects the loads you can bolt onto a lithium-ion battery.
The typical 12 Volt 100 Ah battery would, for example, have a limit of 100 Amp continuous output current. That translates to (roughly) 12 x 100 = 1,200 Watt. Connect a 4,000 Watt inverter and you are guaranteed to never reach that level of output power! After 1,200 Watt the BMS will intervene and switch the battery off. It would take at least 4 of these batteries to be able to drive an inverter of this size and actually reach full continuous output!
Besides continuous output limits, there is a more insidious issue: Inverters have input capacitors on the battery side, to smooth out and handle surges in output power on the AC side. Large inverters have large input capacitors, and large capacitors cause very large currents to flow when they are connected to the battery. For large inverters, around 3 kW and up, this can easily reach in the hundreds of Amps! While most lithium-ion batteries have a large limit for brief surges, this capacitor charge-up current can still exceed that limit, causing the BMS to switch the battery off. To make it possible to even connect these large inverters, the input capacitors need a chance to slowly charge up, and to do that a 4.7 kOhm 5 Watt resistor can be connected over a switch or breaker in line with the positive wire between battery bank and inverter. The resistor makes it so a little current flows into the inverter, slowly charging the capacitors, and by the time the switch or breaker is moved to the “on” position they are already charged up and there is no large current surge.
While it is important to understand the above in case problems arise, in practice we have found lithium-ion batteries to behave quite well. Even when connected in series. Just follow the guidelines we just talked about and you will likely be just fine!