“Drop-in” batteries are pre-built LiFePO4 (LFP) lithium (Li) batteries that are more or less the same size and voltage as lead-chemistry 12v batteries. 100Ah is the most common capacity, but larger and smaller batteries do exist.
The other approach to lithium is DIY2), where one selects cells, BMS, and other components and builds it themselves. That topic is beyond the scope of this article. People interested in DIY might want to read the DIY LiFePO4 Battery Banks subforum over on DIY Solar. You can also see the Lithium Battery Summery article for high points of a debate on lithium suitability for long term use.
Note: there are many myths about charging LFP.
There are two main approaches to charging a drop-in:
Some chargers are labeled “lithium-compatible”. This can mean:
If you already own a fully-configurable charger then you probably don't need to buy a lithium-specific one. [my settings are here - secessus]
LFP are typically fully charged at 13.6v, so Absorping at 13.8v for gentle overvolting and floating at 13.2-13.4v are common setpoints for folks who want to get maximal life from the bank. Observe cell voltages and return to mfg-spec voltage if/when balancing is needed.
Andy from Off-Grid Garage found at moderate charging rates like ~C/5:
Manufacturers need simple instructions that will still allow the batteries to meet their advertised lifetime and reduce customer support issues. In this scenario jigher charging voltages have the following benefits to the seller:
Bottom line, stay within the manufacturer recommended specs, and you should be fine, go beyond that (more conservative) and you should be extra fine. – Dzl
In this section we are thinking about “extra fine”. One way to baby the bank is to treat it like the manufacturer did when getting the much larger cycle numbers like 8,000. These tend to be:
The overall idea is to treat the bank like there is no BMS, no safety net. Charging rates/voltages are conservative and charging takes longer. Note this only works if one has enough time for gentle charging; if you only have a 2-hour charging window hard-and-fast is the only option and we accept the shorter life trade-off.
If there is sufficient charging time, a lower charging voltage may offer these advantages:
At gentle charge rates like C/5, the following patterns emerge:
Some drop-in BMS only start top-balancing at 14.2v10) but increasing voltage to that level tends to cause imbalance. Catch-22.
If charging at lower votlages the initial charge (and occasional charges thereafter) might be to mfg spec voltage. This might allow the BMS to reset “full” and top-balance to the degree that such balancing works.
Also see: Will Prowse's Lithium Battery Longevity: Double or Quadruple the Life of Your Lithium Battery
BMS are often configured to limit charging to 1C (100A for a 100Ah battery) as an absolute maximum. Manufacturer's who actually warranty the battery often recommend .5C (50A) or even .2C (20A) for longevity. Charging at too high a rate can cause permanent damaging to the battery.11), and the effect may be worse at low temperatures.
Charging Li at very high rates may also strain the alternator.
Lead batteries require fully charging to 100% state of charge (SoC) but lithium batteries do not. Charging them fully at higher voltages can cause cells to become further out of balance. Leaving them at 100% SoC can cause degradation.12)
Having said that, there are valid reasons for charging to 100%:
Depends on the charger and how your Li wants to be charged. Most fully-configurable chargers can be used to charge Li.14) Note that some so-called “lithium compatible” chargers may have presets that do not match the requirements of your particular battery, so read the specs.
Here is the order of operations:
Armed with a full understanding, here is one approach to thinking about lead battery charger setpoints:
Li certainly doesn't need Float voltage (Vfloat) the way lead-chemistry batteries do, but the Float setpoint is still useful for Li battery banks.
Reminder: lead requires Float because
Neither of these is true for Li, which dislikes sitting at 100% SoC and has vanishingly-low self-discharge rates.22) So with lithium Float is used for a different purpose, as a voltage floor. It is a voltage below which the charger shouldn't let the bank fall while charging is present. Without no Vfloat (or a very low one) the bank would charge then fall until the next morning when charging starts again. After initial charging loads would run off the battery instead of the charging source. Having a sane Vfloat allows Li to “relax” after charging while retaining the desired amount of Ah/Wh capacity.
What Vfloat setpoint should actually be is a matter of some discussion and experimentation. Each setup (and use case) is different, but in general
Depends on both the battery, the alternator, and even the isolator.
Here an order of operations one might use to assess whether or not isolator charging will work in a given install:
Make the first test run a brief one and with the Li fairly well charged.24) Start the engine and see if the charging current and voltage is acceptable. If that works test it with longer drives and/or with the Li at lower states of charge.
Caveats: Only alternator charge while driving (no idling). Pay attention while charging from alternator to keep from overcharging the Li. You may want a manual disconnect or HVD to shut off alternator charging.
mega-thread on DIYsolarforum
Microcyling means bouncing between two25) voltage setpoints. The concern here is that each one of these bounces would count as a cycle and subtract from the rated 3,000 cycles or whatever the manufacturer claims. Those who take this position suggest setting Float voltage to the same as Absorption voltage to avoid microcycling. The drawback to this approach is two-fold:
The 2nd point takes a bit of explaining. A solar charge controller completes Absorption then falls to into Float where it will remain as long as the sun26) cooperates. Absorption can be re-triggered if voltage falls below the Absorption Reconnect setpoint, but that setpoint is even lower than Vfloat. If that happens the solar charging has already been overtasked and we will get a “microcycle” during that day in any case if the sun comes back.
Lithium cells can hurt themselves if left unsupervised. A BMS (Battery Management System) is device that tries to keep them out of trouble. It's like a fence around the yard that won't prevent skinned knees but should keep the kids out of the street where they could get really hurt.
Typical BMS protections include:
Charging is disabled for a few different reasons: temperature extremes, high cell voltage, overcurrent.
People who camp in cold weather may want to select a battery that has “low temperature cut-off”, which disables charging near freezing.27). The belt-and-suspenders solution is both low temperature cutoff and a way to keep the batteries warm. Some pricier batteries have internal heating, or you may DIY a heater.
Discharge is disabled for similar reasons as charging: extremes in temperature, cell voltage, or current.
The low temperature discharge cut-off is typically much lower (like -20C) than for charging.
Cell balancing attempts to keep all the cells in the same voltage ballpark. The method used in most drop-ins is passive balancing, which applies a small amount of braking a bit on runaway cells.28)
The image on the right shows the LFP voltage curve: a lower knee (below ~3.2v) on the left, a wide, flat section in the middle, then the upper knee (~3.4v) on the right. Between the knees the voltage stays relatively flat and cells stay in relative balance. The first cell[s] that hits a knee will race in that direction ahead of the others.
This matters because voltage extremes, high or low, are not good for the cell. Cell imbalance can change effective capacity, since a wayward cell could trigger cell voltage BMS protection before the bank is fully charged or discharged.
Consider these two batteries, using made up round numbers to illustrate the point. Cells are arranged in series so the volts add up:
In both cases the overall battery voltage is 13.6v, but in the unbalanced battery cell #1 is lagging (reducing capacity) and cell #4 is too high. The balancer will attempt to rein in #4 but the effect is tiny. Using a typical balancing current of 50mA, if you are charging at 20A that means the cells are receiving 5A except cell #4 which gets only 4.95A, about a 1% difference. Tiny balancing currents and the propensity of cells to race away explains why balancing is so gradual.
It is more practical to minimize imbalance than to try to rebalance them after the fact. Here are some approaches:
Some batteries have BMS with bluetooth or other forms of connectivity. With these you would be able to see more information, like individual cell balance, statistics, setting and the status of any protections.
Charge/discharge rates are expressed in C. C/2 for a 100Ah bank is 50A (100Ah/2). Heated models typically use the charger to run internal heat until the battery comes up to safe temp.
[Please verify stats and specs before ordering. This is a best-effort listing but things change fast in this segment - secessus]
With cold cutoff. BB also has a line of heated LFP.
These batteries have cold cutoff. The batteries themselves do not have bluetooth, but are intended to be connected to other Victron gear that can talk to the batteries. Victron's value proposition is increased inter-device communication when installing a Victron ecosystem.
Charge 14.2v @ C/2, Float 13.6v.33)
With cold cutoff. Will Prowse teardown video
No cold cutoff. Will Prowse teardown video
Renogy's original “Smart Lithium” line has no cold cutoff. They are releasing a new line of temp-protected, bluetooth enabled batteries.
Charge 14.4v @ max C/2.38)
Unheated versions appear to lack cold cutoff. No bluetooth.
Charge 14v - 14.6v, 1C up to 100A max. Float 13.3v - 13.8v.39)
Base models with no cutoff or bluetooth and “Smart” models with 5A internal heating and bluetooth (BT demo).
Will Prowse teardown video of base model; he notes they are similar inside to Ampere Time.