Lithium batteries have some characteristics that you must take into account when designing offgrid installations to avoid problems. Some installers have experienced problems during commissioning and, in most cases, the only solution has been to increase the battery capacity at the high cost that this represents. Please read this document carefully to avoid such problems.
1: pre-charge of internal capacitors
In order to keep solid and robust devices, Studer inverters/solar chargers have large internal capacitors. During startup, these capacitors demand a high charging current (Over 1000 A) from the battery for a very short time, just few milliseconds. Lead-acid batteries can handle these spikes, so this has not been a problem with this old but reliable battery technology.
However, lithium battery modules may not handle these spikes by themselves and systems could not startup. To solve the issue it is necessary to increase the number of battery modules, so they share this inrush current.
For that reason, some lithium battery manufacturers have a “minimum configuration list”. Where they indicate, depending on the inverter model and number of units, what it is the minimum number of battery modules you must install for a proper functioning. Please check for the latest version of these documents.
Example: BYD BBox Pro 2.5. Detail of minimum modules in offgrid single-phase system
2: limited peaks current (pumps, motors..etc)
Other characteristics of lithium batteries are:
- Nominal charge current (A)
- Nominal discharge current (A)
- Peak discharge current per module (A)
Studer inverters use the highest quality. One of our unique features is that our devices can handle peaks up to 3 times its nominal power. Example: xth 8000-48; Nominal power: 7kVA & Peak power (5s): 21kVA. This is especially important in industrial applications, where pumps, fridges and motors can be found. The benefit of this feature is to avoid unnecessarily oversizing the inverter.
In order to take advantage of the full potential of our system, you need to select a lithium battery that can deliver the peak current the load is demanding.
Example: In an industrial application there are 2 x 3kVA pumps. The peak demanded by these 2 pumps is 20kVA and they will be working up to 2h per day. We select inverter/charger xth 8000-48 (Nominal power: 7kVA; Peak power: 21kVA).Now you have to choose between these 2 battery models. What configuration would give a good performance.
| Capacity kWh | Nom. Charge Current | Nom. Discharge Current | Max. Discharge current per module | |
| Battery A | 5kWh | 110A | 110A | 200A |
| Battery B | 15kWh | 216A | 216A | 375A |
Solution: daily energy needed: 6kVA x 2h = 12kWh. If we decide to discharge the battery up to 20%SOC. Then battery capacity should be at least 15kWh.
1 x “Battery B” seems to be the easy choice as its capacity is 15kWh. But what about peak current? These pumps could demand up to 20kVA during startup. Can 1 x “Battery B” supply this current? Unfortunately, No (Peak current = 20kVA/48Vdc = 416Adc). This battery could protect itself due to the high current demand.
In this case the right option is 3 x “Battery A”. You have 15kWh (3 x 5kWh) energy available and can handle up to 600A peak power (3 x 200A) . Other option is add 2 x “Battery B”. Not recommended.
3: low temperatures: reduced charging current
When we talk about a lithium battery with communication, the BMS controls/defines the maximum charge current. When the battery temperature is below 10-15ºC (depending on the manufacturer), the BMS will reduce the charging current significantly (Example: 0.2C). And if the temperature falls below 0ºC, it will forbid the charge of the battery.
This entails that someone who is trying to charge a battery with a generator, through an xtender (Studer Inverter / Charger) could have the generator working for hours without almost charging the battery. Or that on a sunny day, the variotrack (Studer MPPT Charge Controller) won’t produce as much as it could. Due in both cases to the limitations of the BMS.
What is the correct approach to avoid this problem? The simple thing is to place the battery in a room that ensures a minimum temperature of 15ºC in the cold months of the year.
What happens, that this solution in an installation in the middle of the mountain, is not something that can be assured, especially when we talk about mountain shelters or very cold places. For this reason, alternative solutions must be sought, such as thermal resistances, which ensure that at the moment of charging, the temperature of the battery rises in order to make the most of the charging current.