CONNECTING LIMITS
R-SERIES MODEL | MAX UNITS SERIES CONNECTION | MAX UNITS PARALLEL CONNECTION |
| 12 VOLT LFP | 4 (48V System) | 4 (6 if not series connected) |
| 24 VOLT LFP | 2 (48V System) | 4 (6 if not series connected) |
| 36 VOLT LFP | 1 (36V System) | 6 (no series connection) |
| 48 VOLT LFP | 1 (48V System) | 6 (no series connection) |
NOTE: Rolls R-Series & S-Series drop-in models cannot be mixed. Batteries should only be combined in the same capacity, and voltage, from the same product line.
CONNECTING IN SERIES
Rolls R-Series and S-Series 12V and 24V LFP batteries may be combined in series strings to achieve higher operating voltages by connecting the positive terminal of one battery to the negative terminal of the next battery. Don’t connect different voltages in series. For example, do not connect a 12V and 24V battery to reach 36V, use 3 12V batteries instead, or a single 36V battery.
| 24V System | 12V batteries in series - Two (2) | 2 x 12.8V = 25.6 |
| 24V battery in series - One (1) | 1 x 25.6V = 25.6V | |
| 34V System | 12V batteries in series - Three (3) | 3 x 12.8V = 38.4V |
| 36V battery in series - One (1) | 1 x 38.4V = 38.4V | |
| 48V System | 12V batteries in series - Four (4) | 4 x 12.8V = 51.2V |
| 24V battery in series - Two (2) | 2 x 25.6V = 51.2V | |
| 48V battery in series - One (1) | 1 x 51.2V = 51.2V |
NOTE: Do not connect batteries in strings above 48V nominal.
CONNECTING IN PARALLEL
You may combine Rolls R-Series or S-Series drop-in LFP batteries of the same model together in up to four (4) parallel strings to increase system capacity. If no series connections are made, up to six (6) batteries of the same model may be connected in parallel.
NOTE: When connecting parallel strings of Rolls R-Series or S-Series drop-in LFP batteries of the same model, the recommended current limit increases proportional to the number of parallel strings.
NOTE: Strings are independent. External connections should be staggered, i.e., the positive lead is connected to string one, whereas the negative lead is connected to string two.
NOTE: Keep cabling the same resistance (gauge and length) between batteries and strings to ensure proper current sharing. Attempt to minimize length to reduce the magnitude of inductive voltage spikes at the battery.
BATTERY VOLTAGE - CONNECTING IN SERIES/ PARALLEL
For initial balancing prior to connecting batteries in series, each battery should be connected in parallel (you may connect above the maximum of four (4) batteries in parallel, but not for regular cycling) and fully charged (or charged individually) using a 2-stage CC/CV charger at a reduced CV voltage corresponding to the low end of the acceptable charge range (see below), leaving the battery at the absorption/CV voltage for at least 24 hours.
| System Voltage | 12V Nominal | 24V Nominal | 36V Nominal | 48V Nominal |
| Recommended Initial Balancing Voltage | 14.0V | Balance initially at 12V Nominal | ||
If you are unable to charge the batteries individually, the voltage of each battery should be within 30mV (0.03V) before putting them in service. This will minimize the severity of a charge imbalance between batteries which results in reduced pack capacity. LFP batteries, even those with similar open circuit voltages may be at drastically different SOC, due to the flat relationship between open circuit voltage and SOC for LFP cells.
Although the BMS provides over-voltage protection to each cell, developing a charge imbalance between batteries is still possible. Rolls recommends disconnecting and fully charging each battery individually once per year if 1 or 2 parallel strings is used, or every 6 months in systems with 3 or 4 parallel strings. If the batteries are cycled frequently at high charge/discharge currents this may be done more often.
Absorption time can also help with balancing. For a single string, absorption times of 20-30 minutes is recommended, or up to 60 minutes for 4 strings.
INITIAL BALANCING
The BMS in Rolls LFP Batteries is designed to keep cells balanced, not to balance them when they are initially connected. This would require substantial thermal management considerations and a better mechanism for this one-time and recommended yearly procedure can be used.
Because this is not a cycling application, the batteries can be connected in more than 2 in parallel ONLY for initial balancing charges. Each battery should be connected in parallel (above the two (2) batteries in parallel, but not for regular cycling) and charged (or charged individually) using a 2-stage CC/CV charger at a reduced CV voltage corresponding to the low end of the acceptable charge range, leaving the battery at the absorption/CV voltage for at least 24 hours.
Before connecting for cycling, verify voltages are within a 15mV (0.015V) range once each battery has been off the charger for 15 minutes. This way all series-connected batteries will be at 100% SOC.
If you are unable to charge the batteries individually, the voltage of each battery should be within 15mV (0.015V) before putting them in service. This will minimize the severity of a charge imbalance between batteries which results in reduced pack capacity. LFP batteries, even those with similar open circuit voltages may be at drastically different SOC, due to the flat relationship between open circuit voltage and SOC for LFP cells.
Although the BMS provides over-voltage protection to each cell, developing a charge imbalance between batteries is still possible. Rolls recommends disconnecting and fully charging each battery individually once per year, at minimum. If the batteries are cycled frequently at high charge/discharge currents this may be done more often.