Hypothetically speaking… if I wanted to have a custom battery pack built that looks and feels similar to the SP140 battery pack… a single piece… what kind of specs should I ask for to be equivalent to having 4 or 8 bonka 7s batteries?
I have to admit, I’m not electric-savvy… especially, when it comes to batteries. I’d love any advice you smart battery folks might have!
The X4 was developed around those Bonaka batteries. The four small props are not as efficient as one large one so more power is required from the batteries and LiPo cells were pretty much the only way to go when it was developed. Originally a ‘nominal’* 48V system was chosen in part, I think, because much over 50V DC is enough to break your skin barrier and deal an unhealthy shock if you touch it. Extra safety is thus required (I think legally required in Europe). Breaking that 48V into 2 packs of 24V allowed the use of widely available chargers in the hobby market. With a custom battery pack you’ll probably have a custom charging (BMS) system built in. With the Bonkas all the battery management - cell balancing etc. - is on the chargers and pilot.
Updated versions of the X4 (batch 4 and up) moved to a slightly higher voltage by using motors with different winding. 7S Bonka packs are nominal 28V each so two packs in series is now 56Volts. I say ‘nominal’ because fully charged voltage is slightly higher and loaded voltage is a fair amount lower. The higher battery voltage allows designing the rest of the system (motor/prop) for the amp draw to be lower for the same total power output. I fly a batch 4 (designed for 56Volts) but use the 6s Bonkas (48Volts) so my lower voltage spins the motors/props slower which means lower amp draw and lower thrust and less climb performance.
Amp Hours - As much as you can carry. At a minimum you want whatever it takes for your batteries to provide the total amperage below. With the Bonka LiPos that’s 2 in parallel or about 44amp hours. The number of cells in parallel you need will depend on the power density or “C” rating of the cells used. With modern LiFe or LFP type ‘round cells’ you can probably get enough power and still be able to lift and handle the weight on your back.
Max Voltage - The later (Batch 5 and up) electronics are designed to handle up to 60Volts. I’d stay well below that for a fully charged battery. For given set of motors and propellers a higher voltage will result in a higher amp draw.
Max Amperage - 300-350 in full power climb depending on the voltage. You’ll pull around 200 in level flight depending on your setup and AUW. With my under powered setup I’m pulling 280 at full power on freshly charged batteries.
I’d get as close to 4kWh total energy as you can. Depending on your terrain a full power climbout could easily exceed 60 seconds so you’ll want to have a constant 12-15kw available. Should be doable but even with a higher power requirement than the SP140 you’ll still have shorter flight times.
A single connector means a single point of failure. I like having 2 switches cross connected. Four (a switch and battery for each motor) might be a bit much. But, yes, originally it was designed for only one. On the X4 the design is such that you need a master switch (or two) you can reach from the harness - i.e. IN FRONT of the props. This is because you need to reach through the prop arcs to plug things in. I’d rather not do that without a physical switch keeping the circuit open.
It can if you know what you’re doing. X4 is low volt high amp while SP-140 uses higher voltage to get the amps low. Therefore, the wires coming from the battery will need to be a really heavy gage to that single connector on the X4. From the connector to the speed controllers it would split 4 ways and those wires can be the same gage as what the system currently uses because each one will carry 1/4th of the current.
Another thing to consider is wire length. Long wires between the battery and speed controllers create induction voltage spikes which will damage the speed controllers. With multiple batteries it was easier to keep the wires short by having them connect to different locations on the unit.