This post is to detail another version of an external BMS charging system built for a fellow La Croix DIYer here in Perth Australia.
The basic specs are
- Digital displays for individual cell voltages ( its for a 10S 6P battery pack)
- LCD Meter on box showing Charging voltage/current/power with cumulative Wh
- Utilising DieBieMS https://www.electric-skateboard.builders/t/diy-6s-to-12s-bms-with-can-diebiems/2639 PCB – but with external resistors to increase balance discharge current
- LEDs to show DieBieMS balancing function
- 10 switches to discharge individual cells
- External power supply and charging module tom provide up to 5A charging
The reason for the external balance resistors is that the DieBieMS only has 15 ohm bleed resistors which is approx. 250mA.
The battery packs on our LA Croix super long range cruiser boards https://www.electric-skateboard.builders/t/la-croix-long-range-cruiser-carving-machine/76960 are approx. 30Ah ( 10S 6P using 51E 21700 cells) and 250mA would take a very long time ( days) to reduce a P pack voltage during balancing.
With the external 6 ohm resistors and the super bright LEDs – the balance discharge bleed current is approx. 1A.
This build requires direct soldering to the DieBieMS – so it not for beginners learning to solder.
So first the schematic for balancing discharging- the blue on this schematic id DieBieMS pcb and the red is the additional wiring for this build. All balance/discharge wiring inside the box is 22awg silicon and all charge wiring is 16 awg silicon .
The DieBieMS balance discharge Mosfet (Q5 TSM2323) is rated at approx. 4.7A continuous or 20A pulsed so there should be no problems switching the 1A balance discharge current.
Basic shopping list for the bits and pieces is as below
36V 11A Power Supply
6 Ohm 5W Resistors
Basic box with cut-outs
On earlier builds using DieBieMS bms , I have had a few problems with the molex balance plug pins. They are bit flimsy with no mechanical strength ( even using the correct crimping tool) so for this build I have soldered balance leads directly to bms pcb using the available tabs on the 15 ohm discharge resistors. This needs to be completed very carefully ! . Tin the ends of the resistor first using a very hot small soldering iron tip.
The charger is external and consist of a fixed 36V 11A LED strip power supply driving a CCCV module mounted on top of the power supply. The output of the CCCV module was set to 45V 5A.
The DieBieMS pcb finally arrived – it was supplied with FW 0.27 and is compatible with latest DieBieMS tool version 0.27 which has a few enhanced features from earlier versions.
The pcb was configured using the DieBieMS tool with key config settings being:
After all wiring was complete - triple checked all wiring and did as much continuity testing as possible without power .
First power on – no magic smoke so all good.!
Connected up the 12-pin balance lead plug and 2-pin charging plug to my board ( The board pack voltage was approx. 38V) and the cell voltage displays , DieBieMS pcb sprung to life.
Connected up charger and charging current started and LCD meter showed charging current and pack voltage .
With the charging current set to 4A and the BMS “Soft Overvoltage” set to 4.18V – the BMS charged and balanced the P packs to about 4.13V . Setting the charging current down to 0.5A enables the final top-up to 4.18V . This feature is kind of handy in that a “normal” charge only charges to 4.13V which should prolong the cell life – but you can top up to 4.18V ( should you want to for a long ride) by reducing the charging current to 0.5A.
After a long charge and pack balance – the pcb discharge mosfets had no detectable temperature rise – probably due to the cell discharge sequencing function and the pulse discharge function of the bms. The external 6 ohms resistors were only very slightly warm.
Tested P pack discharge switches with BMS off and all works OK.( the switch basically shorts out the DieBieMS discharge mosfet)
So overall the external BMS build was successful and achieved objectives using the worlds best bms pcb DieBieMS.
Happy to answer any queries