09/08/2026
🔋 BATTERY MYSTERY SOLVED: WE HAVE A WINNER! 🔋
Thank you to everyone who commented and to those who sent us their theories privately through Messenger and email. We received some excellent guesses and several people identified parts of the answer.
Nobody explained the complete sequence perfectly, but the closest answer came from Graham Millar.
Graham correctly identified that the voltage difference between the batteries was insufficient to overcome the forward-voltage drop of the relevant MOSFET body diodes.
Congratulations Graham, you’ve won the $100 Muller Energy voucher! 🎉
So, what was actually happening?
The two batteries were connected in parallel:
🔋 Battery A: approximately 13.28 V
🔋 Battery B: approximately 13.35 V
There was no charger and no external load. The difference between the batteries was only 0.07 V.
Both batteries use the same BMS, containing opposing banks of NCEP018N85LL N-channel MOSFETs. Each MOSFET also contains an internal body diode.
Initially, charging was enabled on both batteries but discharging was disabled. With the discharge MOSFETs switched off, current entering the lower-voltage battery would have needed to pass through their body diodes.
Those body diodes need around 0.7 V before meaningful current begins to flow. The 0.07 V difference between the batteries was nowhere near enough, so both displays showed 0 A.
We then enabled discharging on the higher-voltage Battery B. This allowed Battery B to supply current, but the switched-off discharge MOSFETs in the lower-voltage Battery A were still blocking meaningful current from entering it.
Finally, we enabled discharging on the lower-voltage Battery A.
This switched on its discharge MOSFETs. When a MOSFET is switched on, its channel can conduct current in either direction with extremely low resistance. Current no longer needed to pass through the body diodes.
A low-resistance path was now open between the batteries, so they began equalising:
⚡ Battery B discharged at approximately 5 A
⚡ Battery A charged at approximately 5 A
That is why enabling “discharge” on the lower-voltage battery allowed it to begin charging. The discharge control operates MOSFETs that form part of the electrical path in both directions.
Graham Millar came closest by identifying that the small voltage difference was insufficient to overcome the body-diode forward voltage.
An honourable mention also goes to Graham Harris, who correctly recognised that the opposing MOSFET arrangement and passive equalisation were involved, although some of the details were not quite right.
Graham Millar, please send us a private message so we can arrange your $100 voucher.
We’ll also contact the first 10 genuine entrants regarding their exclusive glow-in-the-dark Muller Energy stubby holders. ⚡
