About this replacement
TD Tech EP631 / EP682 — 3.8V Li-ion Replacement Battery (BTY3500LI20)
This is a 3.8V, 3450mAh Li-ion battery for the TD Tech EP631 and EP682 portable two-way radios. It replaces OEM part BTY3500LI20 directly. Capacity figure is drawn from product data, not estimated.
- EP631 and EP682 platform fit: Both models share the same battery bay dimensions, connector pinout, and BMS handshake protocol — that is why one pack covers both. The 3.8V nominal rail matches the RF front-end voltage requirement on this TD Tech platform.
- Bench tested on actual hardware: We cycled this pack through transmit loads on the EP631 dock, monitoring BMS cutoff thresholds under sustained PTT draw. The protection circuit tripped correctly at overcurrent and recovered without lockout on release.
- First insertion into the EP631 dock: If the dock LED shows a fault on first insertion, remove the pack and wipe the gold contact strip with a dry cloth before reseating. The TD Tech platform runs a BMS handshake on contact closure — a film of oxidation or packaging residue on the contacts is enough to fail that check and block charging.
Why the EP631 cuts out mid-transmission on a new BTY3500LI20
A new cell ships at storage voltage — typically 3.6–3.7V, not full charge. The EP631's RF output stage draws a sharp current spike at the moment PTT is pressed. If the cell has not been through at least one full charge cycle, that spike can pull the voltage below the BMS undervoltage threshold and trip the protection circuit. The radio cuts audio mid-transmission and the display may blank briefly. Run a full charge before the first shift, not a partial top-up.
EP631 bar indicator showing one fewer bar than expected after fitting the BTY3500LI20
The EP631 reads battery state through simple voltage-threshold steps — each bar corresponds to a voltage band, not a percentage tracked over time. A cell at storage voltage sits at roughly 3.65V, which maps to the second or third bar rather than full. This is not a fault with the replacement pack. Charge the battery fully to 4.2V on the dock and the indicator will advance to the top bar on the next power cycle.