The rig has been on disposal for a while now and I searched for info about the input voltage vs. output power and found very little, just a few videos at the nominal 13,8 voltage, or battery powered and not many technical reviews.
Only the review from VA7OJ / AB4 OJ was a deep evaluation and testing report.
I got from him the numbers of the input voltage / output power and I prepared a graph showing this data:
I think ICOM realised too late in the design phase, that they should have included an ATU, despite that being the loudest comment made about the IC705 prototype when they presented it.
Other manufacturers learnt from this e.g. the later released Xiegu X6100 has both an internal battery and an internal ATU (albeit this is only an HF radio, not HF/VHF/UHF as in the case of the IC-705). I believe the KX3 can have an internal battery and ATU as well.
The later release of an external ATU by ICOM would have been better if it could have been made smaller and lighter - but that is what aftermarket ATU products are for I guess.
Hi Peter, indeed a voltage dropper is recommended for 4S LIPOs
With my 2m “QRO” rig I use a 4S 3Ah LIPO with a voltage dropper made using a 20A bridge rectifier - that gives you 2S2P diodes for around 1.3V drop idle and 1.5V on a 10A load. I built it onto a small box and included an LED digital voltmeter module.
No ATU in the 705, but look at everything else that Icom fit into that box! A “Swiss Army Knife” has never been as good at any one thing as a single-purpose tool. It’s only in recent years non-resonant antennas with ATUs have become favored, so I think the engineers made a reasonable choice.
A couple different battery chemistries are being mentioned, and possible confused.
LiFePO4 has an absolute max of 3.65v/cell, or 14.6v for a 4S pack, and falls to 3.0v/cell (12.0v for a 4S pack) at 10% remaining capacity. I have two, a 20 Ah and a 100 Ah, which I take out with my FT-891, and they work fantastically for a nominal 12v application. That voltage range is well within the 705’s spec.
The older “lithium-ion” chemistry (such as the 18650) which contain cobalt and manganese are 4.2v/cell or 16.8v for 4S at full charge, well above the 705’s specs, and most recommend not discharging below 3.0v/cell. These would absolutely need something to reduce voltage at full charge to stay under 15.8v for the radio.
One would assume however that ICOM’s designers were told to create a “portable version of the IC7300” and that came with a built-in ATU. Of course when size is reduced things have to go and the ATU got the chop!
Still a nice radio which I was thinking of buying, until I found it will never cover 4 Metres as the 2m and 70cm transverter stages feeding the SDR use a 68-72MHz I.F. I believe. Originally the European version was being advertised as going to have 4m coverage.
Anyway, this is getting off subject. A 4S LifePO4 battery would seem, as others have said, to be the best option for the 705.
Has anyone tried the larger capacity Icom BP-307 (either OEM or 3rd party)? I’d like the option of not needing to plug in an external battery and was wondering if anyone had any stats on longevity with the higher capacity cell?