IC-705 battery for full power

No, I don’t have an IC-705. But I just thought that it would be interesting to share this info in the SOTA reflector.

The point is I found this question in other forum: what battery do you recommend for an IC-705 to get 10 watts out?

As you probably know, the built in battery (7,4 volts) only provides 5w out. In order to get full power you should use an external battery, which is a shame.

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:

By looking at the data, you should stay over 12,5 volts to get around 10 watts.

A drop to 11,5 Volt provides 8,1 watts, while 11 volt cuts to 5,4 Volt.

A regular 3S LiPo or a 3S LiIon would probably be enough for a 8-9 watts power out. Meanwhile a 4S LiFePO4 (14,4 V) would probably be a better choice for this rig.

If you’d like to use a 4S LiPo you’d need a voltage regulator or some Diodes in series to get to a lower and safe voltage.

It would be interesting to compare this feature for some of the alternative rigs. Feel free to provide me with them, I wouldn’t mind doing the tests :wink: :stuck_out_tongue_closed_eyes:

73 de Ignacio.



The manual specifies 13.8V ± 15%, so you should be safe up to around 15.8V maximum, although the heat dissipation is likely to be higher at that voltage



I use this battery pack with my KX2 and have been very pleased with the performance:

Personally, I think the space given over to an internal battery on the 705 would have been better used for an internal ATU.
73 de OE6FEG


Another good choice




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.

73 Ed.

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One of the reasons you will never see a 18650 cradle in a radio, so that users can source their own cells. How cool would that be?
73 Matt

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You’d need to be careful using 4S LiPo as when they are fully charged they can be 4.2v per cell for a short time, although this drops off quick quickly when being discharged.

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There’s data around for the FT817: The KA7OEI FT-817 pages - Optimizing power consumption when operating on batteries
Can’t find anything for the FT818, I’d expect it to be similar but with the line shifted up slightly.

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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.


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4s lifepo4 4200mah zippy flight max when wanting 10w otherwise 2200mah 3s lifepo4 for longer activations at 5w lighter weight.


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.

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Hello Paul, thanks for your contribution.

Right, that’s why I suggested that in the case of using a Li-Ion (18650 , 21700, etc) or a LiPo I would rather go for a 3S instead of a 4S, despite having a 12,6 max V only.

In the other hand, those using 4S for such chemistry should add a voltage dropper (i.e, diodes…).

73 Ignacio


I have 3 rigs to compare, so when are you popping over to GW land to test them out on different summits Ignacio?? Joint activations?? :rofl:

73, GW4BML. Ben

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Great, I thought I’d never get a real offer!
Sure I wouldn’t mind a joint activation Ben, I just need to get some days off working, and free from family duties, I’m afraid… :innocent:

73 Ignacio

Ps You can do the other way round and come to Pamplona bringing your stuff to our summits, better in summertime, hi.



Many thanks for the graph in your first post. I have a 705 and run it from a 4S LiFePo4 without problems so far.

I hope to visit Spain next year and may come near Pamplona. Details vague at the moment, we might get the ferry to Santander or drive through France from Roscoff.

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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.

73 Ed.

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Hi John,
It will be good to meet next year if we have a chance.
Keep me updated with any plan you do.
73 Ignacio


I will see what I can do for next summer and be in touch - you have better wx than us :grinning:

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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?

Regards, Mark.


A quick search on the FB Icom IC-705 group threw up a few comments about the eBay clones which said they work well and are a third the price of the Icom original.

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