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Author Topic: Using LiPo batteries for portable radio  (Read 12963 times)

Posts: 11

« on: November 01, 2010, 07:43:58 AM »

I almost know the answer to this one, but I thought I would air it for some opinions.....

I have just bought a 4 cell, 5ah LiPo battery pack.  The issue I have is that the fully charged voltage can be almost 17 volts, reducing to 12.6 volts when discharged (minimum level if you know LiPos). The radio I intend to use is the small Yaesu FT817 or similar and my question is;

The max specified voltage is 16volts for the FT817, would 17 volts harm the radio? (or is there a tolerance)

Perhaps I should have bought the 3 cell version that has a nominal 11.1 volts but the 4 cell looked good at a nominal 14.8 volts.

Alternatively, can anyone suggest a simple Dc-Dc reg that would reduce the voltage to 13.8 volts where the operating voltage is reasonably central to the specification range without wasting too much battery power.


Posts: 7718

« Reply #1 on: November 01, 2010, 11:10:55 AM »

16 volts is the maximum. 17 volts is too high.

A very simple way to drop the voltage is to add five silicon diodes in series with the +12 V lead. 1N4001 type, although rated for 1 amp, will work given the duty cycle of the FT-817.

Posts: 83

« Reply #2 on: November 01, 2010, 01:11:06 PM »

Even at 16 volts you are dangerously close to causing a problem.

1) You should opt for the 3 cell packs next time. 
2) Anything above about 9 volts is a waste in the FT817.  If you step down your voltage to only 13.8 the FT817 is also going to step is down as well. explains it much better than I can.
3) Texas Instruments makes some very nice DC-DC switching regulators that are about +90% efficient. Much better than the 3 leg LMxxx series voltage regs that are only about 45% efficient.  The trouble is going to be to find one that only drops 3-4v. Most take a 18v (or more) input.

Of course, you CAN still use an LM317 as your regulator but you are going to waste a lot of battery power to heat.  Make sure you get an LMxxx that can handle at least 2amp of power.




Posts: 11

« Reply #3 on: November 02, 2010, 01:17:25 AM »

Many thanks for the excellent replies, (WX7G) I had not thought of series diodes. I will string up some and check the voltage across them aiming for approx 12.5volts at the radio.

 (KE7SAK) Really good link, I like the voltage table that provides efficiency and 17 volts = don't!!

Obviously, the simple solution is to buy a 3 cell pack as the radio operates down to 9.75 volts without any serious performance effects.  But will try out the diode solution as this does not drain valuable current when up the hills.

Thanks again, 73  Steve


Posts: 625

« Reply #4 on: November 15, 2010, 12:32:28 PM »

A better idea would be to add a three terminal low dropout regulator set to 13.8V.  You can still run it below that voltage but it is designed to have a low saturation voltage (PNP) device as the series pass element.

To really take advantage of the full output power you need a switching regulator.  You could use a buck step down and let it go to 100% duty cycle when the battery got below 13.8V.  If you use a FET switch, the full on drop would be low.

I will leave it to you to weigh the difficultly in EMI shielding and so forth.  It is not impossible since many modern radios have switching supplies.  It just takes a little care in packaging.

I think the diode Idea is just slightly better than a dropping resistor.

Posts: 875

« Reply #5 on: December 24, 2010, 02:10:43 AM »

I converted an old lead acid battery charger to a 12V psu for use with my FT817ND.
I used the LM1085IT-12 low dropout 3 amp three terminal voltage regulator.
With a dropout voltage of just 1V (0.5V typically) you could run the voltage down to 13V before it would stop giving 12V output.
Just use a good heatsink ( I used one I scavenged from the CPU of an old laptop computer).
This regulator is very good as it has all the usual overvoltage and short-circuit protection you would expect without extra circuitry.
To make extra sure I am protecting my FT817, I use a 3 amp fuse in the output with a 1 amp 15V zener directly wired across the
output terminals - a simple crowbar protection.
If the output should rise to 15V, the zener will conduct and blow the fuse very quickly.

I would not use the series 1 amp diodes - I have tried it, and the diodes get really really hot.
You can put them in parallel to share the current but it takes a lot of diodes and they get really hot without a heatsink.
Some people think the diodes will not get hot because the voltage is dropped by a semiconductor junction, but the power
dissipated is still I x V , so at 2 amps and 0.7V each diode will dissipate 1.4W the same as a similar resistor.
Also, if you use diodes, use 3 Amp diodes - it is just too risky using 1 Amp diodes, when the current could easily reach 2.5 amps for
extended times with the FT817.
In addition, if the diodes short circuit, there goes the FT817.

Just use the regulator, its simpler and much more protection.
Another tip is that if you decide to put a really big electrolytic capacitor on the output (not necessary), put a 1 amp power diode
reverse biased from the output to the input of the regulator.

Three terminal regulators do not like having their output voltage greater than the input, and if you turn off the supply, a big capacitor on the output may hold the voltage on the output terminal for long enough to blow the regulator.
It really sounds more difficult than it is - but don't be deterred, it really only needs the LM1085IT-12 or similar, a couple of .01uF caps from the input and output to ground, a 1uF tantalum on the output, a good heatsink and you are done.

You could build the regulator into a small box with heatsink and use it inline from whatever voltage source you had, so giving you a versatile general purpose low dropout voltage 12V regulator.
As the FT817 does not take full power at all times, it will run fairly cool most times anyway.
I have abused my poor PSU and the output is still rock steady on 12V.
Oh, and I am also using some old toshiba LiPo 5 amp batteries and they work well with the FT817ND in portable mode, much lighter,smaller and cleaner than lead acid.

73s and cheers

Posts: 1555

« Reply #6 on: December 24, 2010, 04:42:48 AM »

Look at the FT-817 voltage conditioner in the "Articles" section of my website at  Leave out the 15V transient diode, or put in an 18V unit if you want.

Phil - AD5X
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