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Author Topic: 14.8 v battery pack using 3.7 volt 18650 batteries?  (Read 30418 times)
W4KYR
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Posts: 1803




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« on: September 06, 2013, 01:07:05 PM »

Has anyone made a 14.8 v battery pack using 3.7 volt 18650 batteries?

Was it worth doing?

How are you charging the pack up?

Do you use some sort of dropping resistor or circuit to keep it from rising higher than 14.8 volts?

What combination did you use?

Did you use just 4 x 3.7 in series?

Or a combination of series/parallel to get higher amp hours?

What radios are you running from this pack?  And what are your results?

Thanks
« Last Edit: September 06, 2013, 01:21:36 PM by W4KYR » Logged

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G6UDX
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Posts: 11




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« Reply #1 on: September 06, 2013, 01:16:19 PM »

What uses 14.8 volts?
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W4KYR
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Posts: 1803




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« Reply #2 on: September 06, 2013, 01:42:53 PM »

Most radios operate best at 13.8 volts.

Only using 3 X 3.7 volt (18650 batteries) will get you 11.1 volts and close to 12.8 fully charged. The Youkits HB-1B uses these 3 of these batteries and it reads 12.8 fully charged.

Most radios operate best at 13.8 volts. Some radios (like the SG 2020) can tolerate up to 18 volts, others up to 16 volts. And the  14.8 volts fully charged could put it into 15 to 16 volts. Using the dropping resistor or circuit will keep the voltage lower 14.4 volts or if need be...below 14 volts.

The other way to keep close to 13.8 volts is to use a voltage booster like MFJ 4416 or the N8XJK 12Volt Boost Regulator which brings an additional piece of equipment into the field.

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KB1GMX
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Posts: 1824




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« Reply #3 on: September 07, 2013, 09:11:08 AM »

forget the dropping resistor. For simple applications that run on constant current or
have small variation in current drain it would work.  For a transceiver no.

You can use a series diode or several as you get .7V per silicon diode.  Use robust devices
with current ratings at least a few times what the expected maximum is.

Most "12V" radio are rated at 13.8V, stop working at something around 11V and universally
all tolerate 16V some more.  Why, thet nominal audio electrical system is 13.8V when charinging
12.6V (more or less) with the engine off and under some circumstances like heavy charging of the
battery may reach well above 15V. 

So a 4 cell pack is only slightly high (freshly charged about 16.7V).  The solution I use is a 10A
series diode.  Instant .7V drop,  and as the cells discharge to their lower limit you still in the
above 11V range.

A 3 cell pack at the 70% point is about  10.8V.  For msot 12V radios that is on the edge of too low.
many QRP rigs do not mind but be careful as soem will RX and tx well but as the voltage gets marginal
chirp and drift are problematic as the voltage regulators may not have enough voltage to operate.

A good example of the above is my KNQ7A, receives great at 10V but the 8V regulator used for the VXO
and Carrier osc is on the hairy edge and a drop to 9.9V during transmit means FMing of the radio with
very bad reports.  There is also a noticeable drop in power.  However the same radio at 16.7V is fine and
has no issues.  So a 3cell pack works at the cost of some power out and the inability to fully discharge it
to the lower safe limit of the battery.  Another way to say that is a 3cell pack does not allow the full
possible operating time where a 4 cell pack of the same AH rating does.  For me the whole point of
using LION/LIPO batteries is maximum operating time for minimum weight.

Lithium ion, and all the related flavors (LIPO, LIFE) are extremely fussy about charging.
How fussy, they catch fire!  You must limit the chrage voltage to NOT more than the
recommended voltage per cell (3.6 to 4.02 depending on what type and who made them).
Current limiting is rarely a problem as you can fast charge them if you watch the temperature.
However trickle charging after reaching full charge is NOT advised.   Also over discharging them
below the recommended voltage is not advised.  Most RC style series connected packs have two
sets of wires one is the high current output and the other is for charge monitoring and
equalization during charge.  I use a lot of those at work to power stuff for portable use and have
had  one 3S 3.7ah pack just fail and another literally catch fire during charging with the vendors
charger.

See Battery University on the net.  http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

The simple solution is to use the recommended charger.  Usually not cheap.

Lithium battery tech is great but has things to be aware of and be careful of.


Allison



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W4KYR
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Posts: 1803




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« Reply #4 on: September 09, 2013, 05:14:59 AM »

Thank you for the excellent explanation. I put a search for 10A 'series diode' and it came back for Rectifier Diodes. Amazon has them 10 for $3.66. http://www.amazon.com/Amico-Molded-Plastic-Rectifier-Diodes/dp/B009IN1KB8
I assume these are the correct diodes?

Thanks for the batteryuniversity link, it contains good solid information. My interest for using these 3.7 18650 batteries came about because I saw that the YouKits HB1B uses 3 X 3.7 to bring the voltage to 12.9 charged. I figured I can use these batteries to power other radios as well.

In addition someone on E-Bay was selling a charger and two 11.1 volt battery packs for $50 stirred my interest. The seller said that these were used for the Elecraft KX3. I passed up the deal because I needed more information about the batteries.

I really wanted a battery that ran close to 14 volts to run some of my radios. How come no one (to my knowlege) ever made a 13 to 14 volt gel cell?  12 volt gel cells are usually fine, but as been noted...some radios run better with what it was designed to run on 13 volts or higher. Has anyone used those 13.2 volt Yaesu FNB78 batteries which run the FT 897 on other radios like the SG 2020? What kind of success has anyone had with them? Someone could use two of these and run the SGC 2020 all day or the FT 817 for several days.

From what I gathered is that these 18650 batteries require a specialized charger. Which isn't a bad thing, but not sure how that would work if someone wanted to charge these batteries in the field using solar power. The issues with these batteries having a potential to catch fire is an issue to take into consideration.

With a 10 watt solar panel and a generic charge controller, charging gel cell and lead acid don't seem to be an issue. And one last note. I noticed that Harbor Freight is running a sale this month on D cell NiMH for $6 for pack of two.  They are rated at 2500 ah. Perhaps these might be an option to build a 13 volt battery pack. Not sure if that can be safely charged in the field by solar power.

Again thanks for the information.
« Last Edit: September 09, 2013, 05:18:10 AM by W4KYR » Logged

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G3RZP
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« Reply #5 on: September 09, 2013, 07:39:36 AM »

The old fashioned liquid electrolyte lead acid battery would run about 13.5 volts on charge: that's where the 13.5volt  rating comes from. But I don't know what gel cells run on charge - maybe a bit less. Professional mobile radios in Europe are tested at 1.1 times the nominal voltage, so that is 13.2 volts for a nominal 12 volt radio.
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KB1GMX
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Posts: 1824




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« Reply #6 on: September 09, 2013, 06:38:24 PM »

HF D cell at 2500mah... not a bargain.   real D nicads or NImh are 10,000mAh.  I have AA sized Nimh that are 2200mah. 
Fishy yes.

FYI a gell cell properly charged is 12.6V (after the initial surface charge dissipates) and holds that down to about 1.9V (near dead)
for 11.4V.  That makes a gell a good battery for 12V gear save for one thing, they are made with Lead, and heavy.

13.8 is nominal for an electrical system with charging.  The only battery that gets close to that is 10 NiCd or NiMh cells.
I know that as my station runs on 10 flooded NiCd (150AH) cells and I also use AA (2200mah) NiMh for portable use.
They float to about 14.2V, discharge to 1.1V (70% discharge)  so that about 14 down to 11V, most commercial 12V gear
I have likes that.  NiCd and NiMh both sit around 1.25V for most of their discharge curve so 10 gives a nice 12.5V.

The alternate is 4 3.7V LIPO cells (18650) at about 16.7V fully charged and at cutoff about 12V.  Add one series diode and
that make it 16 down to 11.3, good enough.  Especially since during the bulk of the discharge cycle they will hover around
3.6V cell (14.4V) with a diode that's 13.7 and near perfect.

Of the three gell cells ( or VRLA SLA) are cheap for high AH, NiCd/NiMh a bit more but smaller, Lion/LiPo is far more expensive
but lighter by a lot.

Allison
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WB6BYU
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Posts: 18527




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« Reply #7 on: September 09, 2013, 07:52:24 PM »

Quote from: W4KYR

...How come no one (to my knowlege) ever made a 13 to 14 volt gel cell?



The basic lead-acid cell is a nominal 2.1V, hence 12.6V for a 6-cell battery (and up to about
14V when fully charged.)  That is a good match for equipment designed for automotive systems
because the starting battery also has 6 cells.  The next step up would be 14.7V, and significantly
higher under charge.

I've seen Gates cells sold as individual units to be assembled into whatever size battery pack
you need, so you could make your own at the higher voltage if you wish.  You can also get
batteries at a nominal 8.4, 6.3V and 4.2V, so could combine a couple (preferably with the
same Ah rating.)


Quote

...I noticed that Harbor Freight is running a sale this month on D cell NiMH for $6 for pack of two.  They are rated at 2500 ah...



First, those batteries are probably 2500mAh, or 2.5Ah.  (2500Ah is a VERY LARGE battery!)
That's about the rating of a typical NiMh AA cell, and if you compare the weight you'll probably see
that what they are selling is a AA cell inside a D-cell case.  Convenient, in that it fits your big
flashlights, and aren't as heavy in the big mag lights with 6 cells, but they aren't going to deliver the
power that you would expect.  (A standard alkaline D cell is rated at something like 20Ah, and
a good NiMh battery will be closer to that.)
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