Thank you for reading!

I am interested in building DC / DIY chargers and battery packs!

I want to create a battery pack and charger for specialized radio battery applications using 18650’s

My basic question is if / how this will work:

I want to use 18650’s to charge radio(s) – of course, these radios have specific power requirements
My idea was to use a DC car charger and cables, cut the leads, and use it to charge from a battery back and/or 18650’s.
I know that I can get the voltage correct or close from attaching 18650 batteries in series, BUT MY MAIN QUESTION IS WILL THIS WORK WHEN CONSIDERING THE AMPERAGE. (see pictures)

The car charger expects 11.7 - 15.9 volts, so no problem – I can make an 18650 x 4 battery pack, but if I use this DIY battery pack to charge my radio batteries, can anyone comment on how the amperage will affect the system?

I understand that a car adapter will have a massive variance in AMPS, but in theory would this work if the charger and wires are designed for 12V car applications?

If the voltage is correct or close and I attach a battery pack to this adapter / charger, would this work and/or would it be a good idea? Clearly, I can get the voltage correct, but what about the amperage?

Thanks,
Dave

 

To clarify, I would be (want to use) a 4 x 18650 battery pack in place of the car battery / alternator / charging system.

This battery back would be behind the adapter pictured, with the current sent to charge the radio(s) ... would this work and/or be a good idea?

Or might it be too different in AMPERAGE?

Thank you for reading and/or for any replies,
Dave

 

You need to start with specifications. The 18650 Li-ion batteries are 3.7V nominal and contain 2200 MaHrs.
Four of them in series will provide 14.8 volts (nominal) which means the voltage will vary according to the state of charge in the Li-ion batteries. At nominal voltage, they are perfect. As the charge declines, they aren't. Is that why you want to use a charger which will function with an input as low as 11.7 volts? (2.925V/ Li-ion cell)

[The radio batteries are what voltage and can contain how many amp hours?]!

The radios seem to be designed for automotive battery voltages.
The charger you posted a photo of requires a voltage input very similar to what a car battery charger would provide: 11.7 to 15.9VDC
Yes, you can use (4) 18650 batteries in series to power the charger. The amps drawn from the Li-ion batteries depends very much on the load and I am not at all clear about what load you are going to connect the charger to.

Using a battery pack to power a charger to charge a radio battery puzzles me. The Li-ion pack is already the right voltage to run a car radio. Why would you run that voltage through a charger to get to the same voltage you started with?

You left out some very important stuff, therefore my (anybodies') answer can not be very good.
I need you to simplify. Right now, the answer looks like you're being silly. Your intent is not clear and your method looks redundant. Please try to explain again.

 

Fully charged lithium ion cells will be 4.1 to 4.2 volts, so your converter could see as high as 16.8 volts at the input. Whether that would be damaging or not is impossible to say without knowing what the circuitry is.

You must use either protected cells or a battery management board for your pack. If you over-discharge the cells you will dramatically shorten their useful life. A good management board that includes charging circuitry would be "best" since it would allow you to safely charge the cells while they are connected in series. Charging cells in series, even if they have individual protection boards, is a bad thing to do without proper control circuitry.

I don't understand what your issue with current actually is. Good quality 18650 cells can certainly provide 2.5 amperes. One thing to consider is that the adapter you show will be a switchmode converter. Switchmode converters have a "negative resistance" input characteristic - if the input voltage falls, the current will rise in order to keep the output regulated. This means as your pack is discharged, the discharge rate increases, so you can go from "everything OK" to "out of power" quite quickly. If there is another charger downstream for the battery in the radio, it is very likely a switchmode converter, too.

The brand-x cells sold on ebay and the like are notorious for being far below claimed capacity. About 1 ampere hour seems to be very common, regardless of the claim. One ampere hour of actual capacity would numerically equate to 24 minutes of operation at 2.5 A, but might in reality give far less time at that discharge rate. If you want good cells, you'll need to buy brand-name types (e.g. Panasonic or Samsung) from a reputable vendor. You can turn good cells into bad cells quite rapidly if you overcharge or overdischarge them.

 

Thank you for the replies, I really, really appreciate it!

I should have been more clear, it's just a little bit hard to explain the build and I am new-ish to this stuff.
I have taken some new pictures, which should help clarify.

The radio(s) are handheld, not the car's radio system - the idea is to use an 18650 battery pack (when away from the car) and cut and use the car charger leads and adapter for this purpose, because it is already designed for DC current. I want to use 18650's because I have hundreds of them, and I am trying to standardize my backpacking / outdoor kit on 18650 power.

The 18650's cannot power the radio(s) directly because of the physical dimensions and/or possible damage, so I am left with a situation where I have to somehow charge the specialized radio battery. (with AC, 12V car, or another system, hence the build).

Initially, the 18650's would be individually / separately charged beforehand, then slotted into a bay in series to charge the radio battery. A dedicated battery pack with a BMS, etc. would be the next step, but individually charging them and installing them in a "mobile" type battery pack would be a good start.

Hopefully this explanation and/or the pictures will clarify the situation and the desired build,
Thank you for any and all replies and information,
Dave

 

Am I seeing a cigar lighter plug, which normally plugs into a car, and then the power from the car goes through the charger to power a radio, which normally contains 7.4 volts worth of batteries containing 11.84 WHr of power? So you want (4) Li-ion batteries to pretend they are a car, have them connected to the cigar lighter leads, to power the charger which charges the 7.4V battery pack which usually lives in the radio?

 

I think @ebp is better aware than I am about how a 18650 will present it's voltage after it is charged to possibly as high as 4.2 volts. (I've never played with Li-ion batteries.) How long will the 18650 batteries hold that 4.2 volts after they are removed from the charger? Will they subside to 3.975 volts each (or lower) in a matter of minutes after they are unplugged from the charger so they are perfectly safe to connect to the cigar lighter leads just because they came off the charger a few minutes ago?

The radio battery pack is labeled at 11.84 WHr and 7.4V. That means 1600 MaHr. The charger is labeled to deliver 11 volts at a maximum of 2.5 amps to the radio. That means the radio is using the 11 volts to charge its original 7.4V battery pack and run the radio at the same time.

Next idea: The battery pack in the radio fits the description of (electrically resembles) a pair of Li-ion batteries a little smaller than 18650's in series. Connecting (2) 18650 batteries (in series) to the chrome looking, radio mounted battery connectors with a pair of alligator leads would probably work just fine if there was a magic guarantee that nobody would ever connect the wires backwards. We don't have any magic so the method offered by tripleJ057 looks reasonable now, right after we are sure the high charging voltage of the 18650's (4.2V max ea x 4 = 16.8V) isn't going to be too high for the 15.9V max input of the charger.

What say you, ebp?

 

Thanks again for all of the information so far! I really appreciate it!

Am I seeing a cigar lighter plug, which normally plugs into a car, and then the power from the car goes through the charger to power a radio, which normally contains 7.4 volts worth of batteries containing 11.84 WHr of power? So you want (4) Li-ion batteries to pretend they are a car, have them connected to the cigar lighter leads, to power the charger which charges the 7.4V battery pack which usually lives in the radio?

Yes, absolutely correct! The 18650 battery pack would "replace" the car charging system (cut the leads, solder) - I have very little need for car charging actually (because the radios are for backpacking) and/or I could buy another car charger after cutting this one and/or direct car charging would be unnecessary with a multitude of battery packs and/or other charging options. (AC, for example).

You are also correct, some folks do run their radios with "directly" with 2 x 18650 with bare leads and/or a dummy battery or similar setup. This seems more risky, but I've entertained the possibility. It won't let me post an Amazon link but if you search for "BAOFENG BL-5 BATTERY ELIMINATOR" there are people that have used 2 x 18650 instead of cigar plug connections.

So I guess I have to decide which route to take - the manual for the radios is contradictory and confusing, to make matters worse. It might be good to try / have both setups - 2 x 18650 direct connection & 4 x 18650 connected behind the car adapter.

From the manual:

•Power supply range is between 5.5–11.5 V DC.
NEVER CONNECT OVER 11.5 V DC directly into
the DC power jack of the transceiver.
•BE SURE to use the CP-19 when connecting a regulated
12 V DC power supply.
• If a battery pack is attached, the voltage of the external
power supply must be within 11.5–16 V DC

I tested some of my 18650 cells today and right off the charger they are all 4 V or 4.05 V and depleted cells are as low as 3 V - I am fairly sure that they are all 1500 mAH (some are unmarked) so I wasn't sure if a higher capacity cell might boost the voltage slightly? The large ones are 3500 mAH ...

Thanks again for all of the advice! Sorry for the long post, but I am trying to figure this out as I go.