replacing battery cells

[Jetronic]

I have a portable radio with a built in LiFePO battery, 7.4V 1000mAh it claims. The external power supply is 9V DC 1500 mAh and is used to run the radio on mains and charge the battery. The radio is rated at 15W, so the power supply is definitely not overkill.

The battery lasts about 2.5 hours with low speaker volume, but I could use 9-10 hours of use without charging.

I also have a surplus 20V 2000mAh battery for power tools, I would like to take 4 cells out of that and put them in the radio to get to 8V 4000 mAh (fully charged) and 7.3V discharged. The cells are LG 18650 HD2. That capacity should get me to about 10 hours. I use 20/40V batteries on all my tools so this one is the odd one that just sits and collects dust

What are the chances this will work with the built in charger? Obviously the charge current would be a bit lower than ideal but if that's an issue I could probably source a 9V 2500 mAh power supply.

I haven't opened the radio yet to check what it actually has inside, and won't if there's 0 chance this will work. Main concern is not starting fires!

 

[HankHill]

This is where I would ask my electrical technician grandfather to talk his magic and give me the answer, I took electricity far too long ago in engineering school. If you don't use it, you really do lose it.

Depending on how much the unit cost, and what you would gain from the modification, you may just want to pass on the idea. Someone will come along and say, yea, no problem, but that may take a while. I wouldn't do it.

 

[Vercingetorix]

Can you buy an inverter for your 20v battery?

 

[Jetronic]

could be an option... https://asset.conrad.com/media10/ad...24-vdc-9-vdc-2000-ma-aantal-uitgangen-1-x.pdf

I could fit 4 cells inside the radio easily but not the whole 20V battery. but this would likely be easier

 

[eljefino]

Lithium chargers are a bit more risky than the old NiMH or lead acid. They have temp sensing and compensation by slowing or stopping the charge rate. You should look into, at least, this.

 

[Dave9]

Something is wrong with the specs, LiFePO4 cells are nominal 3.3V so a series of two should be rated as 6.6V pack. 7.4V looks more like consumer grade Li-Ion 3.7V/cell.

If it is not LiFePO4, this is a good thing making your plan possible without more work, simply reconnect any sensing wires between the series of cells if it has a balancing charger. Don't substitute a higher current charger if the design uses a low current capable charging IC as it may fry it or cause an overheat, shutdown condition. You can find the IC, identify it and consult the datasheet. This could be incorrect, depending on design it may use a constant current and then a higher input PSU current capacity is better. Again consult the datasheet though the charge current could be controlled by a resistor value so some reverse engineering of the circuit could be needed or measure the charge current yourself.

However, I might go a different route, to make an adapter for your tool battery and keep it intact at 18V/20V then use a buck switching regulator to drop it to 8.4V. Depends on how portable vs bulky it ends up. For example something like this, though there are tons of variations of buck switching adapters on ebay and other sites, just make sure it's spec'd for at least the 21V peak charge of a 18V/20V Li-Ion battery and >= 1.5A:

2pcs XL4015 5A DC Buck Step Down Voltage Converter Constant Current Power Module | eBay

This module combines the high efficiency XMLSEMI XL4015 DC-DC buck converter IC with a TI LM358 Op-Amp for stable current regulation. Current limiting function requires at least 5.6V input. 60W Buck/Boost w/ Constant Current.

www.ebay.com

 

[y_p_w]

Lithium chargers are a bit more risky than the old NiMH or lead acid. They have temp sensing and compensation by slowing or stopping the charge rate. You should look into, at least, this.

Once I took apart a Wal-Mart On-branded USB power pack that stopped working. I saw that it was using a couple of cylindrical Li-ion cells in parallel where the ground terminals were connected via a metal strap that was spot welded to the battery terminals. And the spot welding was in a lousy location right at the edge where drops had caused it to crack and make intermittent contact. It was fairly easy to just wedge a piece of folded over aluminum foil with electrical tape to reestablish contact with the ground strap.

The circuit board was tiny and didn't seem to have more than one tiny charging IC. Most of the board was for the USB ports, indicator lights, and control button. It had a single control button that would flash the charge level lights or could be used to start the output (it didn't automatically output anything).

I don't think it was terribly sophisticated either, like with a smartphone battery. I suspect that it didn't even have any kind of onboard diagnostics with a couple of simple cylindrical cells. My guess was that the charging circuitry was very conservative to reduce the chances that it would catch fire, even if that was at the expense of not using the full capacity of the cells.

 

[Jetronic]

Lithium chargers are a bit more risky than the old NiMH or lead acid. They have temp sensing and compensation by slowing or stopping the charge rate. You should look into, at least, this.

Yes, but the charge rate is a lot lower than the cells allow for, with the power supply only 1500 mAh and the charger for the tool battery 4000 mAh

 

[Dave9]

^ Yes, there are lots of products that use a simple, single chip solution to charge, without cell temperature detection, just internal temperature protection for that chip's own die temperature, for example products that charge via USB (often it's a generic clone of an LTC4056, possibly also DW01 or similar protection IC), but since you want to have at least two cells in series to get 7.4V or more, you instead need a balancing charger circuit that senses the voltage of each cell so neither over-charges or over-discharges.

That should already be built-in to your radio since it already has two cells in series. You just have to be sure to hook back up the sensing wire between the two original cells, in between your new array of 2-cell series , and connect their mid-points together. Maybe easier to show with a crude drawing.

The exception is if they used misleading specs, that it really does have a LiPoFE4 battery and they are stating 7.4V when should really be 7.2V, as the FULL charge voltage. You can charge the radio then measure the cells yourself to see which is the case, normal 18650 tool battery would be 4.2V each at full charge instead of 3.6V.

 

[Jetronic]

I checked the sticker on on the device and it says LiPo 7.4V