DIY high capacity battery pack for Logitech's Squeezebox Radio

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If you enjoy the Squeezebox line of products like I do, you probably already know about Logitech's Squeezebox Radio.  I've just added a third radio to my collection (shown here with cat 1 of 2).  However, if you don't yet have a battery pack for your Squeezebox Radio you are missing out on one of it's coolest features (it's portable!).  Of course, it still must be in range of a wireless signal (all the more reason to get a really great wireless home router).  Unfortunately, the battery pack must be purchased separately for $50.  Logitech does bundle it with a mini-remote to help make it a bit more tempting.

Since the accessory pack wasn't released until well after the radio, many adventuresome owners went ahead and built their own.  Now that the pack is available, there is less reason to do it yourself.  You can still build a pack cheaper yourself (depending upon the cost of the batteries), but you might not save that much money (and it is a hassle).  The main reason for doing it yourself now (IMO) is that you can build a higher capacity pack.  Logitech claims 6 hours of battery life on their pack, but people have reported up to two hours more - not bad.  However with my pack, I've gotten just under 11 hours!   More importantly, doing it yourself is fun!

Here is what I did:

Please note: if you follow these steps, do so with care.  Standard disclaimer:  I'm not responsible for any damage or injury that might occur...

Getting Started

Besides basic electronics tools, wire, solder, etc., you will need a battery kit and some batteries.

The kit for building the battery pack consists of the battery connector, a thermistor (for measuring temperature), and instructions.  You can order the kit from Small Green Computer for $7.99 (including shipping).  These aren't expensive parts, but this seems a very reasonable price.  Plus, they sent me two connectors so I could actually build a second pack if I bought another thermistor myself.

Which batteries to use?

The pack needs 10 rechargeable AA NiMH (Nickel Metal Hydroxide) batteries.  Logitech's original pack contains batteries that are rated at 2000 mAh (milliamp hours).  mAh is simply a measure of battery capacity (more is better obviously).  Logitech's batteries are also a special type called LSD (read all about low self-discharge here).

LSD NiMH batteries (first introduced in 2005) are just like regular NiMH batteries except that they will tend to hold their charge much longer when not used (a very good thing!).  This effect is much more pronounced as the batteries age (have more charging cycles).  LSD batteries are reported to be able to hold a significant charge even after a year (when regular Nimh batteries would probably be nearly dead).  Regular Alkaline batteries don't have this problem (though they do leak eventually!).  Now you know why regular rechargeable batteries are a bad choice for an emergency flashlight!

In addition to LSD NiMH batteries, there are alternatively much higher capacity NiMH batteries (up to 2700 mAh).  However there doesn't yet seem to be a battery that is both high capacity and LSD.  So you need to choose one or the other.

For a really great discussion of NiMH batteries that compares the various brands and actual measured performance see this NiMH battery shootout over at candlepowerforums.

A good example of high quality LSD battery is the Sanyo Eneloop.  These cells are rated at 2000 mAh.  The best price I found was a 16-pack new version Sanyo Eneloop for $40.  I suppose you can always use extra batteries around the house so this is a good value.  If you go with the Sanyo Eneloop purchased elsewhere make sure it is the new version (which was just released).

A good non-LSD higher capacity battery is the regular Sanyo 2700 mAh.  The cheapest place I found them was from BestByte.net at $5.99 per 2-pack (where I bought mine).  You can also order them from Amazon in 2-packs, or 4-packs.

Finally, if you are going to build a battery pack, they do make special versions of batteries that have solder tabs (pre-attached wires).  This makes it much easier (and safer) to wire the batteries together.  However, finding the battery you want, cheaply enough, with solder tabs is a challenge.  Here is one source for the Sanyo 2700 mAh batteries available with solder tabs (Batteries America at $3.75 per cell).  If you buy from them, be sure to select solder tabs as they aren't the default.

You also have the option of buying off-brand batteries that are probably significantly cheaper.  I have nothing against this, but keep in mind your time and effort in putting together the pack.  For this reason alone I wanted the best high quality battery I could buy (for a reasonable price)!

Finally, you might also be able to find a complete pack already built up (many battery stores on the net have 10-packs available).  You might need to modify it some, but this is an option too.  However, I couldn't find one that was both cheap enough and had the batteries I wanted.

I ended up selecting the regular Sanyo 2700 mAh batteries because I don't really care about the LSD aspect of these batteries for my purpose (I'm sure the radio will be mostly plugged in).  I'm more than willing to trade off LSD for longer battery life.  Finally, I went with batteries without tabs simply because they were cheaper and I knew I'd be VERY careful soldering them (more on that in a minute).

Based on what I read about LSD NiMH and higher capacity batteries, I was willing to gamble that the charger in the SqueezeBox Radio would still work.  What is the fun of a project without a little risk?

My Completed Pack

No it isn't "pretty", but it will be hidden from view once installed.

Let me point out some non-obvious things about the pack before I get into how to build it.

The red arrow shows the thermistor that measures the battery pack's temperature.  This is important as the charging circuit (algorithm actually) needs to make sure the pack doesn't get too hot.  Also, some charging algorithms use the temperature to determine when the pack is fully charged (more on that in a minute).

You can't see the thermistor here because it is taped to the side of the battery in the middle of the back (to better measure the "core" temperature.

Here is another shot of the pack from the other side:

Note the red arrows again.  These show two additional battery "taps" measuring intermediate voltages in the pack (reported by the Radio as vmon1 and vmon2).

Other than the thermistor and two battery taps, the pack is simply 10 batteries connected in series to deliver 12 to 14 volts to the Radio.  Note that a NiMH batteriies' voltage varies with their state of charge (just like any battery).

Note that I've read that the original battery pack has a fuse of some kind (possibly current or temperature actuated).   I couldn't find any details on this fuse and it wasn't mentioned in the kit or by any of the other DIY packs.  Frankly, this does worry me a bit.  If anyone has any details on this please let me know!  I'll add it later if I can.  Safety is important!

Next, you have the connector.  This is a tiny 5 x 2 pin connector with crimp on pins.  Did I mention that it was TINY?  I had a heck of a time crimping on the wires, but your eyes may be better than mine.  Just take your time.  I also soldered on the wires to the pins to make extra sure that I had a good connection.  Just be careful as too much solder will melt the insulation and prevent the pin from going in the connector.  Soldering wouldn't be necessary if you are better at crimping than I am.

Finally, there is a REALLY TINY tab on each of the pins that locks them into the connector.  Be sure not to smash this down as you crimp the pin (I did this on the first pin) otherwise it won't stay on the connector.  Fortunately, my kit came with extra pins.

Lastly you have the challenge of soldering to the batteries themselves.  I chose to use plain old solder wick as a convenient jumper (thanks Kevin for this and other tips!).  Solder wick is easy to solder (obviously), conducts well, and probably helps keep the soldering iron heat off of the battery.  It also looks just like a ground strap.

To make the soldering easier, I filed (roughed up) the surfaces of each side of the battery.  I then quickly pre-applied a bit of solder to the battery at medium heat.  I found that if the heat was too low the solder took too long to flow.  Then I soldered the solder wick to the battery by applying the iron for a couple of seconds (or less).   Use at little time as possible to avoid damaging the battery.

NOTE:  The top of the battery (positive side) has vents along the button at the edges.  BE SURE not to fill these with solder or rosin.  This is a battery safety mechanism that prevents too much pressure from building up in the battery.  Closing this off would be most likely VERY BAD.  If this happed by accident, I'd throw away (recycle) the battery instead of using it.

TIP:  Practice doing the soldering on cheap or dead alkaline batteries first until you feel comfortable that you can solder them quickly and without excessive heat on the battery.

IMPORTANT SAFETY TIP:  Wear eye protection when soldering.  You might have your eyes up really close to the batteries.  If something bad were to happen because of the heat, you want your eyes protected!

Other assembly tips

• The thermistor just has bare leads so I used heat shrink tubing to prevent shorting.
• I used 3M double stick tape to get the batteries to stick together and electrical tape to wrap the whole pack together.
• I routed the wires through the tape to create a crude strain relief for the solder connections.  It isn't bullet proof, but sufficient as long as you are careful.

All done

Here is the battery pack all tucked into it's compartment.  Note that there isn't much room to spare (which is why you can't use much more than tape to hold the pack together.

Note that I didn't bother insulating the sides of the battery pack itself.  The battery compartment of the Radio is plastic so it won't short anything out.  Since the pack will spend it's entire life there, I saw no need for extra insulation.  Besides, if I have to take apart the pack later it won't be covered in goo from old tape.

Charge it!

What would be the fun of this project if you couldn't get all kinds of GEEKY numbers from the charging circuit / algorithm itself?   On the squeezebox radio simply go to Settings->Advanced->Diagnostics->Power and you will see a screen with tons of info on the battery.  It reports battery voltage, wall voltage, battery tap 1 and 2 voltage, battery temperature, power mode, battery capacity (as a graphic) and the charging state.  Is this cool or what?

Even though we don't have control over the charging circuit, it is really interesting to read about how sophisticated NiMH chargers have become.  Turns out that detecting when a battery is full isn't so simple.  The charging circuit must look at the temperature, change in temperature v.s. time, voltage, change in voltage v.s. time, and/or the charging current.  There are also fast chargers and slow (trickle) chargers.  Often the faster you charge a battery, the less life (charge cycles) you will get.  Also, if you over charge your batteries (or get them too hot), you also reduce their life (or risk them exploding).  Batteries do have safety mechanisms built in (like the vents on the top of the cells), but they aren't perfect.

The developers at Logitech have hinted that there is a fair amount of code dedicated to the charging circuit so I have no doubt that the algorithm is pretty sophisticated.

Here are some good links for reading about how some NiMH chargers work at PowerStream and BatteryUniversity.  This is great background info for interpreting the data that the Squeezebox Radio reports via it's diagnostic screen.

So, how did my pack perform?

First of all, I had pre-charged the batteries before assembling the pack.  After installing the pack (without A/C power), the Radio booted right up and worked (always a good sign).  All of the voltages, etc. reported by the radio looked fine.  I then plugged in the A/C adapter and the battery started charging (topping off since the battery was mostly charged).

The next morning I started a playing music on battery only.  I streamed FLAC files continuously from my local PC to see how long it would go.  All day long it played without a problem.  It finally shut off after about 10 hours and 49 minutes (not too bad!).   I did notice that the battery indicator dropped down quickly to about 1/4 full within a few hours.  However, it just kept playing.  I'm assuming that the battery graphic is calibrated to the lower capacity 2000 mAh battery pack so may not be entirely accurate anymore.   From what I can tell from 2700 mAh batteries, their voltage drops quicker (initially), but holds longer than a 2000 mAh equivalent.

After the Radio died, I plugged it in to see how the charging worked on a (nearly) dead pack.   I'm sure the batteries weren't completely dead as the Radio's circuit would protect against damaging the pack.   Anyway, charging went very slowly over several hours (slow charging is better for the battery life).   The packs' temperature eventually rose to about 45C (113F), at which point the charging was paused by the software (presumably due to a temperature threshold).  The pack wasn't yet fully charged, but based on the reported voltage of 14.4, I'm assuming it was pretty close.

Afterwards, charging resumed and the radio reported a full charged pack.  I'm assuming the lower capacity Logitech pack might have fully charged in a single cycle, but I don't know that.  All in all I'm impressed with the intelligence of the radios' charging circuit and that it is working so far with my custom pack.

Reference

There are lots of threads on  forums.slimdevices.com that discuss DIY battery packs, but this one has the most info "Sqeezebox Radio Battery Kit Available".