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Lithium-ion batteries - some hands-on testing

by Peter Whatley
Lithium-ion batteries are nothing new, but in the past couple of years their capacities have increased and pricing dropped significantly, at least as far as eBay supply is concerned.   After chatting to one of my "battery power only" friends, I decided to take the plunge and acquire a generic 12 volt Li-ion "block" battery from one of the numerous Chinese vendors on eBay.   I describe the item as a "block" since that's what it looks like.  There are also Li-ion 3.7 volt cells on sale which look much more like a conventional AA 1.5 volt cell.  Please note that you MUST acquire a dedicated charger for Li-ion batteries.  It is unsafe to use a charger designed for other types of battery (NiCd/ NiMH) to charge Li-ion cells.
Typically, the items on offer on eBay are decribed as 12 volt cells for CCTV applications, rated at 4800 or 6800 milliamps (mA).   I decided to acquire a 6800mA item complete with charger at a price (May 2011) of GBP14.69, which amazingly included airmail postage to the UK.  The odd price is due to conversion from the supplier's quoted price in Australian dollars, which I guess shows the primary target market!   Delivery was quoted as 11-18 days from China and the item duly arrived in 15 days.   Due to the item's low value, no UK customs duty or taxes were due on the item. 
The pictures illustrate the size of my "block" compared to an equivalent voltage of NiMH low self discharge cells:
The "block" weighs 195 grams, compared to 310 grams for the NiMH cells, which obviously is a benefit if used in a trailing vehicle.  Its shape is equally clearly a bit of a poser to fit into a loco, though it's certainly small enough to fit into some LGB single-motor locos, even if only in the cab.  I should add that this particular battery is in a rigid plastic case which is claimed to be waterproof.  I don't intend to dunk it in a bucket to test that, but it certainly looks resistant to the odd drop of rain.  The keen eyed will note that it comes ready fitted with an on-off rocker switch.  That is illuminated by an LED when the battery is switched on.  My battery has a single socket for both normal use and charging, which would mean fitting a DPDT switch to create two leads (one r/c feed, the other to a charging point) if the battery was to be hidden in a loco. Other varieties of Chinese Li-on cells have two leads to spearate the feeds for charging and r/c.
On arrival, the battery was measured at 10.78 volts.  It was then charged overnight.  The supplied charger has an output of 350mA and the instructions state that charging will take "at least 8 hours".  Fortunately, the charger switches off automatically when the battery is fully charged.   During charging, neither charger nor battery became hot, in contrast to typical NiMH cells.
The testing train was then assembled, with the "block" fitted in the trailing van in place of the NiMH cells.  I should mention that my line includes R1 (300mm radius) loops at each end and a lengthy 1 in 20 (5%) gradient.
Testing has exceeded all my expectations.  The test train ultimately ran for 8 hours and 25 minutes, with four breaks for weather.   The battery voltage as measured at various times was as follows:
Test start (fully charged) - 12.60 volts
3 hours 30 minutes run - 11.38 volts
6 hours 00 minutes run - 11.14 volts
7 hours 20 minutes run - 10.77 volts
8 hours 25 minutes run - engine stopped!  (battery output showed 2.97 volts)
As a comparison the 10 NiMH cells, rated at 2100mA, can power the same train for 3.5 hours.   On flatter, less curvy layouts, I'd expect both battery types to permit useful work for somewhat longer.
After the test, the battery was placed on charge again.  I confess that I forgot about it (!) so all I can say is that the charger automatically shut itself off after somewhere between 6 and 10 hours charge time.  After this second charge, the battery voltage was back up to 12.63 volts.  All entirely satisfactory and with no drama whatsoever.
In summary, this lithium-ion battery has so far proven to be a remarkably cheap and highly effective means of getting battery power.   I commend it to you!