Saturday, 28 September 2013


As I've mentioned before, I have a very nice GRP battery box from Draganfly, made to look like a Lucas GU11E battery. Here it is:

Now, the original GU11E apparently has a 20 Ah capacity, which is the peculiar way in which our industry has chosen to illustrate the energy capacity of a battery - it means it is capable of delivering a current of 20 Amps for an hour, or 40 Amps for half an hour, or 10 amps for 2 hours - you get the idea.

Now an Ampere, or Amp for short, is a unit of current, and current is the rate of flow of electrical charge - measured in coulombs. So an Amp is a way of describing coulombs per unit of time - 1 amp represents 1 coulomb per second of flowing charge. So, if you multiply Amps by time (As in Amp x hour = Ah) , you get back to charge again - and units of charge are what is stored in our batteries.

So, in order to get my bike to run for a reasonable period of time without being charged (for when I choose to ride at 10 mph in a traffic queue on the M25 (when did you last see a Square Four do that?)) I need a battery of sufficient capacity to deliver sufficient current for the time I am riding so slowly that the battery is not charging, without running flat and bring the bike to an undignified halt.

You see, according to this chart I draw from the data in the C35SD dynamo instructions combined with some details of the Square Four's performance (and assuming the C35SD runs at engine speed, which it does, more or less) the dynamo does not start charging until you are moving at about 20 mph. So, go slower than this and the battery is draining.

But how much current do I use? Let's work it out:
  • my headlamp will use up to 55W, which at 6V needs just under 10 Amps to deliver the power (Power, W = Potential Difference V x Current A)
  • my tail lamp will use 5W, which needs another Amp
  • Lets assume the ignition coil will need 20 W, or about 3 Amps
Which gives us around 15 Amps, allowing for infrequent use of the stop light and horn. A 20 Ah battery will be flat in 1.334 hours, or 1 hour and 20 minutes. Reasonable? Lets hope so.

So we know what we are aiming at - a 6 Volt battery with capacity of around 20 Ah that will fit in my neat little fibre glass box. Since a year or so into having the Bantam, Beattie has been using a sealed lead-acid battery - the electrolyte is treated to form a gel, so it can't flow out, and the chemistry is altered somehow (I'm not that clever) so that when the battery is charged and would normally produce free oxygen & hydrogen, the gases recombine as water, replacing the lost liquid in the cell. This is really neat for a motorcycle, because it means you can fit them wherever you want (they are really small), they don't drop nasty acid all over your bike (how many rusty battery carriers have you seen?) and because they are made for things like burglar alarms, they are really cheap.

The downside is you cannot abuse them as you can a traditional wet battery - if you overcharge them and lose the electrolyte, you can't replace it.

The thing is, I came to use one of these things the first time because I realised the rudimentary (putting it kindly) charge control system on the Bantam was boiling the original wet battery - so I added a modern solid state regulator/rectifier (from the marvelous Rex Caunt Racing) and was able to use a gel battery. The one on the Bantam is tiny - about 3.4 Ah I think, at 12 V, and has coped perfectly for several years. It even remains charged through the winter, something the old wet battery could never do.