Lithium Batteries for 6V Vintage Bikes

By LoneStar

 

Most British bikes prior to 1960 were supplied originally with hard-rubber case 6V batteries from makers such as Lucas, Exide and Varley. These are usually visible, unlike on later bikes. Original batteries are difficult or impossible to find these days, and simply installing a modern one looks a bit goofy. A common solution is to use either a gutted original battery or a replica case, with a modern battery inside.

This has been my practice, and it works reasonably well. As the modern battery has to be smaller, to fit in the case, it usually has lower capacity than the original - but this rarely causes problems. I've used the Powersonic PS-682 (6V 9AH) with good results, and it fits easily into a standard hard rubber case. This is an Absorbent Glass Mat (AGM) sealed battery, mostly used for alarm systems.

While AGM batteries hold their charge better than traditional flooded lead-acid ones, they still require periodic recharging if the bike isn't ridden. If you forget this and the voltage drops too low, the battery will not recover. Also, vintage bikes sometimes vibrate a bit (!), which can cause internal damage. This happened to me recently on a BSA Gold Star - no power, but it would come back briefly if you slapped the battery case and restored the internal connection.

So, I thought it would be interesting to explore an alternative to the AGM, namely lithium iron phosphate (LiFePO4) batteries. The idea was proposed on the VOC forum by user "oexing". Note these are a different chemistry to the lithium ion batteries used in cell phones and elsewhere.

Important differences:

  • Lithium ion batteries are known to catch fire if abused; this is much less likely with LiFePO4
  • Lithium ion batteries are 3.7V per cell - so the closest you can get to 6V would be 7.4V (2 cells in series). This is too high for motorcyle electrical systems. LiFePO4 are 3.2V per cell, so 2 in series will be 6.4V - just right.

There are several advantages to LiFePO4 batteries over AGM:

  • More robust, with no internal lead plate connections to break
  • Lower self-discharge, so can be left longer without trickle charging
  • Not damaged by deep discharge, like an AGM battery
  • Greater capacity (AH) for a given size
  • More stable output during discharge - voltage does not decline rapidly as with AGM batteries
  • Lighter weight

All very promising, but I was unable to find a suitable 6V LiFePO4 battery. Antigravity makes one that might be squeezed into a vintage hard-rubber case - but a 6V 10AH version retails for $180. Being cheap, I decided to make my own.

I chose LiitoKala 70A 32700 LiFePO4 cells - 32mm diameter x 70mm long. These are 3.2V and claimed to be 7AH capacity, though a more conservative estimate would be 6AH. An array of 4 cells wired as sets of 2 in parallel, with 2 of those sets in series, results in a 6.4V 12AH battery - perfect. Note you could probably get by using only two cells instead, wired in series. The resulting 6AH would be adequate for most use on an old bike with limited electrical draw. But with plenty of room in the case, and not much extra expense, I opted for the higher capacity of 4 cells.

The 32700 cells are available in the US, but scarce - I found it easier to buy them from the "liitokala Official Store" on AliExpress in China. Four cells with shipping cost $26 (as of late 2023). While you can buy individual cells, I chose an array of 4 connected in parallel, thus rated as 3.2V 28AH. The advantage is that the cells are connected with welded-on nickel strips, making it easier to wire them.

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Construction

The battery consists of two pair of cells wired in series, each pair consisting of 2 cells wired in parallel with the original nickel strips.
  • Cut each nickel strip in the original array in the center, leaving two sets of two cells each still connected by strips at each end
  • Arrange the cells as shown, noting polarity: positive is indicated by a white ring
  • Connect two nickel strips together with a short wire (16ga or heavier) as shown
  • Connect a lead to each of the remaining two nickel strips, for the output

I inserted a small tube in the center of the four cells, to hold them in a square alignment, then wrapped the assembly in 150mm PVC heat shrink tubing (available on Amazon).

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Battery Management

Standard practice with lithium batteries is to incorporate a Battery Management System (BMS). These are small PC boards wired into the battery to protect it from overcharging or total discharge. I chose not to use one of these, since the bikes involved have electronic regulators (V-Reg2B). This unit limits charging voltage to 7.2, below the safe limit for LiFePO4 cells. Maximum safe charging current for two of the cells in parallel is 13A, well in excess of the 10A maximum output of a Lucas or Miller 6V dynamo. So, the only benefit of a BMS would be if the battery voltage fell to 4V - at which point it would be automatically disconnected, to prevent further discharge. I decided this was an acceptable risk.

If you prefer to use a BMS, they're also available on AliExpress for around $3 - look for one labeled "Lifepo4 2S 6.4V 15A".

Does it Work?

So far, yes. It's installed on a Vincent Black Shadow, and I now have several hundred miles in. The dynamo is an original Miller D6, rated 50W, with a V-Reg2B 6V electronic regulator. Electrical draw while riding is pretty low - an LED headlamp (10W) and a tungsten tail lamp (8W). The ammeter settles precisely on zero and stays there, indicating a fully-charged battery and all current being supplied by the dynamo.

What about 12V?

Many people have converted to 12V, to support electric start or higher-power lighting. The latter is less incentive now that bright 6V LEDs are available, using much less current.

The scheme outlined here could be used to build a 12V LiFePO4 battery, but I haven't tried it. There isn't room in a hard-rubber case for 8 32700 cells, so it would be necessary to use 4 cells in series for 12.8V with 6AH capacity. This might be fine for many setups, though certainly not for an electric start. Or possibly smaller LiFePO4 cells can be found, to fit 8 cells in the box (4 series x 2 parallel) and give greater capacity.

 

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Copyright 2023 by Dave Hartner