Existing batteries: 5 units of 12V 20Ah sealed gel lead acid.
Replacement battery: 60V 18Ah lithium ion battery pack made up of 18650 lithium ion cells of 3.6V each. I have assumed of maximum 20A discharge rating of these 18650 cells (to be confirmed by lipotek)
The 60V 18Ah lithium ion pack was unable to work on its own with the motor and controller on the Vortex TWA and kept cutting out. This is probably due the 18650 cells unable to maintain a constant 60V under heavy load conditions.
My solution is to use the lithium ion pack in parallel with the sealed lead acid batteries.
I have fitted a switch to allow each set of batteries to be charged independently. The obvious overhead with this that you have to charge two different batteries with two different chargers (not shown in the diagram). The lithium ion 60V battery pack has to be charged at a very specific constant voltage hence you cannot use
the 60V acid battery charger and the reason for installing the switch to isolate the two batteries from each other during the charge process. See diagram below.
Note: I tried a 10A diode before I installed the switch. It worked in principal. However on one long journey to SÃ£o Bras do Alportel it became very hot and exploded. I was not best pleased with the walk home. Fortunately there was a lot of downhill bits so this helped. It maybe possible to use a diode that has and integral heat sink but this would have to be mounted on part of the metal frame to conduct the heat away. The switch is a simpler solution and doesn't have any internal resistance once the contacts are closed.
I think it is possible to install much smaller lead acid batteries as they are only necessary to prevent a sudden voltage drop under initial load take-up. My current 20 Ah lead acid batteries are chemically exhausted and the bike can only run 3 km on these. I intend to replace them with 5Ah lead acid batteries and save on the weight.
In conclusion , the lithium ion battery pack weighs less than 7 kg. Each of five 12V 20 Ah lead acid batteries weigh 7 kg. each. With the 18 Ah lithium battery pack I can easily do a 30 km trip. So I am overall very pleased with this battery pack built by lipotec.