Basically just after opinions on using LiPo or LiFePo4 for building a diy motorcycle battery. My current lead acid battery just died and i'm sick of replacing them after 1-2 years.
Background info - Bike is a 2007 BMW F800S with a 400W alternator and 14.4V peak charge voltage. From what I can gather online, the system puts out closer to 13V at idle and has enough current to run everything off the alternator alone at idle.
My problem - LiFePo4 has low current ratings, and LiPo has impractical cell voltages.
Doing some research the lithium motorcycle batteries you can buy online all use LiFePo4 as a 4 cell gives the right voltage range, but they seem to most commonly have a 4 series/2 parallel setup (A123 cells) which gives about 140A peak - this is below the current lead acid battery which does 200A.
LiPo on the other hand will easily do >200A from a small pack (easy to get packs with C ratings higher than 100) however 3 cell is impractical as the fully charged voltage is 12.6V (lower than alternator output) while a 4 cell pack would mean the resting voltage per cell is around 3.5V, which is close to the discharge voltage on these packs.
Use a LiFePo4 pack in 3 parallel config (along with a passive balancer) to give the current. The pack would remain connected at all times as the chemistry is relatively safe. this would mean 12 cells however which is getting expensive compared to lead.
Use a 3 cell LiPo with diode isolator. Essentially use a diode to connect the battery only when the alternator is not running, and along with this use a simple regulated trickle charge balance circuit to keep the pack topped up.
Use 4 cell LiPo and keep the cells at low charge all of the time by using a LiFePo4 passive balancer to keep them balanced at the lower cell voltage.
At this point im leaning towards trying with A123 in 2 parallel and see if it has the cranking current to start the engine easily - this is the tried and tested solution - but would be very keen to give it a go with LiPo.
The bike runs well and the alternator is fine, it just does not get ridden much at all so the lead battery discharges itself. I have a battery charger and can easily sit the battery on charge when it is not being used, but the battery sits behind a large fairing which is a pain to remove, so would rather not have to do this ever time I want to use it
As far as I am aware, the existing alternator should easily provide the current needed to keep a lithium pack at full charge, as long as a balance circuit is used (passive current shunt). I have started the bike many times off a 3 cell LiPo and to start the bike uses very little of the pack (2200mAh) so I doubt the pack would ever see a high depth of discharge