As we're on batteries, here are some thoughts. Technical stuff first, and then some concrete products to look at
So the Shorai lithiums and lithium starter batteries in general are LiFePO₄ chemistry, or lithium ferrophosphate. Sometimes called lithium-iron but that's confusing IMO as it reads too similar to lithium-ion. Iron (Fe) vs. ion. (Even hard to type it correctly one after another!)
To distinguish from LiFePO₄: Lithium-ion is what's used in laptops, multicopters, electric cars etc., where charge capacity is supremely important. Li-Ion are not suitable for starter batteries in general. (Edit - LiFePO₄ is actually a subtype of Li-Ion, so in some confusing cases LiFePO₄ batteries are sold as Li-Ion :/ ... And to further add to the confusion you CAN use the types commonly known as li-ion to start a vehicle ...) About LiFePO₄
LiFePO₄ is similar to lithium-ion overall, but lithium-ion aren't suitable as starter batteries as they are too sensitive - mechanical or internal electrical failure can and will cause fire in lithium-ion batteries. LiFePO₄ has significantly less charge-per-weight than lithum-ion (Ah per kg) and somewhat less max-current-per-weight than lithium-ion (burst Amperes per kg), but LiFePO₄ still has MUCH higher max-current capacity and also higher charge capacity than any form of lead-acid batteries.
LiFePO₄ batteries also in theory
are specified for much
more charge cycles before they go kaput compared to lead-acid, meaning that the charge-discharge lifetime is something like 5-10x greater - in theory
. This means that LiFePO₄ should
be cheaper in the long run. However they still aren't really proven
in the context of reliability as starter batteries in a hostile environment (on a dirt bike in the cold) ...
One further advantage of LiFePO₄ is that they have only very little self-discharge, meaning they mostly stay charged on a shelf or in a bike. IOW don't need a maintenance charger as badly as the lead-acid batteries.
Besides LiFePO₄ batteries not being FULLY proven in the field yet, they get sluggish in the cold. When it's quite cold they'll be slow to start. However! They warm up when current is drawn from them, and I've read multiple reports that simply having the headlight on for a short while is enough to warm the battery to where it will put out its rated current. (Charge is preserved, a cold battery just won't give it out as fast.)
Finally, like most other battery types, LiFePO₄ don’t like to go under a certain voltage threshold (undercharge). Significant undercharging can/will kill cells. Additionally it’s considered ideal to balance the voltage of the cells in a LiFePO₄ battery (not considered strictly necessary as they seem to stay in balance pretty well, but would prolong lifetime in cases where they don’t).
Often LiFePO₄ batteries are distinguished by a series/parallel scheme. xSyP means that the battery is x cells in series (x cells “high”) and y cells in parallel (y “wide”). For our purposes series height determines voltage and the parallel width determines current output. A 4S LiFePO₄ battery is the correct voltage for a motorcycle ~12V lead-acid charging system. What width you want is determined by how much current you need to start your bike, and what the cells your battery is made of are capable of. Usually for ‘Bergs you’d go for 4S2P to exceed OEM current capacity with significantly less weight. If you calculate things a bit then you can in some cases go to 4S1P to save further weight. The difference is so small that I’d go with the nice 4S2P overkill. The P value isn’t the arbiter of what the battery can do - you want to look at the “burst discharge current” rating, or similar. (Not the constant current output! This is a much lower number. If the burst current is not specified, look elsewhere.) Summary - LiFePO₄ vs. lead-acid
So to sum up - the three advantages of lead-acid batteries are lower up-front costs, fully known properties and behavior, and somewhat more peace of mind in very cold environments.
The LiFePO₄ chemistry's chief advantages are MUCH less weight for the same starting current - something like one-fifth? - plus they need less maintenance charging.
Slightly more tenuous advantages are the theoretical lower lifetime costs due to the higher charge-cycle count. And that LiFePO₄ cells can also have a greater charge capacity per weight if you're prepared to pay for that. Could be good for e.g. long-range adventure bikes.
But from what I know, I’m going to try LiFePO₄ next. Specific manufacturers, cells, and batteries
Making a REALLY GOOD LiFePO₄ cell takes some pretty advanced manufacturing capacity and R&D. As far as I know they’re actually made with nanoscale wire fleece coated with very precisely formulated materials. A123 Systems are reputed to make by far the best cells - being more reliable and have more output current and charge capacity per weight. Shorai batteries are not made with A123 cells, they are Shorai’s own (and Shorai use ‘pouch cells’ which make for a slightly smaller battery but according to specs I’ve read they aren’t as good electrically).
There are two battery series that I know of that are made with A123 cells: Full Spectrum Power’s Pulse series, and a123rc.com’s batteries. Pulse P1 | Light Weight Lithium Motorcyle Battery | Full Spectrum | fsbattery http://www.a123rc.com/
Full Spectrum Power use officially-sourced A123 cells, pack them in a strong case, and have battery protection electronics included - a board that cuts the battery out when undercharged, and they have a facility for balancing the cells (nice to have). All very good ideas according to what I know. They’re somewhat expensive though! BUT if these batteries will actually last as long as LiFePO₄ batteries are supposed
to last - 5x as long iirc? 10x? - then they’re good value. The Pulse P1 batteries are 4S1P, somewhere around 125 CCA. Pulse P2 is double that. Given that these are high-quality cells, you can probably trust the CCA rating pretty well.
A123RC are supposedly selling not-quite-A-grade batteries that have failed the most exacting tests at the factory. This means slightly less capacity, 5% iirc? That’s OK anyway as you’re already tens of percent above OEM lead-acid current capacity to begin with with most LiFePO₄ batteries … And still, they’re A123 cells which are very well made to begin with. They can be had in a hard case, but they lack undervoltage protection and don’t come with a cell balancing tap. (A123RC do make custom batteries though, so maybe they can be had with a balancing tap if desired … I’m not 100% convinced it’s worth it. If it cost little, then yes.) The A123RC’s are quite cheap! The site seems down atm, but iirc they had 4S1P ~125 CCA batteries for something like $50-60! Which is cheap enough to try … Just need to make sure they fit the battery box - I’m pretty sure they would, as LiFePO₄ batteries are quite small to begin with. Can be a little tallish though, so need to check specs.
Hope this is useful to someone