This does seem like a great idea, but for couple possible concerns:
- Charging a lead acid battery is less efficient than lithium. IIRC only about 80% of the charge to a lead acid is actual charge, and that number lessens when reaching the float stage. And if you're not floating a lead acid, it's life is going to be shortened.
- Do you know that the 5S Amazon BMS is a good one? There are a lot of cheap BMS out there that fail on short notice. And then since you're relying on the BMS to manage the battery, if it goes kerplunk, you likely won't know of any battery issues until it's too late. Obviously LiFePO4 is much more robust but it won't be good for the life of the battery. From what I've seen, a good 4S-5S BMS is anywhere from $40-$70. Then again if it were to fail, it would be much cheaper to replace the battery than to spend $ on a higher quality BMS.
My thoughts are:
Why not just replace the lead acid with the lithium, and use a switch for the alternator like you've been doing? There would be fewer charging inefficiencies due to going from battery to battery, and you'd also save some weight by eliminating the lead acid battery altogether.
....but that's just me being an extremist ecomodder, where if I do an ecomod I have to go all the way for efficiency and leave nothing on the table (80 PSI tires..
). Other than that it seems like a great idea. Maybe replace the normal 12V starter battery with a small 12V lithium battery in addition to this? but then most small lithium batteries don't have the cranking amps available.
This would definitely be a much simpler way of doing it, although I surmise that the efficiency gained would be slightly less than doing a direct lithium replacement and alternator cutoff. But since we're talking about, what, a 5%-10% gain from deleting the alternator, the difference between these methods is likely <1%.
Here is a 32.95V 2.9ah pack
with a built in BMS for $25. Considering 90% usable capacity @ 14.4V gives around 6ah of capacity. Two of them brings you to $50 for 12ah@14.4V and you don't have to worry about rigging up your own BMS. A cheap DC-DC converter to trickle charge your 12V battery from the larger pack.
I'm not too sure about a PWM voltage regulator. I really don't know anything about them other than that I've read that PWM solar charge controller's lower the voltage but don't up the amps (i.e. a 18V/2.75A/50W solar panel through a PWM charger puts out 2.75A@14V (38.5W), which wastes over 20% of the energy as heat. Now I have no basis for assuming that other PWM voltage regulators work like that, but that's just what I've read. A small, 100W DC-DC converter isn't that expensive and usually much more robust.