Charging lithium from the Alternator

[mistericeman]

From memory Average tick over speed of the alternator is usually around 2500 rpm

Most are rated to spin up to 15,000

 

[colinm]

Isn't that what he said Colin?

Yes, I was halfway through posting when I got a phone call and hit reply too early, soo to finish my post.

A smaller pulley on the alternator turns it at a higher speed, to work out the speed differance you need to compare the circumference, next time I'm under the bonnet if I remember I'll measure the pulley's to get an idea of speed.

 

[merl]

Having done the research rather than relying on widespread belief I can say that charging a lithium battery via an alternator or lead acid mains charger in your motorhome is usually completely OK.
It's widely touted that
'Lithium could cause excessive current because the internal resistance is massively lower than lead acid'
This is completely false, a 100Ah LFP battery is typically only around half the resistance of a new 100Ah lead acid, this means that 2 new 100Ah lead acid batteries would have a similar internal resistance of a 100Ah LFP battery, do people question or fear their alternator or wiring burning out when fitting 200Ah of new lead acid batteries into their MH? NO! It's been done thousands of times.
Secondly the difference in internal battery resistance is massively outweighed and swamped by the wiring resistance because MH wiring resistance is typically at least 10 times higher than battery resistance.
Thirdy, whilst charging, lithium battery voltage is nearly always higher than LA, this voltage increase actually reduces charge current in LFP compared to LA, so typical alternator charging currents are lower for lithium than new lead acid.
WRT damaging lithium by leaving them connected to a charger or alternator while they are fully charged (float charging lithium). Again the widely held belief and 'expert' advice is in fact massively overblown and inaccurate and 'floating' lithium with an alternator or mains charger would have very little negative effect over time considering typical MH use.

 

[Geek]

Thirdy, whilst charging, lithium battery voltage is nearly always higher than LA, this voltage increase actually reduces charge current in LFP compared to LA, so typical alternator charging currents are lower for lithium than new lead acid.

Although I think your other points and your conclusions are correct, this point is not, according to my experience.
I have a li-ion battery running in parallel with two agm batteries. They are charged by the same b2b charger, but I monitor the battery terminal voltages and the currents in and out of each battery bank separately.

Typically, when they are on charge, the Li-ion battery takes the lion's share of the charge, but due to voltage losses in the charging cables (they're only 25mm) the li-ion battery is at a lower voltage than the other two, not higher.

When the charger gets past the bulk stage, the Li-ion BMS starts to have an influence and the battery voltages tend to even out.

After the charging stops, there is generally a bit of current flowing from the Li-ion into the AGMs, and when charging resumes, the Li-ion gets more current again.

They work really well together. The Li-ion tends to do almost all of the day-to-day work, but after a long time (several days) of draining with no charge, when the Li-ion is getting empty, its voltage drops to where the AGMs are able to help.

Note that my 12v use is all low current loads. Nothing big. The biggest is probably the water pump, which doesn't run often.
It might be different if I used a big inverter or a 12v fridge.