All the self-proclaimed geniuses here can continue living in their safe spaces of delusion; for the rest:
Alternator Considerations for LiFePO4
If you decide on LFP, it is best designed and installed as a
system. A good system design will almost always include an alternator installation that is suitably designed for that part of the charging system.
Due to the extremely low internal resistance of LFP batteries, and the extremely flat voltage curve, LFP banks can easily tax an alternator to death, if not properly installed. The high acceptance rate of LFP batteries will force the alternator to be in BULK charge mode for the vast majority of the charge cycle, (depends upon amperage/size) before even attaining absorption voltage.
With a high current alternator on lead acid, you can hit limiting/absorption voltage as low as 50% SOC where the alternator begins to catch a break. Alternators for lead acid batteries don’t always need the same level of protectiong from burning out than do alternators charging LiFePO4. With LFP your alternator simply won’t catch a break.
If you cycle the LFP bank to 80% DOD this means you are in BULK charge mode for approx 75% or more of the capacity of the entire bank bank before any sort of voltage limiting even begins.
BULK CHARGE:
This means the alternator has not brought the terminal voltage of the battery bank up to ABSORPTION or the
limiting voltage. In BULK the alternator is working
flat out,
as hard as it can in what is referred to as BULK or CC.
(CC = constant current). Once the bank comes up to ABSORPTION VOLTAGE we switch to ABSORPTION or CV
(CV = constant voltage). In absorption/CV is where voltage is held steady and current begins to taper off based on what the battery can accept at that SOC and voltage. This is where the alternator finally catches a break but with LFP this duration is very short and only at teh very top of charge.
In BULK / CC the alternators capacity/ability is your limit.
In ABSORPTION / CV the battery determines how much current can flow at a specific SOC and terminal voltage.
Take a 400Ah LFP bank at 80% DOD, that means 320Ah that need to go back in. With a 130A rated alternator running hot, at about 100A, this means BULK charging will be about
three hours long. There is no *small-case alternator on the planet I know of or have tested that can run at full bore for three straight hours, into an LFP bank, inside the typical engine room on a boat. *Unless the rectifier has been mounted externally with its own cooling fan.
Let’s assume you’re a marathon runner, and you can do the 26 miles at a pretty good jog. This is similar to a high capacity alternator feeding a large lead acid bank. You start out strong (BULK/CC) but as the race goes on you plateau & settle in at a sustainable pace (ABSORPTION/CV).
An alternator feeding an LFP bank is like trying to SPRINT the entire 26 mile marathon. Not going to happen….
FACTORY ALTERNATORS
Some factory alternators have a built in temp compensator and it resides in the voltage regulator circuitry to reduce current / voltage as the alternator heats up. This really defeats the purpose of “charging fast” or even having an LFP bank if you want to capitalize on the fast & efficient charging LFP batteries can offer.
While this alternator temp compensation feature is
self protective of the alternator, in theory at least, it is really a very poor regulation choice for an LFP bank. The other conundrum is that the voltages and temp protection features in these internal regulators are based on lead acid voltages, not LFP. With LFP they can literally cut back so much, due to heat, that little to no current can flow into the LFP bank. We have measured Yanmar / Hitachi alternators so hot they have reduced the voltage output to 13.2V. Considering the resting voltage of an LFP bank is higher that, well. This means little to no charging. Discharge the bank deep enough and even these internally temp protected alternators will literally cook themselves. Bottom line? Do it right and include a performance alternator, regulator, temp sensors and pulley kit (for anything over 100A) as part of your “
system“..
DIY LiFePo4 On Boats Last edit;3/26/23:The base of this article was written a number of years ago (2010) but this does not mean the information here is outdated. We have been keeping it updated and have added to it when ever we had the time. This article deals
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